The role of H2S in low temperature-induced cucurbitacin C increases in cucumber.

KEY MESSAGE: In this study, we first linked the signal molecule H2S with cucurbitacin C, which can cause the bitter taste of cucumber leaves and fruit, and specifically discuss its molecular mechanism. Cucurbitacin C (CuC), a triterpenoid secondary metabolite, enhances the resistance of cucumber plants to pathogenic bacteria and insect herbivores, but results in bitter-tasting fruits. CuC can be induced in some varieties of cucumber on exposure to plant stressors. The gasotransmitter hydrogen sulfide (H2S) participates in multiple physiological processes relating to plant stress resistance. This study focused on the effect of H2S on low temperature-induced CuC synthesis in cucumber. The results showed that treatment of cucumber leaves at 4 °C for 12 h enhanced the content and production rate of H2S and increased the expression of genes encoding enzymes involved in H2S generation, Csa2G034800.1 (CsaLCD), Csa1G574800.1 (CsaDES1), and Csa1G574810.1 (CsaDES2). In addition, treatment at 4 °C or with exogenous H2S upregulated the expression of CuC synthetase-encoding genes and the resulting CuC content in cucumber leaves, whereas pretreatment with hypotaurine (HT, a H2S scavenger) before treatment at 4 °C offset these effects. In vitro, H2S could increase the S-sulfhydration level of His-Csa5G156220 and His-Csa5G157230 (both bHLH transcription factors), as well as their binding activity to the promoter of Csa6G088690, which encodes the key synthetase for CuC generation. H2S pretreatment enhanced the cucumber leaves resistance to the Phytophthora melonis. Together, these results demonstrated that H2S acts as a positive regulator of CuC synthesis as a result of the modification of proteins by S-sulfhydration, also providing indirect evidence for the role of H2S in improving the resistance of plants to abiotic stresses and biotic stresses by regulating the synthesis of secondary metabolites.