Exogenous melatonin confers enhanced salinity tolerance in rice by blocking the ROS burst and improving Na+/K+ homeostasis

Melatonin (MT; N-acetyl-5-methoxytryptamine) has been reported to enhance the salt tolerance of rice seedlings and other crop species, yet its role in regulating rice grain yield formation and the underlying mechanism have received little attention. In this study, exogenous MT markedly mitigated the adverse effect of salinity on final rice grain yield, which was increased by 4.0 %-13.0 % compared with that under the same salt concentration in the absence of MT. Under the MT treatment, the number of rice green leaves was significantly (P < 0.05) increased, accompanied by increased leaf photosynthesis and enhanced photosynthetic pigment contents. MT protected leaf cell membranes by scavenging H2O2 and O2.- that accumulated (whose contents decreased by 6.5 %-28.9 % and 1.1 %-5.1 %, respectively, at heading stage) and enhanced the activities of superoxide dismutase (SOD), peroxidase (POD) and catalase (CAT) (whose average activity increased by 5.0 %-12.8 %, 14.9 %-27.0 % and 3.4 %-12.2 %, respectively). The leaf K+ content and expression of OsHAK genes didn't show significant changes under low salt concentration (0.1 % NaCl) but both parameters were greatly increased under relatively high salt concentrations, especially after MT application. Thus, we concluded that the mechanism through which MT provided enhanced rice salinity tolerance was different under various salt concentrations. MT prioritized activation of the ROS scavenging system to improve salinity tolerance in rice under relatively low salt concentration but alleviated damage from salinity both by blocking the reactive oxygen species (ROS) burst and by increasing K+ retention in leaf mesophyll cells via regulation of OsHAK expression (especially that of OsHAK5) under relatively high salt concentrations.