Salinity-induced alteration in expression of antioxidant genes
Excess Na+ accumulation in the plant causes ionic imbalance which results in ion toxicity, oxidative stress, and generation reactive oxygen species (ROS) (AbdElgawad et al., 2016). To combat the ROS induced cellular damage plant induces several antioxidant enzymes such as superoxide dismutase (SOD), catalase (CAT) and peroxidase (POD) (Reddy et al., 2004). In this study, the observed higher expression levels of the different SOD isoforms might protect the plants at the onset of oxidative stress much before the cellular antioxidant signaling cascade initiates its function under high salinity. According to Gill & Tuteja, (2010) the H2O2 produced through SOD activity, provides an additional advantage to the plant by strengthening cell wall through lignin biosynthesis (Figure 7). Also, higher CAT4 expression levels in roots might help the plant to reduce the higher levels of H2O2 under high salt conditions. The lower expression levels of CAT4 in the leaves may reflect the dynamic and tissue specificity of the antioxidant system under high salt environment (AlHassan et al., 2017). Increased expression of POD in leaves and roots were well correlated with the respective lignin depositions. The classIII apoplastic peroxidases play a major role in maintaining the structural integrity of the cell wall under salt stress (Kim et al., 2012). In addition, the expression levels of several other cell-wall modifying and lignin biosynthesis genes were monitored which include, PAL1, PAL2, CAD1, CAD1-like, CAD2, and CAD6. The increased expression levels of both PAL1 and PAL2 in 500 mM NaCl treated roots might permit the plant to preserve the cell wall integrity under high salt environment (Lee et al., 2007). The consistent increase in the CAD1-like gene expression suggests its possible role in reinforcing the strength and rigidity of the cell wall to enhance the selectivity and permeability of cell wall for solute and water transport under high saline environment (Zhao et al., 2013).