Salicylic acid alleviates copper toxicity in rice (Oryza sativa L.) seedlings by up-regulating antioxidative and glyoxalase systems.

The present study investigated the effect of salicylic acid (SA) on toxic symptoms, lipid peroxidation, reactive oxygen species generation and responses of antioxidative and glyoxalase systems in rice seedlings grown hydroponically under copper (Cu) stress for 48 h. Exposures of 75 and 150 μM Cu(2+) caused toxicity symptoms (chlorosis, necrosis and rolling in leaves), sharp increases in malondialdehyde (MDA), hydrogen peroxide (H2O2) contents and lipoxygenase (LOX) activity with concomitant reductions of chlorophyll (Chl) and relative water content (RWC). Both levels of Cu decreased ascorbic acid (AsA), glutathione (GSH), non-protein thiol (NPT) and proline contents in roots but rather increased in leaves except that AsA decreased in leaves too. These results together with overaccumulation of superoxide (O 2 (•-) ) and H2O2 in leaves revealed that Cu exposures induced oxidative stress. Contrary, SA-pretreatment (100 μM for 24 h) reduced toxicity symptoms and diminished Cu-induced increases in LOX activity, H2O2, MDA and proline contents while the levels of RWC, Chl, AsA and redox ratios were elevated. Higher levels of GSH and NPT were also observed in roots of SA-pretreated Cu-exposed seedlings. SA-pretreatment also exerted its beneficial role by inhibiting the Cu upward process. Studies on antioxidant enzymes showed that SA further enhanced the activities of superoxide dismutase, ascorbate peroxidase, glutathione reductase and glutathione peroxidase, and also elevated the depressed activities of catalase, dehydroascorbate reductase and glutathione S-transferase particularly at 150 μM Cu(2+) stress. In addition, the activity of glyoxalase system (glyoxalase I and II) was further elevated by SA pretreatment in the Cu-exposed seedlings. These results concluded that SA-mediated retention of Cu in roots and enhanced capacity of both antioxidative and glyoxalase systems might be associated with the alleviation of Cu-toxicity in rice seedlings.