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).