Plants constantly encounter harsh environmental conditions throughout their lifespan. Among these conditions, drought stress is the primary abiotic factor hindering plant growth, development, and productivity, threatening crop production (Martignago et al., 2020; Bashir et al., 2021). Drought is often defined in a meteorological sense to mean a period of below normal precipitation that restricts plant growth and productivity in a natural or agricultural system (Boyer, 1982). However, drought stress (more relevant to plant physiology and referred to herein) is a different concept which refers to a plant water status when there is reduced water available for the plant, due to a decrease in water potential (ψw, or the free energy of water) and turgor, that is enough to disrupt the normal plant physiological functioning (Kramer and Boyer, 1995; Osmolovskaya et al., 2018; Fradera-Soler et al., 2022). Therefore, drought stress responses often mean responses to an altered plant water status due to reduced plant available water (as water acquires a lower free energy state in relation to unstressed conditions) (Verslues et al., 2006; Juenger and Verslues, 2023). Similar to plant cell turgor, drought stress is often quantified in pressure terms (as a decrease in ψw) (Kramer and Boyer, 1995; Verslues et al., 2006), permitting an assessment of the two to be made to establish the plant-water relationship and the direction of water movement along the soil-plant-continuum (Juenger and Verslues, 2023). Meanwhile, decreased ψw complicates the ability of the plant to take up water, which in turn prompts a repertoire of responses that enable the plant to avoid water loss, permit continual water uptake at decreased ψw, or help the plant to tolerate a state of reduced tissue water content (Verslues et al., 2006; Osmolovskaya et al., 2018). The severity of drought stress on a crop depends on its intensity and duration, genotypic capacity of species to resist, the plant developmental stage at occurrence, and the plant tissue affected (Zia et al., 2021). Whilst “drought” naturally causes drought stress over time, a short period (of just few days or even one day) without water may be sufficient enough to cause water deficit (drought stress) that negatively impact yield (Shao et al., 2009; Cluzel, 2024; Vadez et al., 2024), especially if it occurs at the critical stage of plant development such as anthesis or grain filling stages in maize (Vennam et al., 2023). The physiological effects of drought stress on plants include decreased leaf water potential, loss of cell turgor, disrupted plant water relations, reactive oxygen species (ROS) over-accumulation, impaired photosynthesis, inhibited cell growth, impaired metabolism of cell wall components, compromised stomatal functioning, etc. (detailed in (Le Gall et al., 2015; Osmolovskaya et al., 2018; Gupta et al., 2020)). In concert, all these factors retard plant growth and development, and decrease crop yields. Therefore, the increased drought incidences and severity associated with climate change (Osmolovskaya et al., 2018; Means, 2023), often causing moderate to severe drought stress that limit crop productivity and threatens global food security (Vadez et al., 2024), motivate the need to develop drought stress-tolerant crops for sustainable food production. This depends on first gaining a mechanistic understanding of how plants respond and adapt to such stress (Zhang et al., 2022).

Plants respond to drought stress by evoking elaborate cellular, physiological, biochemical, and anatomical changes, including cell wall modifications, root architectural and biochemical adjustments, phytohormonal elicitation, etc (Gupta et al., 2020; Seleiman et al., 2021). These morpho-physiological responses are tightly regulated by genetic and molecular mechanisms (Bashir et al., 2021; Liu and Qin, 2021). In particular, the cell wall is a complex and dynamic entity whose properties are tightly regulated via cell wall remodeling (see Table 1 for definitions), which refers to controlled modification, rearrangement, degradation or/and reconstruction of the cell wall in both growing and mature cells in response to various cues (Barnes and Anderson, 2018; Fradera-Soler et al., 2022). These modifications include cell wall loosening, cell wall stiffening, etc. and can be in response to biotic (Bacete et al., 2018; Swaminathan et al., 2022; Molina et al., 2024a) or abiotic stresses (Moore et al., 2008; Tenhaken, 2014; Oliveira et al., 2020; Jardine et al., 2022; Li et al., 2024) and/or developmental cues. For instance, cell wall components such as cellulose, hemicellulose and pectin are dynamically remodeled as an immune response against pathogen infection (Wan et al., 2021). Upon infection, cell wall hydrolysis induced by cell wall degrading enzymes releases carbohydrates (gylcans) that are sensed by plant receptors as alert signals to trigger plant immune response (Molina et al., 2024a; Molina et al., 2024b). Thus, infection-induced cell wall modification crucially mediates plant defense signaling and pathogen resistance (Wan et al., 2021; Molina et al., 2024a). Besides modulating biotic stress response, cell wall remodeling essentially mediates abiotic stress resistance in plants (Tenhaken, 2014; Houston et al., 2016; Jardine et al., 2022). In some instances, pathogen-triggered cell wall-related immune responses may partially overlap with abiotic (eg. salinity) stress-induced adaptive mechanisms, offering an exquisite machinery for combined stress resistance (Gigli-Bisceglia and Testerink, 2021), Thus, cell wall remodelng is an indispensable plant stress adaptation strategy.

Essentially, drought stress results in loss of cell turgor, decreased leaf and root water potentials, osmotic adjustment, decreased stomatal conductance, etc (Osmolovskaya et al., 2018; Hemati et al., 2022). Since turgor pressure drives cell expansion and growth (dependent upon cell wall extensibility), drought stress-triggered reduction in cell turgor pressure leads to reduced or ceased growth, due to reduced cell wall extensibility and cell expansion (Le Gall et al., 2015). In response, plant cell walls undergo dynamic mechanical and chemical composition modifications to cope with these drought stress effects (Tenhaken, 2014; Le Gall et al., 2015). Besides, drought stress, just like other abiotic or biotic stresses, induces cell wall damages, such as cellulose damage or reduction, or pectin breakdown, etc., which compromise the cell wall integrity (CWI) and proper functioning of the cell. These changes prompt appropriate cell wall damage responses, including cell wall stress sensing and signaling, cell wall remodeling mechanisms, and activation of downstream gene expressions and physiological responses, to ensure CWI maintenance and plant adaptation to drought stress (Le Gall et al., 2015; Vaahtera et al., 2019). In some instances, the plant cell wall elasticity is critical in facilitating differential root cell wall responses, such as loosening and stiffening within different root zones, necessary for continued root growth under drought stress conditions (Wu and Cosgrove, 2000; Cosgrove, 2016a). Furthermore, the guard cell wall is dynamically remodeled, for instance, through differential thickening and orientation of cellulose microfibrils, to permit continual stomatal opening and closing necessary for adaptation to different drought stress episodes (Amsbury et al., 2016; Jaafar and Anderson, 2024).

Cell wall remodeling processes are regulated by several loosening and stiffening enzymes/proteins, including pectin methylesterases (PMEs) which catalyze cell-wall esters hydrolysis, α-expansins (EXPAs), β-glucanases, peroxidases (PODs), etc (Tenhaken, 2014; Chebli and Geitmann, 2017; Wu et al., 2018; Perrot et al., 2022), with the resultant physicochemical shifts being critical for proper cell and tissue morphogenesis and stress adaptation (Chebli and Geitmann, 2017; Jardine et al., 2022). The activities of these cell wall remodeling-involved enzymes are tightly controlled, spatio-temporally (Barnes and Anderson, 2018). Besides, different phytohormones such as auxins (Jobert et al., 2023) and brassinosteroids (BRs) (Rao and Dixon, 2017) precisely regulate and potentiate the transcriptional output, actions and modulation of genes encoding cell wall remodeling-involved enzymes (Jobert et al., 2023). Meanwhile, several papers have uncovered the role of cell wall remodeling in enhancing plant abiotic stress tolerance, including salinity (Gigli-Bisceglia and Testerink, 2021; Dabravolski and Isayenkov, 2023), cold (Bilska-Kos et al., 2022; Kutsuno et al., 2023), cadmium (Loix et al., 2017), and heat (Wu H-C. et al., 2017; Wu et al., 2018), among others (Le Gall et al., 2015; Ezquer et al., 2020; García-Angulo and Largo-Gosens, 2022). For example, negatively charged pectin effectively sequester cadmium whereas lignification immobilizes cadmium to enhance cadmium tolerance in plants (Loix et al., 2017). Cell wall integrity (CWI) maintenance, lignin accumulation and amplified ascorbate-mediated antioxidant defense help plants adapt to salinity (Rui and Dinneny, 2020; Liu J. et al., 2021; Dabravolski and Isayenkov, 2023). Increased expression of lignin biosynthesis-related enzymes (such as peroxidases), and other cell-wall related proteins (including expansins) enhances thermotolerance acquisition (Wu and Jinn, 2010; Tenhaken, 2014; Wu et al., 2018). Thus, apt stress-induced cell wall compositional shifts constitute an effective strategy regulating plant abiotic stress adaptation (Oliveira et al., 2020). Despite all these examples revealing the role of cell wall remodeling in plant abiotic stress resistance, relatively less focus has been placed on its role in drought tolerance. Thus, our knowledge on how stress-induced cell wall modifications mediate drought tolerance remains fragmented.

More recently, an increased number of researches have revealed the role of cell wall remodeling as an important strategy for drought stress response and adaptation in several plant species. For example, cell wall O-acetylation (through enhanced O-acetyl esters, but down-regulated methyl ester hydrolysis) under drought stress in poplar (Populus trichocarpa) fine-tunes cell wall elasticity, regulates proper vascular tissue functioning, and influences growth-stress response trade-offs (Jardine et al., 2022). In soybean (Glycine max), increased cell wall plasticity and crosslinking under drought contribute to improved hydraulic conductance, water use efficiency, photosynthesis performance, and sustained higher plant growth under stress (Coutinho et al., 2021). Meanwhile, environmental-acclimation-triggered leaf cell wall modifications in Vitis vinifera have been suggested to crucially regulate leaf physiology by markedly affecting photosynthesis and water relations in an environmental condition-dependent manner (Roig-Oliver et al., 2020). These examples buttress the significant role of cell wall remodeling as a plant drought response strategy; hence, a budding area for research, and a promising trait for enhancing crop drought tolerance and yield (Ganie and Ahammed, 2021; García-Angulo and Largo-Gosens, 2022). Therefore, a better understanding of the stress-induced plant cell wall modifications may help us create novel strategies for enhancing crop drought tolerance (Piccinini et al., 2024).

In this review, we parse together the recent insights on how selected cell wall remodeling mechanisms, including cell wall demethylesterification, cell wall loosening and stiffening, stomata guard cell adjustment, cell wall lignification and root cell wall suberization orchestrate plant drought tolerance, revealing a potential target area for drought tolerance improvement in crops. First, we describe the primary structural composition of cell wall, cell wall-related sensing and signaling systems governing abiotic stress response, and forms of stress-induced cell wall modifications. We also review phytohormonal regulation of cell wall modifications that orchestrate drought tolerance in plants. Further, we highlight the recent advances in cell wall imaging techniques that now facilitate direct visualization and real-time quantification of native cell wall chemical composition and dynamics. We sum up by proffering future prospects related to the topic.

Plant cell wall is a complex assembly, primarily constituted by polysaccharide polymers (cellulose, hemicelluloses, and pectins), comprising distinct monosaccharides moieties joined by different type of bonds. The polymers may possess different biochemical decorations/modifications (such as methylations, acetylation, calcium bound ions) and may form connections with other cell wall components such as structural proteins (glycoproteins) and phenolic compounds (eg. polyphenolic lignin) (Cosgrove, 2005; Loqué et al., 2015; Höfte and Voxeur, 2017; Anderson and Kieber, 2020; Delmer et al., 2024). These different complex materials are woven into a strong, dynamically organized, and adaptable polymer structure primed for different functions (growth, pathogen defense, stress response, etc.) depending with the situation (Miedes et al., 2014; Wu et al., 2018; Zhang et al., 2021b). Although specific plant cell wall constituents and overall composition vary with plant species, tissue type, and tissue developmental state (Braidwood et al., 2014; Loqué et al., 2015), generally, polysaccharides constitute more than 80% of the total composite volume, with structural proteins and other minor components (eg., minerals) filling up the balance (Höfte and Voxeur, 2017). The plant cell wall is multi-layered or organized from the outside towards the plasma membrane (PM) as the middle lamella and primary cell wall (PCW), or/and secondary cell wall (SCW) in some species (Loqué et al., 2015; Höfte and Voxeur, 2017; Wu et al., 2018) ( Figure 1 ). The dynamic PCW is formed in young dividing cells and functions to provide flexibility and basic structural support, protecting the cell and facilitating cell to cell interactions, whereas the thicker and more durable SCW is located between the PCW and the PM and is deposited beneath the primary wall of some specific cell types at a later stage after the cell has ceased growing and dividing (Hamann, 2012; Houston et al., 2016; Li Z. et al., 2016). The SCW is considered a vital adaptation characteristic that enables upright growth and environmental stress endurance in land plants (Houston et al., 2016).

Sandwiched between the middle lamella and the plasma membrane, the thin layered primary cell wall is built up of polysaccharides cellulose, hemicelluloses and pectin ( Figure 1A ). The PCW lacks lignification, and is pervious to small molecules. At the same time, it is elastic and extensible to enable the growth of the cell via the acid growth mechanism (Cosgrove, 2005; Braidwood et al., 2014; Dabravolski and Isayenkov, 2023). Depending with the relative amounts (composition) of different polysaccharides, the PCW can be categorized into type I and type II cell walls. Type I PCWs are found in dicots, non-commelinoid monocots and gymnosperms (such as conifers), whilst type II PCWs are limited to commelinoid monocots, including grasses (Carpita, 1996; Vogel, 2008; Sarkar et al., 2009; Barnes and Anderson, 2018). Grass cell wall type and dicots cell wall type also exhibit further differences in types of lignin and ferulic acid esterification they possess (Sarkar et al., 2009). Both type I and type II PCWs possess same amounts of cellulose, but differ in proportion of hemicelluloses (Carpita and McCann, 2020). Whereas type I PCWs contain a lesser proportion of hemicelluloses and a greater pectin content, there is predomination of hemicellulose but less pectin content in type II PCWs (Vogel, 2008; Barnes and Anderson, 2018). Additionally, type I PCWs have a greater proportion of structural proteins as compared to type II PCWs. Furthermore, the two PCW types have different hemicellulose composition, whereby type I PCW hemicellulose is mainly comprised of xyloglucan (XyG) and minor amounts of glucomannans, arabinoxylans and glucuronoxylans. Contrariwise, type II PCW hemicellulose largely contains glucuroarabinoxylans (GAX) and less of mixed-linkage β-(1,3)-(1,4)-_D-glucan (MLG) and XyG. Besides, type II PCWs possess hydroxycinnamic acids that crosslink with GAX (Carpita, 1996; Vogel, 2008; Sarkar et al., 2009; Le Gall et al., 2015; Barnes and Anderson, 2018; Carpita and McCann, 2020).

The thick layered, stiff, and often waterproof, secondary cell wall forms beneath the PCW in some specific cell types, and is constituted by cellulose, xylan hemicelluloses and lignin (Kang et al., 2019; Dabravolski and Isayenkov, 2023) ( Figure 1B ). Lignin is a major structural component of plant secondary cell walls. It is a complex polymer composed of covalently joined monolignol subunits (coniferyl, sinapyl, and hydroxyphenyl alcohols) derived from amino acid L-phenylalanine linked via free radical coupling. The monolignol subunits are subjected to redox-mediated polymerization to produce guaiacyl (G), syringyl (S), hydroxyphenyl (H) units of lignin, respectively (Vanholme et al., 2010). Gymnosperm lignins are almost entirely made of G units, whilst angiosperm lignins are composed of mainly G and S units, with much reduced percentage of H units (Vanholme et al., 2010). Due to their assemblage by non-enzymatic polymerization, lignin chains lack a definite structure (Delmer et al., 2024). Lignin is sometimes covalently connected to ferulated xylan side chains, and renders apoplastic cell wall domains stiff/firm (Swaminathan et al., 2022). This increases the plant tissue`s structural robustness or mechanical strength (Polo et al., 2020), and facilitates its better resistance to pathogen attack and acclimatization to environmental changes (Khasin et al., 2021; Yadav and Chattopadhyay, 2023). SCWs of certain boundary tissue layers of plants such as root endodermis may be enriched with suberin, a lipophilic polymer composed of phenolic-derived glycerol, fatty acids, and aromatics, that provide structural support to these SCWs (Woolfson et al., 2022). Besides, suberin may act as a protective barrier, controlling water and ion transport (Vishwanath et al., 2015) (discussed in detail later under section 4.5).

Cellulose constitutes the major polymer in most plant cell walls, and is comprised of unbranched β -(1,4)-linked glucan chains (Zhang et al., 2021b). Despite the simplistic structural nature of cellulose, its bundling into multiscale cellulosic fibrils creates a complex nanostructure with high tensile strength and an important load-bearing function (Zhang et al., 2021b). Each cellulose fibril is composed of fundamental units called microfibrils, of approximately 35 Å width, corresponding roughly to a 6×6 array of chains. These native cellulose chains (cellulose I) are in a parallel arrangement as has been revealed by diffraction patterns analyses, high-resolution electron microscopy, and atomic force microscopy (AFM) (see (Delmer et al., 2024) and references therein). The microfibrils are modelled in a crystalline form, with the parallel chains oriented in cellulose Iα or Iβ lattices, with hydrogen-bonded sheets of chains running diagonally across the rectangular cross section (see (Jarvis, 2017)). These microfibrils are synthesized by the cellulose synthase (CeSA) enzyme complexes (Turner and Kumar, 2017; Wilson et al., 2021). The cellulose synthase complexes are thought to be assembled in the Golgi apparatus and transported to the PM via vesicle trafficking (Zhang et al., 2021b).

Hemicelluloses comprise of xylans, xyloglucans, β-(1,3;1,4)-glucan, mannans, and glucomannans. They all contain β-(1,4)-glycosyl connected backbones with the same equatorial arrangements (Zhang et al., 2021b). Except for xylans (whose backbone is synthesized by the GT47 and GT43 family Type II membrane proteins (Wu et al., 2010)), backbones of most hemicelluloses are synthesized by the GT2 family cellulose synthase-like proteins (Scheller and Ulvskov, 2010). However, their glycosyl residues vary. The backbones are frequently replaced by different glycosyl residues with unique patterns, which explains their architectural and physiochemical differences (reviewed in (Zhang et al., 2021b)). Xylan is the most abundant hemicellulosic polysaccharide in vascular plants (most common in type II primary cell walls and eudicot secondary walls), with a β-(1,4)-linked xylosyl sugar backbone decked with non-/methylated or non-/feruloylated arabinose and glucuronic acid residues as side chains. Xyloglucan hemicellulose is the principal polysaccharide in type I (dicotyledonous) primary walls, possessing a β-(1,4)-linked glucan sugar backbone decked with side chains of xylose, fucose, and galactose residues (Zabotina, 2012). Other hemicelluloses include mannans and glucomannans. Mannans are commonly found in gymnosperms, and possess linear glycan chains with a β-(1,4)-linked mannose backbone. Glucomannan is just mannan with intercalary β-(1,4)-linked glucose in its backbone (Carpita and Gibeaut, 1993; Ebringerová, 2005; Anderson and Kieber, 2020).

Pectin is a highly complex and heterogeneous polysaccharide, with d-galacturonic acid (GalA) residues linked via α-1,4-glycosidic bonds (Mohnen, 2008). Pectin is synthesized in the Golgi apparatus in methylesterifed form (with homologalacturonan methylesterified at the C-6 position) and exported into the wall where it is demethylesterified by the action of PMEs (Pelloux et al., 2007; Mohnen, 2008; Harholt et al., 2010; Wu et al., 2022). Pectin is composed of four main types of domains, viz., homogalacturonan (HG), rhamnogalacturonan I (RGI), RGII, and xylogalacturonan (XGA), covalently linked to create a pectin matrix (Willats et al., 2001; Mohnen, 2008; Ropartz and Ralet, 2020). HG, or the pectin smooth region, is the dominant component of pectin, and comprises α-1,4-linked d-GalA chains, and contributes to structural elasticity (Ridley et al., 2001; Willats et al., 2001; Ropartz and Ralet, 2020). HG is synthesized by galacturonosyl transferases (GAUTs) of the GT8 family (Atmodjo et al., 2013). PME can act upon HG, in both random and blockwise demethylesterification, thus playing dual roles in both cell wall loosening and stiffening (Du et al., 2022). In the former, HG can become susceptible to the actions of other cell wall degrading enzymes such as polygalacturonases and pectate lyases, resulting in cell wall loosening. In the later, HG demethylesterified in a blockwise fashion exposes carboxyl groups which can cross-link with Ca²⁺ ions to form a pectate gel network, known as an “egg box”, which contributes to cell wall stiffening (Pelloux et al., 2007). RGI backbone is built upon galacturonic acid and rhamnose residues, laced with arabinan, arabinogalactan, and galactan side chains; whereas RGII comprises of HG backbone decked with side chains of several sugar units and diverse glycosyl linkages (Anderson and Kieber, 2020; Malacarne et al., 2024). Pectin makes up the main constituent of primary cell walls, in both dicotyledonous (~ 35%) and monocotyledonous (2-10% in Gramineae) species, playing essential roles in cell adhesion and cell wall plasticity (O’neill et al., 1990; Willats et al., 2001; Mohnen, 2008; Wu et al., 2018). Thus, pectins control cell and organ growth, development, cell-wall porosity, and response to environmental cues (Willats et al., 2001; Caffall and Mohnen, 2009; Wolf and Greiner, 2012; Wu et al., 2022). Despite also being localized in the PCW and SCW, pectins are more enriched in the middle lamella, and their biosynthesis requires different kinds of transferases, such as glycosyl-, methyl-, and acetyltransferases (Ridley et al., 2001; Mohnen, 2008; Caffall and Mohnen, 2009; Yang and Anderson, 2020).

Besides, cell wall-related proteins and enzymes, including expansins (α-expansins, β-expansins, EXPLA, EXPLB), xyloglucan endo-β-transglucosylases/hydrolases (XET/XTHs), endo-1,4-β-D-glucanase (EGase), extensins, proline rich proteins and glycine-rich proteins, as well as minor components (eg., minerals, small metabolites, etc.) are also important constituents of cell walls (Jamet et al., 2006; Qiu et al., 2021). Cell wall-related proteins regulate cell wall extensibility, which modulates cell enlargement and expansion (Le Gall et al., 2015). Expansins induce wall creep and wall relaxation, via loosening of the linkages between cellulose microfibrils (Cosgrove, 2000; Sampedro and Cosgrove, 2005; Cosgrove, 2016b) (discussed in detail hereafter in section 4.2.1). Extensins (eg. leucine-rich repeat extensins) and hydroxyproline-rich proteins (HRGPs) are involved in regulating cell wall expansion, cell growth, and cell wall integrity sensing (Herger et al., 2019). Other cell wall-modifying enzymes crucially regulate cell wall plasticity/rheology (Cosgrove, 1999; Le Gall et al., 2015). These cell wall-modifying proteins actively participate in cell wall remodeling processes in response to different growth and stress stimuli (Höfte and Voxeur, 2017; Anderson and Kieber, 2020). In summary, plant cell wall is a complex and highly dynamic entity whose components and structure are constantly and appropriately modified during growth and development and in response to environmental cues.

Plant cells possess an effective CWI sensing mechanism that monitors functional and structural changes to the cell wall to ensure a balance between cell wall biosynthesis and turgor-driven cell wall extension/growth without compromising the CWI (Vaahtera et al., 2019; Rui and Dinneny, 2020). Most of the work on CWI sensing in plants is based on Arabidopsis thaliana (Arabidopsis), but its elucidation in other species is gaining traction. CWI signaling pathway is initiated by PM-localized cell surface sensors, including several members of the receptor-like Ser/Thr protein-kinase (RLK) and receptor-like protein (RLP) families, and relayed downstream via the mitogen-activated protein kinase (MAPK) cascades among other signaling modules (Bashline et al., 2014; He et al., 2018). The molecular mechanisms of CWI-modulated stress signaling share commonalities with the osmotic signaling cascade, that may be drought- or salinity-induced. For instance, alterations in cell water potential and turgor pressure are common among these stresses (Verslues et al., 2006; Novaković et al., 2018). In fully hydrated (> 60%) primary cell walls, reduced cell water status alters the gel-like matrix, which affects the organization and interaction of cell wall polymers and the wall-plasma membrane physical connection (Thompson and Islam, 2021). In ion toxicities, for instance high salinity-induced, monovalent ions (eg., Na⁺ and K⁺) may displace Ca²⁺ ions and disrupt the pectin “egg box” matrix structure (Peaucelle et al., 2012; Engelsdorf et al., 2018). These stress-induced alterations in cell wall mechanical properties yield CWI changes that are perceived by the CWI sensors such as Wall Associated Kinases (WAKs), Feronia (FER), and Receptor-Like Protein Kinases (RPKs) (Ringli, 2010; Hamann, 2012; Novaković et al., 2018; Vaahtera et al., 2019; Liu J. et al., 2021; Baez et al., 2022). Meanwhile, cold stress may cause the ice crystallization of the apoplastic space, which may result in cell wall deformation (Rajashekar and Lafta, 1996). Cold and drought stresses may similarly affect cellular exchanges through a decrease in membrane permeability or a decrease in water content and plasmolysis. Osmotic adjustment becomes critical in response to both stresses to ensure osmotic potential and a protective layer around cell structures and macromolecules (Charrier, 2021). Thus, plants have developed similar molecular response pathways for these stresses, mediated, in part, by abscisic acid (ABA) and Dehydration-Responsive Element (DRE) cis-acting element or C-Repeat (CRT) (Yamaguchi-Shinozaki and Shinozaki, 1994; Stockinger et al., 1997; Nakashima et al., 2014; Charrier, 2021; Kim et al., 2024); these pathways regulate responses to osmotic stress (Charrier, 2021). Besides, both drought and cold stresses induce stomatal closure, although, in cold stress, this mechanism seems to be ABA-independent (Wilkinson et al., 2001; Kim et al., 2024). It will be crucial to understand how these combined stresses are perceived by the cell wall and get integrated to the CWI maintenance pathway to produce a robust stress response. This will create a possibility to enhance plant drought tolerance via cell wall modification (Barbut et al., 2024).

The cell wall-perceived stress signal is transduced into the cell, prompting an eventual repertoire of responses to be coordinated (Ringli, 2010; Smokvarska et al., 2020). Cell wall-mediated stress responses encompass CWI sensing, ROS generation, and phytohormonal signaling pathways converging (Rao and Dixon, 2017; Novaković et al., 2018; Vaahtera et al., 2019) ( Figure 2 ). Hyperosmotic stress triggers ROS accumulation, which act as secondary messengers for inducting several plant responses (Martinière et al., 2019). Ca²⁺ influx evokes Respiratory Burst Oxidase Homologues (RBOHs), triggering the production of ROS (H₂O₂ and OH•−) in the cell wall (Suzuki et al., 2011). The osmotically triggered ROS buildup requires Rho-of-plants 6 (ROP6), an upstream activator of RBOH D or/and F (Martinière et al., 2019). The ROP6 creates osmotic stimuli-dependent nanodomains within the plasma membrane, ensuring signal specificity (Smokvarska et al., 2020). When combined with POD activity, ROS production fuels radical coupling of extensins and signaling, resulting in pectate buildup and wall stiffening (Francoz et al., 2015) ( Figure 2 ). However, limited POD activity or H₂O₂ induces OH•− radicals’ formation, triggering the severing of polycarbohydrate sugar bonds, and consequent cell wall loosening (Kidwai et al., 2020).

Several receptor-like kinases, including WAKs family, Catharanthus roseus Receptor-Like Kinase1-Like (CrRLK1L), FER, Leucine-Rich Repeat Receptor-Like Kinases/Protein Kinases, etc. have been known to mediate cell wall stress sensing and signaling (Ringli, 2010; Steinwand and Kieber, 2010; Wolf and Greiner, 2012; Osakabe et al., 2013; Galindo-Trigo et al., 2016; Wolf, 2022). WAKs are the most studied RLKs and are highly conserved in Arabidopsis in five transmembrane protein families (Steinwand and Kieber, 2010). They harbor a cytoplasmic serine threonine kinase, a transmembrane domain, and an extracellular domain (Kohorn and Kohorn, 2012). WAKs, such as WAK1, seem to directly bind to polysaccharides in the wall, through their extracellular N terminus firmly attaching to Ca²⁺ cross-linked pectin-derived oligogalacturonides (He et al., 1996; Decreux and Messiaen, 2005), to initiate cell wall perception and signaling through MAPKs (such as MAPK3, MAPK6, etc.) and modulate vacuolar invertases and turgor maintenance (Kohorn et al., 2006; Kohorn and Kohorn, 2012). Meanwhile, WAK1 has also been shown to interact with the glycine-rich protein AtGRP-3, which is a cell wall-localized structural protein (Park et al., 2001). WAKs are involved in cell expansion (Wagner and Kohorn, 2001), and are also induced by, and participate in the response to, pathogen attack and several stresses such as wounding, heavy metals, etc. (reviewed in (Ringli, 2010; Kohorn and Kohorn, 2012)). Disruption of WAK expression using WAK2 antisense RNA led to reduced leaf cell expansion (Wagner and Kohorn, 2001), whereas wak2 loss-of-function mutants and WAK4 antisense-RNA-expressing seedlings showed impaired root cell elongation (Kohorn et al., 2006). The growth performance of wak2 loss-of-function mutants exhibited a dependence on extrinsic sugars, signifying that possibly WAKs provide a cell-wall-sensing function, mediated by pectins and sugar metabolism (Kohorn et al., 2006; Kohorn and Kohorn, 2012).

Other members of the RLK family also participate in CWI sensing. Theseus1 (THE1), a member of the CrRLK1L family, was identified as a suppressor of the short-hypocotyl phenotype in the cellulose-deficient procuste1-1 (pcr1-1) mutant (Hématy et al., 2007).Under control (non-stress) conditions, knockout mutants (the1-1, the1-2, the1-3 and the1-6) did not exhibit any phenotypic defects, signifying that THE1 is only activated upon CWI being compromised, reinforcing the idea that THEI functions as a CWI sensor (Hématy et al., 2007; Baez et al., 2022). Additionally, the1 attenuated hypocotyl growth inhibition in other cellulose-deficient mutants, such as cesa3eli1, cesa1rsw1, etc (Hématy et al., 2007; Hématy and Höfte, 2008), suggesting that THE1 is activated by cellulose synthesis perturbation, and may act as a CWI sensor, which in turn evoke the expression of downstream candidate genes that regulate cell elongation (Bashline et al., 2014; Novaković et al., 2018). Meanwhile, Feronia (FER) is a PM-localized receptor kinase and most characterized CWI sensor from the CrRLK1 family (Galindo-Trigo et al., 2016). FER synergizes with different Rapid Alkalinization Factor (RALF) peptide ligands to function in several growth, development, and stress response processes, including CWI maintenance in growing-tip or elongating root-tip cells (Duan et al., 2010; Stegmann et al., 2017; Feng et al., 2018; Cheung, 2024). FER acts as a receptor for numerous RALFs, including RALF1 and RALF34 (Li C. et al., 2016; Gonneau et al., 2018). FER-RALF1 interaction enhances FER phosphorylation ability but inhibits across PM proton transport directed by H⁺-ATPase (Li C. et al., 2016), which possibly influences cell wall remodeling according to the acid growth theory (Haruta et al., 2014; Baez et al., 2022). FER and its RLK relatives possess extracellular domains that interact with cell wall carbohydrate moieties to sense cell wall perturbations and initiate appropriate cellular responses (Li C. et al., 2016; Moussu et al., 2023). For example, the FER extracellular domain directly interacts with pectin to sense salinity-induced wall defects, and trigger corresponding stress responses (Feng et al., 2018). In addition to the PM and cell wall, the RALF-FER signaling cascades interact with molecules in the cytoplasm and nucleus to modulate a complex intertwined signaling network (Cheung, 2024). FER also interacts with ROP-GEF [Rho-of-plants (ROP)-guanosine nucleotide exchange factors (GEF)] to facilitate the ROP2 exchange of GDP to GTP (Igisch et al., 2022), RBOH activation, and ROS production (Galindo-Trigo et al., 2016; Novaković et al., 2018). Disruption of FER function decreases ROPs levels, hinders ROP-mediated and RBOH-reliant ROS synthesis (Duan et al., 2010). More recently, it has been shown that FER controls the accrual and nano-scale compartmentalization of phosphatidylserine in the PM, to regulate Rho GTPase signaling in Arabidopsis (Smokvarska et al., 2023). Overall, considering the functional diversity of FER and the mechanistic complexity of the FER-anchored signaling modules, FER provides a rich ground for research that could help uncover new insights on plant abiotic stress response and signaling (Li C. et al., 2016; Cheung, 2024).

FEI1 and FEI2, leucine-rich repeat RLKs, belong to RLK subfamily XIII, which is different from the WAK and THE1 subfamilies (Xu et al., 2008; Bashline et al., 2014). They are important for the non-uniform expansion of different root cells in Arabidopsis, as well as cell extension in Arabidopsis stamen (filaments) and etiolated seedling hypocotyls (Xu et al., 2008). FEI1 and FEI2 gene mutations interrupt the non-uniform (anisotropic) cell expansion, hinders biosynthesis of wall polymers, and fortify cellulose biosynthesis repressors (Xu et al., 2008). The fei1 fei2 roots with expansion defects were rescued by the disruption of only 1-aminocyclopropane-1-carboxylic acid (ACC) synthase (an ethylene biosynthesis-related enzyme involved in conversion of Ado-Met to ACC), and not the entire ethylene (Et) response pathway, suggesting that FEI kinases crucially mediate a signaling pathway that integrates cell wall biosynthesis and ACC synthase in Arabidopsis (Xu et al., 2008). More recently, FEI1, FEI2, and Altered Root Hydrotropic Response 1 (ARH1), the three closely linked RLKs, have been shown to exhibit polar localization at the PM regions of Arabidopsis root tips (Chang et al., 2024). Overexpression of these three genes greatly reduced root hydrotropism, but their corresponding loss-of function mutants showed an increased root hydrotropic response tips (Chang et al., 2024). Additionally, the triple mutant arh1-2 fei1-C fei2-C showed cell wall, cutin, and wax (CCW) biosynthesis impairments in its root tips, suggesting that the root tip cell wall integrity, cutin and wax status mediate a balance between root hydrotropism and osmotic tolerance (Chang et al., 2024); this will need further exploration as it may also crucially regulate root responses to drought stress.

A PM-localized receptor-like protein, RLP44, mediates the response to pectin modification through activation of brassinosteroid (BR) signaling pathway (Wolf et al., 2014). RLP44 mediates this activation via direct connection with the BR co-receptor BAK1 (Brassinosteroid-Insensitive 1(BRI1)-Associated Receptor Kinase 1), to integrate cell wall surveillance with hormone signaling, and regulate CWI sensing and growth in Arabidopsis (Wolf et al., 2014). BAK1 can be activated upon both abiotic and biotic stresses, and can also act as a co-receptor for several RLPs mediating DAMP (Damage-Associated Molecular Patterns) and PAMP (Pathogen-Associated Molecular Patterns) recognition (Yasuda et al., 2017; Novaković et al., 2018; Molina et al., 2024a). DAMPs and PAMPs are unique CWI sensors that detect plant cell wall damage (eg. to cellulose and other polysaccharide components such as pectins) caused by pathogen infection, wounding, or other stresses, to activate RLKs or receptor kinases (RKs) that initiate stress signaling cascades (for reviews, see (Molina et al., 2024a; Molina et al., 2024b)). At the same time, these stress-induced cell wall damages prompt cell wall remodeling to ensure CWI maintenance (Hématy et al., 2007; Novaković et al., 2018). In case of DAMPs, the caused by these stresses leads to the release of carbohydrate-based wall molecules (glycans) that are recognized/perceived by the extracellular ectodomains (ECDs) of pattern recognition receptors (PRRs) as DAMPs to actuate pattern-triggered immunity (PTI) response and disease resistance (Molina et al., 2024b). For PAMPs, specific ECD-PRRs of RKs (eg., RKs with leucine-rich repeat and Malectin domains within their ECDs, LRR-MAL RKs (Martín-Dacal et al., 2023)) or RLPs (eg., lysine motif, Lys-M OsCERK1 (Shimizu et al., 2010)) can recognize oligosaccharide/polysaccharide molecules emanating from pathogens in the apoplast as PAMPs (Novaković et al., 2018; Molina et al., 2024b). Examples of PAMPs include chitooligosaccharides from fungi, flagellin from bacterial flagellae, etc (Boller and Felix, 2009; Novaković et al., 2018; Molina et al., 2024b). Likewise, specific ECD-PRRs recognize glycans emanating from the walls and extracellular layers of microbes interacting with plants as microbe-associated molecular patterns (MAMPs), evoking corresponding PTI responses (Boller and Felix, 2009; Molina et al., 2024a). Despite not directly linked to abiotic stress response, further elucidation of these unique glycan-based recognition mechanisms may reveal any possible similarities with abiotic-stress-specific cell wall sensors, which could help in identifying novel abiotic stress cell wall sensors in plants.

Meanwhile, another key receptor, Arabidopsis Receptor Like Protein Kinase 1 (RPK1), is only responsive to abiotic stress, and its overexpression enhances tolerance to drought, heat, salinity, and cold stresses (Osakabe et al., 2010; Novaković et al., 2018). Other RLKs involved in cell wall sensing, including PERK4 (Proline-rich Extension-like Receptor Kinase 4), are discussed in recent reviews ( (Baez et al., 2022; Wolf, 2022). The exact functions of these several RLKs and RLPs are yet to be elucidated. Nonetheless, we expect that as research on CWI sensing and stress signaling gathers much pace, new details will emerge that will help us assign receptor/gene functions and clarify the complex signaling crosstalk involved in abiotic stress response (Novaković et al., 2018). Downstream of these receptor complexes, MAPK cascades are among the key signaling modules which integrate signals and regulate diverse cellular and physiological responses via phosphorylation of several downstream targets (reviewed in (He et al., 2018)).

NAC [no apical meristem (NAM), Arabidopsis ATAF1/2, and CUC2 (cup-shaped cotyledon)] and MYB (myeloblastosis) TFs crucially modulate the induction of downstream SCW-biosynthetic or stress-responsive genes (Wang and Dixon, 2012; Cao Y. et al., 2020), with the transcriptional regulation exhibiting a high plasticity to abiotic stress, which helps plants to appropriately adapt to the stress (Nakano et al., 2015; Yoon et al., 2015; Houston et al., 2016; Choi et al., 2023). For instance, transcriptional actuation of SCW-biosynthesis-related genes by rice and maize secondary wall NACs (SWNs, that is, OsSWNs and ZmSWNs) rescued an Arabidopsis snd1 nst1 double mutant that had secondary wall thickening defects (Zhong et al., 2011). Overexpressed OsSWNs and ZmSWNs significantly activated several SCW-related TFs and biosynthetic genes in Arabidopsis, simultaneously increasing cellulose, xylan, and lignin accrual (Zhong et al., 2011). Additionally, OsMYB46 and ZmMYB46, the functional orthologs of AtMYB46/AtMYB83, effectively activated the whole SCW biosynthesis system after overexpression in Arabidopsis. OsSWNs and ZmSWNs activated OsMYB46 and ZmMYB46 by directly binding to the SCW NAC-binding elements (SNBEs) at their (OsMYB46 and ZmMYB46) promoters (Zhong et al., 2011). Reasonably, these NAC and MYB TFs could be candidates for manipulation to influence lignin biosynthetic genes expression changes that possibly modify plant cell walls, via increased lignin accumulation, to enhance plant drought tolerance (Miyamoto et al., 2020; Han et al., 2022).

Recently, SHORT-ROOT (SHR) transcription factor, a master regulator of endodermal development, has been observed to mediate a transcriptional interplay between lignification and suberization, integrated to stress signaling (Xu H. et al., 2022). Additionally, 13 key MYB TFs (including MYB74, MYB68, MYB36, MYB122, MYB41, MYB39, MYB52, MYB53, etc.) that form multiple sub-networks mediating feedback or feed-forward loops to balance this interplay were uncovered (Xu H. et al., 2022). Among them, sub-networks involving nine MYB TFs were shown to interact with ABA signaling to integrate stress response and root development, suggesting that SHR and these key MYB TFs crucially modulate and integrate multiple developmental and stress signals, and are key targets for genetic engineering for enhancing plant stress adaptation (Xu H. et al., 2022). Equally, the function of SUB TF (partly discussed in section 3 above) crucially modulates transcriptional networks related to suberin, lignin, and phenylpropanoid biosynthesis, as well as phytohormonal signaling (Cohen et al., 2020); hence, its characterization and potential targeting for genetic engineering may be a key step towards enhancing cell wall remodeling-mediated drought tolerance in crop plants (Cohen et al., 2020).

Photo-sensitive Leaf Rolling 1 (PSL1) gene encrypts a cell wall-localized polygalacturonase (PG) that alters cell wall structure and enhances rice drought tolerance (Zhang G. et al., 2021). A psl1 mutant, exhibiting ‘napping’ phenotype and reduced growth, displayed significant cell wall composition modifications as compared Wt plants (Zhang G. et al., 2021). Such cell wall composition alterations improved the mutant`s drought tolerance, through decreasing osmotic and drought stress-induced water loss. Collectively, these results suggested that PSL1, acting as PG, modifies cell wall biosynthesis, plant development, and enhances rice drought stress tolerance (Zhang G. et al., 2021). Equally, Curled Leaf and Dwarf 1/Semi-Rolled Leaf 1 (CLD1/SRL1) gene, that encrypts a glycophosphatidylinositol (GPI)-anchored protein (GAP) involved in controlling other growth and development facets in rice, functionally regulates rice leaf rolling by influencing cell wall synthesis, epidermis integrity, and water homeostasis (Li et al., 2017). A cld1 mutant shows substantially decreased cellulose and lignin contents in leaf SCWs, signifying that deterred CLD1/SRL1 function impacts cell wall development (Li et al., 2017). Additionally, CLD1/SRL1 function deficiency results in leaf epidermis defects (eg., formation of bulliform-like epidermal cells), which reduce the water retention capacity and causes water deficit in cld1 mutant leaves – the main contributor to leaf rolling. Due to the accelerated leaf water loss and reduced leaf water content, cld1 mutant shows decreased water deficit stress tolerance (Li et al., 2017). Overall, CLD1/SRL1 may play an essential role in improving plant drought tolerance, by functionally regulating leaf-rolling and minimizing leaf transpiration (Li et al., 2017), and thus, can be targeted for genetic engineering to enhance crop drought tolerance.

Cellulose synthase gene AtCesA8/IRX1 participates in SCW synthesis, and influences plant drought and osmotic stress tolerance (Chen et al., 2005). Disruption of AtCesA8/IRX1, combined with other two allelic mutants, leaf wilting 2-1 (lew2-1) and lew2-2, improved drought and osmotic stress tolerances in mutant plants as compared to Wt Arabidopsis (Chen et al., 2005). The lew2 mutant accrued greater ABA, proline, and soluble sugar contents in comparison to Wt plants, revealing that knocking down of LEW2 enhances drought tolerance, whereas cellulose synthesis is essential for plant response to osmotic and drought stresses (Chen et al., 2005). Other cell wall remodeling-involved genes important for improving plant drought tolerance are provided in Table 2 . Leveraging on modern genomics tools such as genome-wide association studies, (GWAS), which now permit high-resolution mapping of QTLs, we can exploit the abundant natural genetic variation present in progenitor species (Bahri et al., 2020) and cultivars (Martínez et al., 2007), to identify and harness desirable cell wall properties for the genetic improvement of drought tolerance in crop plants (Yoshida et al., 2021).

Drought stress response-associated cell wall plasticity conceivably contributes to cell turgor maintenance (Martínez et al., 2007), and as such, can be connected with plant drought tolerance (Le Gall et al., 2015). In common beans (Phaseolus vulgaris L.), for example, a comparative analysis of six genotypes showed that more drought tolerant genotypes had a huge drop in elasticity modulus (ϵ), coupled with greater cell wall elasticity, which enabled them to maintain better their turgescence (Martínez et al., 2007). In comparison, drought susceptible genotypes did not display any significant decrease in ϵ or cell wall elasticity, suggesting that cell wall elasticity may be critical for cell integrity maintenance and drought stress tolerance (Martínez et al., 2007; Hessini et al., 2009; Le Gall et al., 2015). Meanwhile, several enzymes, proteins and ions such as PMEs, expansins, β-glucanases, PODs, Ca²⁺ ions, etc. regulate cell wall remodeling-related loosening and stiffening processes that are critical for inducing growth changes, and drought tolerance (Tenhaken, 2014; Wu et al., 2018; Ganie and Ahammed, 2021; Qiu et al., 2021). Here, various forms of these cell wall modifications are discussed.

Phytohormones regulate cell wall properties, growth, development, and cell wall stress signaling, as well as transcriptional output of some genes encoding cell-wall remodeling enzymes. In turn, the cell wall modulates the homeostasis of these phytohormones (Wolf et al., 2012), creating an interplay essential for stress response (Jobert et al., 2023). For instance, ABA, as already briefly highlighted above, ABA regulates suberization, AQP activity and drought tolerance in rice roots (Kim and Sung, 2023). Besides, endodermal ABA signaling mediates suberization responses to nutrient stress in Arabidopsis roots (Barberon et al., 2016). ABA signaling transduction mediates suberin lamellae formation (Wang et al., 2020), and cell-wall biosynthesis (Dalal et al., 2018) in drought-exposed roots, as has been observed in rice (Henry et al., 2012), wheat (Dalal et al., 2018), Arabidopsis (Wang et al., 2020), and barley (Kreszies et al., 2019). Similarly, ABA-treated wheat exhibited increased root suberin lamellae, whereas Arabidopsis showed accelerated suberin deposition under drought conditions (Grünhofer et al., 2021), suggesting that drought induces suberin biosynthesis, and ABA-signaling may coordinate root suberization and drought tolerance in such species (Wang et al., 2020; Kim et al., 2022) ( Figure 6 ). However, the exact mechanism underlying ABA-mediated root suberization under varying water potentials remains unexplored (Kim and Sung, 2023).

BR-mediated modulation of stress-triggered cellulose synthesis seems to be evolutionarily conserved in certain species (Kesten et al., 2017). For instance, increased levels of BRI1, CESA3, and other BR signaling-associated genes were observed in progenitor species Agropyron elongatum than in common/cultivated wheat genotypes in response to water stress (Placido et al., 2013). When introgressed into cultivated genotype, the wheat translocation line exhibited enhanced drought tolerance and greater root and shoot biomass in comparison with the control under stress (Placido et al., 2013), suggesting that the enhanced BR-signaling pathway of Agropyron elongatum could contribute to its higher drought stress adaptation, via improved root and shoot biomass, that possibly facilitate enhanced water extraction under stress (Placido et al., 2013; Rao and Dixon, 2017). Additionally, BRs mediate several cell wall remodeling processes, including cell wall damage responses, cell wall signaling, CWI maintenance, cell wall loosening, cellulose deposition, lignin accumulation, pectin modification, etc. (Zurek and Clouse, 1994; Rao and Dixon, 2017; Novaković et al., 2018; Rao and Dixon, 2018) ( Figure 6 ). BR hormone signaling pathway also modulates the induction of several cell wall-associated genes (Wolf et al., 2012). In Arabidopsis, BR-activated TF BZR1 and its homology BZR2/BES1 directly bind to the promoter regions (such as the CANNTG-E motifs) of numerous cell wall-associated and BR-responsive genes such as the cellulose synthase A (CeSA) genes (Xie et al., 2011; Li et al., 2018), and NAC and MYB TFs linked to lignin biosynthesis pathways (Zhao and Dixon, 2011; Rao and Dixon, 2017; Rao and Dixon, 2018). However, Brassinosteroid Insensitive 2 (BIN2), a protein kinase, directly phosphorylates Arabidopsis CESA1, negatively regulating BR signaling, CESA activity and cellulose biosynthesis (Sánchez-Rodríguez et al., 2017) (Bottom left insert of Figure 6 ). CESA genes, such as CESA1, are transcriptionally and post-transcriptionally modulated via the BR-mediated signaling to regulate cellulose synthesis in Arabidopsis (Novaković et al., 2018). Besides, BRs regulate the expression of genes encoding cell wall loosening EXPAs and XTHs (Zurek and Clouse, 1994; Novaković et al., 2018). XTHs (eg., AtXTH22, AtXTH24, etc.) and EXPA genes have been shown to be considerably up-regulated by BR treatment in Arabidopsis and soybean (Zurek and Clouse, 1994; He et al., 2003), and in response to salinity and drought stress in halophytic plant Salicornia europaea (Tiika et al., 2021).

BRs also modulate stress-induced pectin remodeling. In particular, BR signaling crucially modulate cell wall modifying proteins, including PMEs (Wolf et al., 2012), whereupon stress-induced feedback mechanism from the cell wall potentiates the output of the BR pathway to ensure cell wall homeostasis and CWI in Arabidopsis (Wolf et al., 2012; Novaković et al., 2018). More recently, BR signaling has been shown to modulate root growth and drought tolerance in Arabidopsis, by repressing the expression of transcription factors PLETHORA 1 and 2 (PLT1 and PLT2, involved in root development, including apical meristem embryonic pattern formation (Aida et al., 2004)) (Zhao et al., 2023) (Top left insert of Figure 6 ). PLT1-OE and PLT2-OE transgenic Arabidopsis exhibited higher water deficit tolerance as compared to their Wt counterparts. Additionally, both in vivo and in vitro experiments have shown BZR1 to bind to PLT1 promoter region and hinder its transcription activation (Zhao et al., 2023). Further, BR signaling regulated PLT1 and PLT2 expression in root development, and PLT2 partly rescued the drought sensitivity of bes1-D mutant (Zhao et al., 2023). It might be probable that these genes could play other essential roles in cell wall remodeling processes linked to drought tolerance, and will need further exploration.

Auxin (Aux) is another phytohormone that regulate the expression and activities of cell wall remodeling-involved enzymes, including EXPAs, PMEs, PODs, etc (Cosgrove, 2015; Nafisi et al., 2015; Cosgrove, 2016b). These enzymes crucially mediate cross-linking of lignins, pectins, and proteins, resultantly influencing stress-induced cell elongation (Passardi et al., 2004; Passardi et al., 2006; Liu X. et al., 2021). Essentially, Aux signaling potentiates the transcriptional output of several pectin remodeling related enzymes-encoding genes, their local activity, and spatio-temporal distribution or modulation of pectin (Jobert et al., 2023). Using atomic force microscopy (AFM) (Braybrook and Peaucelle, 2013), established a link between Aux signaling and PME activity that modulates cell wall biochemical and mechanical changes necessary for organ formation in the Arabidopsis shoot apical meristematic region. Auxin also triggers low pH, which activates PME and hinders PMEI (see (Majda and Robert, 2018)). Besides, indole-3-acetic acid (IAA) orchestrate several plant growth and development processes (Sánchez-Rodríguez et al., 2010; Zhao, 2010), including the canonical acid growth cell expansion theory (McQueen-Mason et al., 1992). Auxin phosphorylates and activates PM proton pumps (H⁺-ATPase) to control apoplastic pH, which triggers expansins and relaxes the cell wall (McQueen-Mason et al., 1992). The transmembrane kinases also directly interact/phosphorylate PM H⁺-ATPases, evoking their activation, and inducing cell-wall (apoplastic) acidification, and hypocotyl cell elongation in Arabidopsis (Lin et al., 2021). Auxin acidification modulates the relaxation of cell wall matrix due to loosening of the binding between cell wall polysaccharides, as controlled by XTHs and cellulases. The expression of these XTHs and cellulases is upregulated by auxin. When cellulase activity is increased by auxin, the load-bearing hemicellulose chains are cleaved, causing modification of interactions between polysaccharides and other cell wall components. This leads to eventual cell wall loosening and extensibility (nicely reviewed in (Majda and Robert, 2018)). However, how these hormonal crosstalks regulate drought stress response and adaptation in crops is yet to be fully understood. It will be interesting to understand, and/or quantify, how cells balance growth/development- and stress-response-related portions of cell wall modifications.

Other phytohormones, including jasmonic acid (JA), salicylic acid (SA), ethylene, etc., also participate in cell wall remodeling-mediated abiotic stress response (Ellis et al., 2002; Novaković et al., 2018), and defense ( (Nafisi et al., 2015). The levels of these hormones are elevated in response to stress-induced cellulose damage or reduction, leading to cell wall compositional and mechanical shifts, including ectopic lignification (Kesten et al., 2017) ( Figure 6 ). In certain situations, these phytohormone (eg., BR and JA) signaling pathways intricately interact with ROS and CWI pathways to regulate stress response and adaptation (Nafisi et al., 2015; Novaković et al., 2018). For instance, on one hand, JA inhibits stress-induced cell-wall-remodeling-related ectopic lignin accumulation in Arabidopsis (Denness et al., 2011). Mutants (eg., allene oxide synthase, aos; and jasmonic acid resistant 1, jar1) with stifled JA production exhibit elevated lignin accumulation prompted by the damage to the cell wall (Denness et al., 2011). On the other hand, mutants with repressed ROS generation and CWI detection (eg., rboh and the1), exhibit elevated contents of ectopic lignin caused by the damage to the cell wall (Denness et al., 2011), revealing the important crosstalk coupling JA, ROS and CWI pathways as necessary for cell wall damage-triggered responses. Decoding such crosstalk helps in clarifying how drought tolerance is regulated by each or combination of the cell-wall remodeling-involved components or pathways, and could facilitate for appropriate cell wall adjustments, or metabolic engineering for drought tolerance in crop plants (Neumann, 1995; Liu S. et al., 2023).

Over the past two decades, a repertoire of plant cell imaging techniques has been developed to aid the functional visualization of plant cell structural architecture and biosynthetic processes at different spatiotemporal scales, offering a better understanding of how the plant cell wall functions under different conditions (Higashiyama et al., 2021; Xu et al., 2021) ( Figure 7 ). Particularly, the deployment of different microscopy models, including polarized light microscopy (Radosavljević et al., 2021), scanning electron microscopy (SEM) (Fromm et al., 2003; Wightman, 2022), field emission scanning electron microscopy (FESEM) (Zheng et al., 2017), etc., revolutionized plant cell/tissue/organ imaging, and significantly enhanced visualization and quantification of subtle cell wall compositional and morphological changes, which helped in resolving some complex biological questions that could not be answered by conventional biochemical assays (Komis et al., 2018; Cui et al., 2023).

Fluorescence-based techniques, including confocal laser-scanning microscopy, total internal reflection fluorescence microscopy, light sheet fluorescent microscopy, etc., have provided insights into plant cell wall composition, structure, and biosynthetic machineries such as lignin and pectin cross-linking (reviewed in (Tobimatsu et al., 2013; Zeng et al., 2017; Kitin et al., 2020; DeVree et al., 2021)). Additionally, several histochemical and immunolabeling tools can be used for direct labeling, targeted analyses, and/or real-time visualization of dynamics of cell wall polysaccharides or other non-polysaccharide components, thereby offering new ways to explore the physiological mechanisms of cell-wall remodeling (Barnes and Anderson, 2018; Rydahl et al., 2018; Voiniciuc et al., 2018; DeVree et al., 2021). These histochemical and immunolabeling techniques include the monoclonal antibodies, small-molecule probes and fluorescently tagged monolignols (detailed in (Rydahl et al., 2018; Ursache et al., 2018)). For instance, 2F4 monoclonal antibody has been successfully used to detect how pectin HGs, one of the major plant cell wall components, can form pectate complexes with divalent Ca²⁺ ions (egg boxes) when GalA residues are blockwise de-esterified (Liners et al., 1989), since this mechanism crucially regulate wall biomechanics and mediates cell-cell bonding (Du et al., 2022). Meanwhile, turgor pressure changes within guard cells during drought stress regulate stomatal opening and closing (Auler et al., 2022), and these are governed by biochemical and mechanical shifts of the GCWs (Amsbury et al., 2016). A histological technique, monoclonal antibody LM20 – with binding affinity to highly methylesterified HGs, has been used to differentiate guard cell wall components from adjacent epidermal cells. Based on the variations in immunofluorescence, Arabidopsis GCWs are distinguished by low level of methylated pectins (Amsbury et al., 2016), thereby providing an effective way for identifying GCW components and understanding GCW modifications during drought stress (Alonso Baez and Bacete, 2023). Recently, an inventive oligogalacturonide-derived molecular probe (modeled on Ca²⁺-dependent attachment of fluorescently-labeled oligogalacturonides to endogenous de-esterified HGs) was developed to competently monitor HG crosslinking dynamics in elongating pollen tube- and root hair cell walls of Arabidopsis in real-time (Mravec et al., 2017), thereby further clarifying the mechanisms by which cell walls are remodeled. Equally (Besten et al., 2025), developed CarboTag, a modular approach for live and functional fluorescence imaging of plant cell walls. It is based on a small molecular motif, a pyridine boronic acid, that undergoes high-affinity binding with diols in the plant cell wall (Besten et al., 2025). CarboTag enabled the authors to develop several cell wall imaging probes (in different colors) for multiplexing, and novel functional reporters for live and quantitative imaging of key cell wall properties such as network porosity, cell wall pH, and the presence of ROS (Besten et al., 2025). This technique opens the way for dynamic and quantitative mapping of cell wall responses to various perturbations, including abiotic stress, at subcellular resolution. Moreover, it is applicable to diverse plant and brown algae species without the need for genetic manipulation of the organism of interest (Besten et al., 2025). Besides, fluorescent proteins (FP) can be conveniently tagged to cell wall-specific fluorophores, permitting simultaneous visualization of FP-labelled proteins and cell wall components (Ursache et al., 2018; Voiniciuc et al., 2018; DeVree et al., 2021). Further, coupling cell wall-directed molecular probes to light microscope-based analysis facilitates high-level-specificity imaging, minimal sample processing, and real time parsing of cell wall assemblies, thereby “simplifying” our understanding of plant cell wall architecture and stress-related response mechanisms (DeVree et al., 2021).

Meanwhile, high-resolution label-free imaging techniques, including AFM (Zhang et al., 2016; Song et al., 2020; Pu et al., 2022; Pu et al., 2025), Fourier-transformed infrared (FTIR) microspectroscopy (Smith-Moritz et al., 2015), hyperspectral imaging (HSI) (Sarić et al., 2022), transmission electron microscopy (TEM) (Fromm et al., 2003), confocal Raman microscropy (µ-Raman) (Gierlinger et al., 2012), etc., now enable direct and powerful visualization, and revelation of molecular and native chemical composition, as well as structural architecture of cell walls (Barnes and Anderson, 2018; Zhao et al., 2019; Alonso Baez and Bacete, 2023). For instance, AFM technique has been used to directly image PCWs and SCWs from fresh maize tissues under near-native conditions (Song et al., 2020). By analyzing cellulose structure in these different cell wall types, individual microfibrils and bundles in these cell walls were measured at nanometer scale, which helped to parse the different mechanisms of cellulose biosynthesis in PCWs and SCWs (Song et al., 2020). Similarly (Braybrook and Peaucelle, 2013), used AFM to reveal a feedback loop between Aux signaling and pectin-mediated cell wall remodeling that underpins organ formation and development in Arabidopsis (Braybrook and Peaucelle, 2013). The latest AFM-based infrared/Raman spectroscopy allows for in situ imaging of the multidimensional structure of the cell wall, revealing the mechanical characteristics of plant tissues or single cells, and specific single-molecule recognition of cell wall-related enzymes (Pu et al., 2025). As already discussed above, stomatal opening determines drought resistance, and guard cell walls have a crucial role in this process (Pirasteh-Anosheh et al., 2016). Since FTIR microspectroscopic imaging technique allows for a detailed analysis of the biochemical composition of specific cell walls in situ, and has already been used to classify cell wall mutants (Mouille et al., 2003), it could be a suitable fit for in-depth analysis of the composition and orientation of guard cell wall (GCW) polymers during drought stress (Alonso Baez and Bacete, 2023).

Hyperspectral imaging (HSI) is a noninvasive, label-free, rapid, and high-throughput plant phenotyping (HTPP) technique that acquires and processes both spectral (λ) and spatial (x, y) information and merges these into a 3D data matrix, referred to as ‘hyperspectral data cube’, with HSI sensors capturing information from the entire wavelength spectrum (UV, near-infrared, visible, and short-wave infrared in the 250–2500 nm regions) (Sarić et al., 2022). Moreover, HSI systems have a finer resolution (<5 nm) with tens to hundreds of spectral bands in a continuous range (Mehta et al., 2018). In comparison, RGB imaging collects light interacting with a sample at only three distinct wavelengths (red – 630 nm, green – 545 nm, and blue 435 nm) and the information on the location (spatial information) from which the light is being collected (Mehta et al., 2018). The additional spectral information provided by HSI facilitates for more accurate analysis and understanding of micro and nanoscale properties of the plant cell walls that are not detectable using RGB imaging (Mehta et al., 2018). HSI is a powerful and important tool for plant cell wall visualization and analysis, and has been applied in determining plant traits (eg., root traits), and detecting plant abiotic and biotic stress (eg., early phases of plant disease) responses (Sarić et al., 2022). For instance (Charrier et al., 2021), used a multimodal scattering near-field optical microscopy (SNOM) technique for in-situ HSI of poplar wood material. This technique facilitated determination of nanoscale properties of PCWs by correlating the local optical, chemical, and mechanical properties at a spatial resolution of 20 nm, which enabled monitoring of the delignification process and different lignin acetylation yields in relation to their structure and location in the PCW of poplar wood (Charrier et al., 2021). It is possible that this same technique can equally be applied to monitoring drought stress-induced cell wall modifications such as acetylation or demethylesterification of pectin. Meanwhile (Barbut et al., 2024), combined RGB and HSI techniques to reveal that the integrity of xylan backbone in SCW affects Arabidopsis response to drought stress. Coupling such advanced techniques to single cell analysis and machine learning approaches now permits for real-time in vivo monitoring of plant stress response, such as stress-related H₂O₂ signaling, and accurate differentiation of different types of stress, ultimately enhancing our understanding of the mechanisms of plant stress signaling (Hu et al., 2025).

TEM is a high-resolution microscopic technique that employs highly energetic electrons for the in situ analysis of the ultrastructure (at atomic scale) and direct visualization of the nanoscale morphologies of cellulosic materials (especially the “nanocelluloses” – nanofibers and nanocrystals) through a highly magnified image (Ogawa and Putaux, 2019). Now, time-resolved in situ TEM enables unprecedented understanding of the ultrastructure of materials and how structure is related to properties and function (Alcorn et al., 2023). Already, TEM technique has been applied in ultrastructural and immunohistological investigations of plant viral pathogens (Zechmann and Zellnig, 2009), and can be deployed for real-time assessment of cell wall remodeling dynamics during drought stress. When coupled to molecular probes, ie., colloidal gold-conjugated antibodies (to form immunogold TEM), TEM immunolocalizes polysaccharides across the cell wall thickness at much higher resolution (Sun et al., 2017). Additionally, immunogold TEM offers information on the relative abundances of target polysaccharides in a cell wall (Liners and Van Cutsem, 1992), thereby providing utility for analyzing the composition and distribution of cell wall polysaccharides at an ultrastructural level (Sun et al., 2017; Majda, 2019).

The latest cutting-edge Raman microscopy (µ-Raman), which integrates the chemical analysis technique, Raman spectroscopy (dependent on inelastic or Raman scattering of monochromatic laser light interacting with biomolecules), with a traditional light microscope (Gierlinger et al., 2012), is a non-destructive label-free technique that facilitates in-situ analysis of chemical composition and direct visualization of the structures of cell wall components such as pectins at micrometer (<0.5 μm) and nano-scale levels (Mateu et al., 2020). µ-Raman based approaches, including the coherent Raman scattering (CRS) microscopy, coherent anti-Stokes Raman scattering (CARS) microscopy, and stimulated Raman scattering (SRS) microscopy (Mateu et al., 2020; Xu et al., 2021), are modelled on detecting biomolecules` major vibrational states (Kumamoto et al., 2018), thereby offering label-free dynamics, rapid, high-specificity, and quantitative microanalysis of cell walls chemical compositions in their native states (Cui et al., 2023). µ-Raman is used in live imaging of cell wall biosynthetic processes (Xu et al., 2021) and cell wall lignification (Schmidt et al., 2010). Thus, µ-Raman spectroscopy is important for elucidating plant cell wall cross-linking chemistry and polymeric architecture (Zeng et al., 2017), improving our fundamental understanding of the dynamic processes involved in stress-induced cell wall remodeling (Zhao et al., 2019). When integrated to super-resolution microscopy, such as structured illumination microscopy, photoactivation localization microscopy, etc., and real-time analyses of dynamic cell wall structural changes (through use of light-sheet fluorescence microscopy), these label-free imaging technologies fine-tune the molecular characterization, and enhance our knowledge of temporal cell wall structural and compositional shifts under stress conditions (Komis et al., 2018; Zhao et al., 2019). This may help us to deconstruct individual constituents of CWI, phytohormonal, and ROS signaling networks, and to decipher cell wall regulation by their crosstalk (Novaković et al., 2018). Meanwhile, the surface-enhanced Raman scattering (SERS)-based nanoprobe is used for the real-time in vivo monitoring of multiple endogenous stress signaling molecules in plants (Son et al., 2023). The nanosensor is placed in the intercellular space, and is optically active in the near-infrared region (785 nm), enabling it to evade interferences from plant autofluorescence (Son et al., 2023). The method has been successfully used to detect abiotic and biotic stress-related molecules, including phytohormone salicylic acid, extracellular adenosine triphosphate (ATP), cruciferous phytoalexin, and glutathione in Nasturtium officinale, bread wheat, and barley species, signposting the possible onset of plant stress, including disease (Son et al., 2023). This paves the way for the possibility of monitoring the commencement or early processes of stress in plants, including the key molecules involved.

Furthermore, the latest label-free interferometric-based technologies, such as biospeckle imaging, optical coherence tomography, etc., can harness the interaction of laser light with biological materials to deduce (from interference patterns) important structural and activity information, permitting measurement of cell structure and temporal intracellular changes (Ebrahimi et al., 2023). For instance, laser speckle contrast imaging has been used to monitor the dynamic changes occurring during the growth of the apical root region of beetroot (Beta vulgaris) (Schott et al., 2022). It is possible that this technique can be harnessed for in situ monitoring of the physiological processes, and in particular the cell wall remodeling mechanisms occurring within the root apical region under drought stress. Besides, interferometric-based technologies have shown high utility in determining seed germination capacity, plant growth, plant disease, etc., and are envisaged to facilitate high-resolution and dynamic imaging of plant cell walls at various spatiotemporal scales (Ebrahimi et al., 2023). This will enhance our knowledge of abiotic stress-triggered cell wall modifications and crosstalk among multiple interconnected pathways.

Despite these recent advances, however, understanding drought stress-induced cell wall remodeling still remain conceptually and technically challenging. This is because of several reasons. First, disentangling the complex crosstalk among CWI maintenance, cell wall stress sensing and signaling, phytohormonal signaling, and ROS production mechanisms remains cumbersome, especially deconstruction of the individual components constituting each of those networks, and grasping how cell wall stress sensors decode or differentiate specific abiotic stress-induced cell wall modifications to institute narrowly targeted responses (Kumar et al., 2016; Novaković et al., 2018). Secondly, the non-quantitative nature of the data output of some of the low-end biochemical and cytological exploration techniques still in use, and their inability to monitor dynamic changes in plant development or biomass transformation limit their overall effectiveness (Zhao et al., 2019; Xu et al., 2021). Fortunately, the recent cutting-edge imaging techniques highlighted above are countering some of these shortcomings. Furthermore, despite the central role of cell wall-localized proteins like EXPAs, XTHs, or PMEs in assembly and spatiotemporal cell wall control, their precise localization or movement within the cell wall, remains largely unresolved (DeVree et al., 2021). Designing specific probes targeting these cell wall proteins will be crucial in understanding the connection between the cell wall protein activity, localization, and formation of cell wall microdomains, and whether engineering of these proteins could be useful in designing cell walls with desired characteristics to enhance plant stress tolerance (DeVree et al., 2021). Going forward, coupling these latest high-resolution imaging techniques to high-sensitivity mass spectromy-based analytical and spatially resolved singe-cell omics approaches (Zhang H. et al., 2023), and advanced data analysis (eg., multivariate) methods (Moore et al., 2020) will immensely improve our understanding of the cell wall composition, structure, biosynthetic machineries, function, and regulation at spatial and time-gated scales (DeVree et al., 2021). Besides, synthetic biology (Shelake et al., 2022) and genome-editing approaches (Zafar et al., 2020) are rapidly “simplifying” deconstruction of complex networks, and facilitating metabolic pathway engineering for enhanced stress tolerance (Liu S. et al., 2023). We anticipate progressive improvement in these technologies to facilitate appropriate cell wall modifications (for instance, via genetic, or in-planta biosynthetic pathways engineering (Yoshida et al., 2021)) necessary for developing more drought-tolerant crop plants tailor-fit for the new climatic environments.

Cell wall interfaces the cell interior with its surrounding environment, regulating stress sensing, signaling and response; therefore, cell wall plasticity under drought stress crucially helps in regulating plant stress acclimation and adaptation. Here, we have synthesized how several cell wall remodeling mechanisms orchestrate drought tolerance in plants ( Figure 8 ). Stress-induced demethylesterification of pectins, mediated by PMEs, permits calcium crosslinking of polyphenolics, which enhances cell wall rigidity and may help in intra-cell water preservation. Lignin accumulation-mediated cell wall thickening increases the plant tissue`s structural robustness, and regulates cell water movement across the cell membrane, which potentiates better plant tolerance to drought. Root suberization creates a hydrophobic and protective secondary cell wall layer that minimizes water loss and enhance drought tolerance. Similarly, PMEs and expansin activities that regulate guard cell wall plasticity are essential in controlling stomatal aperture in response to drought stress, and are excellent targets for genetic engineering of drought tolerance in crops. At the same time, highly plastic transcriptional regulation of secondary cell wall biosynthetic or stress-responsive genes, modulated by several TFs such as MYBs, NACs, WRKYs, etc., orchestrates plant drought tolerance. Meanwhile, phytohormones such as BRs, auxins, etc., crosstalk with ROS and cell wall integrity pathways to modulate stress response and transcriptional output of some cell-wall remodeling-associated genes. However, fine-tuning plant cell wall properties to engineer stress tolerance requires a deep understanding of these cell wall biosynthesis and regulation mechanisms, and then deploy genome engineering techniques to tailor the desired modifications. Harnessing modern high-resolution plant cell wall imaging techniques, single cell omics, genome editing and synthetic biology approaches could help us achieve this feat.