Members of the Arabidopsis HRT/RPP8 family of resistance genes confer resistance to both viral and oomycete pathogens

Turnip crinkle virus (TCV) inoculation onto TCV-resistant Arabidopsis leads to a hypersensitive response (HR) controlled by the dominant gene HRT. HRT is a member of the class of resistance (R) genes that contain a leucine zi pper, a nucleotide binding site, and leucine-rich repeats. The chromosomal position of HRT and its homology to resistance gene RPP8 and two RPP8 homol ogs indicate that unequal crossing over and gene conversion may have contri buted to HRT evolution. RPP8 confers resistance to an oomycete pathogen, Pe ronospora parasitica. Despite very strong similarities within the HRT/RPP8 family, HRT and RPP8 are specific for the respective pathogens they detect. Hence, the HRT/RPP8 family provides molecular evidence that sequence chang es between closely related members of multigene families can generate novel specificities for radically different pathogens. Transgenic plants express ing HRT developed an HR but generally remained susceptible to TCV because o f a second gene, RRT, that regulates resistance to TCV. However, several tr ansgenic plants that overexpressed HRT produced micro-HRs or no HR when ino culated with TCV and were resistant to infection. Expression of the TCV coa t protein gene in seedlings containing HRT resulted in massive necrosis and death, indicating that the avirulence factor detected by the HRT-encoded p rotein is the TCV coat protein.