Enhanced cell-wall carbohydrates, carbohydrate alcohols, organic
and fatty acids maintain cellular osmotic balance under salt stress
Pongamia exhibited significant changes in the metabolite level including
carbohydrates, amino acids, organic acids, polyol, and fatty acids.
Interestingly, the rise in mannitol level (~12 fold),
observed in both leaves and roots of salt treated plants at 8DAS,
suggest that mannitol may serve as a major osmoticum in Pongamia. The
increased levels of carbohydrate alcohols not only regulates the
cellular osmotic potential but also provides non-enzymatic ROS
scavenging protection under abiotic stress conditions (Abebe et al.,
2003; Hossain et al., 2017; Dumschott et al., 2019). High levels of
carbohydrates were recorded in both leaves and roots of treated plants
at 8DAS. These carbohydrates are known to contribute to secondary call
wall synthesis, while their accumulation alters the cell wall
composition in Pongamia (Gilbert et al., 2009; Geilfus et al., 2017;
Zhao et al., 2019). The rise in carbohydrates level may serve as
osmolytes and immediate source of energy for a cell (Abdallaha et al.,
2016).
The metabolic profile also indicate that enhanced levels of free amino
acids such as β-Ala, Val, Leu, Ala, Thr, Cys and Phe in leaves and roots
of treated plants at 8DAS. The increased levels of these free amino
acids may provide continue nutrient and water uptake to support plant
growth under salt stress condition and also contribute to tolerance by
regulating several biological processes including biosynthesis of cell
wall components, to protect the membrane protein integrity as well as
stability of cellular macro-structures ((Cao et al., 2017; Nasir et al.,
2010). Interestingly, the accumulation of serine and glycolate are
active components of photorespiration, which play a crucial role in
protecting the photosynthestic apparatus by limiting the deposition of
toxic photo-inhibitory metabolites (Hossainet al., 2017). Lactic acid
accumulation may involve in the regulation of cytoplasmic pH, and
production of pyruvate to maintain glycolysis under salt stress
condition (Felle et al., 2005; Hossain et al., 2017).
Increased accumulation of saturated fatty acids might protect the
membrane fluidity to protect the cell and cellular organelle from
Na+ toxicity and ion leakage (Shu et al., 2015; Atikij
et al., 2019). Our results also showed an increase in coumarate and
benzoate in both leaves and roots, while significant increase was
observed in caffieate and ferulate levels only in roots of salt treated
plants. Increased levels of these metabolites could be beneficial to the
plants for lignin biosynthesis as well as salicylic acid production,
which may play defensive role under salinity stress (Chen et al., 2019).
An increase in the accumulation of free amino acids Thr, Ala, Cys, and
Leu might also involve in the production of pyruvate to maintain TCA
cycle. Consistent with these results, we observed a marginal induction
of the GABA shunt. The induction of GABA shunt may serves as an
alternative source for carbon in TCA cycle and support respiratory
carbon metabolism under salt stress (Che-Othman et al., 2019;
Seifikalhor et al., 2019). Further, we observed marginal increase in
metabolites of glycerol pathway, which are essential in providing carbon
source to glycolysis and triglyceride pathways. Moreover, glycerol is
known to function as intra-cellular osmoticum under salt stress
(Bahieldin et al., 2013; Igamberdiev & Kleczkowski, 2018).