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The Transcriptional Response of Hybrid Poplar (Populus trichocarpa x P. deltoids) to Infection by Melampsora medusae Leaf Rust Involves Induction of Flavonoid Pathway Genes Leading to the Accumulation of Proanthocyanidins

July 2007 , Volume 20 , Number  7
Pages  816 - 831

Manoela Miranda , 1 Steven G. Ralph , 2 Robin Mellway , 1 Rick White , 3 Michele C. Heath , 1 Jörg Bohlmann , 2 , 4 and C. Peter Constabel 1

1Centre for Forest Biology and Department of Biology, University of Victoria, Victoria, BC, Canada; 2Michael Smith Laboratories, University of British Columbia, Vancouver, BC, Canada; 3Department of Statistics, University of British Columbia, Vancouver, BC, Canada; 4Departments of Forest Sciences and Botany, University of British Columbia, Vancouver, BC, Canada

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Accepted 13 February 2007.

The transcriptional response of hybrid poplar (Populus trichocarpa × P. deltoids) to poplar leaf rust (Melampsora medusae) infection was studied using the Populus 15.5K cDNA microarray. Pronounced changes in the transcriptome were observed, with approximately 20% of genes on the array showing either induction or repression of transcription within the 9-day infection timecourse. A small number of pathogen-defense genes encoding PR-1, chitinases, and other pathogenesis-related proteins were consistently upregulated throughout the experimental period, but most genes were affected only at individual timepoints. The largest number of changes in gene expression was observed late in the infection at 6 to 9 days postinoculation (dpi). At these timepoints, genes encoding enzymes required for proanthocyanidin (condensed tannin) synthesis were upregulated dramatically. Phytochemical analysis confirmed that, late in the infection, proanthocyanidin levels increased in infected leaves. Strongly M. medusae-repressed genes at 9 dpi included previously characterized wound- and herbivore-induced defense genes, which suggests antagonism between the tree responses to insect feeding and M. medusae infection. In this highly compatible plant-pathogen interaction, we postulate that the biotrophic pathogen evades detection and suppresses early host responses.

Additional keywords: condensed tannins, forest tent caterpillar, fungal pathogen, phenylpropanoid metabolism.

© 2007 The American Phytopathological Society