| How pathogens cause disease in plants and how plants mount immune responses to defend themselves and survive pathogen infection are central questions in plant pathology. For the last two decades, the Pseudomonas syringae pv tomato DC3000 (Pst DC3000)- Arabidopsis thaliana interaction has been used as a model pathosystem to study plant immunity and bacterial pathogenesis. Current studies suggest that plant immunity involves two main types: pathogen-associated molecular pattern (PAMP)-triggered immunity (PTI) and effector-triggered immunity (ETI). PTI and ETI share overlapping immune outputs, such as production of reactive oxygen species and accumulation of anti-microbial compounds. In Chapter 1 of the thesis, I overviewed the current knowledge of Pst DC3000 pathogenesis in relation to plant immunity.;The ABC transporter protein PEN3 in Arabidopsis was previously shown to be required for fungal penetration resistance. PEN3 focally accumulates at the fungal infection site, and is hypothesized to transport anti-microbial compounds against fungal pathogens. A definitive role of PEN3 in bacterial resistance, however, was lacking. In Chapter 2 of the thesis, I described my research, in collaboration with Dr. Kinya Nomura and Dr. William Underwood, to determine the role of PEN3 in Arabidopsis resistance to Pst DC3000. We found that the Arabidopsis pen3 mutant had enhanced susceptibility to Pst DC3000 and the ACEL mutant (a mutant of Pst DC3000 in which a conserved effector locus was deleted). In addition, we observed that PEN3 protein forms focal accumulation in response to defense elicitation by flg22 and Pst DC3000 infection during an early stage of (5 h post inoculation). Interestingly, PEN3 accumulation was suppressed at a later stage of Pst DC3000 infection (10 h post-inoculation) in a type III secretion system-dependent manner. Further analysis suggests a requirement of multiple type III effectors to effectively suppress focal accumulation of PEN3 during Pst DC3000 infection. Taken together, this study established a positive role of PEN3 in bacterial resistance and revealed the dynamic subcellular localization of PEN3 protein during immune activation and bacterial pathogenesis.;Type III effectors are key virulence factors in bacterial pathogenesis, but the functions of most type III effectors remain unknown. In Chapter 3 of my thesis, I described my efforts to understand the virulence function of AvrE, one of the most important type III effectors found in diverse P syringae strains, in collaboration with other lab members. Several complementary approaches were taken, including serial deletion analysis to study the structure-function relationship of AvrE, confocal microscopy to determine AvrE localization in the plant cell, protein pull down to identify plant AvrE-interacting proteins, and global gene expression profiling to study host gene regulation by AvrE. It was found that AvrE contains two physically interacting halves that are separable and AvrE is localized to the plasma membrane in planta, in a punctuate pattern. Protein pull down experiments showed that AvrE is associated with "lipid raft/membrane microdomain" proteins in Arabidopsis. Our microarray analysis revealed that AvrE and a functionally redundant effector, HopM1, co-regulate a small set of Arabidopsis genes, including an immunity-related NDR1/HIN1 family gene, NHL13. This part of my research contributed to the understanding of the molecular action of AvrE in the plant cell, revealed new components of plant immunity, and provided a foundation for further experiments to identify the long-sought-after host target(s) of AvrE. |
