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).