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

Summary

The transcriptional response of hybrid poplar (Populus trichocarpa x P. deltoides) 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 time course. A small number of pathogen defense genes encoding PR-1, chitinases, and other pathogenesis-related proteins were consistently up-regulated throughout the experimental period, but most genes were affected only at individual time points.

The largest number of changes in gene expression was observed late in the infection at 6 to 9 days post-inoculation (dpi). At these time points, genes encoding enzymes required for proanthocyanidin (condensed tannin) synthesis were dramatically up-regulated. 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.