Methyl Jasmonate Was Involved in Hydrogen Sulfide-Alleviated Cadmium Stress in Cucumber Plants Through ROS Homeostasis and Chlorophyll Metabolism.

Cadmium (Cd), as one of the most toxic nonessential elements, severely prohibits plant growth and development. Hydrogen sulfide (HS) and methyl jasmonate (MeJA) play essential roles in plant response to abiotic stress. However, the potential mechanism of HS and MeJA in alleviating Cd stress in plants remains unclear. In the current study, the importance and crosstalk of HS and MeJA in the Cd tolerance of cucumber seedlings have been investigated. Our results revealed that Cd stress obviously prohibited the growth of cucumber seedlings. Optimal concentrations of HS donor sodium hydrosulfide (NaHS) or MeJA treatment, respectively, or in combination, significantly enhanced seedling growth under Cd stress. However, the positive effects of HS during seedling growth were obviously reversed by the application of MeJA biosynthesis inhibitors, which implied that MeJA might be involved in the HS-improved growth of cucumber seedlings under Cd stress. Moreover, Cd stress resulted in the increase in hydrogen peroxide (HO), superoxide radical (O) accumulation, and impaired the functioning of the ascorbate-glutathione cycle. Both HS and MeJA decreased the reactive oxygen species (ROS) level and ameliorated the negative effects of Cd stress through significantly increasing the ratio of ascorbate (AsA)/dehydroascorbic acid (DHA) and reduced glutathione (GSH)/oxidized glutathione (GSSG). Besides that, the expression level of ROS scavenge genes was significantly upregulated by the application of exogenous HS or MeJA treatment. Moreover, HS and MeJA significantly enhanced the chlorophyll concentration and inhibited chlorophyll degradation through decreasing the expression levels of chlorophyll catabolic enzymes. Additionally, exogenous HS and MeJA obviously enhanced the chlorophyll fluorescence. However, MeJA biosynthesis inhibitors significantly suppressed the positive role of HS. The above results suggested MeJA is involved in HS-induced Cd stress alleviation in cucumber seedlings through enhancing ROS-scavenge capacity and improving the photosynthesis system.