Pseudomonas syringae pv. tomato DC3000 causes bacterial speck disease on tomato and Arabidopsis. DC3000 pathogenesis is dependent on the type III secretion system (TTSS), which translocates many effector proteins into plant cells. An important function of some effectors, such as AvrPto, is suppression of basal resistance (innate immunity) in plants. P. syringae also secretes several proteins through the TTSS that appear to be targeted to the plant apoplast rather than the cytoplasm. These "apoplast Hops" (Hrp outer proteins) include two proteins with a lytic transglycosylase domain: HopP1 and CEL (conserved effector locus) ORF1. The latter is encoded adjacent to the hrp/hrc genes encoding the TTSS. A P. syringae hrp/hrc cluster, cloned in P. fluorescens, enables this nonpathogen to translocate AvrPto into plant cells and to suppress basal resistance. Remarkably, P. fluorescens expressing the TTSS and either HopP1 or CEL ORF1 suppresses basal resistance in the absence of any effector. The specific objectives of this project are to: (i) identify all P. syringae tomato DC3000 apoplast Hops and test their ability to either elicit or suppress the hypersensitive response and basal resistance; (ii) determine the basis for the innate ability of the P. syringae TTSS to suppress basal resistance when expressed in a nonpathogen; (iii) construct mutations in P. s. tomato DC3000 and test the functions of apoplast Hops during interactions with host plants. This project should yield new insights into the TTSS and innate immunity, two factors important in bacterium-host interactions involving both plant and animal pathogens. The project has ongoing outreach activities with high schools and primarily undergraduate institutions.
