Over-expression of heat shock factor AcHsfA2-1 upregulates transcripts of multiple genes and enhances heat tolerance of kiwifruit plants

The role of heat shock factor AcHsfA2–1 in heat tolerance in kiwifruit plants was investigated. Exposure of plants to high temperature (50 ℃) for 1 h resulted in high H2O2 accumulation in leaves, while 2 h at this temperature caused irreversible foliar damage. AcHsfA2–1 (Acc28668) was the only transcription factor in the top four differentially expressed genes (DEGs) that was up-regulated by heat shock stress. Stable overexpression of AcHsfA2–1 in kiwifruit plants enhanced resistance to 50 ℃ heat shock stress for at least 2 h. AcHsfA2–1 could bind to promoters of three AcHsp20s (Acc16656, Acc25875 and Acc11386), which were highly expressed after heat shock, to trans-activate them more than 50-fold. A total of 68 DEGs were regulated by the over-expression of AcHsfA2–1 both directly and indirectly, based on the results of multiple tissue comparisons. In addition to three AcHsp20s, the promoters of six other DEGs could be directly trans-activated by AcHsfA2–1, related to ‘posttranslational modification, protein turnover, chaperones’ (Acc06344, Acc13870, Acc26097, Acc18330), zinc metallopeptidase EGY3 (Acc21153) and an uncharacterized protein (Acc31563). Other DEGs, such as the BAG domain-containing proteins (Acc19817, Acc09180), showed no response to AcHsfA2–1 directly, suggesting an indirect regulatory mechanism. This study enhances the understanding of the mechanism of heat stress response and demonstrates the importance of HsfA2 in perennial crop plants.