Abstract
Proanthocyanidins (PAs) are important phenolic compounds and PA biosynthesis is regulated by a ternary MBW complex consisting of a R2R3-MYB regulator, a bHLH factor and a WDR protein. In this study, a tobacco R2R3-MYB factor NtMYB330 was characterized as the PA-specific regulator in which the PA biosynthesis was promoted in the flowers of NtMYB330-overexpressing lines while decreased in the flowers of ntmyb330 mutants. NtMYB330 can interact with flavonoid-related bHLH partner NtAn1b and WDR protein NtAn11-1, and the NtMYB330-NtAn1b complex is required to achieve strong transcriptional activation of the PA-related structural genes NtDFR1, NtANS1, NtLAR1 and NtANR1. Our data reveal that NtMYB330 regulates PA biosynthesis in seeds and affects seed germination, in which NtMYB330-overexpressing lines showed higher PA accumulations in seed coats and inhibited germination, while ntmyb330 mutants had reduced seed coat PAs and improved germination. NtMYB330 affects seed germination possibly through two mechanisms: modulating seed coat PAs to affect coat-imposed dormancy. In addition, NtMYB330 regulates the expressions of abscisic acid (ABA) and gibberellin acid (GA) signaling-related genes, affecting ABA-GA crosstalk and seed germination. This study reveals that NtMYB330 specifically regulates PA biosynthesis via formation of the MBW complex in tobacco flowers and affects germination through adjustment of PA concentrations and ABA/GA signaling in tobacco seeds.
Highlights
Proanthocyanidins (PAs), known as condensed tannins, are a class of polyphenols found in a wide variety of plant species (Dixon et al, 2005)
Our results reveal that NtMYB330 regulates PA biosynthesis in tobacco flowers, and the regulatory function of NtMYB330 relies on the interaction with a basic helix-loop-helix (bHLH) partner
The deduced amino acid sequence of NtMYB330 was aligned to other MYB factors regulating flavonoid biosynthesis in dicots and monocots (Figure 1A)
Summary
Proanthocyanidins (PAs), known as condensed tannins, are a class of polyphenols found in a wide variety of plant species (Dixon et al, 2005). NtMYB330 Regulates Proanthocyanidin Biosynthesis dihydroflavonol 4-reductase (DFR) and leucoanthocyanidin dioxygenase (LDOX/ANS), leading to the productions of leucoanthocyanidins and anthocyanidins, respectively. These two substrates are converted to (+)-catechin and (−)epicatechin, catalyzed by leucoanthocyanidin reductase (LAR) and anthocyanidin reductase (ANR), respectively (Bogs et al, 2005). For the regulation of PA and anthocyanin biosynthesis, MYB factors interact with basic helix-loop-helix (bHLH) and WDrepeat (WDR) co-activators to form a ternary complex to control gene expression (Ramsay and Glover, 2005). PA-specific MYB regulators placed in two subclades are distinct in gene structures, conserved C-terminal motifs and targeted cis-motifs in promoter regions. PA-clade 1 regulators recognize the MYBCORE element, while PA-clade 2 regulators mainly target AC elements in the promoter regions of PA pathway genes (Hichri et al, 2011b)
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