Sodic alkaline stress mitigation by exogenous melatonin in tomato needs nitric oxide as a downstream signal

The present study was designed to determine the interactive effect of exogenous melatonin and nitric oxide (NO) on sodic alkaline stress mitigation in tomato seedlings. It was observed that exogenous melatonin treatment elevated NO levels in alkaline–stressed tomato roots. However, exogenous NO had little effects on melatonin levels. Importantly, melatonin–induced NO generation was accompanied by increased tolerance to alkaline stress. Chemical scavenging of NO reduced melatonin–induced alkaline stress tolerance and defense genes’ expression. However, inhibition of melatonin biosynthesis had a little effect on NO-induced alkaline stress tolerance. These results strongly suggest that NO, acting as a downstream signal, is involved in the melatonin–induced tomato tolerance to alkaline stress. This process creates a new signaling pathway for improving stress tolerance in plant.