Genome-wide analysis of the B3 family in apple (Malus domestica Borkh.) and functional identification of MdREM19 involvement in salt stress

The B3 gene family plays a vital role in plant development and stress response. Despite the significance of these genes, an extensive genome-wide analysis of the B3 family in apple (Malus domestica Borkh.) has not yet been conducted. A total of 100 MdB3 genes were identified in apple, and their homologous relationships, chromosome positions, gene structures, and promoter cis-acting elements were analyzed. These MdB3 genes were divided into five subfamilies: ARF, HIS, REM, RAV and ABI3. Analysis of the promoter indicated that the apple MdB3 genes may be closely associated with hormone signaling and stress response mechanisms. qRT-PCR analysis was performed on 30 MdB3 genes. Under salt stress, MdB3 genes expression was diversified, up-regulated, or down-regulated. Salt stress treatment induced the expression of several MdB3 genes, among which MdREM19 was significantly expressed and found to be positively correlated with the salt tolerance of different apple rootstocks. Further analysis showed that overexpression of MdREM19 improves the salt tolerance in Arabidopsis. The above results provide a general understanding of the characteristics, evolution, and function of the apple B3 family, laying the foundation for further improving apple’s salt tolerance.