Integrative genomic and functional analyses identify LjbZIP36 as a candidate regulator of β-carotene accumulation during floral color transition in Lonicera japonica

Abstract Background Floral color transition is a characteristic developmental process in Lonicera japonica , during which flowers gradually change from white to golden yellow during flower maturation owing to carotenoid accumulation. Although carotenoid biosynthetic genes have been identified in L. japonica , the transcriptional regulatory mechanisms underlying carotenoid accumulation during floral color transition remain poorly understood. Basic leucine zipper (bZIP) transcription factors are important regulators of plant growth, development, and secondary metabolism; however, their potential involvement in carotenoid biosynthesis in L. japonica has not been systematically investigated. Results Using a recently released chromosome-level reference genome, we performed a comprehensive genome-wide analysis of the bZIP transcription factor family in L. japonica and identified 68 LjbZIP genes, which were classified into 13 conserved subfamilies. Integrating phylogenetic analysis, expression profiling across floral developmental stages, and transcriptome–metabolome association analyses, we identified LjbZIP36 as a candidate regulator associated with carotenoid accumulation during floral color transition. The expression of LjbZIP36 increased markedly during late floral development and exhibited a positive correlation with β-carotene accumulation. Subcellular localization analysis demonstrated that LjbZIP36 is a nucleus-localized protein. Transient overexpression of LjbZIP36 in Nicotiana benthamiana promoted carotenoid accumulation, particularly β-carotene, and enhanced the expression of several carotenoid biosynthetic genes. Furthermore, yeast one-hybrid and dual-luciferase assays indicated that LjbZIP36 can interact with and activate the promoters of LjPSY , LjPDS , LjZDS , and LjLCYB , key genes involved in carotenoid biosynthesis. Conclusions This study provides the genome-wide characterization of the bZIP transcription factor family in L. japonica and identifies LjbZIP36 as a candidate positive regulator associated with carotenoid accumulation during floral color transition. These findings expand our understanding of the transcriptional regulation of carotenoid metabolism in honeysuckle and provide valuable genetic resources for future functional studies and molecular breeding of ornamental and medicinal cultivars.