CmNAC25 targets CmMYB6 to positively regulate anthocyanin biosynthesis during the post-flowering stage in chrysanthemum.

ABSTRACT

BACKGROUND: Anthocyanin is a class of important secondary metabolites that determines colorful petals in chrysanthemum, a famous cut flower. 'Arctic Queen' is a white chrysanthemum cultivar that does not accumulate anthocyanin during the flowering stage. During the post-flowering stage, the petals of 'Arctic Queen' accumulate anthocyanin and turn red. However, the molecular mechanism underlying this flower color change remains unclear. RESULTS: In this study, by using transcriptome analysis, we identified CmNAC25 as a candidate gene promoting anthocyanin accumulation in the post-flowering stage of 'Arctic Queen'. CmNAC25 is directly bound to the promoter of CmMYB6, a core member of the MBW protein complex that promotes anthocyanin biosynthesis in chrysanthemum, to activate its expression. CmNAC25 also directly activates the promoter of CmDFR, which encodes the key enzyme in anthocyanin biosynthesis. CmNAC25 was highly expressed during the post-flowering stage, while the expression level of CmMYB#7, a known R3 MYB transcription factor interfering with the formation of the CmMYB6-CmbHLH2 complex, significantly decreased. Genetic transformation of both chrysanthemum and Nicotiana tabacum verified that CmNAC25 was a positive regulator of anthocyanin biosynthesis. Another two cultivars that turned red during the post-flowering stages also demonstrated a similar mechanism. CONCLUSIONS: Altogether, our data revealed that CmNAC25 positively regulates anthocyanin biosynthesis in chrysanthemum petals during the post-flowering stages by directly activating CmMYB6 and CmDFR. Our results thus revealed a crucial role of CmNAC25 in regulating flower color change during petal senescence and provided a target gene for molecular design breeding of flower color in chrysanthemum.