The role of melatonin in reinforcement of antioxidant system of rice plant (Oryza sativa L.) under arsenite toxicity

Arsenic (As) is a toxic metalloid, which enters into the human food chain mainly through rice. One imminent toxic effect of As is oxidative stress. Melatonin is potentially an antioxidant compound. The aim of the present study was to evaluate the efficiency of melatonin to decrease arsenite (AsIII) toxicity in rice (Oryza sativa L.). Twenty one day old rice plants were treated with or without 25 µM AsIII, 0.25 µM melatonin, and a combination of both, for 48 h in hydroponic culture. Arsenite toxicity was manifested as reduced chlorophyll, soluble sugars and protein contents, and decreased growth of the plants (48%, 29.1%, 23.6% and 87.5% of the controls, respectively). In addition, AsIII increased the content of H2O2, the activity of catalase (CAT), the contents of total non-protein thiols (TNP-SH), glutathione (GSH), and phytochelatine (PCs) (2.5%, 926%, 191.5%, 12.3% and 361.4 of their corresponding controls, respectively). In the presence of AsIII, however, melatonin ameliorated oxidative stress on roots by reducing As content, increasing the contents of TNP-SH, GSH, and PCs which led to decrease of H2O2 and malondialdehyde (MDA) content. The results also indicated that melatonin facilitated the transportation of As from roots to shoots, resulting in turn in higher accumulation of As in shoots and suppression of antioxidant system. Further studies are needed in order to clarify the mechanism(s) by which melatonin accelerated transportation of As from root to shoot of rice plant.