Photosynthesis is a prime physiological process that can be the principal target of drought- and salinity-induced imbalance (Chaves et al., 2009) ( Figure 1 ). A high amount of water is required in sugarcane during tillering and the grand growth phase to sustain the physiological activity as well as growth and development, and in such a way, sugarcane is more vulnerable to drought stress at this phase of development (Dinh et al., 2017). Sugarcane, as a C₄ crop, has a unique CO₂ concentrating system that gives it an advantage over C₃ crops in terms of reduction of photorespiration and maximization of its water use efficiency (Ghannoum, 2009; De Souza et al., 2018). Reduced leaf water potential decreased photosynthetic enzyme activity such as ribulose-1,5-bisphosphate carboxylase-oxygenase (RuBisCo), phosphoenolpyruvate carboxylase (PEPC), nicotinamide adenine dinucleotide phosphate-malic enzyme (NADP-ME), and pyruvate, phosphate dikinase (PPDK). The decrease in activity was most pronounced in PPDK (Du et al., 1998). Sugarcane has two distinct modes of C₄ metabolism depending on their decarboxylating enzymes, namely, NADP-ME and phosphoenolpyruvate carboxykinase (PEPCK), and PEPCK is more prevalent than NADP-ME (Calsa and Figueira, 2007).

A substantial drop in physiological attributes during drought stress in both susceptible and tolerant genotypes of sugarcane can be noticed, while the severity of the adverse effect was pronounced in susceptible genotypes. Higher biomass accumulation, increased photosynthetic rate, and antioxidant enzyme activities were exhibited by tolerant genotypes (Zhang et al., 2020). Physiological processes and the yield of sugarcane were significantly affected under drought stress (Silva et al., 2008; Basnayake et al., 2012). The decline in the photosynthesis rate led to the consequent reduction in the yield during drought (Zargar et al., 2017). Plants can reduce their stomatal aperture in response to drought situations, and when the stress condition persists, impairment in the leaf photochemistry and carbon metabolism results in negative consequences on photosynthesis. Reduced stomatal aperture prevents or minimizes water loss, and this protection system can be considered an adaptive response toward drought onset (Saradadevi et al., 2017).

Drought stress-induced non-stomatal constraint has been described as a major obstacle that inhibits the photosynthesis in sugarcane (Ribeiro et al., 2013), under severe drought, or prolonged moderate drought conditions in sugarcane production systems (Basnayake et al., 2015). Under mild water deficiency, changes in the activity and amount of the RuBisCo were reported at the transcription level (Zhang et al., 2013). Moreover, ROS production in the chloroplast under a stress state reduces the adenosine triphosphate (ATP) production by decreasing the activity of ATP synthase, consequently lowering the regeneration capacity of substrate ribulose-1,5-bisphosphate (RuBP). Early onset of drought stress in sugarcane led to the reduction in the efficiency of photosystem (PS)-II by lowering the maximum quantum yield (Leanasawat et al., 2021). Physiological traits like the maximum quantum yield of PSII efficiency (Fv/Fm), stomatal conductance, and net photosynthetic rate have been investigated in sugarcane during drought stress and recommended for the improvement in yield under drought (Jangpromma et al., 2010; Cha-um et al., 2012).

Even though plants have complex machinery to adapt under osmotic and ionic stresses induced by high salt through an osmotic adjustment with the aid of elevated levels of compatible solute accumulation, drastic retardation of growth at various stages is substantiated due to the highly sensitive nature of sugarcane (Plaut et al., 2000). Reduced photosynthetic efficiency under salt stress conditions debilitates productivity and quality (Akhtar et al., 2003), resulting in a reduction in stalk sucrose concentration (Rozeff, 1995). Reduction in the photosynthetic rate in sugarcane under salinity stress is probably a response against stress in which loss of moisture is prevented through the partial closure of stomata, followed by stomatal conductance reduction, reduced transpiration rate, and, subsequently, limitation in the internal stomatal CO₂ concentration (Sharma et al., 2021). However, non-stomatal factors such as degradation of chlorophyll and reduction in photosynthetic enzyme activity could also play a major role (Vasantha et al., 2010). Additionally, salt stress influences other components of the photosynthetic machinery, such as chlorophyll and other accessory pigments, biosynthetic enzymes, and carbon fixation competency of the RuBisCo (Demetriou et al., 2007). The susceptibility of light-mediated photosynthetic reactions has been investigated against salt stress and was found to be extremely high. Salinity-induced photosynthesis inhibition is somewhat related to the PSII complex. Reduced PSII activity and a decrease in electron transport quantum yield alter the pigment–protein complex of photosynthesis (Demetriou et al., 2007). Reduction in maximum quantum yield of PSII efficiency (Fv/Fm) in sugarcane under salinity stress due to lesser uptake of water lowers the ATP synthase electrochemical potential as well as photo-system-I, which can further hinder the ATP and NADPH production through interference in photosynthetic apparatus (Silva et al., 2018). Variation in the photosynthetic complex parameters relies on the duration and severity of stress, with plant species being the main factor.

The leaf chlorophyll content of the plant plays a significant role in its photosynthetic capacity. Under salinity or drought conditions, a reduction in chlorophyll content caused by photooxidation of photosynthetic pigments and degradation of chlorophyll is supposed to be a consequence of oxidative stress. Sugarcane is an isohydric plant, capable of maintaining water potential under water-deficit conditions (Meinzer and Grantz, 1990), and this characteristic of the plant helps to use water more efficiently under mild-to-moderate osmotic stress. However, under severe stress conditions, the chloroplast structure of plants is affected due to their chlorophyll content, which leads to a reduction in photosynthetic rate. Different studies have reported that sugarcane chlorophyll concentration decreases in response to osmotic stress (Tang et al., 2021). Sharma et al. (2021) reported a reduction in SPAD (soil plant atmosphere device) reading of sugarcane under salt stress by 56% as compared to their non-stressed counterparts. Under extreme drought stress conditions, the tolerant sugarcane varieties had higher SPAD values (Zhang et al., 2020). Reduction in total chlorophyll content was noticed in both tolerant and susceptible sugarcane clones exposed to salinity, and the level of reduction was less in tolerant clones. Additionally, phenolic and anthocyanin syntheses increased (Wahid and Ghazanfar, 2006). During salt stress, the chlorophyll content of sugarcane becomes reduced, and this impacts the plant’s photosynthetic capacity. Commercial varieties, which have higher chlorophyll content during salt stress, exhibit higher activity of aminolevulinic acid synthase (ALA synthase) or lower chlorophyllase (Dhansu et al., 2022). The interruption in the photosynthetic potential caused by lower photosynthetic pigments reduces the primary production efficiency of the cane crop.

The recognition of the stress event initiates cascades of signal transduction; further interaction with the phytohormones transduction of signal via pathway takes place (Harrison, 2012). Moreover, their important role in plant growth endogenous phytohormone assists in the adaptation of plants to drought and salt stresses through manipulation of their molecular and physiological responses (Fahad et al., 2015; Wani et al., 2016). A diverse group of phytohormones such as ABA, cytokinins (CK), SA, indole-3-acetic acid (IAA), jasmonic acid (JA), and gibberellins (GA), biosynthesis, accumulation, and their further distribution are all influenced under drought and salt stress conditions (Eyidogan et al., 2012; Gujjar and Supaibulwatana, 2019). Rodrigues et al. (2011) reported the differential expression of genes associated with hormone metabolism, stress response, signal transduction, and photosynthesis under different levels of drought in sugarcane. Under drought- and salt-induced stress, signal perception triggers the synthesis of phytohormone, abscisic acid, which plays a significant role in the stress response. Stress perceived by plants triggers the ABA synthesis principally in roots. Moreover, synthesis of ABA can also take place in the leaf cells; further, it can be translocated throughout the plant and can regulate the physiological activities such as stomatal aperture, activities of channels, and upregulation of ABA response-associated genes (Gujjar et al., 2014; Fahad et al., 2015; Ullah et al., 2018). Li et al. (2014) suggested that the endogenous synthesis of ABA as well as its exogenous application in sugarcane imparts drought tolerance through enhanced expression of its antioxidative system. A steady decline in the transpiration rate as well as stomatal conductance, associated with increased ABA content, was observed in sugarcane during rising drought conditions introduced for up to 9 days (Li et al., 2016).

Endogenous ABA, as well as exogenous application, regulates the stomatal closing and opening (Wilkinson and Davies, 2002) via one of the various signaling pathways (Neill et al., 2008) involving a network of alternative intermediate molecules such as secondary metabolites and mineral ions (An et al., 2016; Li et al., 2016). ABA-regulated gene in sugarcane, SoNCED, upregulated under drought stress, which encodes an enzyme 9-cis-epoxy carotenoid dioxygenase, which commits the rate-limiting steps of ABA biosynthesis (Li et al., 2013). Bundle sheath cell’s water status regulation is linked to SoDip22 (sucrose-phosphate synthase) (Sugiharto et al., 1997). The effect of IAA is extensively accepted for the invigorations of plant growth and their development (e.g., apical dominance, cell elongation, and vascular tissue expansion) (Fahad et al., 2015). This phytohormone appears to carry out growth correspondence during drought and salt stresses (Eyidogan et al., 2012; Iqbal et al., 2014), induce the expression of genes responsible for the initiation of the root meristem, accelerates the root branching, and improves stress tolerance in plant. Li et al. (2016) found an increase in both ABA and IAA contents in sugarcane in response to drought stress.

Salicylic acid, as a phenolic compound, is generally associated with pathogen-mediated protein expression and regulation (Miura and Tada, 2014). Additionally, various reports highlighted the importance of SA in combating drought and salt stresses and defending the plant against them (Fahad and Bano, 2012). Almeida et al. (2013) reported the differential expression of sucrose-phosphate and trehalose-6-phosphate under drought stress in sugar cane following the foliar application of SA.

Carbon assimilation and conversion into the glucose and other sugars are carried out into the source organs (exporter of photosynthates, e.g., completely developed leaves), and photo assimilates are exported to the sink organs (photo accumulation importers, e.g., stems, root, fruits, and seed) for the growth and development of plant organ (Yu et al., 2015). During plant growth phases, communication between organs of source and sink influences the assimilation of carbohydrates and their partitioning/allocation, both of which are closely related to photosynthesis. Reduction in the photosynthetic activity under drought and salt stress conditions exacerbates the effect on carbon flow in sink organs (Courtois et al., 2000; Lebon et al., 2006). Drought and salinity stresses influence phloem loading and sugar metabolism and have been extensively studied (Pattanagul and Thitisaksakul, 2008; Lemoine et al., 2013). Reduced photosynthesis leads to the alteration in carbohydrates at the source and inconsistency in the translocation pattern between source and sink levels under drought and salinity, particularly in sucrose-translocating plants (Pattanagul and Thitisaksakul, 2008). Furthermore, the reduction in demand caused by stress-induced growth limitation raises the sugar concentration.

During drought stress, the expression of various genes including gluconeogenic enzymes fructose-bisphosphate aldolase (Cramer et al., 2007), soluble sugar phosphorylating enzyme hexokinase (Whittaker et al., 2001), and enzyme associated with the biosynthesis of the raffinose family oligosaccharide galactinol synthase (Taji et al., 2002) enhanced, as exhibited by their abundance at the transcript level in the source leaves. Enhancement in the activity of sucrose-phosphate synthase over ADP glucose pyrophosphorylase in sink tissues under drought stress augmented sucrose synthesis over starch by interfering with the ADP glucose pyrophosphorylase (Geigenberger et al., 1997). Maintenance of cell turgidity under drought can be regulated through a higher content of sugar found in the cytosol, which lowers the osmotic potential (Razavi et al., 2011). Subsequently, a decline in photosynthetic activities as well as leaf senescence can be observed. An increase in plant sugar levels under drought stress is probably an attempt by plants to balance their metabolism for the further maintenance of osmotic homeostasis (Giné-Bordonaba and Terry, 2016). Since sugar concentration acts as a differential sensor for the process like, leaf development by the route of senescence may become affected; however, it can be utilized in carbon mobilization and subsequent reallocation to assist the host plant in mitigating drought-induced negative effects (Whittaker et al., 2001).

Irrigation with poor quality water commonly leads to the salinity stress that initially affects the physiological traits of the plants as influenced under drought stress, specifically the early response of the plant, since both the stresses interrupt the water absorption via the root system through osmotic effects (Navarro et al., 2008). Moreover, prolonged exposure to particular stress affects differently, and the transport of sodium into the plant tissues by the route of the transpirational pull leads to sodium toxicity that can be considered as an initial response to stress. The involvement of K⁺ channels in Na⁺ recirculation via leaf phloem to roots led to a decrease in Na⁺ concentration in leaves (Berthomieu, 2003). The distinct mechanisms utilized by plants to alleviate salt stress are either reduction in cytoplasmic salt concentration or curtailed entry of ions into the plant tissues.

Plants developed complex oxidation–reduction reaction in which ROS are used as markers to regulate normal and stress-related biological processes (Mittler et al., 2011). The activation of the antioxidant defense system is an important chemical process to oxidative stress with respect to plant adaption. Thus, the positive regulation at the transcriptional and post-transcriptional levels of the antioxidant defense system acts as an important marker for stress such as drought and salinity (Gill and Tuteja, 2010). The important ROS scavengers in plants are APX, CAT, and GPX. In comparison to the upregulation of CAT and GPX, the upregulation of APX occurs strongly at the post-transcriptional level. APX is a marker enzyme for the cytosol, chloroplast, and peroxisomes and is found in all plant species. Mittler and Zilinskas (Mittler and Zilinskas, 1994) found higher activity of APX during drought stress in peas. In Arabidopsis, alx8 mutant (altered expression of APX2) has increased tolerance toward drought stress (Estavillo et al., 2011). Transgenic tobacco with overexpressed APX (peroxisomal/cytosolic) from poplar has increased plant performance during drought (Rodrigues et al., 2009). Rodrigues et al. (2009) observed enhanced expression of a peroxidase gene in a drought-tolerant sugarcane cultivar, and a decrease in peroxidase activity is considered to be a limiting step to sugarcane ROS deactivation (Chagas et al., 2008).

CAT is a tetrameric heme-containing enzyme. In peroxisomes, the reaction of catalase involves the dismutation of H₂O₂ into H₂O and O₂•⁻. An isomeric form of catalase (CAT2) is important during severe drought stress. CAT activation occurs at the post-transcriptional level. The complex regulation of CAT activity involves gene expression, translation, and protein turnover when plants are exposed to severe drought (Sofo et al., 2015). The activities of APX and CAT control redox levels in cells and may contribute to the increased capacity of some sugarcane cultivars to decompose H₂O₂ under drought conditions (Jain et al., 2015; Sales et al., 2015). SOD is a class of metalloenzyme. It catalyzes the dismutation of two molecules of O₂•⁻ into molecular oxygen and H₂O₂. The higher activity of SOD isoforms (Mn-SOD, Fe-SOD, Cu, and Zn-SOD) counteracts O₂•⁻ accumulation in different cell organelles during drought stress. Transgenic plants that are more drought-tolerant expressed higher activity of Cu-Zn SOD (Wu et al., 2016). SOD activity in sugarcane is regulated by water-deficit conditions (Jain et al., 2015). Moreover, drought-tolerant sugarcane cultivars exhibit maximum SOD activity under water deficit (Jangpromma et al., 2012). Different SOD isoforms in sugarcane cultivars with different expression patterns have a major impact on antioxidant response during stress conditions (Jain et al., 2015).

However, low-molecular-weight compounds such as glutathione-S-transferase (GST), thiol peroxidases, and GPX engage in an antioxidant defense system. Drought increases the activity of thiol peroxidases such as NADPH-thioredoxin reductase (NTR), ferredoxin-dependent TRX reductase (FTR), and GSH/GRX systems. To overcome the drought stress activity of MDHAR, DHAR, GST, and GR transcripts were also expressed (Vaseghi et al., 2018). PRX plays a major role during redox signaling and information transmission in the cell, which might be their predominant function under drought stress (S.-Z. Zhang et al., 2006; Thalmann and Santelia, 2017). To protect plant cells from oxidative damage, non-enzymatic antioxidant molecules can work in tandem with the enzymatic ROS scavenging system.

Plants in a drought environment must integrate stress signaling and osmoprotective mechanisms. The biotechnological implication for improving abiotic stress tolerance involves metabolic pathway engineering for a number of osmolytes such as glycine betaine, sorbitol, mannitol, and proline (Gujjar et al., 2018; Gujjar et al., 2019). Transgenic plants engineered with osmoprotectant molecules have improved resistance to drought stress, high salinity, and cold stress (Suprasanna et al., 2016; Gupta et al., 2022). Abiotic stress increases the activity of sucrose catabolic enzymes such as invertase (INV) and sucrose synthase (SuSY). Invertase catalyzes sucrose into glucose and fructose, while sucrose synthase gives uridine diphosphate (UDP)-glucose and fructose. Trehalose is a non-reducing disaccharide sugar that acts as an osmolyte and stabilizes membrane lipids (Suprasanna et al., 2016). Sugars homeostasis is a dynamic process in which sucrose–starch interconversion occurs as per the cellular requirement. Starch metabolism and its associated enzymes perform important roles in alleviating the effects of abiotic stress in plants (Thalmann and Santelia, 2017). In sugarcane, the accumulation of osmolytes in drought conditions prevents damage associated with ROS generation (Guimarães et al., 2008). Proline is an efficient scavenger of ROS. Furthermore, proline can function as a compatible osmolyte and molecular chaperone. During water-deficit conditions, proline is accumulated in plants mainly due to increased synthesis and reduced degradation (Das and Roychoudhury, 2014).

In the present climatic change scenario, frequent occurrences of abiotic stress such as drought and salt stresses in various regions are major constraints for the sugarcane production system that leads to a decline in tonnage. Despite recent progress in conventional breeding as well as transformation technique, the development of drought- and salt-tolerant sugarcane is still a major challenge. These limitations are due to the complex sugarcane genome, complicated plant responses to water-deficit and salinity conditions, and difficulty in the identification of morpho-physiological traits that could be utilized during the selection processes for the commercial production of drought- and salt-tolerant varieties. The parental selection is very important from the viewpoint of the development of commercial cultivars and depends on the progeny performance (Breaux, 1984). The response of clones to various biotic and abiotic stresses, in addition to cane yield and sucrose content, should be the main criteria for selecting good parental lines under changing climatic conditions. Two types of selection approach, i.e., individual and family selection, are utilized in sugarcane breeding programs. Individual selection approach is used when characters have high heritability, and family selection is used when the heritability of the family mean is higher than that of a single plant (Falconer et al., 1996).

Hybridization techniques include biparental crossing and poly crossing involving two and more than two parents, respectively (Cox, 2000). Hybridization and selection techniques are mainly used in sugarcane breeding programs to generate new recombinant clones with high yield, sugar content, and resistance to stresses. Generally, the parents for breeding for peninsular zones and north Indian plain zone are to be selected on the basis of water regimes, rainfall patterns, and irrigation facilities. As far as yield and sucrose are concerned, the studies suggest that in sugarcane breeding, more importance has to be given to the selection of the female lines than to the male lines (Misra et al., 2020). Drought is a complex trait and involves multiple dynamic interactions. During breeding for drought tolerance, selections for traits such as stalk number, height, diameter, and weight are critical along with the cane weight. Integration of suitable physiological traits in the selection program will be valuable in improving the genotypes against drought and salinity stresses. In the past, traditional breeding methods have been fruitful in the development of drought-tolerant cultivars such as Co-87 and Co-263 (Sreenivasan and Amalraj, 2001). Saccharum spontaneum, Narenga, and Erianthus serve as donor varieties in drought conditions when used as a parent for breeding the variety (Wahid and Ghazanfar, 2006; Patade et al., 2008).

The essential guiding force for adaptive responses to drought and other stresses is genetic variability. Conventional breeding is regulated by Mendelian genetics, which means that traits are passed down from parent to offspring. Plant breeders cross-pollinate parental lines with desired traits to produce progeny with desired characteristics. Conventional breeding has primarily been used to increase genetic variability in sugarcane in order to improve variety. During the grand growth phase, cane formation and elongation occur very actively (Jaiphong et al., 2016), and water stress causes crop production losses of up to 60% (Basnayake et al., 2012). Moderate stress, in contrast, has a positive effect on sucrose yield during the maturity stage. Imposing stress at specific crop stages is an effective method for screening abiotic stress and recording observations on physio-biochemical parameters at different crop intervals. Drought has the greatest impact on complex traits such as shoot, leaf, and root parameters; however, their genetic control varies between genotypes. Under water stress, highly exploitable genetic variation for cane and sugar yield was observed (Hemaprabha et al., 2006; Meena et al., 2013; Basnayake et al., 2015; Meena et al., 2020). Through conventional breeding, varieties that have high yield and high sucrose content and are drought tolerant are produced.

Imposing stress at a specific crop stage is an effective approach for screening abiotic stress and recording the observation of physio-biochemical parameters at various crop intervals. Therefore, screening for these important parameters under the drought stress environment is of paramount importance for drought tolerance breeding. The selection of drought-tolerant genotypes through an indirect selection of physiological traits can also be integrated into the breeding program for the improvement of sugarcane (da Silva et al., 2012; Basnayake et al., 2015). Physiological traits are used for drought-tolerant genotype selection. S. spontaneum, Narenga spp., and Erianthus species are used in the sugarcane breeding program for the incorporation of drought tolerance in sugarcane (Meena et al., 2020). Alternatively, drought tolerance can be increased in the plants through exposure to water stress during the early stage of the life cycle (Marcos et al., 2018; Leanasawat et al., 2021). For the selection of desired clones, clones should first enter zonal evaluation trials after those distinct stages of selection; i.e., ground nursery stage, first clonal stage (C1), second clonal stage (C2), and pre-zonal varietal trial stage (PZVT) are used for selection of desired sugarcane clone. For screening crops for abiotic stresses like drought, waterlogging, and salinity, these stresses are applied at C2 and PZVT stages to identify plants that are high yielding and vigorous and perform better in a particular specific climate.

The screening of drought-tolerant cultivars showing low heritability and high interactions between genotype and environment (G × E) is difficult because of the complexity of traits and genes (Cattivelli et al., 2008). Drought-tolerant lines of some selected crops were produced through conventional plant breeding, but this method is time-consuming, labor-intensive, and costly (Ashraf, 2010). Crop physiology, marker-assisted breeding, GWAS, gene editing, and omics (genomics, transcriptomics, proteomics, metabolomics) are all being used to provide knowledge and tools for plant improvement (Oladosu et al., 2019). Despite the importance of quantitative trait loci (QTLs) and GWAS in identifying genomic regions associated with drought-related traits, genetic variants associated with drought-related traits are largely unknown (Liu and Qin, 2021). Given this, there is an urgent need to integrate modern breeding techniques with multi-omics platforms and high-throughput phenotyping to vastly improve our understanding of drought stress response in sugarcane.

Sugarcane parent species have a complex genome that evolved from a large breeding pool of five closely related genera: Saccharum species (S. officinarum, S. spontaneum, and Saccharum robustum), Erianthus species, Miscanthus species, Narenga species, and Sclerostochya species (Scortecci et al., 2012). Several Saccharum spp. and related genera, including Erianthus spp., Saccharum barberi, Saccharum sinense, and S. robustum, are thought to be good sources for instilling salinity tolerance in commercial cultivars (Rao and Sreenivasan, 1985). The ploidy level of Saccharum species ranges from 5× to 16× (Manners et al., 2004). Modern sugarcane varieties are produced by interspecific hybridization between S. officinarum and S. spontaneum. The tropical cane, which is the noble cane S. officinarum, has a thicker stem, higher sugar content, and basic chromosome number x = 10. S. spontaneum is a wild species tolerant to biotic and abiotic stresses with basic chromosome number x = 8 (MaChado et al., 2009; Sica, 2021). The varied species of these genera can be classified into various gene pools based on their cross-ability with the cultivated sugarcane. In the primary gene pool (GP-I), the varied species are grouped, which can be easily crossed to produce a fertile hybrid. In the secondary gene pool, the species are crossable with certain difficulties and tend to be sterile. S. spontaneum, S. barberi, S. sinense, and S. robustum can be viewed as a secondary gene pool. However, the number of generations of breeding required for the transfer of traits in varietal improvement will be more. In the tertiary gene pool, Narenga, Miscanthus, Erianthus, Sclerostachya, Sorghum, and Zea are included, and the crossing of these species with cultivated sugarcane is difficult to achieve and needs the techniques of embryo rescue and tissue culture ( Figure 3 ). The produced hybrids will be weak, lethal, or completely sterile (Sica, 2021).

Molecular methods are suitable for the development of cultivars for tolerance to one or more traits at once. In sugarcane, the Agrobacterium-mediated transformation of Arabidopsis tubes pyro-phosphatase (AVP-1) was performed to achieve tolerance against drought and salinity stresses (Kumar et al., 2014). New breeding and genomic approaches have been used in recent decades to improve genotypic performance under abiotic stress conditions. Introgression of genes from wild species and related genera for abiotic stress tolerance traits in sugarcane is important in the development of several stress-tolerant varieties. The QTL mapping work has been performed for the disease resistance traits, soluble solid content, and yield traits (Balsalobre et al., 2017); however, to date, there is no report on mapping QTL(s) related to drought and salinity. The sources of genes/QTLs for drought and salinity tolerance need to be explored at a greater pace than the earlier, already reported species of sources such as wild relative S. spontaneum, and members of the related genera such as Narenga spp. and Erianthus spp. (Meena et al., 2020) serve as valuable resources for the same. The utilization of wild and related species in the breeding program is further made difficult due to genome complexity and polyploidy. A successful cross was made to generate an inter-generic hybrid between Erianthus arundinaceus and S. spontaneum (Lekshmi et al., 2017).

Some of the hybrids have higher root volume and polyphenol component, thereby influencing the extent of drought tolerance (Fukuhara et al., 2013). The transcriptomic analysis of the drought-tolerant and sensitive genotypes led to the identification of several differentially expressed genes (DEGs), and it was noted that the genes such as MYB, E3 ubiquitin ligase, small ubiquitin-related modifier-protein (SUMO-protein), SIZ2, and aquaporin are drought-responsive genes and transcription factors (Belesini et al., 2017). When salt stress is introduced, an osmotic adjustment mechanism begins to maintain cell turbidity, resulting in slow growth in stressed plants. Changes in morphology, anatomy, water relations, photosynthesis, hormones, ion distribution, and biochemical adaptation may occur depending on genotype, developmental stage, stress intensity, and duration (Liang et al., 2018). Identifying QTLs associated with salt tolerance is an important step toward improving salt-tolerant varieties (Nakhla et al., 2021) and increasing crop production in saline soils. The various QTLs associated with salt tolerance have been identified in cultivated crops such as soybean (Glycine max L. Merr.) (Cho et al., 2021), maize (Zea mays L.) (Luo et al., 2019), and rice (Oryza sativa L.) (Singh et al., 2021), but reports on identified QTL(s) in sugarcane are still lacking. Understanding the magnitude of the impact of salinity on sugarcane crop growth and yield, as well as mapping the salinity stress QTL, should be the primary focus. The list of candidate genes suitable for the improvement of sugarcane for tolerance against salinity and drought is presented in Table 1 .

Transcriptomics involves the study of total mRNA synthesized by the genome under specific conditions; therefore, it is important to study transcriptome profiling during different stress conditions. Furthermore, alterations in climatic conditions have posed a major threat to agricultural production, which severely impacts the food requirement of the population. In case of severe abiotic stress, plants have developed various kinds of molecular approaches to combat natural stress conditions, in particular the transcription factors (TFs), to deal with different abiotic stress in sugarcane and other crops that can be studied using advanced transcriptomics technology. The sugarcane plant belongs to the Poaceae family, which produces sucrose and is greatly used in agro-based enterprises in different tropical and subtropical regions. To that end, the sugarcane transcriptome profile under stressed conditions must be investigated.

The NAC (no apical meristem (NAM), ATAF1/2, Arabidopsis transcription activation factor 1/2, and cup-shaped cotyledon) proteins are a class of TFs that are useful in plant developmental activities and help in providing resistance to various kinds of stresses in different crops. Recently, Belesini et al. (2017) studied the transcriptomic activity of two different sugarcane varieties, “SP81-3250” and “RB855453”, grown under different drought levels with the aid of Illumina HiScanSQ platforms. They observed the upregulation of several genes such as ascorbate peroxidase, E3 SUMO-protein ligase SIZ2, MYB, key enzymes involved in the flavonoid biosynthesis like coenzyme A ligase, and aquaporins, which are responsible for drought tolerance. Furthermore, Belesini et al. (2017) identified various kinds of receptor-like protein kinases (RLKs) and their elicitation upon onset of drought in drought-sensitive varieties; these RLKs are a major player in drought sensing, bHLH TFs (basic helix-loop-helix), and 1-aminocyclopropane-1-carboxylic acid oxidase (ACC oxidase) generated in ethylene biosynthesis and different unknown genes. These TFs play crucial roles in abiotic stress resistance (Guo et al., 2017). However, Pereira-Santana et al. (2017) employed a next-generation sequencing technique to unravel the transcriptome profile of the cultivar Mex 69-290 against osmotic stress in Mexico. They observed that enhancement in the expression of genes is related to transcriptional regulation, oxide reduction, carbohydrate breakdown, flavonoids, and distinct kinds of secondary metabolites in the tolerant cultivar. Additionally, genes related to ABA biosynthesis, water regulation, and heat stress were also upregulated.

Proteomics is an advanced study of the proteome that gives useful information about proteins in plant cells at various stages of growth under specific climatic conditions; therefore, it is necessary to study the sugarcane proteome, which helps in determining the drought-resistance mechanism. Sugiharto et al. (2002) isolated and identified a drought-inducible gene SoDip22 in a drought-stressed cultivar of sugarcane by using two-dimensional gel electrophoresis (2-DE) and reported that SoDip22 functions in drought stress adaptation in the cells of bundle sheath, which is where ABA-mediated signaling path is induced. An 18-kDa protein was extracted and purified, and 2-DE methodology was applied to identify that protein that was present in leaves of sugarcane subjected to drought conditions (Jangpromma et al., 2007). Various proteins useful in the photosynthesis process and enzymes associated with anti-oxidative injury were isolated and characterized by 2-DE as well as liquid chromatography–tandem mass spectrometry (LC–MS/MS) (Ngamhui et al., 2012). The overexpression of gene EaDREB2 (dehydration responsive element-binding 2), which was transferred from E. arundinaceus, in combination with pea helicase gene PDH45 (pea DNA helicase), increased drought and salinity tolerance in transgenic sugarcane (Augustine et al., 2015). To understand the impact of drought on protein profiling two contrasting cultivars of sugarcane, RB 72910 (resistant) and RB 943365 (susceptible) were grown under water-stressed conditions for 30 days. The water deficit-associated proteins were identified by using 2-DE and mass spectrometry. Several types of proteins related to photosynthesis, signaling pathways, and regulation processes were either upregulated or downregulated in RB 72910; alternatively, these proteins were downregulated in RB 943365.

Khueychai et al. (2015) used 2-DE accompanied by LC–MS/MS to characterize different types of proteins that were responsive to drought in two different cultivars: K86-161 (resistant) and B34-164 (susceptible). Their findings demonstrated that gene expression of fructose bisphosphate aldolase, O₂-liberating enhancer proteins, and SOD increased to a greater extent in various parts of K86-161; alternatively, these proteins were found in lower quantities in B34-164 under drought conditions. Furthermore, Salvato et al. (2019) quantified the protein of drought-stressed sugarcane stalk nuclei using filter-aided sample preparation (FASP) and LC–MS/MS techniques. Their result exhibited that most of the 74 exclusive proteins found in control plants are associated with cell wall metabolism, indicating that cell wall metabolism is negatively regulated under drought. Similarly, 37 TFs that were related to different protein domains, e.g., NAC, C2H2 (Cys2-His2), bZIP (basic leucine zipper), C3H (Cysteine3Histidine), LIM (LIN-11, Isl-1, and MEC-3), Myb-related (myeloblastosis viral oncogene homolog), HSF (heat shock factor), and auxin response factor (ARF), were characterized. These TFs are known to be present in nucleus and are synthesized by plant in response to drought conditions.

Moreover, salinity present in the soil is an important problem that affects the sugarcane growth and development process. Pacheco et al. (2013) used 2-DE and LC-MS to analyze the differentially expressed proteome in sugarcane root against the salinity stress in different cultivars and concluded that most proteins accumulated in response to stress are involved in developmental process, carbohydrate metabolism, ROS pathway, protection of protein, and membrane steadiness in resistant cultivar after 2 h of salinity appearance, whereas their presence in a susceptible cultivar was noted after 72 h of salinity stress.

Ionomics is the study of trace elements and mineral nutrients in plant systems. During salinity stress, ion concentrations in different cells are disturbed, but plants can adapt to drought and salinity through osmotic adjustment. Various approaches have been identified in response to highly saline and drought conditions in crop plants. Physiochemical changes in buds of sugarcane were reported in the canes exposed to salinity stress (Rasheed et al., 2016). Salinity causes overproduction of hydrogen peroxide, high amount of Cl⁻and Na⁺ in sugarcane plant tissue, and decreased amount of K⁺ and Ca²⁺, as well as Ca²⁺:Na⁺ and K⁺:Na⁺ proportions, and is useful in the production of different osmolytes in sugarcane plant cells.

Metabolomics is an advanced technique used for exclusive profiling of all metabolites present in plant cells; therefore, it has been actively used in describing the mechanism of salinity and drought stresses in sugarcane. In recent reports, salt-tolerant and salt-sensitive sugarcane varieties were grown under salinity and drought conditions to identify the various secondary metabolites involved in salinity tolerance. Accumulation of a high amount of proline and lower Na⁺ in leaves of salt-tolerant variety was noticed (Chiconato et al., 2019). Similarly, enhanced levels of phenolic acid, anthocyanin content, and flavone content were beneficial in providing resistance to drought and salinity conditions in sugarcane plants (Molinari et al., 2007; Ali et al., 2019). Furthermore, in order to investigate the physiological and developmental changes in sugarcane-developing buds, which were subjected to salt stress, Rasheed et al. (2016) undertook an experiment, according to which salinity increases the production of hydrogen peroxide, increases the tissue limit of Cl⁻ and Na⁺, decreases the K⁺ and Ca²⁺, and Ca²⁺:Na⁺ and K⁺:Na⁺ ratios, and increases osmolyte synthesis in growing sugarcane buds. Similarly, Vital et al. (2017) in their metabolomics studies revealed that drought and salinity stresses led to a reduction in sucrose content and an increase in the reducing sugar such as glucose and fructose in different sugarcane cultivars. Sugarcane varieties like RB867515 showed significantly higher glucose content when subjected to drought stress. Xylose and inositol sugars also increased during drought and salinity stresses. Moreover, the level of organic acids increased as compared to the levels of pyruvate and isocitrate, which sharply decreased during drought conditions. Several amino acids, which include tryptophan, phenylalanine, tyrosine, leucine, valine, proline, glutamine, lysine, isoleucine, asparagine, and glycine, accumulated in different cultivars of sugarcane like RB867515 and RB855536.

Production of different osmolytes by microbes protects the plant from drought stress. Increasing the root–shoot biomass through PGPB-mediated IAA production may significantly contribute to coping with drought stress by the plant. Production of aminocyclopropane-1-carboxylate deaminase (ACCD) by rhizospheric bacteria suppresses the ethylene signaling pathway to negatively regulate root drying under water stress conditions. There is also a report of increased proline content in plant leaves and root after injection with drought-tolerant bacteria, thus achieving better plant growth (Yuwono et al., 2005). PGPBs also increase the availability of some chemicals, which play a role in growth promotion and provide micronutrients to the host plant. Production of ESP plays an important role in protection from desiccation (Vanhaverbeke et al., 2003). Secretion of SA by microbes acts as a signaling molecule under drought stress, which triggers genes that act as heat shock proteins (HSPs), antioxidants, and chaperones and also activates genes synthesizing secondary metabolites (El-Daim et al., 2019). It is reported that microbes increase the metabolites like pyruvic acid (PA), thiamine pyrophosphate, uridine diphosphate, succinic acid, and dihydroxyacetone, which helps in combatting drought (El-Daim et al., 2019).

PGPBs can reduce the effects of salt stress through both direct and indirect mechanisms (M’piga et al., 1997; Gupta et al., 2021b), and it was reported that PGPBs act against different phytopathogens by inducing different defense-related enzymes like POX, chitinase, β-1,3-glucanase (GLU), and phenylalanine ammonia lyase (PAL). Extracellular polymeric substances (EPSs) produced by PGPBs bind with positively charged ions such as Na⁺ and reduce the accessibility of toxic ions (Upadhyay et al., 2011). EPS around roots increases the water potential, providing a physical barrier to toxic ions, and improves plant nutrient uptake by plants (Dodd and Pérez-Alfocea, 2012; Zulfiqar et al., 2020). At higher salt concentrations, a greater influx of Na⁺, Cl⁻, Ca²⁺, Mg²⁺, SO₃²⁻, or CO₃ ²⁻ leads to ion toxicity. PGPBs maintain high K⁺/Na⁺ ion ratios and regulate toxic ion homeostasis. It reduces the accumulation of ions like Na⁺ and Cl⁻ in the leaves, increases ion exclusion by the root system, or modulates the ion transporter expression (Zhang et al., 2008; Ha-Tran et al., 2021). A plasma membrane protein, high-affinity K⁺ transporter (HKT), facilitates Na⁺ ion transport in plants, which prevents the over-accumulation of Na⁺ ion concentration in shoots (Zhang et al., 2008). Host–microbe interactions influence the tissue-specific regulation of some genes like HKT-type genes during salt stress to maintain ion homeostasis.