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 zipper, a nucleotide binding site, and leucine-rich repeats. The chromosomal position of HRT and its homology to resistance gene RPP8 and two RPP8 homologs indicate that unequal crossing over and gene conversion may have contributed to HRT evolution. RPP8 confers resistance to an oomycete pathogen, Peronospora 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 changes between closely related members of multigene families can generate novel specificities for radically different pathogens. Transgenic plants expressing HRT developed an HR but generally remained susceptible to TCV because of a second gene, RRT, that regulates resistance to TCV. However, several transgenic plants that overexpressed HRT produced micro-HRs or no HR when inoculated with TCV and were resistant to infection. Expression of the TCV coat protein gene in seedlings containing HRT resulted in massive necrosis and death, indicating that the avirulence factor detected by the HRT-encoded protein is the TCV coat protein.