Abstract
The flavonoid compounds, proanthocyanidins (PAs), protect plants from biotic stresses, contribute to the taste of many fruits, and are beneficial to human health in the form of dietary antioxidants. In this study, we functionally characterized two Malus crabapple R2R3-MYB transcription factors, McMYB12a and McMYB12b, which co-regulate PAs and anthocyanin biosynthesis. McMYB12a was shown to be mainly responsible for upregulating the expression of anthocyanin biosynthetic genes by binding to their promoters, but to be only partially responsible for regulating PAs biosynthetic genes. In contrast, McMYB12b showed preferential binding to the promoters of PAs biosynthetic genes. Overexpression of McMYB12a and McMYB12b in tobacco (Nicotiana tabacum) altered the expression of flavonoid biosynthetic genes and promoted the accumulation of PAs and anthocyanins in tobacco petals. Conversely, transient silencing their expression in crabapple plants, using a conserved gene region, resulted in reduced PAs and anthocyanin production a green leaf phenotype. Meanwhile, transient overexpression of the two genes and silenced McMYB12s in apple (Malus domestica) fruit had a similar effect as overexpression in tobacco and silenced in crabapple. This study reveals a new mechanism for the coordinated regulation of PAs and anthocyanin accumulation in crabapple leaves, which depends on an auto-regulatory balance involving McMYB12a and McMYB12b expression.
Highlights
IntroductionMembers of each class of flavonoid (flavonols, anthocyanins and proanthocyanidins) are synthesized via a multi-step enzymatic reaction branching from the common core flavonoid pathway (Fig. 1)
Members of each class of flavonoid are synthesized via a multi-step enzymatic reaction branching from the common core flavonoid pathway (Fig. 1)
We investigated the transcriptional regulation of PAs biosynthesis by functionally characterizing the Malus crabapple transcription factors (TFs), McMYB12a and McMYB12b, which were identified as potential targets because of their similarity to identified PAs regulators in grape
Summary
Members of each class of flavonoid (flavonols, anthocyanins and proanthocyanidins) are synthesized via a multi-step enzymatic reaction branching from the common core flavonoid pathway (Fig. 1). VvMYB5a and VvMYB5b have been partially characterized and are thought to regulate structural genes involved in the general flavonoid pathway at different stages of berry development[32,33] Transient expression of these two TFs in tobacco (Nicotiana tabacum) resulted in the activation of several flavonoid pathway genes, and when overexpressed, the biosynthesis of anthocyanins, flavonols, tannins and lignins in reproductive organs was increased[32,33]. MdMYB3 is involved in transcriptional activation of several flavonoid pathway genes This TF regulates the accumulation of anthocyanin in the skin of apple fruits, but it is involved in the regulation of flower development, that of pistil development[39]. We investigated the transcriptional regulation of PAs biosynthesis by functionally characterizing the Malus crabapple TFs, McMYB12a and McMYB12b, which were identified as potential targets because of their similarity to identified PAs regulators in grape
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