Petal Color-Transition in Lonicera japonica is Mainly Associated with Increase of the Carotenoid Content and Carotenoid Biosynthetic Gene Expression

Background: Plants have remarkable diversity in petal color through the biosynthesis and accumulation of various pigments. To better understand the mechanisms regulating the petal pigmentation, we used multiple approaches to investigate the changes of carotenoids, gene expression dynamics and endogenous hormones in Lonicera japonica during petal color transitions, i.e. green bud petal (GB_Pe), white flower petal (WF_Pe) and yellow flower petal (YF_Pe). Results: Metabolome analysis showed that YF_Pe had a much higher content of carotenoids than white petals, with violaxanthin identified as the major carotenoid compound in YF_Pe. Comparative transcriptome analysis revealed that the key differentially expressed genes (DEGs) involved in carotenoid biosynthesis, such as phytoene synthase, phytoene desaturase and ζ-carotene desaturase, were significantly up-regulated in YF_Pe. However, expression levels of the carotenoid degradation-related genes, including abscisic-aldehyde oxidase 3 and carotenoid cleavage dioxygenase 4, were significantly down-regulated in YF_Pe. The results indicated that upregulated carotenoids concentrations and carotenoids biosynthesis-related genes promote the color transition. Furthermore, enrichment analysis of DEGs was mainly associated with the metabolic pathways of hormone signal transduction during petal color transitions. The DEGs were mainly involved in the auxin, cytokinin, gibberellin (GA), brassinosteroid (BR), jasmonic acid and abscisic acid (ABA) signal pathways. Accordingly, analyses of changes in indoleacetic acid, zeatin riboside, GA, BR, methyl jasmonate and ABA levels indicated that the color transitions are regulated by endogenous hormones. Conclusion: Our results provide new insight into the regulatory mechanisms underlying the petal color transitions during flower development process in L. japonica.