Hydrogen sulfide and nitric oxide are involved in melatonin-induced salt tolerance in cucumber.

Hydrogen sulfide (HS) is a novel gaseous signaling molecule in response to adversity stress. Melatonin (MT) is a multifunctional molecule that plays an important role in regulating plant stress resistance. However, the interactions between HS and MT are still unknown. Therefore, the role of HS in MT-induced salt tolerance was elucidated in this study by measuring the antioxidant defense system and photosynthetic characteristics of cucumber. In addition, the crosstalk among HS, NO, and mitogen-activated protein kinase (MAPK) was investigated. Results showed that MT induced the production of HS by significantly increasing the activity of L-/D-cysteine desulfhydrase, thereby regulating photosynthetic efficiency, antioxidant enzyme activity, and antioxidant enzyme gene expression in cucumber, thus alleviating reactive oxygen species burst by salt stress. In this process, the HS and NO induced by MT were inhibited by NO scavenger (cPTIO) and HS scavenger (HT) but not affected by MAPK inhibitor (U0126). Intriguingly, the expression of MAPK3/4/6/9 was inhibited by HT and cPTIO. These results suggested that HS may act as downstream of MT, interact with NO and MAPK cascades, and jointly participate in the process of MT mitigating salt stress in cucumber. In addition, HS and NO are upstream signaling molecules of the MAPK cascades.