A Conserved Network of Transcriptional Activators and Repressors Regulates Anthocyanin Pigmentation in Eudicots

Abstract

Plants require sophisticated regulatory mechanisms to ensure the degree of anthocyanin pigmentation is appropriate to myriad developmental and environmental signals. Central to this process are the activity of MYB-bHLH-WD repeat (MBW) complexes that regulate the transcription of anthocyanin genes. In this study, the gene regulatory network that regulates anthocyanin synthesis in petunia (Petunia hybrida) has been characterized. Genetic and molecular evidence show that the R2R3-MYB, MYB27, is an anthocyanin repressor that functions as part of the MBW complex and represses transcription through its C-terminal EAR motif. MYB27 targets both the anthocyanin pathway genes and basic-helix-loop-helix (bHLH) ANTHOCYANIN1 (AN1), itself an essential component of the MBW activation complex for pigmentation. Other features of the regulatory network identified include inhibition of AN1 activity by the competitive R3-MYB repressor MYBx and the activation of AN1, MYB27, and MYBx by the MBW activation complex, providing for both reinforcement and feedback regulation. We also demonstrate the intercellular movement of the WDR protein (AN11) and R3-repressor (MYBx), which may facilitate anthocyanin pigment pattern formation. The fundamental features of this regulatory network in the Asterid model of petunia are similar to those in the Rosid model of Arabidopsis thaliana and are thus likely to be widespread in the Eudicots.

The MYB182 Protein Down-Regulates Proanthocyanidin and Anthocyanin Biosynthesis in Poplar by Repressing Both Structural and Regulatory Flavonoid Genes

“…Overexpression of FaMYB1 causes the suppression of anthocyanin synthesis in tobacco (Nicotiana tabacum), and suppression of FcMYB1 by transient RNA interference in strawberry fruit increased the accumulation of anthocyanin. Likewise, RNA interference suppression of PhMYB27 increases the accumulation of anthocyanin in petunia flowers and vegetative tissues (Albert et al, 2014). In phylogenies, these flavonoid R2R3 repressor MYBs all cluster together within MYB subgroup 4 (Stracke et al, 2001), and all share the C2 motif (pfLNLD/ ELxiG/S) with a core consensus sequence of LxLxL or DLNxxP (Aharoni et al, 2001;Dubos et al, 2010).…”

“…Elegant work with PhMYB27 has begun to illuminate the mechanism of action of the subgroup 4 R2R3 repressor MYBs. In yeast two-hybrid assays, PhMYB27 interacts directly with bHLHs of both the GL3 and TT8 subgroups, although with different affinities (Albert et al, 2014). Furthermore, deletion of the DLNxxP-type EAR motif reduced PhMYB27 repressor activity; the function of this motif was not defined but may involve chromatin remodeling (Kagale and Rozwadowski, 2011).…”

“…In petunia, the R3-MYB factor MYBx acts as a negative regulator of anthocyanin accumulation in parallel to PhMYB27 (Albert et al, 2014). These R3-MYB proteins do not contain the repressor motif but act as passive repressors by binding bHLH proteins required for the formation of MBW complexes (Albert et al, 2014). A distinct type of repressor from Arabidopsis, AtMYBL2, also contains only a single repeat but otherwise is more closely related to the R2R3-MYBs than other characterized R3-MYBs (Dubos et al, 2008;Matsui et al, 2008).…”

“…These include strawberry FaMYB1 and FcMYB1 (for Fragaria chiloensis) and petunia PhMYB27 (Aharoni et al, 2001;Salvatierra et al, 2013;Albert et al, 2014). Overexpression of FaMYB1 causes the suppression of anthocyanin synthesis in tobacco (Nicotiana tabacum), and suppression of FcMYB1 by transient RNA interference in strawberry fruit increased the accumulation of anthocyanin.…”

“…In petunia, the R3-MYB factor MYBx acts as a negative regulator of anthocyanin accumulation in parallel to PhMYB27 (Albert et al, 2014). These R3-MYB proteins do not contain the repressor motif but act as passive repressors by binding bHLH proteins required for the formation of MBW complexes (Albert et al, 2014).…”