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

Abstract

Trees in the genus Populus (poplar) contain phenolic secondary metabolites including the proanthocyanidins (PAs), which help to adapt these widespread trees to diverse environments. The transcriptional activation of PA biosynthesis in response to herbivory and ultraviolet light stress has been documented in poplar leaves, and a regulator of this process, the R2R3-MYB transcription factor MYB134, has been identified. MYB134-overexpressing transgenic plants show a strong high-PA phenotype. Analysis of these transgenic plants suggested the involvement of additional MYB transcription factors, including repressor-like MYB factors. Here, MYB182, a subgroup 4 MYB factor, was found to act as a negative regulator of the flavonoid pathway. Overexpression of MYB182 in hairy root culture and whole poplar plants led to reduced PA and anthocyanin levels as well as a reduction in the expression of key flavonoid genes. Similarly, a reduced accumulation of transcripts of a MYB PA activator and a basic helix-loop-helix cofactor was observed in MYB182-overexpressing hairy roots. Transient promoter activation assays in poplar cell culture demonstrated that MYB182 can disrupt transcriptional activation by MYB134 and that the basic helix-loop-helix-binding motif of MYB182 was essential for repression. Microarray analysis of transgenic plants demonstrated that down-regulated targets of MYB182 also include shikimate pathway genes. This work shows that MYB182 plays an important role in the fine-tuning of MYB134-mediated flavonoid metabolism.

The Transcriptional Repressor MYB2 Regulates Both Spatial and Temporal Patterns of Proanthocyandin and Anthocyanin Pigmentation inMedicago truncatula

“…In a phylogenetic analyses of repressor MYB proteins based on N-terminal protein sequences encompassing the R2R3 domain or R3 domain (for the R3-MYB group) or the entire protein sequences ( Figure 1B; Supplemental Figures 3 and 4 and Supplemental Files 1 and 2), MYB2 was clustered with a group of proteins that have been identified as negative regulators of anthocyanin and/or PA biosynthesis in other plant species (Aharoni et al, 2001;Albert et al, 2014;Yoshida et al, 2015;Cavallini et al, 2015 Zimmermann et al, 2004) are highly conserved and all of the proteins except Fa MYB1 additionally contain the C1 motif (LlsrGIDPxT/SHRxI/L; Shen et al, 2012) (Supplemental Figure 5). More importantly, a putative EAR-like repressive motif (LxLxL) is present near the C-terminal end of the proteins.…”

“…In Arabidopsis, MYBL2 promoter activity is expressed in the endothelium of the seed coat, and expression of MYBL2 driven by the TT8 promoter produced seeds with reduced PA content (Dubos et al, 2008). Overexpression of Fa MYB1, grapevine (Vitis vinifera) Vv MYBC2, or poplar (Populus trichocarpa) Ptr MYB182 resulted in reductions of PA or both PA and anthocyanin biosynthesis (Paolocci et al, 2011;Huang et al, 2014;Yoshida et al, 2015;Cavallini et al, 2015). However, information on how MYB repressors may be involved in fine spatio-temporal regulation of pigmentation is still limited, especially for PA deposition, and requires analysis of lossof-function phenotypes.…”

Flavan-3-ols Are an Effective Chemical Defense against Rust Infection

“…Three candidate genes encoding LARs and two genes encoding ANRs were discovered in the P. trichocarpa genome (Tsai et al, 2006), and a subset of these genes have been genetically characterized in Chinese white poplar (Populus tomentosa; Yuan et al, 2012;Wang et al, 2013). Furthermore, the biosynthesis and accumulation of PAs have been shown to be transcriptionally regulated either positively or negatively by MYB transcription factors that have been characterized in poplar (Mellway et al, 2009;Yoshida et al, 2015;Wang et al, 2017). However, biochemical characterization of poplar LAR or ANR proteins catalyzing the two different branches of flavan-3-ol biosynthesis, and regulatory studies of the corresponding genes, have not been attempted so far in black poplar (Populus nigra).…”

Poplar MYB115 and MYB134 Transcription Factors Regulate Proanthocyanidin Synthesis and Structure

“…VvMYBPA1 enhances PA accumulation when overexpressed in transgenic grapevine hairy root cultures, but also acts in conjunction with the TT2-like VvMYBPA2 (Terrier et al, 2009). The regulation of PAs is further complicated by the activity of repressor-like R2R3 MYBs, such as VvMYBC2 in grapevine and MYB182 in poplar (Huang et al, 2014;Yoshida et al, 2015). These repressor MYBs interact with bHLH cofactors to inhibit flavonoid gene expression.…”