Salicylic acid-induced abiotic stress tolerance and underlying mechanisms in plants

1Abiotic stress has now been recognized as the biggest and potential threat for agricultural productivity all over the world. Nevertheless, anthropogenic activities in the developmental era have aggravated the degradation of agricultural system and its productivity due to major abiotic stresses such as such as metals/metalloids (hereafter termed as ‘metal/s’), salinity, ozone, UV-B radiation, extreme temperatures, nutrient (deficiency and excess), and drought. In fact, the imposed abiotic stresses can potentially influence almost all physiological, biochemical, and molecular processes in plants from the early stage of seed germination to maturity, and eventually cause severe losses in the economic yield of crop plants. It has been projected that abiotic stresses may adversely affect 70% yield of staple food crops. In one of the estimates of U.S. Environmental Action Group, the toxicity caused by varied metals has been one of the major concerns to the agriculture system and health of more than 10 million people in eight countries (such as China, Dominican Republic, India, Kyrgyzstan, Peru, Russia, Ukraine, and Zambia; ). In addition, increasing salinization of arable land is expected to result in 30% land-loss by the end of 2028 and 50% by mid of the 21st century. Notably, the rise in CO2 concentration by about 500–1000 ppm by the year 2100 has been projected to cause an increase in the mean temperature by approximately 3°C. Hence, together with salinization of arable land, drought, nutrients-unavailability, metal toxicity, and climate change have been expected to significantly aggravate the problem.

2Plant growth regulators play important roles in the regulation of plant developmental processes and signaling networks as they are involved either directly or indirectly in a wide range of biotic and abiotic stress responses and tolerance in plants. Salicylic acid (SA) is a phenolic compound involved in the regulation of growth and development of plants, and their responses to biotic and abiotic stress factors. SA is involved in the regulation of important plant physiological processes such as photosynthesis, nitrogen metabolism, proline (Pro) metabolism, production of glycinebetaine (GB), antioxidant defense system, and plant-water relations under stress conditions and thereby provides protection in plants against abiotic stresses. Apart from its involvement in the induction of defense-related genes and stress resistance in biotic stressed plants, SA has been shown to improve plant tolerance to major abiotic stresses such as metal, salinity, osmotic, drought, and heat stress. Exogenously sourced SA to stressed plants, either through seed soaking, adding to the nutrient solution, irrigating, or spraying was reported to induce major abiotic stress tolerance-mechanisms. SA influences plant functions in a dose dependent manner, where induced or inhibited plant functions can be possible with low and high SA concentrations, respectively. For example, in Matricaria chamomilla, 50 and 250 μM SA concentrations were reported to, respectively, promote and inhibit growth. In another instance, 0.1 and 0.5 mM SA promoted photosynthesis and growth of Vigna radiata but an inhibited growth was evidenced with 1.0 mM SA. Besides the concentration of SA, the duration of the treatment, plant species, age, and treated plant organ can also influence the SA-effects in plants. Recent molecular studies have established that SA can regulate many aspects in plants at gene level, and thereby can improve plant-abiotic stress tolerance. SA was reported to induce several genes responsible for encoding chaperone, heat shock proteins (HSPs), antioxidants, and secondary metabolites [sinapyl alcohol dehydrogenase (SAD), cinnamyl alcohol dehydrogenase (CAD), and cytochrome P450; ]. Additionally, SA-involvement in mitogen-activated protein kinase (MAPK) regulation, and in the expression and activation of non-expressor of pathogenesis-related genes 1 (NPR1) has been evidenced. Nevertheless, the transcriptional reprograming that occurs during the plant defense response against biotic and abiotic stress was reported to be modulated by SA, where the transcription of different sets of defense genes can be controlled in a spatio-temporal manner via SA-mediated mechanisms.

3In order to develop ideotypes for sustainable agriculture as well as to improve overall plant performance in the conditions of the changing climate and the increased severity of abiotic stresses, it would be imperative to exploit the information available on the involvement of SA in abiotic stress tolerance in plants. In the current effort, in addition to overviewing historical background and biosynthesis of SA in plants under both optimal and stressful environments, the role of SA in plants exposed to major abiotic stress conditions is critically discussed, potential mechanisms controlling SA-induced plant abiotic stress-tolerance are cross-talked, and major aspects so far unexplored in the current context are briefly highlighted.

4Centuries ago Americans, Indians, and Greeks used willow tree bark and leaves to cure aches and fevers, but it has been documented that Hippocrates prescribed the compound to relieve pain for women during child birth and fever was later recognized as SA. Ancient texts also indicate that Babylonians, Assyrians, and Chinese used willow (Salix sp.) bark and leaves for medicinal purposes. The importance of willow (Salix sp.) bark has also been shown in 1763 when the Reverend Edward Stone informed the Royal Society that it contained substances that relieved the symptoms of “ague” (probably malarial fever) effectively. Until 19th century the active principle of willow was not known, but later on the salicylates, methyl salicylate (MeSA), saligenin (alcohol of SA), and their glycosides were isolated from extracts of willow and other plants. The SA was isolated and purified from willow and meadowsweet in the first half of the 19th century and was chemically synthesized by the carboxylation of sodium phenoxide in 1860. In 1928, Johann Buchner, a German scientist for the first time isolated SA from willow bark under the name “salicin” (the glucoside of salicyl alcohol), which was the major salicylate in willow bark. The name SA is originated from the Latin word Salix and was given to the active ingredient of willow (Salix sp.) bark by Raffaele Piria in 1838. The first commercial production of synthetic SA began in Germany in 1874. Aspirin, a trade name for acetylsalicylic acid, was introduced by the Bayer Company in 1898 and rapidly became one of world’s best-selling drugs and it replaced the use of SA by producing less gastrointestinal irritation yet has similar medicinal properties. With the further advancement in scientific research in 20th century, uses of SA as a treatment for acne, psoriasis, warts, and calluses became common. The use of SA as a skin softener, for removal of dead skin cells, dirt, oil and debris and cleaning of pores is widespread. In spite of the fact that the mode of medicinal action of salicylates is a subject of continual debate, they are being used to treat human diseases ranging from the common cold to heart attacks. As aspirin undergoes spontaneous hydrolysis to SA, the two compounds have similar effects in plants.

5Salicylic acid is a seven carbon (C) containing, naturally occurring phenolic compound and endogenously synthesized signaling molecule in plants. The shikimic acid pathway and the malonic acid pathway are the two main pathways known to be involved in the synthesis of plant phenolics. The shikimic acid pathway takes part in the biosynthesis of most plant phenolic compounds. It simply converts simple carbohydrate precursors derived from glycolysis and pentose phosphate pathway to the aromatic amino acids including SA precursor, phenylalanine. The most common pathway in plants for SA synthesis is phenylalanine pathway; however, SA biosynthesis may also be accomplished by isochorismate pathway. SA is produced after a series of chemical reactions catalyzed by many enzymes. The hydroxylation of benzoic acid catalyzed by enzyme benzoic acid 2-hydroxylase synthesizes SA. Benzoic acid is synthesized by cinnamic acid either via β-oxidation of fatty acids or a non-oxidative pathway. Cinnamate 4-hydroxylase (C4H) catalyzes the second step in the pathway during the conversion of cinnamic acid to coumaric acid. Cinnamic acid is produced from phenylalanine by the action of enzyme phenylalanine ammonialyase (PAL). Cinnamic acid is hydroxylated to form coumaric acid followed by oxidation of the side chain, and further hydroxylated and form SA. In the third pathway, SA biosynthesis in plants has been reported from shikimic acid via chorismic acid and comaric acid (;; Figure 1).

6Multiple abiotic stress factors have been evidenced to modulate major enzymes involved in plant-SA biosynthesis. Overproduction of SA via enhanced activity of SA biosynthetic pathway enzymes (mainly isochorismate synthase, ICS; PAL) in plants helps in their protection against environmental stresses. These enzymes are key regulators of SA functions and are known to be modulated by different abiotic and biotic stress factors. In Arabidopsis, ICS has been found to be involved in the biosynthesis of SA during plant defense process. Synthesis of SA following exposure to ozone in Arabidopsis was also suggested to involve ICS. SA application can positively regulate ICS1 and improve drought tolerance in plants such as Arabidopsis thaliana. In addition, mutant study on Arabidopsis (ics1 and ics2) confirmed that in the absence of ICS1, ICS2 can encode a functional ICS enzyme but in limited amounts. Under salt stress, high expression of ICS and C4H enzymes can be correlated to the level of applied SA concentration. To this end, with 1.0 mM SA, induction of ICS and C4H was higher than that observed with 0.1 mM in Carthamus tinctorius. The isolated genes of ICS and C4H in C. tinctorius were considered significant for both salt stress tolerance and pathogen resistance. The involvement of enhanced PAL activity in heat and chilling tolerance was documented in Musa plants. Both Cd and Pb were reported to induce mRNA coding for PAL in Glycine max. Isolation and profiling of PAL and CHS in salinity exposed C. tinctorius plants suggested that these two enzymes were highly responsive to 1.0 mM SA (vs. 0.1 mM SA; ). Recently, both water deficit and UV-B radiation were reported to cause SA accumulation as a result of increased activity of PAL and benzoic acid hydroxylase (BA2H; ).

7Metal/metalloid-accrued stress has become a subject of great concern to sustainable agriculture and environmental system. Agricultural soils can receive metal pollutants from multiple pathways including industrial effluents, burning liquid and solid fuel, urban run-off, sewage waste disposal, agricultural toxic chemicals run-off, domestic garbage dump etc. Literature is full on the significance of SA in the minimization of metal-accrued stress-impacts. Exogenously applied SA was reported to improve growth and photosynthetic traits in several crop plants including lead [Pb-(0.05, 0.15, 0.25 mmol L-1)] exposed Oryza sativa, Cd (10, 15, and 25 μM)-exposed Zea mays, and Cu (0.05, 0.10, 0.15, and 0.20 mM)-exposed Phaseolus vulgaris. Applied SA was evidenced to modulate antioxidant system-components and significantly decrease membrane lipid peroxidation in to Cu-exposed P. vulgaris and Pb-exposed O. sativa. Recent evidences also suggested that SA is an important regulator of photosynthesis, photosystem II (PSII), photosynthetic pigments, and the activity of enzymes such as Rubisco and carbonic anhydrase under metal stress. Recently, an increased tolerance of Linum usitatissimum to Cd was attributed to SA-mediated control of H2O2 accumulation. Ni-impact-mitigation role of SA can differ in plants exhibiting a differential Ni-accumulation. To this end, SA-biosynthetic pathway compounds and its derivative metabolites (phenylalanine, cinnamic acid, salicyloyl-glucose, and catechol) were elevated in the hyperaccumulator Thlaspi goesingense compared to the non-accumulators A. thaliana and T. arvense. Further, the presence of SA enhanced the activity of sulfur (S) assimilation pathway enzyme (serine acetyltransferase; SAT), glutathione (GSH) content and eventually increased Ni-resistance. Exogenous SA (3.0 mM) enhanced OsWRKY45 gene expression and increased endogenous content of SA. It has been shown that increased endogenous SA level prevented membrane damage by lowering H2O2 content in Cd-exposed O. sativa. Involvement of SA (0.5 mM) in the phytochelatins (PCs)-mediated protection of Z. mays was evidenced against Cd-toxicity. SA can also significantly inhibit Fe-deficiency-caused chlorosis in plants.

8It has been estimated that about 45 million hectares of irrigated land have been damaged by salinity stress worldwide and considerable area of land affected by salinity is increasing day by day worldwide. In fact, the loss of plant productivity due to salinity stress is a consequence of imbalance in cellular ionic and osmotic balances. Major adverse effects of salinity stress include increased ion toxicity, osmotic stress, and nutrient-acquisition and homeostasis/deficiency, impaired stomatal conductance, increased cell-turgor loss, decreased reduction in leaf water potential, altered physiological/biochemical processes, and elevated ROS-caused oxidative stress. The role of SA in strengthening salinity stress-tolerance mechanisms has been extensively evidenced in many crops including Vicia faba, Brassica juncea, Medicago sativa, and V. radiata.

9Salicylic acid was reported to induce salinity tolerance and increased biomass of Torreya grandis as a result of enhanced chlorophyll content and the activity of antioxidant enzymes that eventually activated the photosynthetic process and alleviated oxidative stress. SA-deficiency in plants was considered as a major reason of salinity-accrued increased damages and diminished activity of antioxidant enzymes in SA-deficient NahG transgenic of Arabidopsis lines. NaCl-accrued oxidative stress in Hordeum vulgare was minimized by SA (50 μM)-mediated decrease in cellular malondialdehyde (MDA, a marker of membrane-lipid peroxidation) and ROS (such as H2O2;; ). SA-priming can be an important strategy for enhancing major GSH-based H2O2-metabolizing enzyme such as GST. To this end, SA mitigated salinity stress-injury in Solanum lycopersicum by causing characteristic changes in the expression pattern of GST-gene family members such as SlGSTT2, SlGSTT3, SlGSTF4. Exogenously sourced SA (0.5 mM) was reported to improve salt tolerance in Triticum aestivum due to an enhanced transcript level of antioxidant genes; GPX1, GPX2, DHAR, GR, GST1, GST2, MDHAR, and GS, and an increased activity of ascorbate (AsA)-GSH pathway enzymes. In another instance, SA-mediated restoration of membrane potential and prevention of salt-induced K+-loss via a GORK channel, and eventually improved salinity-tolerance were evinced in A. thaliana.

10Ozone is among the major components of photochemical air pollution responsible for causing significant damage to both cultivated plants and forest trees. Ozone enters to mesophyll cells via stomata where it immediately interacts with water and other cellular components to generate phytotoxicity mainly by elevating the generation of ROS (such as, H2O2,.OH, and 1O2), and triggering a series of signaling cascades and plant defense responses. Extensive reports are available on the key regulatory roles of SA in plant-ozone stress tolerances. SA can work as a signal molecule and promote molecular and physiological changes in ozone-exposed plants. Moreover, SA is required to potentiate the antioxidant response, maintain cellular redox state, and activate processes against hypersensitive cell death and ozone-sensitivity. SA was reported to maintain plant growth, development and cellular redox system by activating the GSH-biosynthetic pathway.

11In ozone-exposed A. thaliana, SA was involved in the accumulation of defense-related transcripts and induced resistance. SA is also involved in signaling network integrating other phytohormones such as JA and/or ethylene in ozone-exposed plants. Notably, both SA and ethylene were evidenced by these authors to act in concert to regulate ozone-induced cell death in A. thaliana. In the same plant, ozone-mediated induced biosynthesis of JA or methyl jasmonate was reported to attenuate SA-dependent lesion-initiation that eventually was considered as a major factor for the decreased lesions caused by ozone. It has also been evidenced that ozone-accrued SA-accumulation can be promoted by ethylene-mediated regulation of the expression of the PAL and chorismate mutase (CM) genes in ozone-exposed Nicotiana tabacum. In another study, ozone-sensitivity in hybrid poplar was correlated with insensitivity to both SA and jasmonic acid, where these phytohormones were associated with the programmed cell death in lesion formation.

12The effects of increased UV-B radiation (280–320 nm) on plant growth and development raised concerns on the need of protection mechanisms. Increased UV-B level can significantly diminish crop productivity by inhibiting PSII, electron transport systems, photosynthetic rate, photosynthetic pigments, nucleic acids, and biomass accumulation and partitioning. Exposure to UV-B radiation can decrease the expression and synthesis of key photosynthetic proteins such as chlorophyll a/b-binding proteins (Lhcb) and the D1 polypeptide of PSII (psbA;; ). Significant induction in SA-accumulation was evidenced in UV radiation-exposed plants.

13Exogenously applied SA was reported to modulate antioxidant levels, detoxify superoxide radicals, prevent oxidative damage, and protect membranes and important metabolic enzymes. SA-mediated activation of antioxidant enzymes was considered as a major factor for SA-mediated regulation of UV-induced oxidative stress in Capsicum annuum leaves. Increased photosynthetic rate, pollen viability, leaf phenolic concentration and yield in UV-B stressed-O. sativa was also reported. SA can decrease UV-B caused chromosome aberration level in the meristematic root tip cells. Exogenously applied SA was reported to significantly improve photosynthetic function and its related variables in UB-B exposed plants. Earlier, UV-C radiation was also reported to upregulate the transcription of the SA induction deficient 2 gene coding for the SA biosynthetic isochorismate synthase 1 enzyme. Additionally, SA-dependent pathway was reported to control the up-regulation of the pathogenesis-related (PR) proteins (PR-1, PR-2, and PR-5) in UV-B-exposed transgenic NahG A. thaliana plants.

14In the current changing environmental scenario, both low (cold and chilling stress) and high (heat) temperatures have become a potential abiotic stress-threat to crop plants. Temperature stress affects many plant-physiological and biochemical processes and induces molecular mechanisms and gene expression to modulate plants responses. SA-supplementation has been reported to differentially benefit several plant species exposed to low/chilling temperatures and high. SA (0.5 mM) modulated antioxidant enzymes (such as ascorbate peroxidase, APX; superoxide dismutase, SOD; guaiacol peroxidase, GPOX; GSH reductase, GR) and improved chlorophyll fluorescence in Z. mays under low (2°C) temperature stress. Exogenously SA can inhibit the activity of isozymes (CAT-1 and CAT-2) of catalase (CAT) which in turn can mediate responses of Z. mays to low temperature stress.

15Salicylic acid (2.0 mM)-mediated increased synthesis of total phenolics and the activity of PAL were reported to improve chilling tolerance in cold-stored lemon fruit (Citrus limon; ). reported that exogenously sourced SA results in cold tolerance by enhancing antioxidant enzymes, ice nucleation activity, and the patterns of apoplastic proteins in H. vulgare genotypes. In another report, SA significantly protected ultra-structures in Musa acuminata seedlings under chilling stress. Least reports are available in literature on the molecular mechanisms underlying SA-mediated improved plant tolerance to cold/chilling temperature. Increased chilling tolerance was evidenced in chilling-exposed S. lycopersicum fruit as a result of low concentrations of (0.01 mM) MeSA-mediated induction in the synthesis of some stress proteins, such as PR proteins. The applied MeSA significantly increased accumulation of PR-2b and PR-3a mRNAs but slightly increased PR-3b mRNA accumulation.

16Salicylic acid-mediated improved plant tolerance to heat stress has also been reported. reported that the transgenic Arabidopsis seedlings showing a bacterial SA-decomposing salicylate hydroxylase were less tolerant to heat stress. SA signaling played an important role in the acquisition of Ocimum basilicum to heat. SA application increased antioxidant system in Vitis vinifera and induced activity of APX, GR, and monodehydro ascorbate (MDHA), increased redox ratios of AsA and GSH, and maintained Ca2+-homeostasis were reported in SA-supplemented and cold/heat treated V. vinifera. SA (1.0 mM) decreased electrolyte leakage and oxidative stress, and improved maximum yield of PSII, Fv/Fm, and the quantum yield of the PSII electron transport in Cucumis sativa seedlings after both heat stress and recovery. Recently, have shown that treatment of 0.5 mM SA can alleviate heat stress in T. aestivum by increasing Pro-production and restriction of the stress ethylene formation under heat stress. Notably, the details of the mechanisms of heat stress mitigation with the application of SA are not available and the area could be an open challenge at both physiological and molecular levels.

17Drought has been considered as one of the most acute abiotic stresses presently affecting agriculture. Drought stress can significantly reduce photosynthesis and stomatal conductance, inhibit photosynthetic pigments synthesis and ultimately lead to reduction in growth of plants (reviewed by ). SA (500 μM)-supplementation to drought stressed H. vulgare resulted in increased net CO2 assimilation rate due to increased stomatal conductance and eventually in increased plant dry mass. Exogenously applied SA can modulate important enzymatic (including monodehydroascorbate reductase, MDHAR; dehydroascorbate reductase, DHAR; GR; GSH peroxidase, GPX) and non-enzymatic (including GSH) components of AsA–GSH pathway, and also glyoxalase system (Gly I and Gly II) and decrease oxidative stress in drought-exposed plants. Foliar application of SA (1.0 μM) strengthened antioxidant defense system in drought-tolerant Z. mays cultivar to a great extent (vs. drought-sensitive cultivar; ). Low membrane lipid peroxidation but increased plant height and dry mass, and less wilting of leaves were reported in drought-exposed and SA (0.5 mM)-supplemented T. aestivum. Recently, SA-biosynthetic enzymes (such as CS and ICS) were not correlated with the SA level, but ortho-hydroxy-cinnamic (oHCA) was correlated with SA biosynthesis and played crucial role in drought tolerance in O. sativa.

18The application of 5.0 μM SA induced the expression of genes in Mitragyna speciosa. Among the analyzed 292 expressed sequence tags (ESTs) randomly, the most of the genes that encode chaperone, HSPs, antioxidants and secondary metabolite biosynthesis, such as SAD, CAD, and Cytochrome P450 (CYP) responded to SA treatment. The authors correlated the gene-expression-responses to SA with signaling pathway in plants under drought stress condition. SA (0.5 mM)-mediated significantly improved growth in drought-exposed T. aestivum seedlings was argued as a result of SA-mediated enhancements in the transcription of GST1, GST2, GR, MDHAR genes. SA-accumulating mutants (cpr5 and acd6) exhibited stomatal closure and improved drought tolerance in A. thaliana by SA-mediated induced expression of PR genes (PR1, PR2, and PR5; ). SIZ1-mediated endogenous SAaccumulation was reported to play an important role in stomatal closure and drought tolerance in A. thaliana (Miura et al., 2013). Potential involvement of SA in the 76 identified proteins was reported in drought-exposed T. aestivum. Theses identified proteins were advocated to perform major physiological functions such as photosynthesis, carbohydrate metabolism, protein metabolism, stress and defense, energy production, signal transduction, and toxin metabolism. Some of the recent studies have shown that SA played significant role at different concentration by regulating many metabolic mechanisms (Table 1).

19In order to enlighten the potential mechanisms underlying SA-mediated improved abiotic stress tolerance in plants, this section briefly appraises recent reports available on the interaction of SA with major osmolytes, mineral nutrients, secondary metabolites, and other phytohormones, and SA-involvement in ROS-signaling and the modulation of antioxidants.

20To counteract the adverse effects of abiotic stresses-induced excess ROS production, plants have developed mechanisms facilitating their adaptation to osmotic and ionic stresses. Nevertheless, to maintain osmotic balance plants have well developed protective mechanism termed as osmoregulation mediated by osmolytes such GB, Pro, soluble sugars, amines etc. These compounds do not interfere with the other plant metabolic processes and contribute to the turgor-maintenance in stressed plants.

21Glycinebetaine is considered as an effective compatible solute for osmotic adjustment and protection against osmotic stress, salt stress, and also to stresses caused by heat and metals. The accumulation of GB in stressed plants adjusts cell osmotic balance, stabilizes membrane integrity, prevents the dissociation of polypeptides from the PSII complex, protects Rubisco activity, and also detoxifies toxic ions. Notably, SA and its analog aspirin can induce GB-accumulation in the range of 0.5–2.5 mM in plants at high levels of NaCl, drought, and cold stresses. The effectiveness of both salicylate and aspirin in the induction of GB-accumulation is a part of systematic acquired resistance and could be an important part of the reason for its induction in plants under NaCl-, drought-, and cold stresses-plants. The induction of GB might have other effects as well, such as activation of protein kinase when aroused by hyperosmotic stress. SA-mediated increase in GB-level can improve overall plant growth. The increase in GB content led to the increase in biomass of Rauwolfia serpentina. Recently, have shown that alleviation of salinity-inhibited photosynthesis and growth by SA involves GB in V. radiata. It was shown that SA (at 0.5 mM) induced GB-accumulation through increased methionine content and suppressed excess ethylene formation under salinity stress. Similar effect was observed by these authors with the application of SA-analog, 2, 6, dichloro-isonicotinic acid on GB accumulation and alleviation of salinity-induced adverse effects on photosynthesis and growth.

22The accumulation of another major osmolyte Pro is one of the adaptive mechanisms that plants operate for survival especially under salinity/osmotic stress conditions. Pro detoxifies excess ROS, adjusts cellular osmotic balance, protects biological membranes, and stabilizes enzymes/proteins. Researches have well documented that SA is involved in increasing Pro metabolism under abiotic stresses. SA (at 0.5 mM) significantly induced activity of Pro biosynthesis enzymes (such as pyrroline-5-carboxylate reductase and γ-glutamyl kinase) under salinity stress along with the increased Pro content. This increase in Pro metabolism was attributed to salinity stress tolerance in Lens esculenta. have shown that up-regulation of Pro biosynthesis enzymes (such as pyrroline-5-carboxylate reductase and γ-glutamyl kinase) and down-regulation of Pro oxidase activity were responsible for increased Pro level. In turn, the increased Pro level was advocated to the maintenance of the cell turgor in R. serpentina under salinity stress. SA-treatment (0.5 mM) alleviated heat stress in in T. aestivum by increasing Pro-production as a result of SA-mediated increase in γ-glutamyl kinase and decrease in Pro oxidase activity. Increased Pro production was argued to improve nitrogen assimilation and to alleviate the heat stress-impact on photosynthesis. In contrast, exogenous Pro-application was evidenced earlier to induce SA production (mediated by NDR1-dependent signaling pathway) and was shown to modulate calcium (Ca)-mediated oxidative burst defense response in plants.

23Accumulation of soluble sugars and sugar alcohol mannitol has also been reported to contribute in plant stress tolerance as osmoprotectants. SA was reported to inhibit valine and sucrose uptake in a concentration-dependent manner (10–200 μM; ). Improved plant health can also be achieved with increased contents of polysaccharides and soluble sugars, respectively, with 100 μmol L-1, and 0.5 and 1.0 mM SA.

24Mineral nutrition is a basic requirement for proper growth and development and survival under different environmental stress conditions. Studies have shown that mineral nutrient status in plants plays a critical role in the alleviation of abiotic stress. SA can significantly modulate the uptake and metabolism of important mineral elements and thereby improve growth and development in abiotic stressed plants. In addition, the protective role of SA in membrane integrity and regulation of ions including nutrients uptake has also been reported.

25Salicylic acid can be involved in the regulation of uptake of several plant-beneficial elements such as Mn, Ca, Cu, Fe, P, and Zn and thereby minimize oxidative stress under Pb stress. SA-mediated changes in photosynthesis were attributed to the nutrients content of N, P, K, and Ca in B. juncea cultivars differing in salt tolerance. In particular, 0.5 mM SA increased photosynthesis under salt stress by decreasing cellular Na+ and Cl- ions, and increasing the content of nutrients. Earlier, strongly inhibited Na+ and Cl- accumulation but stimulated N, Mg, Fe, Mn, and Cu concentrations were reported in SA-supplemented and salinity stressed Z. mays. In another work, SA (0.5 mM)-mediated maintenance of higher K+/Na+ and Ca2+/Na ratios was considered as a major factor underlying SA-assisted improved growth, gas exchange, yield, and salinity tolerance in Z. mays. Recently, demonstrated that SA can improve salinity tolerance in B. juncea by upregulating the assimilation of S. Exogenously sourced SA (seed-soaked or soil-incorporated) can improve overall plant growth under B-toxicity and salinity stress by stimulating the accumulation of mineral elements including K, Mg, Mn, N, and P.

26Calcium is one of the important nutrient elements known for various structural roles under both optimal and stressful conditions in plants. Literature supports a close link between SA and Ca (and Ca-signaling) in stressed plants. Interaction between SA and Ca2+ signaling might be involved in defense mechanisms induced by stress and maintenance of K+/Na- ion selectivity. SA and Ca (alone or in combination) can markedly improve salinity tolerance in plants via increasing Pro level. Involvement of Pro in the induction of Ca-mediated oxidative burst and SA signaling has also been reported. A range of abiotic/environmental stresses (such as high temperature, UV-B stress, or salt stress) can induce the expression of Genes encoding calmodulin-binding protein (Ca-containing protein). On the other hand, SA was reported to induce calmodulin-binding proteins in abiotic stressed A. thaliana. Ca-dependent protein kinases (CDPKs) have been shown to be involved in abiotic stress responses and may also be induced by SA. reported that SA-induced a rapid and transient generation of superoxide anion followed by a transient increase in cytosolic free Ca ion concentration in N. tabacum. reported that SA failed to induce Ca-Cyt oscillations in guard cells; whereas, K+ channel activity was suppressed by SA and lead to the stomatal closure.

27The generation and scavenging of varied ROS such as, H2O2, and •OH are usual in the normal aerobic metabolism in plants. Notably, important signal transducing roles and triggering and/or orchestrating plant responses to varied stress factors can be possible with the minimal levels of ROS. However, (abiotic) stresses cause imbalance between generation and scavenging of ROS, and eventually lead to a physiological condition known as oxidative stress. Thus, a number of consequences such as oxidative modification of vital biomolecules, cell death, and the arrest of plant growth and development can be possible under uncontrolled oxidative stress.

28Apoplastic-ROS have been found as regulators of cell death through their interplay with several other signaling pathways including SA-mediated signaling pathways. Both endogenous and exogenous SA was evidenced to play roles in antioxidant metabolism and have a tight control over cellular ROS. However, the coordination of SA-dependent and independent signaling components with ROS-signaling provided an appropriate defense response. SA can act as a signal for the development of the systemic acquired resistance, and can also induce the activation of a protein kinase. SA was evidenced as a major signaling molecule required for agonizing ozone-induced GSH-based defense gene expression in A. thaliana. A. thaliana plants were able to recognize the response of ROS–SA interaction via an antagonistic action of SA and SA-signaling on apoplastic ROS-signaling. SA-signaling except from the well-documented role in the regulation of defense responses was also evidenced to be involved in the regulation of light-acclimation processes and thereby influencing photosynthetic function.

29Involvement of SA in the modulation of antioxidant metabolism has been widely reported to control plant-tolerance to major abiotic stresses including ozone, UV-B, heat, metal, and osmotic stress. SA-pretreatment was evidenced to alleviate the adverse effects of salinity stress on photosynthesis and growth in V. radiata through enhancing the activities of antioxidant enzymes including SOD, CAT, GPX, APX, and GR. Activities of H2O2-metabolizing enzymes (such as CAT, POD, and APX) and superoxide-dismutating enzymes (SOD) were also modulated with SA in plants exposed to drought and cold. SA-application (at 0.5 mM) increased activity of enzymes of AsA–GSH pathway resuled in the increased tolerance of B. juncea to salinity stress. AsA and GSH, as redox active compounds have been extensively reported to maintain a homeostatic balance of the cellular redox state, and are involved in protective mechanisms against both abiotic and biotic stresses. In Cd-exposed T. aesticum varieties, SA-signaling was correlated with GSH-related mechanisms. Exogenous SA (0.5 mM) significantly improved salinity (250 mM NaCl) tolerance in T. aestivum by markedly increasing the pools of AsA and GSH.

30Salicylic acid-mediated differential regulation of the transcript levels of the genes encoding AsA–GSH cycle enzymes GPX (GPX1), phospholipid hydroperoxide GPX (GPX2), and DHAR (DHAR), GR (GR), GST (GST1 and GST2), MDHAR (MDHAR), and GSH synthetase (GS) was advocated as a major mechanisms underlying previous role of SA in salinity tolerance via AsA and GSH. Recently, in S. lycopersicum, SA-priming exhibited a concentration-dependent modulation of GST-supergene family, where SA-mitigated the salinity stress-injury and caused characteristic changes in the expression pattern of SlGSTs only at 10-4 M concentration. In particular, SlGSTF4 displayed a significant up-regulation in the leaves, while the abundance of SlGSTL3, SlGSTT2, and SlGSTZ2 transcripts were enhanced in the roots of plants primed with high SA-concentration. SA-mediated induction in SOD and GSH-based H2O2metabolizing enzymes namely GPX and GST was argued to improve in ozone tolerance in plants. SA (500 μM) improved Cd-tolerance and photosynthetic capacity in hemp (Cannabis sativa) by enhancing both SOD and POD. In another instance the applied SA-induced SOD activity accompanied an increase in shoot Ca2+ (a second messenger) and caused a transient increase in H2O2 which in turn was argued to induce antioxidant enzymes and eventually to decrease in cellular ROS.

31Secondary metabolites, substances biosynthetically derived from primary metabolites, produced by plants as defense chemicals and are not involved in plant metabolic activity. Examples of secondary metabolites include including terpenes, phenolics, and compounds with N (alkaloids, cyanogenic glucosides, non-protein amino acids) and S (GSH, glucosinolates, phytoalexins, thionins, defensins, and allinin; ). Secondary metabolites have also been established as beneficial for plants tolerance under biotic and abiotic stress.

32Extensive reports are available on the direct and indirect involvement of SA in the induction of the secondary metabolite synthesis in plants. SA-mediated induction of glucosinolates production was reported in Brassica napus. Exogenous application of acetyl-SA to tumor lines of Catharanthus roseus elevated therein the production of important secondary metabolites such as alkaloids, phenolics, furanocoumarins, and anthocyanins. Recently, in the same plant SA-mediated restoration of Ni-induced inhibition was attributed to SA-assisted improved the content of alkaloids such as vincristine and vinblastine. Role of exogenously applied SA was reported in UV-B exposed T. aestivum, where SA increased the accumulation of anthocyanin and tocopherol, and also modulated the expression of PR proteins. Eliciation in the PAL activity and subsequent increased vanillin production were reported in SA-supplemented in C. chinense. Foliar application of SA at the rate of 50 mg L-1 induced the level of secondary metabolites such coumarins, sterols, xanthoproteins, cardiac glycosides and saponins, and ameliorated water stress in Simarouba glauca. Earlier, SA-mediated improved thermo-tolerance in V. vinifera was advocated as a result of SA-induced accumulation of PAL mRNA, the synthesis of new PAL protein, and significant accumulation of phenolics. A schematic presentation of SA-mediated metabolisms responsible for abiotic stress tolerance is shown in Figure 2.

33Many factors are involved in a complex process of plant growth and development. SA can regulate various aspects of plant responses under both stressful and optimal environments through signaling cross-talks with other phytohormones. The interaction between SA and other phytohormones such as auxin, cytokinin, gibberellins, abscisic acid, ethylene, nitric oxide (NO;; ), and brassinosteroids has been established under both normal and stressed conditions.

34The outcomes of the SA-interaction with other phytohormones can be either from synergistic or antagonistic relation therein under optimal and stressful conditions. SA-mediated repression of auxin signaling was demonstrated by. The authors showed that the expression of the PR1, a SA-induced gene in tir1 afb2 (auxin perception small family of F-box proteins) mutant plants plays a significant role in auxin-signaling in stressed plants. Furthermore, in these mutants, PR-1 was induced to a great extent as compared with SA-treated wild-type plants, where the negative effect of SA on auxin signaling indicates the occurrence of an antagonism between auxin and SA-signaling pathways. SA triggered the accumulation of ABA under both normal and salinity stressed plants which in turn helped in the osmotic adaptation and improved photosynthetic pigments and growth attributes in S. lycopersicum. ABA may alter the SA-related abiotic (cold) stress response in plants. To this end, in chilling-exposed Z. mays, ABA treatment induced changes in the endogenous SA and the content of oHCA, and suggested that SA related stress responses may overlap with ABA-induced cold-acclimation.

35The increased ethylene production under environmental stress referred as stress ethylene is believed to induce oxidative stress. Reports suggest an antagonism between SA and ethylene. Under stressful conditions, the application of SA could inhibit ethylene biosynthesis by restricting the conversion of 1-aminocyclopropane carboxylic acid (ACC) to ethylene. Exogenous SA was reported to alleviate heat stress by increasing Pro-metabolism and restricting ethylene formation in heat-stressed plants to an optimal range by inhibiting activity of ACC synthase (ACS; ). Increased methionine and GB-accumulation in V. radiata plants was concomitant with improved photosynthesis and growth, and the suppression of ethylene formation as a result of SA (0.5 mM)-mediated inhibition in ACS activity under salinity stress. Salinity stress was evidenced to induce cell death mainly by ethylene-induced ROS-production; however, ROS generated by SA was not controlled by ethylene in Lycopersicon esculentum cell suspension. In contrast, suggested a synergistic cross-talk between ethylene and SA for combating environmental stress-impact in plants. In a similar study, an increased ethylene formation was observed with SA-addition in the MS medium; whereas, it was decreased in regenerants exposed to AgNO3 in Prunus persica rootstock. In contrast, the accumulation of polyamines under salinity stress was prevented by SA-application along with the increased ACC content. Further, SA-induced inhibition in the lipid peroxidation and polyamines in nodules of M. sativa can be an indication of SA-mediated activation of ethylene-dependent hypersensitive response.

36Nitric oxide, a major ubiquitous signal in plant systems, plays significant roles in a wide range of responses to environmental and endogenous cues. SA is among the major phytohormones to which NO interacts, plays role as a second messenger, and controls stomatal movement in higher plants. NO has been advocated to act as downstream of SA-signaling in the reduction of induced oxidative damage in osmotic stressed T. aestivum seedlings. SA can also stimulate the synthesis of NO via enhancing the activity of NO–synthesizing enzymes. Under Ni stress, addition of SA or SNP partially reduced the toxic effects of Ni. However, Ni-stressed plants supplemented with SA+NO exhibited an improved growth and photosynthetic pigments in canola (B. napus; vs. Ni-treated plants; ). Recently, the combined application of SA and NO benefitted Arachis seedlings (in terms of high Fe-uptake and less leaf interveinal chlorosis) maximally compared to exogenous individual SA and SNP under Fe-deficiency. The stress tolerance conferring ability of brassinosteroid in plants was advocated to occur in part in its interactions with other stress hormones such as SA. It was observed that NPR1 gene, a master regulator of SA-mediated defense genes is a critical component of 24-epibrassinloide-mediated increase in thermo- and salinity tolerance in A. thaliana. There occur antagonistic interactions between SA and JA at the levels of MAPKs signaling and biosynthesis. The antagonistic relationship between SA and JA can also modulate the expression of PR protein genes, where induction and inhibition of PR genes can be possible with SA and JA, respectively,.

37Abiotic stress has been recognized as a major threat to the agricultural system. To cope up the adverse effects of abiotic stress, plants induce several physiological processes and molecular mechanisms. Investigations have shown SA as a strong and potential tool in reducing or alleviating the adverse effects of abiotic stress in plants. Application of SA has been shown to be beneficial for plants either in optimal or stress environments. SA can regulate various plant metabolic processes and modulate the production of varied osmolytes and secondary metabolites, and also maintain plant-nutrient status hence, to protect plants under abiotic stress conditions. Literature appraised herein confirmed the focus of the studies related with SA in abiotic stressed plants mainly on unveiling various physiological/biochemical processes.

38There are still a few of interesting questions awaiting further investigation. For example, NPR1, 3, and 4 are discovered as the SA receptor in plants. However, it is required to perform exhaustive molecular studies cross-talking NPR1, NPR3, and NPR4, and their cumulative potential role in SA-perception in orchestrating defense gene expression in abiotic stressed plants. Unveiling intricacies of the complete SA-signaling in abiotic stresses, and the relationship between distinct facets of SA in plant immunity and abiotic stresses responses would be fascinating and rewarding. Meanwhile, more genomics and proteomics studies are expected to broadly reveal SA-responsive genes and proteins upon stresses. Since SA influences the plant functions in a dose dependent manner and a high SA-concentration doesn’t profit stress tolerance, abiotic stress-regulated SA-catabolism maybe a field worth further investigation. Molecular dissection is also required to unravel insights into the SA-mediated control of other the production and/or signaling of specific plant hormones/metabolites, and their feed-back influence in the machinery responsible for controlling SA-endogenous levels. Overall, finer details of SA-mediated defense networks/plant-immunity as well as further insights into the cross-talk of SA with other defense signaling pathways in abiotic stressed plants can be uncovered through adopting an integrated approach incorporating genetics, molecular biology, biochemistry, genomics, bioinformatics techniques, and computational biology.

39The 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.