The cytokinin oxidase/dehydrogenase MdCKX4 regulates drought tolerance in Malus domestica

Drought is a major abiotic stress that significantly limits the growth and yield of apples (Malus domestica). Cytokinin oxidase/dehydrogenase (CKX) has been reported to mediate the degradation of cytokinins to regulate cytokinin levels, and participate in the process of plant adaptation to environmental stress. In apple (Malus domestica), we identified and functionally characterized MdCKX4, contains conserved FAD-binding and cytokinin-binding domains, and localizes to the endoplasmic reticulum (ER). The expression of MdCKX4 is induced by drought stress and abscisic acid (ABA) treatment. MdCKX4 positively regulates root growth and reactive oxygen species (ROS) scavenging of apple seedlings under drought stress. In apple callus, overexpressing MdCKX4 exhibit enhanced drought tolerance and reduced ABA sensitivity. In Arabidopsis, overexpression of MdCKX4 increased root growth, survival rate, and seed germination rate of under drought stress, and reduced relative electrolyte leakage. Mechanistically, the AP2 transcription factor MdSHINE3 involved in stress response directly binds the MdCKX4 promoter and activator its transcription. Therefore, this study elucidates the role of MdCKX4 in drought stress, providing novel insights into the regulatory mechanisms underlying drought adaptation in fruit tree crops.