Conservation of pathogen recognition mechanisms in different plant species

For plants to defend against pathogens they must be able to recognize attempted invasions, and plants often employ specific disease resistance (R) genes to detect pathogen proteins important for colonizing the host. While these proteins betray a pathogen's presence to resistant plants, the same proteins generally enhance pathogen growth on susceptible plants, such that a single protein can act as an elicitor of both defense responses and virulence. The AvrB bacterial protein from Pseudomonas syringae is detected by R genes in two plant families: RPM1 in the model plant Arabidopsis, and Rpg1b in cultivated soybean. Previous experiments showed that these two resistance genes evolved independently to recognize AvrB, raising the question of whether they use the same or different molecular processes to detect a single pathogen protein. I discovered that avrB mutants are detected similarly on both plants, and avirulence mutants were generally impaired in virulence functions as well. Nine single-site mutations in AvrB mapped to a solvent-accessible cleft in the protein structure, indicating that AvrB possesses an enzyme activity important for recognition and virulence enhancement in both plants. Three of four AvrB mutants tested failed to bind RIN4, an Arabidopsis protein required for AvrB recognition. Furthermore, we isolated three RIN4 homologs from soybean and showed that two of them bind AvrB and avrB mutant proteins in the same manner as Arabidopsis RIN4. All three soybean RIN4 homologs are cleaved by another bacterial protein that targets Arabidopsis RIN4. We also showed that soybean RPG1b can functionally replace Arabidopsis RPM1 in the recognition of AvrB. Our experimental findings suggest that soybean and Arabidopsis share molecular mechanisms for recognition of the AvrB protein, and that these recognition events depend on detection of alterations in the AvrB virulence target RIN4.