Disease resistance in plants is attributable in part to a specific recognition process dependent upon resistance genes. Recognition of an invading pathogen results in the induction of cellular defense responses in the plant. These induced defense responses function to impede further invasion of the tissue by the pathogen. In plant pathogenic bacteria, such as Pseudomonas syringae, induction of defense response in resistant plants has been linked to the expression of hrp and avr genes. Efforts in my laboratory are focused on understanding the function and regulation of hrp genes in P. syringae strains.

Hrp genes have been shown by my group and others to encode for a novel protein secretion system that is responsible for the overall pathogenicity of the bacterium as well as the initiation of defense responses in resistant plants. Similar secretion systems in other bacteria, known as type III secretory systems, are associated with injection of bacterial proteins into host cells and are best known for their role in the secretion of virulence factors by enteric bacteria pathogenic to mammals. We have recently shown that the activity of bacterial host range determinants, called avr genes, are dependent upon the hrp-encoded secretion system. This observation provides the basis for further experiments on the molecular mechanisms involved in resistance gene-mediated defense responses. A second facet of our work is focused on characterizing the multicomponent regulatory cascade that coordinates the production of the components of this complex secretion system and mediates the environmental regulation of the hrp genes. We have identified several components of the regulatory system, including one of the founding members of a new family of sigma factors and two other unusual regulatory proteins. We are currently working to understand the mechanism for transducing the various environmental signals that affect hrp gene expression and the interaction between secretion activities and expression of the genes. A third facet of our work is attempting to understand the mechanism for assembly of the hrp-encoded secretion system in bacteria.