Two R2R3‐MYB proteins are broad repressors of flavonoid and phenylpropanoid metabolism in poplar

Summary The phenylpropanoid pathway leads to the production of many important plant secondary metabolites including lignin, chlorogenic acids, flavonoids, and phenolic glycosides. Early studies have demonstrated that flavonoid biosynthesis is transcriptionally regulated, often by a MYB , bHLH , and WDR transcription factor complex. In poplar, several R2R3 MYB transcription factors are known to be involved in flavonoid biosynthesis. Previous work determined that poplar MYB 134 and MYB 115 are major activators of the proanthocyanidin pathway, and also induce the expression of repressor‐like MYB transcription factors. Here we characterize two new repressor MYB s, poplar MYB 165 and MYB 194 , paralogs which comprise a subgroup of R2R3‐ MYB s distinct from previously reported poplar repressors. Both MYB 165 and MYB 194 repressed the activation of flavonoid promoters by MYB 134 in transient activation assays, and both interacted with a co‐expressed bHLH transcription factor, bHLH 131, in yeast two‐hybrid assays. Overexpression of MYB 165 and MYB 194 in hybrid poplar resulted in greatly reduced accumulation of several phenylpropanoids including anthocyanins, proanthocyanidins, phenolic glycosides, and hydroxycinnamic acid esters. Transcriptome analysis of MYB 165 ‐ and MYB 194 ‐overexpressing poplars confirmed repression of many phenylpropanoid enzyme genes. In addition, other MYB genes as well as several shikimate pathway enzyme genes were downregulated by MYB 165 ‐overexpression. By contrast, leaf aromatic amino acid concentrations were greater in MYB 165 ‐overexpressing poplars. Our findings indicate that MYB 165 is a major repressor of the flavonoid and phenylpropanoid pathway in poplar, and may also affect the shikimate pathway. The coordinated action of repressor and activator MYB s could be important for the fine tuning of proanthocyanidin biosynthesis during development or following stress.