Genome-Wide Identification of Apple Expansins and Functional Evidence for MdEXPA17 in Postharvest Fruit Ripening

Expansins are crucial cell wall-loosening proteins that play a vital role in various plant developmental processes, including fruit ripening and softening. However, a comprehensive genome-wide analysis of the expansin family in apple (Malus × domestica) and the specific functions of its members in postharvest fruit ripening remain to be explored. In this study, we identified 51 expansin genes in the apple genome and classified them into four subfamilies (EXPA, EXPB, EXLA, and EXLB). Cis-element analysis of the promoters of apple expansin genes showed that these promoters are rich in various hormone-responsive elements, including abscisic acid (ABA)-responsive elements (ABREs) and ethylene-responsive elements (EREs), suggesting potential hormonal regulation of expansin genes. Expression profiling identified six ripening-associated expansin genes. Among them, MdEXPA5, MdEXPA17, and MdEXPA23 were positively regulated by both ethylene and ABA, while being suppressed by the ethylene action inhibitor 1-MCP. Further functional characterization demonstrated that transient overexpression of MdEXPA17 accelerated fruit softening, skin yellowing, ethylene production, and increased total soluble solid (TSS) content. Conversely, silencing of MdEXPA17 significantly delayed these ripening processes. Our study provides a systematic overview of the apple expansin gene family and supports a role for MdEXPA17 in promoting postharvest fruit ripening and softening. These findings offer valuable insights into the molecular mechanisms of apple fruit ripening and provide potential targets for genetic improvement of fruit quality and shelf life.