<scp>BAG2</scp> mediates HsfA1a‐induced thermotolerance by regulating heat shock proteins in tomato

The heat shock transcription factors (Hsfs) and Bcl-2-associated athanogene (BAG) are crucial in response to heat stress. However, the relationship and regulation mechanism between Hsfs and BAGs in plants are largely unknown. Here, we demonstrated that the HsfA1a-BAG2 module mediated thermotolerance through regulating heat shock proteins (HSPs) in tomato. Overexpression of HsfA1a in tomato increased thermotolerance and enhanced the expression of HSP70, HSP90, and BAG2, but compromised in hsfa1a mutant plants. Yeast one-hybrid, dual luciferase, electrophoretic mobility shift assay, and chromatin immunoprecipitation coupled with qPCR assays found that HsfA1a directly bound to the promoter of BAG2 to activate its expression. BAG2 interacted with HsfA1a and BAG5b both in vitro and in vivo. Importantly, BAG5b facilitated the interaction between BAG2 and HsfA1a, resulting in enhanced transcriptional activation capacity of HsfA1a to HSP70 and HSP90. Either overexpression of BAG2 or BAG5b increased thermotolerance, concomitant with sustained expression levels of HSP70 and HSP90 genes. By contrast, knockout of BAG2 or BAG5b displayed the opposite results. Furthermore, silencing of BAG2 or BAG5b in HsfA1a overexpression plants attenuated HsfA1a-induced thermotolerance and HSPs expression. Thus, HsfA1a mediated thermotolerance through transcriptionally regulating BAG2 and forming a complex with BAG2 and BAG5b to ultimately induce the expression of HSPs in tomato. Our findings provide new insights into the regulatory network of Hsfs on HSPs under heat stress in plants.