Cucumber CsHsfA2 improves thermotolerance through self-activation and activation of CsHsp70-1

Heat stress poses a serious threat to crop growth and development, yield, and quality. The function of heat shock factor A2 (HsfA2) to heat tolerance and its regulatory genes, heat shock proteins (Hsps), has been characterized in many plant species. However, the function of CsHsfA2 in heat tolerance of cucumber (Cucumis sativus L.) and its directly regulated genes is still unclear. In this study, CsHsfA2 was cloned from cucumber and its protein possessed typical characteristics of HsfA2 from other dicots. CsHsfA2 could be rapidly reduced by heat treatment within 30 min. The CsHsfA2 protein was localized in the nucleus and exhibited transcriptional activation activity. Furthermore, transient overexpression of CsHsfA2 in cucumber improved thermotolerance and stimulated the expression of CsHsp70-1. Virus induced gene silencing and ectopic expression in Arabidopsis confirmed the key role of CsHsfA2 in thermotolerance. Then, CsHsp70-1 was found to be the downstream gene directly activated by CsHsfA2. The function of CsHsp70-1 in thermotolerance was also confirmed through transient overexpression and virus induced gene silencing and ectopic expression in Arabidopsis. In addition, we discovered that CsHsfA2 can directly bind to its own promoter, which activates its own expression and establishes a positive autoregulation loop. Taken together, our study displayed the key roles of CsHsfA2 and CsHsp70-1, providing candidate genes for thermotolerance improvement of cucumber.