*2.2. Phylogenetic Analysis and Classification of RsAP2 Genes*

A phylogenetic tree of AP2 genes in *Rhododendron* and *Arabidopsis* was constructed to analyze the evolutionary relationship. A total of 195 AP2 genes were classified into 5 subfamilies, and the DREB and ERF subfamilies were further divided into 11 subgroups, as shown in Figure 1. Seventeen RsAP2 genes were classified into the AP2 subfamily with two tandem-duplicated AP2 domains. The DREB and ERF subfamilies both have only one AP2 domain. Forty-four RsAP2 genes with the 14th and 19th amino acids of the domain being valine and glutamic acid, respectively, were confirmed to be DREB subfamily members. Fifty-three RsAP2 genes with the 14th and 19th amino acids of the domain being alanine and aspartic acid, respectively, were confirmed to be ERF subfamily members. Four RsAP2 genes with an AP2 domain at the C-terminus and a B3 domain at the N-terminus were recognized as RAV subfamily members. The remaining two RsAP2 genes with an AP2 domain but with large differences in gene structure from other subfamily members were classified into the soloist subfamily. As shown in the figure, AP2 genes with the same structural features were accurately clustered into a certain subfamily, which indicated that the AP2 gene family has high homology across different species.

**Figure 1.** Phylogenetic tree of AP2 genes from *Rhododendron* and *Arabidopsis*. The identifiers colored in black are RsAP2 genes and the AP2 genes from *Arabidopsis* are colored in red. Subfamilies of AP2, DREB, ERF, RAV and soloist are distinguished by backgrounds shown in red, light blue, dark blue, green and gray, respectively.
