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Journal of Stress Physiology & Biochemistry, Vol. 17, No. 4, 2021, pp. 93-110 ISSN 1997-0838 Original Text Copyright © 2021 by Nath and Ghosh
REVIEW
Mechanism of Phytohormone Responses Against Salt
Stress: a Review
Payel Nath and Sisir Ghosh*
Department of Botany, Sreegopal Banerjee College, Mogra, Hooghly-712148, West Bengal, India
*E-Mail: sisirghosh1981@gmail.com
Received July 12, 2021
Abiotic stress is defined as the negative impact of non-living factors on living organisms in a specific environment. An unfavourable environmental condition comprising extreme low and high temperature, salinity, drought, water logging, heavy metals etc. pose a complex set of stress condition. Plant responses to those environmental stresses are also complex. The effects of stress are usually measured in terms of plant survival, crop yield, growth (biomass) or primary assimilatory processes which are related to overall growth of plants. Various physiological stimuli and/or stresses control the synthesis of phytohormones in many ways. Again all the molecular biological phenomenon including growth and development of the plants are controlled by the phytohormones at very low concentration. During abiotic stress the biosynthesis and accumulation of different molecules thought to have protective functions in the cells. Some plant growth promoting rhizobacteria (PGPR) may exert a direct stimulation on plant growth and development by providing plants with some of the phytohormones. Among the all abiotic stresses salinity limits the crop's growth and productivity worldwide. Salinity affects many of the physiological processes starting from seed germination, enzymatic activity, food production to DNA and protein synthesis. Many of the researchers work on the effect of salinity on the physiological activity of the plants, but the mechanism of phytohormones response against salinity are still not assembled in a systematic manner. An attempt is made to establish the comprehensive mechanism of phytohormones responses against salt stress and to know about the adaptation/tolerance of plants in the molecular level as well as systematic approaches during this post genomic era with 164 references.
Key words: Abiotic stress, salinity, phytohormones, physiological activity, molecular mechanism
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Plants are constantly confronting a variety of abiotic stresses (e. g. drought, salinity, extreme high and low temperature, metal toxicity etc.) that negatively influence plant growth, productivity, reproductive capability or survival. Plant responses to that stresses are complex and involve numerous physiological, molecular and cellular adaptations. When plants are exposed to a variety of abiotic stresses such as salinity, drought, temperature, the growth of the plants and biomass production are unexpectedly changed (Ahmad et al.,
2019). Among abiotic stresses, salinity limits crop growth and productivity worldwide (Abdel Latef et al.,
2020). Many physico-chemical processes in plants including seed germination and seed establishment (Dash et al., 2001), enzymatic activities (Seckin et al., 2009), transcription and translation (Anuradha et al., 2001) are obstructed by salinity. According to some scientist, one-third of the world's food producing areas are salt-affected (Gregory et al., 2018). Saline areas are expanding at a rate of 10% annually which would be amplified by increased global warming and climate changes (Shrivastava et al., 2015). Initially osmotic stress in plants is caused by salinity and the metabolic processes are directly affected (Khan et al., 2019). Later the over-accumulation of NH+ + Cl - ions in cells cause ionic toxicity due to salinity (Khan et al., 2019). Water insufficiency and nutrient such as Ca2+, K+, Fe2+ and Zn2+ deficiency within plants due to salinity caused to disruption of photosynthesis and oxidation stress (Rahman et al., 2019). Salinity reduces leaf area, stomatal conductance and chlorophyll levels in the plant and generates reactive oxygen species (ROS) (Khan et al., 2018); it also reduces germination percentage, length of root-shoot and fresh weight of plant tissues (El-Shaieny, 2015; Nasri et al., 2017). In plants, there are various modified and adaptive methods of salt tolerance to win the worst conditions of high soil salinity (Wang et al., 2019). Some researchers used plant growth regulators (PGRs) to alleviate the adverse effect of salinity on plants (Khan et al., 2019; Khan et al., 2020). In both non-stress and stressful conditions, plants synthesized phytohormones, also known as PGRs, which are small bio active compounds that can work
locally or transport to the distant sites of the plant body to accelerate growth and development (Peleg et al., 2011; Iqbal et al., 2014). PGRs can mitigate the negative effects of salt stress by increasing seed germination, growth, development and yield (Bielach et al., 2017). Auxin, Cytokinin (CK), Gibberellic acid (GA), Abscisic acid and ethylene play crucial roles during high-salinity exposure and their responses are supported by other hormones like Salicylic acid (SA), Jasmonate (JA) Triazoles (Fahad et al., 2015).
This review discusses about the mechanism of phytohormone actions as well as responses of plants under salt stress condition.
Response of auxin against salinity
Auxin plays a crucial role as a plant development propeller of various physiological processes (Teale et al., 2006; Zhao, 2010); it regulates root-shoot architecture, lateral and adventitious root formation, controlling of root-shoot meristems, establishment of apical dominance, leaf morphogenesis, flowering, and senescence (Aloni et al., 2003; Okushima et al., 2005).
Most of the plants produce IAA from tryptophan via IPA and IAN. Salinity can affects some the enzymes activity in this pathway (Fig. 1). Various studies revealed that salt stress conditions reduce auxin levels and decreased expression of auxin transporter (Park, 2007; Du et al., 2012; Liu et al, 2015) (Fig. 1). Auxin efflux carriers regulate meristem size during slats stress (Liu, 2015). Exogenous applications of indole acetic acid (IAA) increased salt-stress tolerance in several commercial crops (Ashraf and Foolad, 2005; Ashraf, 2010). In Pisum sativum L., foliar applications of auxin has enhanced photosynthetic capacity reduce loss of water; mitigate negative regulation induced by salt stress (Husen et al., 2016). IAA treated corn plants enhanced membrane permeability and increased nutrient uptake (Kaya et al., 2010). In wheat seed, the effect of delayed germination caused by high salt levels can be reversed by IAA pretreatment (Ashraf and Foolad, 2005). Salinity increased the Na+ content and reduces K+, Ca2+, Mg2+ content in root, stem, leaf, and seed of faba bean plants. IAA treated faba bean plant reduced the accumulation of Na+ content and improves
the accumulation of K+, Ca2+, Mg2+ content under salt stress. (Abdel Latef et al., 2021). Foliar application of IAA has changed expression pattern of proteins for better salt adaptation (Abdel Latef et al., 2021).
Similarly exogenous application of IAA has shown positive effects in different crops under salt stress such as Zea mays (Keya et al., 2010), Oryza sativa (Javid et al., 2011), Solanum lycopersicum (Alam et al., 2020), Solanum tuberosum (Khalid and Aftab, 2020) Carica papaya (Sa et al., 2020). Changing of the protein expression pattern showed the ability of a plant's tolerance to salt stress and plants synthesize specific polypeptide under salt stress for adaptation (Bavei et al., 2011). Compared to non-stressed plant, protein related to salt stresses show a positive indicator of salinity stress. (Sobhania et al., 2016). Several researchers showed that over expression of indole pyruvic acid (IPyA) pathway in several species increase salt tolerance (Kim, 2013; Yan, 2016).
There are large number of auxin responsive genes which were identified and characterized from different plant species, such as soybean, Arabidopsis sp and rice (Hagen and Guilfoyle, 2002). In cucumber, YUCCA genes are expressed at high and low temperature in response to salinity. Over expression of CsYUC11 gene has increased higher salinity tolerance (Yan, 2016). These auxin responsive genes have been divided into three gene families-i) Auxin (AUX/IAA), ii) GH3 and iii) small auxin-up RNA (SAUR) gene families (Guilfoyle et al., 1993). These novel genes have involved in salt stress responses and by which researchers can set further genetic strategies to improve more stress tolerance cultivars (Zhu, 2002).
Response of gibberellins against salinity
Gibberellins help seed germination, leaf expansion, stem elongation and flowering (Magome et al., 2004). During abiotic stress condition, plants accumulated gibberellic acid (GA) rapidly (Lehmann et al., 1995). GA3 was helpful to enhance the growth of wheat and rice under saline conditions (Parasher and Varma, 1988; Prakash and Prathapasenan, 1990). Stomatal resistance in tomato was reduced and water use in plant was enhanced by GA3 treatment at low salinity (Maggio et al., 2010). With poultry manure, application of GAs
has improved the growth and salinity tolerance of piper (Capsicum annuum) plants (Al Taey, 2017). Wheat seeds treated with GAs increased germination and seedling parameters under salt stress (Abido et al.,
2019). Tomato plant treated with GA3 increased leaf water content, stomatal density and chlorophyll content by alleviating salinity stress (Jayasinghe et al., 2019). Maize seed treated with GAs (5 mgL-1) increased the root-shoot length and water content in tissues under salt stress (Ghodrat and Rousta, 2012). Conformational changes of gibberellin insensitive dwarf (GID1) was induced by binding of bioactive GA to GID1 and has recruited DELLA growth repressor protein to form a GA-GID1-DELLA complex and interaction of E3 ubiquitin ligase F-box protein SLEEPY 1 (SLY1) with DELLAs, 26s proteosome degraded DELLA activity (Bao et al.,
2020). DELLA protein SLR1 was survived under salt stress by inhabiting plant growth (Achard et al., 2006). Plants have increased their salt tolerance ability through retarding the growth by the activity of related genes like AtGA20X7 (Magome et al, 2008), 0SGA20X5 (Shan et al., 2014) OSMYB92 (Zhu et al, 2015) responsible for GA metabolism. Over expression of GA catabolic gene CYP7D8L in transgenic plants was reduced GA accumulation, increase high soluble sugar and chlorophyll content to enhance plant tolerance to salt stress (Zhou et al., 2020). Under salt stress, rice plants have increased their tolerance by ectopic expression of PtCyP714A3 (Populus trichocarpa) encoding unique group of CyP mono-oxygenase which regulates deactivation of GA (Wang et al., 2016). In Arabidopsis, ectopic expression of G(h)PLATZ1 was helped quicker germination than in wild type and has suppressed the transcription of ABI4 (Zhang et al., 2018). Under salt stress, GhPLATZ1-mediated germination was depended on GA: ABA ratio (Zhang et al., 2018).
Response of cytokinin against salinity
Cytokinin (CKs) promotes cell division, affects apical dominance, auxiliary bud growth, chloroplast biogenesis, nutrient mobilization, leaf senescence, shoot differentiation, Photo-morphogenic development, anthocyanin production, vascular differentiation (Mok and Mok, 2001; Davies, 2004). CKs has helped to increase plant resistance power to salinity and high
temperature (Barciszewski et al., 2000). Seed treated with cytokinin has increased plant's salt tolerance (Iqbal et al., 2006a). By interacting with other plant hormones, cytokinin has enhanced salt tolerance in wheat plants (Iqbal et al., 2006b). Exogenous application of kinetin has inhibited negative effect of salinity stress on the growth of wheat seedlings (Naqvi et al., 1982). Prior to salt stress, if the potato plants are treated with kinetin, it alters salt-related growth inhibition (Abdullah and Ahmad, 1990). Under salt stress, Na+ accumulated in plants to make the disorder of ion homeostasis, the imbalance of K+ / Na+ ratio causes oxidative stress (Song and Wang, 2015; Guo et al., 2018; Liu et al., 2017). Ion stress and oxidative stress caused leaf senescence (Han et al., 2011; Li et al., 2012) in plants. Cytokinin has the role against the negative effect of salt on plants such as radish and tobacco (Vnakova et al., 2010). Some studies have shown that tomato seedling measures cytokinin level under salt stress (Keshishian et al., 2018). If INCYDE is sprayed on tomato under salt stress to increase antioxidant enzymes activity, it enhances plant salt tolerance (Aremu et al., 2014). On the other hand when 42 bp in promoter region of IPT5 has deleted, the cytokinin content of apple rootstock "robusta" has enhanced salt tolerance capacity (Feng et al., 2019). It has been established that cytokinin receptor AHK 1 is a positive regulator of salt stress response (Tran et al., 2007).
Response of brassinosteroid (BR) against salinity
BRs regulated many physiological processes like growth, leaf abscission, seed germination, rhizogenesis and senescence (Sasse, 1997). Exogenous application of BRs has enhanced growth and yield in many economically useful plant species under salinity. BRs promoted the number of ears along with their length and weight of kernels per year in cereals (Ali et al., 2008). BRs removed negative effect of salinity to help seed germination and seedling growth in rice. BRs enhanced nitrate reductase, maintained chlorophyll level under salt stress, and played an important role to supply nitrogen, growth and productivity of plants especially in cereals (Bajgur and Hayat, 2009). The impact of salt stress on
rice growth was altered by BRs as a result level of pigment was restored and nitrate reductase activity was increased (Anuradha and Rao, 2001). If leaf segments of barley were incubated in either BR solution or water and then incubated in 0.5 M NaCl solution in presence/absence of BR, there was no effect of BR on the leaf cell ultrastructure under normal condition and damage on nuclei; chloroplast caused by salt stress is altered by treatment of BR (Krishna, 2003). Some key enzymes which are associated with BR synthesis help plant to adopt in salt stress. Due to the over expression of a BR biosynthesis gene of Spinacia oleracea, SOCYP85A1 gene was amplified and increased the longevity of plant against high salinity (Duan et al., 2017). Under salt stress condition, tomato plants deficient of BR biosynthesis genes were more affected, but it has been altered by exogenous BR application (Zhu et al., 2016). Plants were sensitive to salt stress due to leak of BRI 1 or BSK5 (Zhu et al., 2016; Li et al, 2012a). BZR1, a marker, directly repressed the expression of BR biosynthetic gene DWF4 which cross the normal level at an early stress stage not soon back to normal levels (Geng et al., 2013).
H2O2 generation and accumulation of ethylene in tomato, exogenous BR treatment on cucumber were regulated positively to promote antioxidant enzyme activities under salt stress (Zhu et al., 2016; Wei et al., 2015). BR also helped to accumulate nitric oxide (NO) and mitigated oxidative damage which was caused by salt stress (Zhu et al., 2016). BR alleviated effect of salt stress on plant through interacting with other plant hormones like ABA (Zhang et al., 2009) SA (Divi et al., 2010) and GA (Wang et al, 2019a). Under NaCl-stressed conditions, oxidative damage in rice and maize seedlings was mitigated when seed has been treated by EBL or HBL (Ozdernir et al., 2004). Seed treatment with EBL reduced chromosomal abnormalities in root tips of barley plants grown in a NaCl containing medium, (Tabur and Demir, 2009). In wheat cultivars, EBL has leaded to increase the growth of seedling and maintain Ca2+/Na+ and K+/ Na+ ratio to promote uptake of Ca2+, K+ and reducing Na+ , which helped to adapt both wheat cultivars under salt stress (Ali et al., 2006). Foliar application of EBL to two strawberry cultivars helped to
overcome the negative effect of salinity stress on plant growth and enhanced shoot and root dry matters, relative water content of leaf, stomatal conductance, leaf chlorophyll value, macro-micro element content in leaf and root of plants (Karlidag et al., 2011). BRs and spermidine can altered the toxic effect of salt stress on Vigna radiata plants (Mir et al., 2015). The presence of EBL in nutritive solution under NaCl stress enhanced plant fresh weight, shoot dry weight, leaf and root water content, leaf area, sugar concentration, photosynthetic pigments, photosynthetic rate and water use efficiency of Cajanus Cajan plants (Durigan et al., 2011).
BR treated faba bean plant reduced the accumulation of Na+ content and improved the accumulation of K+, Ca2+, Mg2+ content under salt stress (Abdel Latef et al., 2021). Under salt stress, foliar application of BR has changed the expression pattern of
protein for better salt adaptation (Abdel Latef et al., 2021).
Response of abscisic acid (ABA) against salinity
The abscisic acid (ABA) helped plants to survive under adverse environmental condition such as salt stress (Keskin et al., 2010). In presence of salt, ABA increased xylem water potential as well as water uptake to the plant (Fricke et al., 2004). When barley roots are exposed to the salt, ions are accumulated in vacuoles of roots stimulation of ABA in roots which was important for adaptation to saline conditions (Jeschke et al., 1997). Exogenous application ABA inhibited leaf abscission and reduced ethylene release under salt stress in citrus by reducing Cl- ions accumulation in leaves (Gomez et al., 2002). ABA inhibited the deleterious effect of NaCl and enhances tolerance of ionic stress in sorghum (Amzallag et al., 1990).
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Figure 1. Tryptophan dependent IAA pathway in plants and bacteria. Salinity affects in the asterisk regions.
Figure 2. Effect of ethylene signaling on seed germination under salt stress.
Figure 3. Interrelationship of phytohormone responses is represented by a schematic diagram under salt stress.
Salinity up regulated stress hormone ABA, promoted gene to mitigate salt and osmotic stress (Wang et al., 2011). The rice seed treated with ABA increased
seedling growth and yield under saline soil by balancing nutrient uptake (Gurmani et al., 2013; Li et al., 2010). Rice treated with ABA at 10-5 M enhance osmoregulation
by reduced Na+ concentration in cells and increased proline and sugar accumulation in rice leaves under salt stress (Gurmani et al., 2011). ABA also helped the tolerance power against salt stress in wheat plant (Zongshuai et al., 2017). The mustard seeds treated with ABA (100 |m) enhanced germination rate by 25% compared to the control under salt stress (Srivastava et al., 2010).
CED1 (9 CIS EPOXYCAROTENOID DIOXYGENASE DEFECTIVE 1) is an important protein in biogenesis of cuticle. A mutant of CED1 protein was unable to promote ABA biosynthesis under osmotic stress (Wang et al., 2011). During salt stress, ABA acts as a signaling molecule to suppress the growth of lateral roots as in quiescent stage. Lateral roots form a thick, well-developed casparian strip which has reduced diffusion of Na+ ions through the endodermis and in presence of Na+ ions, endodermal cells enhanced ABA signaling and inhibited growth of lateral roots under high saline environment (Duan et al., 2013). LEA proteins produced in an ABA-dependent manner, were highly hydrophilic small proteins, have an osmoprotectant role against cellular dehydration during late embryogenesis. LEA proteins also played an important role in salt stress tolerance (Bhardwaj et al., 2013). In Arabidopsis, 51 LEP proteins were identified from nine different groups (Hundertmark et al., 2008). Arabidopsis can able to mitigate salt stress by over activation of salt stress inducible genes such as RD29B through over expression of AtLEA 14 which belongs to the LEA group-2 proteins (Jia et al., 2014).
Response of ethylene against salinity
In many plant species level of ethylene and its precursor ACC are induced by salinity and other abiotic stresses (Morgan and Drew, 1997). On application of ethylene or ACC has enhanced plant tolerance to high salinity (Cao et al., 2007). Ethylene was an essential positive mediator of salinity stress tolerance in maize (Freitas et al., 2018), tomato (Gharbi et al., 2017), grapevine (Xu et al., 2019). It is reported in grapevines that melatonin involves in enhancing salinity stress tolerance by inducing MyB108A-mediated ethylene biosynthesis (Xu et al., 2019). Ethylene played positive effect under salinity stress by osmotic adjustment, water
use efficiency, maintaining stomatal conductance in Solanum chilense (Gharbi et al., 2017). Under salinity stress, different components of ethylene signaling have played either positively or negatively at the time of seed germination (Fig. 2) and seedling growth (Cao et al., 2008). For examples, in Arabidopsis, ETR1 and ETR4 have inhibited seed germination, but ETR2 acted as a positive regulator involves in enhancing seed germination during salinity stress conditions (Wilson et al., 2014). Under salinity stress, ethylene has maintained Na+ /K+ homeostasis to improve tolerance against salinity stress (Jiang et al., 2013). Stylosanthes humillis, a forage legume grows naturally in saline soils and ethylene produced in seeds of the plants mitigated the effect of salinity stress (Lovato et al., 1999; Silva et al., 2014; Silva et al., 2018). ACC deaminase in plant growth-promoting Pseudomonas fluorescens strain improved salinity tolerance in plants and enhanced seed germination in wheat during salinity stress (Safari et al., 2018). Over expressing ethylene response factors (ERF 95 and ERF 96 ) in seeds of the transgenic line promoted better germination and seedling establishment by comparing with the wild under salinity stress conditions (Wang et al., 2017).
A novel ethylene responsive transcription factor from Lycium clinense Lch ERF helped to improve salinity tolerance in transgenic tobacco in the time of seed germination and vegetative growth (Wu et al., 2014). ERF 109, an ethylene responsive transcription factor gene, has retarded programme cell death (PCD) and enhances adaptation under salinity in the wild type tobacco plant (Bahieldin et al., 2016). Ethylene along with other phytohormones like auxin (IAA), cytokinin (CK), abscisic acid (ABA) helped the plants to overcome the adverse effect of salinity stress (Tuteja et al., 2010).
Response of jasmonates (JAs) against salinity
Methyl jasmonate (MeJA) and its free form jasmonic acid (JA) both are collectively called as Jasmonates (JAs) which regulate many developmental processes like seed germination, fertility, root growth, ripening of fruits and senescence (Wasternack and Hause, 2002). Jasmonates enhanced defense mechanisms in plants under environmental stresses, such as low temperature,
drought, salinity and also in response to various pathogens, insects driven wounding (Cheong and Choi, 2003). In tomato cultivars JAs level changed in response to salt stress and in salt tolerant cultivar HF (Hell frucht Fruhstamm). It was observed that JAs level increased from the beginning of salinization, while salt sensitive cultivar Pera treated with salt in 24h, JAs level decreased (Pedranzaini et al., 2003). Salt tolerant cultivar plants have high concentration of JAs than salt sensitive cultivar plants (Kang et al., 2005). In rice root, MeJA level was significantly increased in 200 mM NaCl (Moons et al., 1997). The inhibitory effect of high salt concentrations on growth and photosynthesis of barley are altered by pre-treatment with JAs (Tsonev et al., 1998). The balance of endogenous hormones such as ABA was changed by exogenous JAs application after salt treatment which gave the protection mechanisms under salt stress (Kang et al., 2005). In barley, the induction of JA-responsive genes played important roles under salinity (Walia et al., 2006). Arginine decarboxylase and apoplastic invertase which are activated by three JA regulated genes, involve in salinity tolerance through JAs (Walia et al., 2007). Symptoms produced by salinity stress in soybean seedling are mitigated by exogenous application of MeJA (Yoon et al., 2009). In tomato, exogenous application of JAs on mitigation of NaCl toxicity by regulating the antioxidant metabolism, accumulation of metabolite, synthesis of osmolyte was reported (Ahmad et al., 2018). The pretreatment of maize seedlings with JAs removed toxic effects of Na2CO3 on photosynthesis and plant growth (Mir et al., 2018). Under salt stress, exogenous application MeJA enhances the growth of Limonium bicolor (Yuan et al., 2018). Through maintaining ROS or ion homeostasis, exogenous application of JAs removed salt toxicity (Qiu et al., 2014; Farhangi-Abriz and Ghassemi-Golezani, 2018). In tomato, with increased K+ accumulation, the high JA-accumulation resistant mutant exhibited adaptation under salt stress (Garcia-Abellan et al., 2015). In tomato and rice, reduced JAs production or accumulation caused hypersensitivity under salinity (Abouelsaad and Renault, 2018; Kurotani et al., 2015), whereas elevated JAs biosynthesis helped to overcome adverse effect of salt stress in wheat and Arabidopsis
(Zhao et al., 2014). Due to lower Na+ accumulation in shoots, two JAs biosynthesis rice mutants, Cpm 2 and hebiba, were resistant to salt but there was no difference to detect in roots (Hazman et al., 2015). MYC 2, the master regulator in JAs signaling, helped plants adaptation to osmotic stress through activated expression of the ABA-inducible genes RD 22 and at ADH1 transcriptionally (Abe et al., 2003).
Response of salicylic acid (SA) against
salinity
Salicylic acid which is phenolic in nature, has regulated many physiological processes like photosynthesis, ethylene production, heat production, flowering, growth, nitrate metabolism (Hayat et al., 2010) and helped the plants to protect from biotic and abiotic stress such as salinity (Kaya et al., 2002). SA increased resistant power of wheat seedlings under salinity (Shakirova et al., 2003). After salt stress, the application of 0.05 mM SA enhanced plant growth, accumulation of ABA and protein in wheat (Shakirova et al., 2003). Exogenous application of SA increased the photosynthetic rate and regulated the membranes stability and the growth under salinity stress in barley plants. (El-Tayeb, 2005). When SA added to the soil during salt stress, Na+ and Cl- accumulation was decreased and also enhanced the survival rate of maize plants (Gunes et al., 2007). SA caused lower lipid peroxidation and membrane permeability in plant during salt stress (Horvath et al., 2007). SA treatment has increased H2O2 level which mitigates the adverse effects on the oxidation damage induced by salt stress in wheat plants (Wahid et al., 2007). SA helped nitrogen fixation in salt environment (Palma et al., 2013). In Arabidopsis application of excess SA (>100|M) exaggerated inhibition of seed germination caused by salt stress, while moderate level of SA (<50|M) treatment alters effect of salt stress on seed germination (Lee et al., 2010). In tobacco, through increasing SA signaling, over expression of AtNPR1 or MhNPR1 increased tolerance under oxidative and salt/osmotic stress (Srinivasan et al., 2009; Zhang et al., 2014). The npr 1-5 mutant did not have NPR1- dependent SA signaling, showing a hypersensitive phenotype to salinity (Jayakannan et al.,
2015). In wheat, SA promoted growth through inhibiting the decline of auxin and CK, maintain high ABA levels to increase plant salt tolerance (Shakirova et al., 2003). In Limanium bicolor, SA enhanced seed germination by maintaining a favorable, GA/ABA balance under salt stress (Liu et al., 2019).
Response of strigolactones (SLs) against salinity
SLs helped symbiotic interactions with fungi and shoot branching (Gomez-Rolban et al., 2008; Zhang et al., 2015). Under salt stress condition a synthetic SL, GR24, has increased plant growth by enhancing antioxidant enzyme activities and photosynthetic characteristics (Ma et al., 2017). In the germination and vegetative stages, the SL-biosynthetic mutants showed more axillary growth (Ha et al., 2013) indicated that SLs were positive regulator of plant salt tolerance. ABA helped the plants to overcome the adverse effect of salt stress through production of SL. Under salt stress, mycorrhiza colonized lettuce roots and ABA content was increased by upregulation of the ABA biosynthetic gene LsNCED2, which was mediated by salt-induced SL production (Aroca et al., 2013). In S. cannabina seedings, concentration of SL was also affected by ABA through the upregulation of CCD7, CCD8 and MAX2 (Rne et al., 2018). In ABA deficient plants, GR24 application restored the salt tolerance (Rne et al., 2018).
From the above discussion we may expect that there must be an interrelationship i.e. the phytohormonal cross-talk within the plants under salt stress (Fig. 3) and plants can mitigate the stressful condition by giving responses in various ways.
Among the all major environmental stresses salt stress limits the growth and productivity of the plants. On the other hand phytohormones can control plant's growth and development. Beside many other physiological functions, production of phytohormones in plants can mitigate the salt stress condition in various ways. The levels of phytohormones are elevated against the salt stress among the many plants. Phytohormones regulate ionic toxicity, osmotic stress, oxidative stress and synthesis of many plant genes by controlling the
transcription factors during salt stress condition. So there might be some molecular signaling which can induce for the production of phytohormones in salinity to combat the stressful situations. In this review we have discussed the responses of phytohormones during salinity, though the detail molecular signaling mechanism during the salt stress condition in plants is warranted for further research.
ACKNOWLEDGEMENT
The authors are thankful to the Principal and the Department of Botany, Sreegopal Banerjee College, Mogra, Hooghly, WB, India.
CONFLICTS OF INTEREST
The authors declare that they have no potential conflicts of interest.
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