Spermine ameliorates prolonged fluoride toxicity in soil-grown rice seedlings by activating the antioxidant machinery and glyoxalase system

The current manuscript presents the first report on the ameliorative roles of exogenous spermine (Spm) during prolonged fluoride-induced toxicity and oxidative damages in the susceptible rice cultivar, IR-64. The application of Spm increased the overall growth in the stressed seedlings by significantly restricting fluoride bioaccumulation within the shoots and roots. The Spm-treated stressed seedlings exhibited low chlorosis and induced activity of pyruvate dehydrogenase and nitrate reductase due to reduced accumulation and localization of reactive oxygen species (ROS) in the shoot and root. Spm-supplementation during stress reduced the levels of molecular damages by lowering malondialdehyde, electrolyte leakage and protein carbonylation, and lipoxygenase and protease activity due to effective detoxification of ROS by the antioxidants like proline, glycine-betaine, anthocyanin, flavonoids, phenolics and higher polyamines like Spm and spermidine. Excessive accumulation of the toxic methylglyoxal was reversed due to the activation of the glyoxalase system (comprising of glyoxalase I and II) and the ascorbate-glutathione cycle. Exogenous Spm also triggered the activity of superoxide dismutase, guaiacol peroxidase, glutathione peroxidase and phenylalanine ammonia lyase, which efficiently scavenged ROS in the stressed seedlings. Overall, Spm treatment mitigated the fluoride-induced injuries in IR-64 by reducing fluoride bioaccumulation and elaborately refining the various defence machineries.