Melatonin enhances Na+/K+ homeostasis in rice seedlings under salt stress through increasing the root H+-pump activity and Na+/K+ transporters sensitivity to ROS/RNS

Salinity stress is one of the most serious factors limiting the productivity of crops. Rice at the seedling stage is extremely sensitive to salt. Studies have shown that melatonin can improve plant resistance to a variety of abiotic stresses, including salt stress. In this study, the mechanism of melatonin in regulating Na+/K+ homeostasis in rice seedlings was explored. The results showed that melatonin pretreatment increased rice dry weight and salt tolerance indices under salt stress. This is because melatonin inhibited the increase of Na+ content and the decrease of K+ content induced by salt stress and increased the ratio of K+/Na+. Melatonin increased the selective absorption indices of rice under salt stress, which indicated that melatonin pretreatment improved the ability of rice to absorb K+ and exclude Na+. The results of root Na+/K+ flux also confirmed this point. Rice pretreated with melatonin showed stronger roots Na+ efflux and K+ influx after salt stress. As signaling molecules, the content of nitric oxide increased in the roots of rice pretreated with melatonin, but the content of hydrogen peroxide decreased. Rice roots Na+/K+ transporters pretreated with melatonin were more sensitive to nitric oxide and hydrogen peroxide, which manifested as the Na+ efflux and K+ influx were stronger after being shocked by nitric oxide and hydrogen peroxide. This was due to melatonin pretreatment increasing roots' ATP content and H+-pump activity. These results indicated that exogenous melatonin could improve root Na+/K+ transporters' response to reactive oxygen species/reactive nitrogen species as well as increase ATP content and H+-pump activity to maintain Na+/K+ homeostasis in rice under salt stress.