Nitric oxide mediates hydrogen peroxide- and salicylic acid-induced salt tolerance in rice (Oryza sativa L.) seedlings

Nitric oxide (NO), hydrogen peroxide (H2O2), and salicylic acid (SA) are well-known signaling molecules that play multifaceted roles in the stress tolerance of plants; however, their interactions during stress alleviation have not been well studied. We investigated the possible regulatory role of NO in H2O2- and SA-induced reduction of oxidative damage in salt-exposed rice seedlings. For this purpose, hydroponically grown 14-day-old seedlings were pretreated with 100 mu M H2O2 or 100 mu M SA in the presence or absence of 100 mu M hemoglobin (Hb, a potent NO scavenger) for 24 h followed by salt stress (200 mM NaCl) for 72 h. Salt stress significantly increased the levels of H2O2, malondialdehyde, and proline whereas H2O2 and SA pretreatment reduced the values of these parameters. H2O2 and SA pretreatment also inhibited salt-induced loss of total chlorophyll and relative water content. Histochemical detection of reactive oxygen species [ROS: superoxide (O (2) (center dot-) ) and H2O2] indicated evident oxidative burst in the seedlings stressed with salt alone. Salt stress modulated the non-enzymatic and enzymatic antioxidants differentially; however, H2O2 and SA treatment prior to salt stress enhanced these antioxidants compared with the salt-stressed seedlings alone. H2O2 and SA pretreated salt-stressed seedlings also showed higher induction of the methylglyoxal (MG) detoxification system. Endogenous NO content was elevated following H2O2 and SA pretreatment over the experimental period. Adding Hb reduced the level of NO and subsequently abolished the beneficial effects of H2O2 and SA. Our results, therefore, suggest that NO might be involved in H2O2- and SA-induced reduction of oxidative damage through the upregulation of the antioxidant defense and MG detoxification systems to confer salt tolerance in rice seedlings. The data are of considerable value in elucidating the biochemical mechanisms of salt-stress tolerance and will augment the goal of developing appropriate and efficient methods for crop protection in saline environment.