PLANT RESPONSE TO SALINITY
According to Munns (2002), plant response to salinity can be divided into two phases. The presence of salt outside the roots elicits the first water stress or osmotic phase. Growth reduction is probably regulated by hormone signals from the roots and during this phase, plant needs to adjust the water potential and turgor to reach osmotic homeostasis, usually, this process is rather quick (within hours or 1 day). The second phase, that is, ionic stress component of salinity stress, is longer and becomes more severe within time. Cell injury is caused by the toxic levels of salt concentration in transpiring leaves, that exceed the ability of the cells to locate them in the vacuoles. During the second phase, the growth in younger leaves is reduced and consequently the supply of carbohydrates to the growing tissues.
Salt tolerance mechanism is principally based on plant attempts to minimize the entry of salt at organ and at cellular level (Munns, 2002). Halophytes can perform both, whereas some glycophytes, even though they exclude efficiently salt, are unable to compartmentalize the up taken salt in the vacuole.
Some mechanisms of salinity response have been clarified by the use of overexpression/loss of function lines, obtained mainly in the model plant
Arabidopsis (Denlein et al., 2014; Forni et al., 2017). Two sensoiy modalities have been evidenced: (1) sensing of the hyperosmotic component, (2) sensing of the ionic Na+ component of the stress. Therefore, successful coping with salt stress depends on the ability of the plants to sense both components of the stress.
Osmotic stress detected after Na+ accumulation in the cytosol is due to the impairment of ionic balance (Denlein et al., 2014). To cope with hyperosmotic imier environment and Na+ component of the stress, signaling plays an important role in eliciting a cascade of responses and in evoking mechanisms counteracting stress (Huang et al., 2012).
After the exposure, the increase of concentration of Ca2+ is detected in different root cell types; it is likely that Ca2+ channels are coupled with plant hyperosmotic sensor is (Knight et al., 1997) calcium-dependent protein kinases, calcineurin В-like proteins (CBLs), with CBL-interacting protein kinases are the protein activated downstream of Ca2+.
The transcription factors (TFs), activated by Ca27calmodulin, are calmodulin-binding transcription activators (Pandey et al., 2013), GT-element-binding like proteins (Weng et al., 2012) and MYBs (Li et al.,
2015). Moreover, Ca2+ signaling is linked to the activity of salt overly sensitive (SOS) genes (Zhu, 2002). SOS pathway has three major components: (1) SOS3, acting as a Ca2+ sensor (Ishitani et al., 2000); (2) SOS2, a serine/ threonine-protein kinase; (3) SOS1, a plasma membrane Na7H” antiporter (Turkan and Demiral, 2009). However, although the increased level of Ca2+ can be considered a hallmark in stress response, it cannot be excluded from the presence of Ca2+ independent osmotic sensory mechanisms. In the response to salinity cytoskeleton play a role, that is, depolymerization and reorganization of microtubules and stabilization of actin filaments are important in successful withstanding salt stress (Wang et al., 2011). Moreover, involvement of the cytoskeleton in Ca2+ influx and in SOS pathway has been reported.
Other second messengers are induced by salt exposure, that is, ROS (Deinlein et al., 2014; Forni et al., 2017). ROS are important molecules acting in signal transduction and mediating cell response to stress. In salt exposed cells, the imbalance between ROS production and scavenging leads to the overproduction of ROS (Miller et al., 2010), a condition that is harmful to cell components, due to the onset of oxidative stress situations damaging severely proteins, lipids, and nucleic acids. To counteract the high level of ROS and to overcome oxidative stress situation, plants have evolved different antioxidant systems, induced by abscisic acid (ABA)-dependent or ABA-independent pathways (Vital et al., 2008).
Plants react to the detrimental effects of the elevated ROS levels through the activation of antioxidant enzymes and the synthesis of molecules with antioxidant properties (e.g., phenolic compounds). Several enzymes are reported to be involved in ROS scavenging (Noctor and Foyer, 1998; Tiirkan and Demiral, 2009; Di Cori et al., 2013; Fomi et al., 2017) whose coordinated activity is fundamental to obtain efficiently a balance between the rate of formation and scavenging of ROS and important in maintenance of hydrogen peroxide at the levels needed for cell signaling (Mumis and Tester, 2008).
Production of specific secondary metabolites, possessing antioxidant activity, can be increased or decreased as consequence of ionic and osmotic stress (Mahajan and Tuteja, 2005). Enhanced synthesis of phenolic compounds after salt treatment has been detected barley (Ahmed et al., 2015) in Brassica napus (Fomi, unpublished results) and other species (Parida and Das, 2005). Increased levels of anthocyanins were detected as response to salt treatment (Parida and Das, 2005), vice versa in salt-sensitive species a decreased concentration of these pigments was determined (Daneshmand et al., 2010).
The reprogramming of metabolic activities involves the fundamental role of different TFs and cis-elements intolerant response, as evidenced in several studies (Balderas-Hemandez et al., 2013; Deinlein et al., 2014; Elfving et al., 2011; Golldack et al., 2011; Li et al., 2015; Nakashima et al., 2009; Nuruzzaman et al., 2013; Sakuraba et al., 2015). The overexpression or suppression of these genes can lead to an improvement of plant tolerance to stress conditions.
Tuteja et al. (2014a, 2014b) reported that DEAD-box RNA helicases are differentially regulated not only during development but also in stress response. At posttranscriptional level, small noncoding micro-RNAs can control and modulate the expression of responsive genes in plant exposed to environmental stress (Jeong and Green, 2013). Under this condition, the activation of osmotic stress-responsive (OR) genes has been detected (Kreps et al., 2002; Seki et al., 2002). The OR genes, usually silent under normal conditions, protect the cells from stress through the synthesis of important metabolic proteins and regulation of the downstream genes involved in signal transduction.
The decrease of water potential, caused by the osmotic component, is counteracted by the upregulation of genes relevant for inorganic ion uptake (Mahajan and Tuteja, 2005) and the synthesis of osmolytes. Enhancement of the synthesis of different osmolytes, such as glycine-betaine, proline, sugar alcohols, polyamines, and proteins from the late embryogenesis abundant, has been reported in species exposed to stress conditions (Aziz et al., 1998;
Nawaz and Ashraf, 2010). They help the cell in overcoming the osmotic stress, allowing the re-establishment of homeostasis (Shinozaki and Yama- guchi-Shinozaki, 1997; Zhu, 2002). The hydroxyl group of sugar alcohols can substitute the OH group of water, mechanism that leads to the preservation of membrane structural integrity, especially the thylakoids (Mahajan and Tuteja, 2005; Yokoi et al., 2002).
Studies on transgenic lines have shown that the expression of genes involved in osmolytes biosynthesis is associated with an enhancement of tolerance to different stresses (Bohnert and Jensen, 1996; Zhu, 2001). Trehalose is reported to act in stabilization of dehydrated proteins and lipid membranes and also in protecting biological structures from damage during desiccation (Redillas et al., 2012); transgenic rice, producing high level of this molecule, showed an improved tolerance to both drought and salt stress (Redillas et al., 2012).
Proline and glycine-betaine are the molecules mostly involved in osmoprotection. The accumulation of glycine-betaine has been related to the protection of plants against abiotic stress via osmoregulation or osmoprotection (for review see Giri 2011). Pyrroline-5-carboxylase synthase is the main proline biosynthetic gene; it has been utilized to increase proline level in transgenic plants in order to improve stress tolerance (Verbruggen and Hermans, 2008; Su and Wu, 2004). However, salt tolerance cannot be always related to significant changes of osmolyte synthesis (Di Cori et al., 2013). Therefore, we can hypothesize that the protective role of the osmolyte depends on different factors, like species, cultivar, growth conditions and developmental stage of the plants, and besides these to mechanism of action, different from osmoregulation or osmoprotection (Ashraf, 2004; Giri, 2011).
The regulation of gene transcription also involves the dynamic changes in hormone biosynthesis. Several hormones are involved in the response to abiotic stress, that is, salicylic acid (SA), ethylene (ET), jasmonates (JAs), cytokinins (CK), brassinosteroid (BR) and gibberellic acid (GA) (Deinlein et al., 2014; Fomi et al., 2017; Ryu and Cho, 2015). In stress conditions, cross-talk among these hormones have been suggested by different authors (reviewed by Peleg and Blumwald, 2011).
The most studied plant stress-signaling hormones are ABAs. In plants, there are ABA-responsive TFs, which expression is induced by different stresses. The stress response can be divided ABA dependent (Takahashi et al., 2004; Geng et al., 2013; Golldack et al., 2014; Yoshida et al., 2014; Tuteja, 2007) and ABA independent (Yamaguchi Shinozaki and Shinozaki, 2006; Tuteja, 2007). Both pathways regulate OR genes expression (Yoshida et al., 2014); while a reduction of water loss via transpiration is obtained by
ABA-inducible gene expression, that causes stomatal closure (Yamaguchi- Shinozaki and Shinozaki, 2006; Wilkinson and Davies, 2010).
An important role in plant development and response to environmental conditions has been ascribed to CKs and CK receptors (Tran et al., 2010; Nishiyama et ah, 2011; Nishiyama et ah, 2012). During growth, as wells as adaptation to environmental stress, CK and ABA may exert antagonistic activities (Javid et ah, 2011).
JA and its active derivates share a positive role in salt tolerance (Qiu et ah, 2014; Zhao et ah, 2014). JA enhances the activities of antioxidant enzymes (Qiu et ah, 2014), pathogenesis-related proteins and salt stress- responsive proteins (Moons et ah, 1997). Furthermore, the application of methyl JA elicits the production of antioxidants, even though the response is species-specific and depends on the concentration of the molecule (Ahmad et ah, 2016).
Stress response may induce the production of ET and SA (Mahajan and Tuteja, 2005). Exogenous application of SA promotes photosynthetic rate enhancement, thus, improving tolerance (reviewed by Hayat et ah, 2010).
Different stress conditions elicit stress ethylene production (Fomi et ah, 2017). Salt imposition causes enhancement of ET evolution from the leaves (Dodd and Perez-Alfocea, 2012). ET can cross-talk with auxin, since members of 1-amino-cyclopropane-1-carboxylate synthase gene family (ACS), encoding rate-limiting enzymes in ET biosynthetic pathways, are regulated by auxin (Tsuchisaka and Theologis, 2004). Improved salt tolerance was related to lower level of ET. In fact, inoculation of plants with bacteria strain containing 1-aminocyclopropane-1-carboxylate (ACC) deaminase gene that reduces stress ET synthesis ameliorates the plant performance in saline conditions (reviewed by Fomi et ah 2017).
BR, mainly by exogenous application, induces the expression of stress- related genes; thus, BR helps the keeping of photosynthetic efficiency, the activation of antioxidant enzymes, the synthesis of osmolytes, and other hormone responses (Divi and Krishna, 2009).
Accumulation of auxin, named auxin maxima, is related to cell elongation, organogenesis, and another physiological process (Lau et ah, 2008). Reduced level of auxin has been reported in salt-stressed tomato (Dunlap and Binzel, 1996). Arobidopsis mutants, defecting in auxin transport, are more sensitive to salt; moreover, inhibition of biomass production was determined in mutants with defects in transcription/receptors involved in the auxin response (Afzal et ah, 2005; Liu et ah, 2015). These results suggest the important role of auxin in the behavior of the plants exposed to salinity.