The most defining concern of the present and future agriculture, and of humanity, is climate change. Human dependence on wild grains began in Halocene—a geological epoch, which further laid the basis of agriculture, as it was characterized by stable and warm temperatures. However, a rapid surge in the atmospheric carbon dioxide levels from 100 ppm to over 400 ppm with an average temperature increase by 1°C, in the past 70 years, could be a steppingstone for unstable environmental temperatures. A further escalation of temperature by 3°C by the year 2100, and even by 8°C or more is also predicted. Such drastic escalations have been identified as antagonists of human civilization (Gowdy, 2020). Similarly, agricultural activities, particularly, the use of chemical fertilizers not only incites diabetes and cancer like chronic diseases in humans, but also deteriorates the environment; thereby aggravating the climate change (Ahmad et al., 2021b). In this regard, European Union (EU) has implemented a “European Green Deal” program that proposes a 20% reduction in the use of fertilizers and a 55% reduction in greenhouse gases by 2030 as compared to 1990 levels (European-Union, 2020). Apart from this, soaring human population that is expected to reach 9.7 billion by 2050, is amplifying the pressure on agriculture to meet the growing food demands globally. Substantially lower crop yield ha^–1 in comparison to increasing world population is already reported by World Food Program (WFP). Likewise, by the year 2100 a decrease in the yield of maize, wheat, and rice, by 20–45, 5–50, and 5–50%, respectively, has been predicted by Food and Agriculture Organization (FAO) if the climate change remains incessant (Arora, 2019). This ever-increasing food demand has provoked intensive agriculture systems with the unprecedented use of chemical fertilizers, pesticides, herbicides, fungicides, along with the exploitation of land and water resources; thereby further aggravating the climate change. If such trends persist, they will not only compromise the global food safety and security but may also provoke a civilization collapse.

Climate change provokes a plethora of abiotic stresses (flooding, drought, salinity, etc.) in plants (Arora, 2019; Gowdy, 2020). These abiotic stresses are one of the major limiting factors for constraining crop production (Ramadoss et al., 2013), for several economically important horticultural crops, and contribute to almost 70% of yield gap (Rouphael and Colla, 2020). Salinity is one such abiotic stress that hampers plant physiological functioning in a number of ways: resulting in lower crop production. It is attributed as a measure of salt amount in soil or water content (Arif et al., 2020), and is classified as primary salinity: caused as a result of natural processes i.e., rain, weathering, wind, etc., and secondary salinity: caused as a result of anthropogenic activity, i.e., excessive irrigation, deforestation, clearing land (Munns and Tester, 2008; Arif et al., 2020). The climate change is directly responsible for primary salinity considering the excessive rains only, for example 100 mm of rainfall year^–1 would deposit 10 Kg ha^–1 of sodium chloride if it contains 10 mg Kg^–1 of salt (Munns and Tester, 2008), whereas indirectly to secondary salinity. Subsequently, intensive agricultural systems and poor agronomic practices, like over-fertilization, desertification, excessive irrigation, etc., have increased the levels of alkalinity and salinity of cultivated soils (Arora, 2019; Li et al., 2021). In 2013, the salinity affected cultivated soils were estimated to be over 800 million ha (Ramadoss et al., 2013) globally that culminated to around 900 million ha in 2020 (Velmurugan et al., 2020). Such a rapid conversion of fertile soils into saline ones can be regarded as a major threat to food security and agricultural sustainability.

Salinity is expressed as the electric conductivity (EC) of the soil solution, and the soil is generally denominated as saline if its EC is 4 dS m^–1 or more, it approximately equals to 40 mM NaCl, producing a 0.2 MPa approximate osmotic pressure (Munns and Tester, 2008). Sodium ions (Na⁺) are considered as major contributors to salinity, whereas Cl^–, Mg²⁺, SO₄^2–, or HCO₃^– are also responsible for soil salinity but to a lesser extent (Munns and Tester, 2008; Zörb et al., 2019). Higher concentrations of these salts trigger ionic and osmotic stress resulting in the generation of reactive oxygen species (ROS), reduced cell/leaf expansion, leaf abscission, reduction in ψ_w, distorted membrane potential, membrane injury, altered rates of photosynthesis, stomatal closure, protein destabilization, altered carbon portioning, cavitation, reduced nitrogen assimilation, ion cytotoxicity, and cell death among others; thereby provoking a drastic reduction in crop growth and yield (Ashraf and Harris, 2004; Munns and Tester, 2008; Acosta-Motos et al., 2017; Farooq et al., 2017; Arif et al., 2020). Therefore, it is indispensable to devise novel strategies for combating salinity.

Use of natural plant extracts (PEs) (or “botanicals”) could be one of the salinity mitigation ecofriendly strategies. PEs are potential alternatives to chemical fertilizers. PEs fall under the umbrella of plant biostimulants, and are used to enhance plant growth (Calvo et al., 2014; Drobek et al., 2019). PEs are the concentrates of plants and could be prepared using any part of the plant, i.e., seeds, roots, stems, leaves, bark, flowers, etc. (Semida and Rady, 2014; Lorenzo et al., 2019; Nessim and Kasim, 2019; Rady et al., 2019b; Zulfiqar et al., 2020b). The application of PEs could be either in liquid form as foliar spray and/or root treatment, or as soil preparations like granules, concentrates, solutions added to soil, or powders (Drobek et al., 2019). PEs can be associated with the amelioration of salinity as they are the sources of prominent phytochemicals like vitamins, carotenoids, amino acids, phytohormones, mineral nutrients, phenolics, and antioxidants (Latef et al., 2017). There are several reports where these compounds, used either individually or in mixtures, were found to be effective against salinity (Calvo et al., 2014; Iqbal et al., 2014; Bulgari et al., 2015; Drobek et al., 2019; Shukla et al., 2019; Zulfiqar et al., 2020b). However, the effect of PEs is often concentration dependent. Similarly, plant part and age of plant used as an extract also influences the PEs overall proficiency.

Currently, there are no extensive studies reported on the particular use of PEs and their subsequent mechanism of action against salinity. Previously reported studies mostly discuss the use of biostimulants that is rather a broader term and even includes microbial and non-microbial formulations, protein hydrolyzates, PEs, amino acid, seaweed extracts, etc. (Du Jardin, 2015). Furthermore, previous studies have discussed the use of biostimulants on abiotic stresses in general (Calvo et al., 2014; Drobek et al., 2019; Shukla et al., 2019; Rouphael and Colla, 2020; Zulfiqar et al., 2020b), whereas no specific study on the use of PEs against salinity is reported. Therefore, the aim of the study was to elaborate the potential of ecofriendly and natural PEs as salinity alleviators, and to underline their possible mode of action with the subsequent physiological changes thus induced. Since an explicit mode of action of PEs remains nebulous, hence this subject has been estimated considering the up or down regulation of signaling signatures, altered photosynthetic rates, and redox metabolism in general.

This review is divided into three sections. Impact of plant based biostimulants under normal conditions is discussed first followed by salinity induced physiological, biochemical, and genetic changes in plants. Subsequently, the use of PEs, including their sources and methods of preparation, as salinity mitigation strategy is discussed. Finally, all this is concluded by identifying the limitations and future perspectives of the use of PEs against salinity.

Use of plant based biostimulants is rapidly gaining popularity in agriculture. Plant based biostimulants, apart from inducing stress tolerance, are also effective in regulating a number of plants physiological processes; thereby improving plant growth and yield (Brown and Saa, 2015). They may comprise of protein hydrolyzates and amino acids, hormone-, amino acids-, or nutrients containing products, vegetable oils, etc. of plant origin (Ikrina and Kolbin, 2004; Kauffman et al., 2007; La Torre et al., 2016; Yakhin et al., 2017). Their mechanism of action might involve phosphorus (P) release from soils, activation of nitrogen (N) metabolism, stimulation of root growth, nutritional and hormonal regulation, or generic stimulation of soil microbial activity. Previous reports have documented that the application of biostimulants has enhanced various physiological processes including plant nutrient uptake and utilization, photosynthesis, water use efficiency, synthesis and concentration of growth hormones (auxins, gibberellins, and cytokinins), germination, and senescence reduction (Parađiković et al., 2011; Bulgari et al., 2015; Nasir et al., 2016; Merwad, 2017; Ur Rehman et al., 2017; Milić et al., 2018; Younis et al., 2018), which in return increase plant production, yield, post-harvest quality, and shelf life of agricultural products.

Among various plant based biostimulants, PEs are economical and easy to prepare. Several studies have reported their beneficial effects on plants growth and yield. For instances, an increase in the growth and hormonal profile was observed in eggplant and snap bean when aqueous garlic extracts were applied (Elzaawely et al., 2018; Ali et al., 2019). Similar results were reported in case of moringa leaf extracts being applied on sword lily, where they improved plant growth and vase life by regulating various physiological processes (Zulfiqar et al., 2020c). Likewise, borage extracts were reported to supplement the primary metabolism, by enhancing leaf pigments and photosynthetic activity, and reduced chlorophyl a fluorescence, by incrementing the number of active reaction centers per cross section, in lettuce plants (Bulgari et al., 2017). Likewise, vine-shoot and oak extracts were found to significantly improve wine yield and quality by triggering amino acids and volatile compounds production (Sánchez-Gómez et al., 2016). In addition, PEs are also responsible for increasing the shelf life and postharvest quality of agricultural products. For example, moringa leaf extracts were found to significantly improve avocado and citrus fruit shelf life and quality by lowering the respiration rate and water transfer from the fruits (Adetunji et al., 2012; Tesfay and Magwaza, 2017). All of these studies indicate the potential of PEs in positively regulating several physiological processes. Therefore, extrapolating the incredible potential of PEs to combat abiotic stresses, especially salinity, would be a promising approach for a sustainable agriculture.

Stress perception and signaling hold an imperative role for subsequent plant behavior. Apoplastic and symplastic pathways are the recognized routes of ions entry into plant that result in salinity (Lamers et al., 2020) (Figure 1). Stress signals are perceived by plant cell surface-based receptors that stimulate the production of secondary messengers like Ca²⁺, ROS, and inositol phosphates, among others (Rao et al., 2016). The most common surface based receptors involved in the cation sensing (Na⁺ and Ca²⁺) are membrane bound proteins or ion channels including glycosyl inositol phosphorylceramide (GIPC) sphingolipids synthesized by MONOCATION-INDUCED [Ca²⁺]_cyt INCREASES1 (MOCA1), Arabidopsis ANNEXIN4 (ANN4), and Arabidopsis HIGH-AFFINITY K⁺ TRANSPORTER1 (HKT1) (Chen et al., 2021). Similarly, ethylene receptors like Nicotiana tabacum histidine kinase 1 (NTHK1) are also reported to modulate stress signaling (Cao et al., 2008). While Ca²⁺ signaling is an important mechanism for salt-sensing and is modulated through SALT OVERLY SENSITIVE (SOS) pathway. SOS pathway consists of SOS1 Na⁺/H⁺ antiporter, SOS2 and SOS2-LIKE PROTEIN KINASE5 (PKB5) protein kinases, and SOS3 and SCaBP8 Ca²⁺ sensors (Lamers et al., 2020). The intercellular Ca²⁺ perturbations trigger Ca²⁺ sensors, e.g., SOS3 and SCaBP8, bringing about conformational changes in them in a calcium-dependent manner. These sensors further interact with their respective partners, e.g., activate SOS2, whereby a phosphorylation cascade is initiated (Quan et al., 2007; Rao et al., 2016). Subsequently, SOS1 is activated carrying out the efflux of Na⁺ ions (Figure 2). It is reported that within 20 s of stress (sodium) application a change in SOS1 exchanger activity can be detected (Lamers et al., 2020). Ultimately these alterations result in the genomic regulations (activation of transcription factors and stress responsive genes) by biosynthesizing metabolites and other compounds needed to combat salinity. There are two categories of stress- responsive genes, i.e., (a) early induced genes: immediate activation on receiving the stress signal with short-term persistence (including transcription factors, interfering RNAs, etc.), and (b) late induced genes: late activation with longer persistence periods (including membrane stabilizing, osmolytes, antioxidants, etc.) (Rao et al., 2016).

Since plants undergo an osmotic stress under saline environment, therefore it is proposed that receptors involved in osmotic, or drought sensing can also be implicated for salinity signal transduction. It is reported that a plasma membrane located channel encoded by OSCA1 regulates the Ca²⁺ influx under plasma membrane tension or extracellular osmotic pressure. Similarly, the receptor-like kinases (RLKs) present on plasma membrane have also been reported to play certain role (Lamers et al., 2020) (Figure 2).

Plants generally vary in their response to salinity. Their response could be cellular- and tissue level, morphological, or physiological. Such response depends on various factors including duration and severity of stress, plant age and its developmental stage, and plant species (Peleg et al., 2011). Therefore, some plants are found to be more tolerant (less sensitive) to salinity than others. For example, barley (Hordeum vulgare) is more tolerant to salinity than rice (Oryza sativa) (Munns and Tester, 2008). Plants are generally categorized as halophytes or euhalophytes based on their genetic adaptability to salinity, whereas they are termed as glycophytes if they are less-tolerant or not adapted to salt stress (Acosta-Motos et al., 2017). The general effects of salt stress in glycophytes occur in the following two forms (Munns and Tester, 2008; Acosta-Motos et al., 2017);

The rapid osmotic phase response of plants begins at the root-soil periphery. The toxic concentrations of salt ions build up osmotic pressure that negatively regulates the rate of leaf expansion, emergence of new leaves, lateral buds- and shoots formation. The second phase is characterized by the higher accumulation rate (i.e., toxic concentrations) of Na⁺ ions in the older leaves that cannot dilute these salts due to lack of expansion, ultimately resulting in their death. Although, some plant species are sensitive to higher Cl^– concentrations. This results in the reduced photosynthetic rates of plants that causes a reduction in their growth rate. Since ionic stress is time taking due to the accumulation of ions, therefore plant growth is affected much later with lesser impact as compared to osmotic stress.

Salinity tolerance in plants is achieved through a series of complex signaling and biosynthetic responses. However, the commonly known mechanisms of salinity tolerance include morphological (roots and aerial parts), metabolic (osmotic regulation, ionic and molecular homeostasis, and hormonal homeostasis), and genetic responses. Comprehensive reports of salinity tolerance mechanisms have been reported previously (Munns and Tester, 2008; Gupta and Huang, 2014; Tang et al., 2015; Acosta-Motos et al., 2017; Farooq et al., 2017; Arif et al., 2020). Here, a description of these mechanisms is presented to elaborate the use of PEs, as they reinforce these salinity tolerance mechanisms.

Salts accumulation around the plants root cause salinity in plants. In order to remove these toxic concentrations of salts so as to gain maximum plant yield, commonly implied strategies include flushing, scraping, and leaching. Of these leaching is most widely used strategy, in which irrigation is sustained over the evapotranspiration rates. The excessive amount of water retains the concentrations of salts below their critical limits. For instance, in a study 20–30% extra irrigated water was found to leach 70% of salts from the maize roots (Plaut et al., 2013; Rao et al., 2016). Various irrigation models (e.g., SALTMED, CWPF, Enviro-Gro, WATSUIT, TETRANS, UNSATCHEM, MOPECO-Salt, etc.) based on the leaching principle have also been suggested as salinity mitigation approach and to improve crop yield (Plaut et al., 2013). But these models either considered the salt concentration as constant for any given time or were limited in their performance under various environmental factors; thereby constraining crop yield.

Soil mulching is another conventional approach for salinity mitigation. For this, soil surface is covered with mulch or plastic sheet to enhance water availability by limiting water evaporation. Previous studies conducted on cotton plants have demonstrated positive impact of mulching against salinity in terms of reduced activity of MDA, decreased accumulation of Na⁺ in leaves and roots, inhibition of lipid oxidation, improved photosynthesis, and higher biomass (Dong et al., 2008, 2009). However, this approach has short term effects and is only efficient to affect the upper soil layer. Irrigation water treatment through aeration and/or magnetic processing is another salinity mitigation technique (Plaut et al., 2013). Nevertheless, this is not widely adopted technique due to associated costs and intricacy of process. Another conventional approach is the cultivation of halophytes in saline soils for eliminating or reducing the accumulated salts to the threshold levels for glycophytes. Some halophytes are reported to have salt glands for this purpose that possess the ability to exclude salts, whereas others are reported to have salt hairs that serve to accumulate salts. Additionally, better agronomic and farm management practices can also improve salinity. For instance, with the drip irrigation a controlled amount of water can be applied to the soil, whereby limiting the soil salination. Also, surface and sprinkler irrigations might prove effective to leach down the excessive salts from root zone. Similarly, crop rotation with perennial crops can be practiced particularly in rain-fed areas. Deep roots of perennial crops might help restore the salt-water equilibrium in the soil (Plaut et al., 2013; Rao et al., 2016).

Selection, conventional breeding, and/or genetic engineering for salinity tolerant crops are also reported as salinity mitigation techniques (Athar and Ashraf, 2009; Plaut et al., 2013). In this regard, halophytes or salinity tolerant genotypes could be bred with desired salinity susceptible crop plants to get salt tolerant progeny. Similarly, salinity susceptible plants can be genetically transformed with salt tolerant genes or can be engineered for having salt glands/hairs. Equally, elicitation of plant bioregulators, osmolytes, antioxidants, or other metabolites biosynthesis has also been regarded as a valuable approach (Ashraf and Akram, 2009; Zulfiqar and Ashraf, 2021a). However, despite of the remarkable potential, these strategies are rather limited due to huge amounts of time required and the associated costs. Equally, salinity tolerance is a complex process that is regulated by a large number of genes that obscures the crop breeding and genetic transformation processes. Therefore, attaining a salt resistant transgenic line with its subsequent adoption in field conditions still remains a challenge.

In the same way, use of microbial inoculants, chemical and organic soil amendments, and electro remediation are other promising salinity mitigation strategies that are gaining increased scientific attention lately (Sahab et al., 2020). As chemical soil amendments pose threats to soil microbiota and indirectly to human life, therefore this cannot be regarded as a sustainable approach. An alternative method of salinity mitigation is the exogenous application of nutrients and metabolites that relieves plants from Na⁺ and Cl^– injury (Munns, 2002; Plaut et al., 2013). Use of natural PEs, in this regard, can be associated with this strategy of salinity mitigation, although PEs do not only have phytonutrients but also other stress relieving metabolites, e.g., GB, proline, melatonin, etc.

Use of PEs to mitigate salinity can be regarded as an environment friendly and sustainable way of fighting abiotic stress, as it contains no synthetic chemicals. Depending upon the parts of plants used to prepare PE, it may contain various amounts of bioactive compounds (flavonols, phenolics, betaines, amino-polysaccharides, sterols, glucosinolates, terpenoids, furostanol glycosides, etc.), phytohormones, mineral elements, photosynthetic pigments, amino acids, nucleotides or nucleosides, lipids, etc. Due to the associated detoxifying and ROS quenching capabilities of these compounds, PEs are frequently used in pharmacological industry for providing protection against neurodegeneration, diabetes, muscular dystrophy, and cancer like chronic diseases (Pehlivan, 2018). Since these natural compounds are also effective in preventing macromolecules like lipids, proteins, and DNA from damage in animal cells (Pehlivan, 2018), therefore it can be deduced that they might also be effective in plants against salinity as it disrupts redox balance in plants. This has been demonstrated in previous studies that PEs contribute to a better growth, development, yield, disease-, and stress-resistance in plants given the presence of aforementioned compounds (Howladar, 2014; Drobek et al., 2019; Desoky et al., 2020; Zulfiqar et al., 2020b). Nevertheless, further scientific evidences are yet to be excavated to ensure that such a wide variety of molecules in PEs is functional or not. Similarly, viability and quality of PEs is also an aspiring research area.

Sources of PEs, methods of application, and their implications against salinity are discussed below.

Plant extracts being amalgams of various biological compounds make it difficult to map out their exact mode of action. Studies undertaken to compare the individual effects of various osmolytes (e.g., GB) or phytohormones (e.g., SA) along with PEs, concluded the supremacy of PEs as being more efficient (Abbas et al., 2010; Rady et al., 2015). Similarly, the role of PEs in signal transduction remains ambiguous as exogenous application of relevant compounds (i.e., ethylene, AsA, etc.) can also elicit the plant response. Either PEs work as elicitors of natural compounds or PEs carried molecule assimilation results in the desired results, needs further investigation. Similarly, the involvement of protein kinases has already been documented in saline environments (Zörb et al., 2019; Arif et al., 2020), but no such studies have been undertaken upon the use of PEs. Likewise, elucidation of the potential role of reactive sulfur species (RSS) (Corpas and Barroso, 2015) and RCS with respect to salinity and its subsequent mitigation through PEs might open new avenues of research. In addition, application of nanoparticles (NPs) coated PEs might improve their efficiency exponentially as various NPs have been documented effective against salinity (Zulfiqar and Ashraf, 2021b). Nevertheless, intensive research is needed for the application of PEs coated with NPs because of the reported toxicities of NPs based on their physiochemical properties and plant species (Ahmad et al., 2021a).

The role of PEs in morphological and anatomical traits like shape and size of palisade and mesophyll cells, integrity of grana and thylakoids, integrity of cristae, the number and size of plastoglobuli, number and diameter of xylem and phloem tissues, width of cortex, suberin and casparian strips development, root-, shoot-apex, endodermis, and exodermis etc. needs more scientific attention, as these factors play crucial role in gaseous exchanges, ions permeability, ψ_w, ψ_s, and energy generation processes (Acosta-Motos et al., 2017). Analogously, comprehensive studies using omics approaches (genomics, transcriptomics, proteomics, metabolomics, and bioinformatics) can further shed lights on positive or negative regulators of morphological-, metabolic-, and genomic-adjustments, target molecules, and the potential receptors activated by the use of PEs. The already identified signaling signatures, genes, and other key metabolites can be used to investigate such processes on the use of PEs. The influence of PEs on interference RNA mechanism to combat salinity also remains an enticing research area.

Use of PEs to combat salinity is a green, ecofriendly, and sustainable approach. This also opens further doors of investigation on the use of invasive plant species to be used as salinity moderator. Another important aspect is that plant response owing to PEs varies greatly from species to species and even within the same species. This might be answered by undertaking further comparative studies. Furthermore, use of salinity to trigger the production of various osmolytes and antioxidants can be utilized as an elicitation approach. Plants subjected to salinity can be used to prepare PEs that might prove more promising against salinity than conventional ones.

Salt stress has become a consistent problem in agriculture over the past few years, and was reported to culminate around 900 million ha in 2020. Plants perceive salinity by sensors, e.g., cell surface-based receptors, protein kinases, etc., resulting in a cascade of phosphorylation that regulates subsequent genetic expression. Salinity results in ionic, osmotic, and oxidative stress, which further disrupts various physiological and metabolic processes in plants. Use of PEs to combat salinity is an efficient, economical, and sustainable approach. Whole plants or parts of plants, i.e., roots, leaves, flowers, bark, seeds, pollens, etc. can be used to prepare PEs through aqueous or organic-solvent extraction techniques. PEs are multicomponent organic mixtures, containing vitamins, carotenoids, amino acids, phytohormones, mineral nutrients, phenolics, and antioxidants, etc., which facilitate stress signaling, genes regulation, redox metabolism, and synthesis of various proteins and metabolites. The degree of impact of PEs depends on various factors like plant species, age of plant, application method, etc. Molecular characterization of the PEs produced effects can pave the way for elucidating their comprehensive mechanism of action.

AA, BB, and VM: conceptualization. AA: study design, data collection and analysis, draft writing and editing, and Illustrations. VM and BB: critical analysis, revision, and supervision. All authors have read and agreed to the published version of the manuscript.

The authors declare that the research was conducted in the absence of any commercial or financial relationships that could be construed as a potential conflict of interest.

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