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.
Highlights
In poplar (Populus spp.), the major defense phenolics produced in leaves are the flavonoid-derived proanthocyanidins (PAs) and the salicin-based phenolic glycosides
Additional genes implicated in PA biosynthesis in Arabidopsis include TT12, encoding a multidrug and toxic compound extrusion (MATE) family transporter (Debeaujon et al, 2001), TT19, encoding a glutathione S-transferase (Kitamura et al, 2004), AUTO-INHIBITED H+-ATPase ISOFORM10 (AHA10), encoding a proton pump involved in vacuolar biogenesis (Baxter et al, 2005), and TT10, a predicted laccaselike polyphenol oxidase functioning in PA oxidation (Pourcel et al, 2005)
The predicted protein encoded by this transcript exhibits high sequence similarity to Arabidopsis TT2 as well as maize C1, both of which belong to the N08 MYB subgroup (Jiang et al, 2004), referred to as subgroup 5 (Stracke et al, 2001) or subgroup C32 (Wilkins et al, 2009)
Summary
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. Poplar leaves typically accumulate several classes of phenolic metabolites, including the salicylate-derived phenolic glycosides (PGs), flavonoids such as flavonol glycosides, anthocyanins, and proanthocyanidins (PAs; or condensed tannins), and numerous small phenolic acids and their esters (Pearl and Darling, 1971; Klimczak et al, 1972; Palo, 1984; Lindroth and Hwang, 1996; Fig. 1). PAs are constitutively produced in poplar leaves, but their biosynthesis is often up-regulated by stresses such as insect herbivory, mechanical wounding, and pathogen infection (Peters and Constabel, 2002; Stevens and Lindroth, 2005; Miranda et al, 2007). This study illustrates the utility of using PA regulatory factors to discover new genes involved in the PA biosynthetic pathway
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