Exogenous melatonin‐mediated regulation of K+/Na+ transport, H+‐ATPase activity and enzymatic antioxidative defence operate through endogenous hydrogen sulphide signalling in NaCl‐stressed tomato seedling roots

Abstract Melatonin (Mel) and hydrogen sulphide (H 2 S) have emerged as potential regulators of plant metabolism during abiotic stress. Presence of excess NaCl in the soil is one of the main causes of reduced crop productivity worldwide. The present investigation examines the role of exogenous Mel and endogenous H 2 S in tomato seedlings grown under NaCl stress. Effect of 30 µ m Mel on endogenous synthesis of H 2 S was examined in roots of NaCl‐stressed (200 m m ) tomato seedlings. Also, the impact of treatments on the oxidative stress markers, transport of K + and Na + , and activity of H + ‐ATPase and antioxidant enzymes was assessed. Results show that NaCl‐stressed seedlings supplemented with 30 µ m Mel had increased levels of endogenous H 2 S through enhanced L‐cysteine desulfhydrase activity. Mel in association with H 2 S overcame the deleterious effect of NaCl and induced retention of K + that maintained a higher K + /Na + ratio. Use of plasma membrane inhibitors and an H 2 S scavenger revealed that Mel‐induced regulation of K + /Na + homeostasis in NaCl‐stressed seedling roots operates through endogenous H 2 S signalling. Synergistic effects of Mel and H 2 S also reduced the generation of ROS and oxidative destruction through the enhanced activity of antioxidant enzymes. Thus, it is suggested that the protective function of Mel against NaCl stress operates through an endogenous H 2 S‐dependent pathway, wherein H + ‐ATPase‐energized secondary active transport regulates K + /Na + homeostasis.