| Summary: | Plants activate innate immune responses upon pathogen invasion. However, successful pathogens overcome host immunity and cause disease. Pathogen effector proteins play critical roles in suppressing host immune responses and modulate the properties of the host environments to favor pathogen reproduction. A bacterial T3S effector, AvrRPS4, from Pseudomonas syringae has been widely used to study plant immune responses. However, the properties and functions are unclear. The avirulence and virulence activities of AvrRPS4 during bacterial pathogenesis in Arabidopsis were investigated. The processing of AvrRPS4 is required for virulence activity but not for avirulence. Transgenic plants expressing AvrRPS4 from a weak promoter in the absence of RPS4 showed enhanced Pst DC3000 growth and suppressed PTI phenotypes. The mechanism involved in the processing of AvrRPS4 seems to be conserved in the Xanthomonas effector, XopO. To develop a high throughput assay system to study the effectors from an biotroph oomycete pathogen, Hyaloperonospora arabidopsidis, AvrRPS4 was used to create T3SS-based delivery effector system. Using this method, I showed the virulence activities of H. arabidopsidis effector ATR13 and found novel resistance triggered by specific alleles of ATR13 in Arabidopsis accession Ws-O. Little is known how pathogen effectors suppress plant ETI. Using a mixed inoculation method, ETI-suppressing activities were identified from several Pseudomonas syringae strains. Most significantly, Pph effector, AvrB4, was identified as a suppressor of AvrRPS4-triggered HR. AvrB4 also showed avirulence activity in tobacco. However, AvrB family T3SEs show overlapping and distinct effector functions depending on host plants tested.
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