Hydrogen peroxide mediates melatonin-induced chilling tolerance in cucumber seedlings.

MT mitigates chilling damage by enhancing antioxidant system and photosystem activities, and cold-responsive genes expression in cucumbers. HO may act as a downstream signaling molecule in the MT-induced chilling tolerance. Melatonin (MT) and hydrogen peroxide (HO) are important endogenous signaling molecules that play multifaceted roles in plant responses to abiotic stress. However, the interactive mechanism by which MT and HO regulate chilling tolerance remains unclear. Here we found that MT exhibited a positive regulatory effect on the chilling tolerance of cucumbers, with an optimum concentration of 100 µM. MT markedly enhanced RBOH1 mRNA expression, activity and endogenous HO accumulation in cucumber seedlings. However, 1.0 mM HO had no significant effect on mRNA levels of TDC and ASMT, the key genes for MT synthesis, and endogenous MT content. Both MT and HO significantly decreased malondialdehyde (MDA), electrolyte leakage (EL) and chilling injury index (CI) by activating the antioxidant system, thereby alleviating chilling damage in cucumber seedlings. MT and HO improved photosynthetic carbon assimilation, which was primarily attributed to an increase in activity, mRNA expression, and protein levels of RuBPCase and RCA. Meanwhile, MT and HO induced the photoprotection for both PSII and PSI by enhancing the QA's electron transport capacity and elevating protein levels of the photosystems. Moreover, MT and HO significantly upregulated the expression of cold response genes. MT-induced chilling tolerance was attenuated by N', N'-dimethylthiourea (DMTU), a HO specific scavenger. Whereas, the MT synthesis inhibitor (p-chlorophenylalanine, p-CPA) did not influence HO-induced chilling tolerance. The positive regulation of MT on the antioxidant system, photosynthesis and cold response gene levels were significantly attenuated in RBOH1-RNAi plants compared with WT plants. These findings suggest that HO may functions as a downstream signaling molecule in MT-induced chilling tolerance in cucumber plants.