Identification and Characterization of the BBX Gene Family in Pomegranate (Punica granatum L.) and Its Potential Role in Anthocyanin Accumulation During Fruit Ripening

B-box (BBX) genes, as zinc finger transcription factors (TFs), play essential roles in regulating plant growth and development. In this study, we identified 23 BBX genes in the pomegranate (Punica granatum L.) genome. These genes were classified into five groups based on the distribution of conserved domains and phylogenetic relationships. Each PgBBX group exhibited similar molecular weights, theoretical isoelectric points (pI), gene structures, and conserved motif distributions compared with BBX members in Arabidopsis and Chinese white pear in corresponding groups. Syntenic analysis revealed segmental duplications of eight PgBBX gene pairs within the pomegranate genome. Additionally, twenty-seven and thirty-one orthologous BBX pairs were identified between PgBBX and AtBBX, and PgBBX and PbBBX, respectively. Promoter analysis revealed the presence of five types of cis-acting elements responding to light, phytohormones, stress, developmental signaling, and potential transcription factors (TFs). GO enrichment analysis confirmed that most PgBBX genes function as TF involved in plant growth and development. RNA-seq data indicated that PgBBX5 was primarily expressed in leaves and flowers, with increased expression in different fruit tissues during ripening. Moreover, PgBBX5 showed a high degree of sequence similarity with anthocyanin-related homologs, including AtBBX24, PhBBX24, FaBBX24, MdCOL4, and PyBBX24. During the ripening of ‘Tunisia’ fruits, PgBBX5 expression was positively correlated with the dynamic changes in anthocyanin content and the expression of key anthocyanin biosynthetic and transport genes. Furthermore, subcellular localization suggested that PgBBX5 encodes a nuclear-localized protein. This study provides a comprehensive characterization of the PgBBX family, offering valuable insights into the mechanisms underlying anthocyanin accumulation during pomegranate fruit ripening.