The Wound-, Pathogen-, and Ultraviolet B-ResponsiveMYB134Gene Encodes an R2R3 MYB Transcription Factor That Regulates Proanthocyanidin Synthesis in Poplar    

Abstract

In poplar (Populus spp.), the major defense phenolics produced in leaves are the flavonoid-derived proanthocyanidins (PAs) and the salicin-based phenolic glycosides. Transcriptional activation of PA biosynthetic genes leading to PA accumulation in leaves occurs following herbivore damage and mechanical wounding as well as infection by the fungal biotroph Melampsora medusae. In this study, we have identified a poplar R2R3 MYB transcription factor gene, MYB134, that exhibits close sequence similarity to the Arabidopsis (Arabidopsis thaliana) PA regulator TRANSPARENT TESTA2 and that is coinduced with PA biosynthetic genes following mechanical wounding, M. medusae infection, and exposure to elevated ultraviolet B light. Overexpression of MYB134 in poplar resulted in transcriptional activation of the full PA biosynthetic pathway and a significant plant-wide increase in PA levels, and electrophoretic mobility shift assays showed that recombinant MYB134 protein is able to bind to promoter regions of PA pathway genes. MYB134-overexpressing plants exhibited a concomitant reduction in phenolic glycoside concentrations and other minor alterations to levels of small phenylpropanoid metabolites. Our data provide insight into the regulatory mechanisms controlling stress-induced PA metabolism in poplar, and the identification of a regulator of stress-responsive PA biosynthesis constitutes a valuable tool for manipulating PA metabolism in poplar and investigating the biological functions of PAs in resistance to biotic and abiotic stresses.

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

“…These MYB transcription factors act as part of the MBW complex that promotes flavonoid gene expression. For MYB134, we demonstrated direct binding to AC-like elements that are found in phenylpropanoid gene promoters (Mellway et al, 2009). A second type of PA MYB regulator was defined by the discovery of VvMYBPA1 from grapevine (Bogs et al, 2007).…”

“…MYB134 is an activator of PA biosynthesis and stimulates the poplar ANR1 promoter in transient expression assays in Arabidopsis (Zifkin et al, 2012;Gesell et al, 2014). Transgenic poplar plants overexpressing MYB134 accumulate up to 50 times more PAs in their leaves, yet they have normal anthocyanin levels and only slightly elevated flavonol contents, suggesting that MYB134 regulates PAs specifically (Mellway et al, 2009). Concurrently, microarray analysis revealed that all known early and late structural genes for PA biosynthesis were up-regulated in these plants (Mellway, 2009).…”

“…TT2-type PA regulators have been characterized in fruit, including DkMYB2 from persimmon (Diospyros kaki), VvMYBPA2 from grapevine (Vitis vinifera), and FaMYB9 in strawberry (Fragaria 3 ananassa; Terrier et al, 2009;Akagi et al, 2010;Schaart et al, 2013). MYBs from the TT2 group also are active in regulating PAs in vegetative tissues, such as LjTT2 from Japanese lotus (Lotus japonicus), TaMYB14 from clover (Trifolium arvense), and MYB134 from poplar (Mellway et al, 2009;Yoshida et al, 2010;Hancock et al, 2012). These MYB transcription factors act as part of the MBW complex that promotes flavonoid gene expression.…”

“…typically contain substantial amounts of phenolic metabolites, including hydroxycinnamate esters, salicinoids, and PAs, which in Populus tremuloides can accumulate to 25% dry weight of leaves (Donaldson et al, 2006). Furthermore, in Populus spp., PA biosynthesis and accumulation can be rapidly induced by stresses including wounding, herbivore damage, pathogen attack, nitrogen deficiency, and UV light (Osier andLindroth, 2004, Miranda et al, 2007;Mellway et al, 2009), suggesting a complex system of regulators. Stress induction of PAs in poplar involves the up-regulation of flavonoid biosynthesis genes.…”

“…Stress induction of PAs in poplar involves the up-regulation of flavonoid biosynthesis genes. This response is mediated by MYB134, a TT2-type R2R3-MYB transcription factor (Mellway et al, 2009). …”