Genome-wide identification and expression analysis of the BBX gene family in Lagerstroemia indica grown under light stress.

B-box proteins (BBX) play pivotal roles in the regulation of numerous growth and developmental processes in plants, particularly the light-mediated biosynthesis of pigments. To elucidate the role of BBX transcription factors in the anthocyanin biosynthetic pathway of Lagerstroemia indica leaves, this study identified 41 BBX genes in the L. indica genome. Using bioinformatics approaches, we predicted their structural and functional characteristics and examined the variations in leaf coloration under varying durations of darkness and the expression profiles of BBX genes. The LiBBX genes were categorized into five distinct subfamilies through phylogenetic analysis, with substantial gene expansion due to segmental duplication events. Promoter analysis demonstrated that the BBX family possesses an abundance of light-responsive cis-elements. Using protein interaction prediction followed by qPCR analysis, we identified 17 interacting partners. Notably, the expression levels of the majority of BBX genes in L. indica 'Ebony Embers' were significantly downregulated in the darkness compared to those in the light. Correlation analyses indicated that the expression levels of most BBX genes were positively correlated with both anthocyanin and chlorophyll contents. Ultimately, we discovered a core BBX protein, LiBBX4, which can interact with LiHY5, LiHYH, and LiCOP1, and verified its involvement in regulating anthocyanin synthesis using VIGS. This study for the first time revealed novel insights into the molecular mechanisms underlying light-induced leaf color changes in L. indica, which could provide a fundamental framework for the genetic improvement of L. indica and enhance its commercial appeal.