Arsenic Toxicity is Counteracted by Exogenous Application of Melatonin to Different Extents in Arsenic-susceptible and Arsenic-tolerant Rice Cultivars

The current study established the protective effects of exogenous melatonin in ameliorating arsenic toxicity in Khitish (arsenic-sensitive) and Muktashri (arsenic-tolerant) rice cultivars. Melatonin highly improved the overall growth performance of arsenic-treated seedlings, more prominently in the sensitive variety, Khitish. Although the level of arsenic increased in both the cultivars, Khitish accumulated comparatively higher arsenic level. However, melatonin supplementation reduced arsenic bioaccumulation and restored physiological growth attributes, as supported by lowering of electrolyte leakage, chlorophyll loss (by inducing RuBisCo), protein carbonylation, malondialdehyde accumulation, lipoxygenase (LOX), NADPH oxidase (NOX) and protease activity, and improvement of membrane stability index. Isoforms of LOX and NOX showed varietal differences during arsenic stress, both in the presence and absence of melatonin. Melatonin reduced methylglyoxal content during arsenic stress, concomitant with down-regulated gene expression and enzyme activity for glyoxalases. The nitrogen assimilation was improved via induced nitrate reductase (NR) activity and NR expression. The variable accumulation of osmolytes like proline, glycine betaine and total amino acids, concomitant with suppressed P5CS and BADH1 expression, and induced PDH was noteworthy. Antioxidant metabolites like anthocyanins, flavonoids, carotenes, xanthophylls and total phenolics were accumulated upon supplementation of melatonin in arsenic-stressed Khitish, supported by the activation of ANS and PSY genes. Melatonin lowered the ascorbic-acid oxidase activity and restored the ascorbate sink in arsenic-affected seedlings. Overall, the study revealed the potential role of exogenous melatonin in mitigating arsenic-induced injuries by strengthening osmolytes and antioxidative machinery, leading to the restoration of growth and metabolism in rice, especially in the susceptible cultivar.