Abstract
Anthocyanins and proanthocyanidins (PAs) are important flavonoids in Chinese bayberry (Morella rubra), which functions in fruit color and exhibits multiple health promoting and disease-preventing effects. To investigate the regulation of their biosynthesis in Chinese bayberries, we isolated and identified a subgroup 4 MYB transcription factor (TF), MrMYB6, and found MrMYB6 shared similar repressor domains with other MYB co-repressors of anthocyanin and PA biosynthesis after sequence analysis. Gene expression results revealed the transcripts of MrMYB6 were negatively correlated with the anthocyanin and insoluble PA contents and also with the gene expressions involved in anthocyanin biosynthesis and PA specific genes such as MrLAR and MrANR during the late ripening stages of bayberries. In addition, overexpression of MrMYB6 in tobacco inhibited the transcript levels of NtCHI, NtLAR, and NtANR2, resulting into a decline in the levels of anthocyanins and PAs in tobacco flowers. We further found that MrMYB6 interacted with MrbHLH1 and MrWD40-1 to form functional complexes that acted to directly repress the promoter activities of the PA-specific gene MrLAR and MrANR and the anthocyanin-specific gene MrANS and MrUFGT. Taken together, our results suggested that MrMYB6 might negatively regulate anthocyanin and PA accumulation in Chinese bayberry.
Highlights
Flavonoids, widespread secondary metabolites, play many important roles in the development of plants (Winkel-Shirley, 2001)
The sequence alignment of MrMYB6 with MYB repressors of other species suggested that their N-terminus contained highly conserved R2R3 domain with a bHLH-binding domain (Grotewold et al, 2000) and their C-terminus included two conserved motifs of MYB subgroup 4 transcription factor (TF), the C1 and C2 motif, of which the C2 motif associated with EAR repressor domain (Jin et al, 2000; Kagale and Rozwadowski, 2011; Shen et al, 2012) (Supplementary Figure 1A)
MrMYB6 was clustered in FaMYB1-like clade, which contained co-repressors in anthocyanin and PA biosynthesis including VvMYBC2 from control 35S:GFP was distributed in both the cytoplasm and the nucleus (Figure 1C)
Summary
Flavonoids, widespread secondary metabolites, play many important roles in the development of plants (Winkel-Shirley, 2001). Anthocyanins and proanthocyanidins (PAs) are two major classes of flavonoids in fruits The former is associated with the wide range of colors including bright orange, pink, red, violet, and blue in fruits, and the latter contribute to the astringency and flavor of fruits. Both anthocyanins and PAs are considered as dietary antioxidants that are beneficial to human health in reducing free radical mediated injury and cardiovascular disease (Middleton et al, 2000). They play important roles in influencing fruit quality. Leucoanthocyanidin reductase (LAR) and anthocyanidin synthase (ANS) can use
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