The CsBLH9-CsOVATE module regulates heat tolerance via chlorophyll degradation in cucumber

Heat stress has become a primary environmental factor that severely threatens crop yield and quality. Despite many genes having been identified to participate in heat response, natural alleles of heat response genes that can be utilized in crop breeding are scarce, and the underlying mechanisms remain enigmatic. In this study, we identified a BEL1-like Homeodomain (BLH) transcription factor CsBLH9, through genome-wide association study (GWAS), and demonstrated its positive role in heat response. The natural variants of CsBLH9 exhibit different levels of heat tolerance and differential response to gibberellin (GA) treatment. Knockout of CsBLH9 resulted in sensitive heat response and reduced chlorophyll levels. CsBLH9 can directly bind to the promoters of chlorophyll catabolic genes stay-green (CsSGR) and pheophorbide a oxygenase (CsPAO) to inhibit their expression. Furthermore, we identified CsOVATE as an interacting partner of CsBLH9 that also functions as a positive regulator of heat tolerance. Csovate mutants showed phenotypes similar to Csblh9 mutants under heat stress, and CsOVATE enhanced the CsBLH9-mediated inhibition of CsSGR and CsPAO expression. Therefore, our findings suggested that CsBLH9-CsOVATE acts as a module that regulates heat tolerance via chlorophyll degradation in cucumber, and the natural alleles of CsBLH9 provide new targets for crop improvement under heat stress.