The regulation of stress-induced proanthocyanidin metabolism in poplar




Mellway, Robin

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Proanthocyanidins (PAs) are polymeric phenolic chemicals produced by many plant species that may contribute to protection of tissues against biotic and abiotic stress conditions. In poplar (Populus spp.) leaves, PA biosynthesis is rapidly activated by insect herbivore damage, indicating that PAs may be an inducible chemical defence. In this study, the expression of PA biosynthetic genes was monitored in poplar leaves exposed to several stress stimuli. The PA pathway was shown to be rapidly activated at the level of gene transcription by stresses such as elevated light, UV-B irradiation and infection of leaves by a biotrophic fungal parasite. A transcription factor gene of the R2R3 MYB type, MYB134, was found to be co-activated with PA biosynthetic genes under these stress conditions. When overexpressed in transgenic poplar, this gene induced a strong, specific activation of the PA pathway, indicating that it might function as a regulator of stress-induced PA metabolism in poplar. MYB134 was shown to bind to promoter fragments of PA biosynthetic genes which contained a conserved DNA cis-element found in the promoter regions of many other phenylpropanoid genes including putative MYB134-regulated genes. A global transcriptome analysis of leaves of the MYB134-overexpressing poplar plants confirmed that the PA pathway activation was complete and specific, and led to the identification of a number of putative novel PA biosynthetic and regulatory genes. These results indicate that stress-responsive PA and flavonoid metabolism in poplar may be regulated by a complex system involving both positive and negative regulation. Preliminary results are also presented related to analysis of PA functions in transgenic plants, and the use of MYB134 to engineer PA metabolism in plants other than poplar. This study provides insight into the regulatory mechanisms controlling stress-induced PA metabolism, and expands our understanding of roles that this biological response may play in poplar.



poplar, diseases and pests