The MdGAMYB-MdHVA22g module confers drought tolerance by mediating γ-aminobutyric acid content and reactive oxygen species scavenging

HVA22 is an abscisic acid (ABA)- and stress-induced protein. However, how it is regulated and whether it plays a role under drought stress are largely unclear. In this study, we found that drought-inducible MdHVA22g plays a positive role under drought stress. Overexpression of MdHVA22g impairs endoplasmic reticulum (ER) morphology. Further analysis demonstrated that MdHVA22g enhances drought tolerance by upregulating MdGAD1 and MdGAD4 expression, thereby leading to increased GAD activity, γ-aminobutyric acid (GABA) accumulation, and enhanced reactive oxygen species (ROS) scavenging. In addition, we identified MdGAMYB as an upstream regulator of MdHVA22g. MdGAMYB, the drought-positive regulator, is able to directly bind to the promoter of MdHVA22g and activate its expression in response to drought, which results in increased GAD activity, GABA biosynthesis, and ROS detoxification. Therefore, the regulatory cascade of MdGAMYB-MdHVA22g enhances drought tolerance by facilitating ER stress-mediated GABA accumulation and subsequent ROS detoxification in apple. Collectively, our findings reveal a novel regulatory factor of MdHVA22g and elucidate the role of the MdGAMYB-MdHVA22g module in drought tolerance.