The transcription factor MYB78 negatively regulates cold stress tolerance in Actinidia chinensis

Previous studies have extensively investigated the key role and regulation of C-repeat binding factor (CBF) in cold stress tolerance. However, the function and regulation of AcCBFs in kiwifruit (Actinidia chinensis) remain largely unknown. In this study, we found AcMYB78 acts as a negative regulator of cold stress tolerance in A. chinensis. Acmyb78 mutants showed enhanced freezing tolerance, whereas overexpression of AcMYB78 resulted in sensitivity to freezing stress. In addition, yeast one-hybrid (Y1H), dual luciferase reporter, chromatin-immunoprecipitation-qPCR, and electrophoretic mobility shift assays indicated that AcMYB78 directly inhibits the expression of AcCBF1 and AcCBF2, while activating the expression of AcRAP2.1, thereby reducing the expression of cold-regulated (COR) genes. Interestingly, AtMYB78 negatively regulated cold stress tolerance in Arabidopsis (Arabidopsis thaliana), similar to AcMYB78. Moreover, the expression of MYB78 was initially down-regulated, followed by significant up-regulation in cold stress response. These findings reveal that the expression of MYB78 is suppressed at the ON stage of cold stress. The inhibition of CBFs and the activation of RAP2.1 by MYB78 were alleviated, resulting in a significant increase in the expression of CORs. At the attenuated stage, the expression of MYB78 was dramatically elevated. MYB78 largely inhibits the expression of CBFs while activating the expression of RAP2.1, thereby suppressing the expression of CORs.