Spectrum of Physiological and Molecular Responses in Plant Salinity Stress Tolerance
Insha Amin, Aditya Banerjee, Abbu Zaid, Mudasir A. Mir, Shabir H. Wani, Nazeer Ahmed, Aryadeep Roychoudhury, and Khalid Z. Masoodi
Salinity stress is regarded as one of the principal environment stresses that retard growth and productivity of crop plants, especially in arid and semi-arid regions of the world (Rozema and Flowers, 2008). According to Munns and Tester (2008), globally more than 800 million hectares of arable lands are severely affected by salinity stress, which corresponds to 50% of all irrigated lands (Sairam and Tyagi, 2004). Salt stress is a physiological condition characterized by increased concentrations of soluble salts inside the cells leading to an imbalance in the cell steady state (Joshi et al., 2016; Khan et al., 2017). Salt stress induces ion toxicity due to increased levels of ions like sodium (Na+), chloride (CL) and sulfate (S042-). Sodium chloride (NaCl) is the most widely present and most soluble salt, and therefore Na+ accounts for the majority of the salt stress-related symptoms in the plants. There can be approximately 40 mM NaCl concentration and electrical conductivity (EC) of 4dS/m in the soils affected by salinity (Acosta-Motos et al., 2017). There is an increased concentration of Na+ ions in the salt-rich soils with a concomitant increase in carbonate/bicarbonate levels making these soils highly alkaline (pH greater than 7). Salinity stress results in an imbalance of ion homeostasis due to an increase in the concentration of Na+ ions as well as the simultaneous decrease in potassium (K+) concentration (Liu et al., 2018). Many plants show a reduced growth, deteriorated quality and a significant decrease in productivity under such salt levels because salinity stress triggers complex signaling pathways to inhibit growth, development and plant physiological processes (Naeem et al., 2012). Salinity stress limits photosynthetic potential as a result of disorganized chloroplast thylakoids (Khan et al., 2014; Fatma et al., 2016) and impairment in the diffusion rate of carbon dioxide (CO,) via decreasing conductance of stomata and mesophyll cells (Flexas et al., 2004). Rasool et al. (2013) conducted an experiment to evaluate the effect of salt stress on growth and some key antioxidants in eight chickpea genotypes which were grown in a hydroponic environment. Their results indicate that salt stress-induced oxidative stress by hampering the growth and physiology of the cells. Salinity stress results in oxidative damage through orchestrating the production of reactive oxygen species (ROS), which can cause cell death by damaging proteins, lipids, RNA and DNA (Gill and Tuteja, 2010; Anjum et al„ 2015; Ahmad et al., 2016). Plants possess intricate mechanisms undergoing complex crosstalks for sensing of environmental stresses (Wani et al., 2013). To survive under such sub-optimal conditions, salt-tolerant plants like halophytes have evolved a well-integrated adaptive response at the molecular, cellular and physiological levels to ensure survival, distribution and productivity (Flowers and Muscolo, 2015). Various salt tolerance mechanisms have been comprehensively depicted in Figure 1.1. These adaptation mechanisms can be associated with detoxification, protein degradation, synthesis of osmoprotectant and antioxidants, overexpression of water and ion channels and accumulation of stress-responsive transcription factors (TFs) like WRKY, NAC, bZIP, MYB, MYC, etc. (Hiz et al., 2014; Banerjee and Roychoudhury, 2017). The TFs upregulate osmotic responsive (OR) genes encoding late embryo- genesis abundant (LEA) proteins, heat shock proteins (HSPs) and antioxidant enzymes (Banerjee and Roychoudhury, 2016). The salt-tolerant genotypes accommodate the stress-mediated low water potential by maintaining a high relative water content (RWC) (Joshi and Karan, 2013). Salinity initiates multi-level regulation of gene expression through complex transcriptional networks (Singh and Laxmi, 2015). At the molecular level, the identification and characterization of candidate genes for the accumulation of ions and movement of water molecules are of paramount importance in dissecting underlying mechanisms of plants’ salt stress tolerance. The salt overly sensitive (SOSI) gene, which encodes an antiporter Na+/H+ in plasma membrane, can play a significant role in deciphering mechanisms related to how Na+ ions are excluded out of a salt-stressed cell and controlled via their long-distance transport from the roots to shoots in Arabidopsis thaliana (Shi et al., 2002). In plants, salt stress is known to increase the expression level of SOS1, which might confer salt stress tolerance (Gao et al., 2016). In a recent study, Liu et al. (2018) studied the growth, ionic response and gene expression analysis in ryegrass under salt stress conditions and observed that salinity tolerance is related to the decreased expressions of SOS1, NHXl and TIPI in the shoots, and increased expressions of NHXl and PIP1 in the roots. These reports suggest that the coordination of genes for regulating the homeostasis of ions might prove beneficial for enhancing plant salinity stress tolerance. Nevertheless, the characterization and incorporation of selected salt-responsive genes like DREB,
FIGURE 1.1 Various salt-tolerance mechanisms in plants.
SOS, HKT, NHX, PMP3, etc., using transgenic technology can help to design salt-tolerant lines and promote agricultural expansion.
Mechanisms Adopted for Salt Adaptations
Ion toxicity and the hyperosmolar interior are the two important factors affecting the plants reared on soils with high salt concentrations. Adaptation to soil salinity is definitely one of the most complicated biological phenomena carried out by plants in order to maintain a steady state inside the cells. Plants adapt to many abiotic stresses at molecular, cellular, biochemical and physiological levels (Adem et al., 2014). Various adaptations include the regulation of ion transport and maintenance of water balance, vacuolar sequestration of Na+ ions, retention of K+ ions, accumulation of compatible solutes for osmotic adjustments and reactive oxygen species (ROS) scavenging at the gene and transcriptional levels.
Effect of Salinity on Plant Growth and Development
Ionic stress is predominant among the various abiotic stresses which act as limiting factors for plant growth and survival (Adem et al., 2014). There is a detrimental effect of salt excess on almost all the developmental parameters like seedling, flowering, chlorophyll content, internodal growth, etc. During salinity stress, there is a change in the concentration of ions in the soil around the root tip causing an imbalance in water potential leading to osmotic stress first and ending in ionic toxicity. This leads to shortening and swelling of the roots due to reduced cell division and proliferation in the root meristematic zone (Li et al., 2014b). It limits leaf extension, photosynthesis and biomass accumulation in plants (Rahnama et al., 2010). A significant reduction occurs in leaf elongation due to loosening of epidermal wall rigidity (Zorb et al., 2015). NaCl changes stem morphology by decreasing the number, diameter and length of internodes leading to stunted plant height (Nja et al., 2018). In olives, high salt levels cause significant reductions in the number and length of roots and an increased root turnover leading to restricted lifespan and development (Soda et al., 2017). Salt stress induces leaf discoloration, wilting, leaf bronzing and necrosis, thus derailing the aesthetic quality of plants (Valdez-Aguilar et al., 2011). Salt stress negatively affects flowering which can lead to drawbacks in the reproductive status by decreasing viable pollen grains (Yu et al., 2017). There is a decrease in root length when capsicum plants are exposed to 150 mM NaCl stress (Shivakumara et al., 2017). However, the transgenic plants overexpressing pea DNA Helicase 45 (PDH45) show a four-fold increase in root length. Meta-analyses of the responsive curves show that salinity markedly affects the leaf area and dry mass per unit area (Poorter et al., 2010).
Comparison of the total biomass of stressed and control plants has shown a relative decrease in plant biomass (RDPB) (Negrao et al., 2017). Dose-responsive curves revealed that growth rates of Arabidopsis plants decreased as a quadratic function of salt concentration by decreasing RGRs (relative growth rate) when NaCl concentrations increased above 25 mM (Claeys et al., 2014) Facilities like plant accelerators have been used to assess the ion-independent component of salt toxicity which inhibited shoot growth from the moment of salt imposition, i.e., even before the accumulation of Na+ in the shoots (Berger et al., 2012; Campbell et al., 2015). Depending on plant adaptations like thick walls and space for ion sequestration, salt stress variably reduces the relative leaf area ratio (RLAR) (Negrao et al., 2017). Salinity affects water potential, hydraulic conductivity and transpiration use efficiency (TUE) in sensitive plants (Negrao et al., 2017). Salinity decreased relative water fraction (RWF) and leaf water fraction (WF) in the susceptible varieties. In spite of reduced water potential, the turgor pressure remains unaffected. This results in significant water losses from opened stomata (Boyer et al., 2008).
Essential physiological processes like photosynthesis, respiration and reproduction are negatively regulated during salinity. Rapid chlorosis resulted in lowering of the soil and plant analyzer development (SPAD) index in stressed plants (Adem et al., 2014). Reduction in the chlorophyll levels has been reported in barley exposed to salt stress wherein the chlorophyll fluorescence (FJFm) value showed the highest correlation with the stress damage index (Chadchawan et al., 2017). Infra-red (IR) thermography has shown a strong genotype dependency between salinity, stomatal conductance and leaf temperature (Sirault et al., 2009). Similar phenomic approaches can be adopted to study salt-induced senescence (SIS) in each leaf rather than in the total shoot as a whole (Ward et al., 2014).
Salinity and the Antioxidant System
Reactive oxygen species (ROS) or free radicals like superoxides, hydroxyl ions, hydrogen peroxides and methylglyoxal are generated due to the degeneration of cell membranes as a result of any external stress stimuli, leading to the inhibition of plant growth and development (Saini et al., 2018; Mir et al., 2018). ROS stimulates peroxidation of membrane lipids producing toxic malondialde- hyde (MDA) degeneration of nucleic acids and proteins, causing uncontrolled apoptosis (Rasouli et al., 2016). Plants increase the concentration of certain NAD(P)+-dependent enzymes which act as ‘aldehyde scavengers’ (Zhu et al., 2014). Transgenic tall fescue plants overexpressing Arabidopsis SOS genes showed resistance to salt exposures and an increase in the activities of many antioxi- dative enzymes like superoxide dismutase, peroxidase and catalase (Ma et al., 2014). Although toxic by nature, ROS have now been recognized as vital signaling molecules in many biological processes like programmed cell death or apoptosis (Schmidt et al., 2013). Genes responsible for aldehyde dehydrogenase activity like Aldhl2A from xerophytic grass, Cleistogenes songorica was found to improve tolerance to salinity and drought in transgenic Arabidopsis plants (Zhang et al., 2014a). Alfalfa plants transformed with the CsALDH gene from Cleistogenes songorica, a desert plant, showed improvement in the phenotypes when exposed to 200 mM of NaCl (Duan et al.,
2015) . This suggests that there exists a significant role played by aldehyde-scavenging enzymes in salt-combating mechanisms by destroying the aldehydes and free radicals generated in salinity stress. The cells of the lentil plant increase the concentration of antioxidative defense enzymes like superoxide dismutase (SOD), ascorbate peroxidase (APX), catalase (CAT) and glutathione reductase (GR) when subjected to salt concentrations of 100 and 200 mM (Cicerali, 2004). Therefore, the antioxidant defense mechanism helps in the maintenance of cellular integrity and cell homeostasis.