Characterization and expression analysis of the MADS-box gene family in Lonicera japonica reveals the role of LjMADS36 in flower coloration

The distinctive flower color change of Lonicera japonica provides the ornamental horticultural plant guidance for its medicinal harvesting time. The variations mainly result from the accumulation of carotenoids and the increased gene expression of carotenoid metabolism. However, the regulatory mechanism of flower coloration involved in carotenoid biosynthesis in L. japonica remains unclear, although MADS-box transcription factors (TFs) usually play essential roles in plant flowering development. This study identified 38 MADS-box genes from the L. japonica genome, including 20 Type I and 18 Type II members. The number of total and the Type II showed an obvious contraction compared to other relatively close species identified. Ten candidate MADS-box TFs showed a high co-expression relationship with carotenoid biosynthetic genes. Therein, LjMADS36 , a FLC subgroup member, upregulated the expression of LjLCYB by binding to its promoter. In addition, the heterogenous over-expression of LjMADS36 lead to the accumulation of carotenoids in Arabidopsis, indicating the potential role of LjMADS36 in the flower color transition of L. japonica . These findings reveal the transcriptional regulation mechanism of flower color change in L. japonica and may contribute to ornamental medicinal plant improvement.