Unknown Functional Domain 506 Protein Regulates Zinc Tolerance in Apple

Zinc (Zn) is an essential plant micronutrient, but excessive soil Zn causes toxicity, inhibiting growth and reducing crop yields. Elucidating plant adaptation mechanisms to Zn stress is critical for improving performance in Zn-rich environments. While multiple genes/proteins regulating plant Zn homeostasis are known, proteins with unknown domains (e.g., the DUF506 family) remain functionally unclear. This study explored the role of MdDUF506 in apple (Malus domestica) responses to Zn stress. Results showed that MdDUF506 overexpression altered the expression of heavy metal stress-related genes. Under Zn stress, MdDUF506-overexpressing apple plants had 32% lower hydrogen peroxide (H₂O₂) and 21% lower superoxide anion (O₂ ^-) content than wild-type (WT) plants. This reduction in reactive oxygen species (ROS) improved growth and significantly enhanced Zn tolerance in both apple plants and their callus. Additionally, upregulated CAT, POD, and SOD in MdDUF506-overexpressing plants may increase ROS-clearing enzyme activity, aiding adaptation to adverse environments. In summary, MdDUF506 may positively regulate apple Zn stress responses by modulating ROS-related gene expression, and its overexpression confers excellent Zn tolerance. This study clarifies DUF506 family function and provides a potential target for improving Zn stress resistance in woody fruit crops.