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.