This project focuses on how plants recognize pathogens, and how the recognition event is translated into a resistant response. To address these questions we take a molecular genetic approach. We use the small mustard Arabidopsis thaliana as our standard host plant, and the bacterial pathogen Pseudomonas syringae as our standard pathogen. Recognition of specific P. syringae strains by Arabidopsis is mediated by specific disease resistance (R) genes of Arabidopsis. These Rgenes are thought to encode receptors that detect a signal produced directly or indirectly by bacterial proteins that are injected into the host cells. The molecular mechanism of this detection step is poorly understood, however. Understanding this mechanism is a major goal in plant biology as it will likely lead to new approaches for engineering disease resistance in plants, as well as provides critical insights into how pathogens evolve to escape recognition and cause disease.

To uncover the molecular basis of pathogen recognition we have focused on identifying genes in both the plant and the pathogen that are required for the recognition event. This has been accomplished by screening for plant mutants that fail to respond to bacteria expressing specific “effector” proteins that are secreted into the plant cells. To date we have cloned two R genes (RPM1 and RPS5) and have identified three additional genes (PBS1, PBS2, and PBS3) that contribute to various steps in the defense induction process. RPM1 and RPS5 belong to a very large gene family in plants. Each member of this family mediates recognition of a specific pathogen molecule. All members of this R gene family contain a nucleotide-binding site (e.g. ATP or GTP) and leucine rich repeats (LRRs). The LRRs are thought to mediate protein: protein interactions, and may possibly participate in binding pathogen molecules or the targets of pathogen molecules. We have recently shown that the P. syringae AvrPphB effector protein functions as a cysteine protease that specifically targets the Arabidopsis PBS1 protein. Cleavage of PBS1 by AvrPphB somehow activates the RPS5 protein, which in turn activates multiple defense responses. These data reveal that plants recognize at least some pathogens by detecting the enzymatic activity of their effector proteins, rather than by binding pathogen molecules directly. This insight has a major impact on the way we model plant-pathogen co-evolution, and may lead to new approaches to engineering stable disease resistance traits in crops.

We are now addressing three important unanswered questions regarding PBS1 and RPS5: How does cleavage of PBS1 activate RPS5? What are the immediate targets of the activated RPS5 protein? and what is the normal function of PBS1 in the plant cell?. Through a combination of biochemical and genetic approaches we expect to answer these questions in the near future.