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Pseudomonas syringae is a Gram-negative bacterial pathogen that infects many crops. A central virulence strategy P. syringae uses to successfully infect plants is the injection of type III effector proteins (T3Es) into plant cells through a type IIII protein secretion system (T3SS). The T3SS is a molecular syringe found in many Gram-negative bacterial pathogens of plants and animals that transport T3Es from the bacterial cytosol into eukaryotic cells. T3Es disrupt host processes in the plant immune system required to restrict pathogen ingress. The plant innate immune system is divided in two branches, pathogen-associated molecular patterns (PAMP)-triggered immunity (PTI) and effector-triggered immunity (ETI). The first branch recognizes conserved molecules found in microbes, known as PAMPs, and the second has the capacity to recognize injected T3Es. T3Es can suppress both PTI and ETI allowing P. syringae to circumvent the plant immune system and multiply in plant tissue. The majority of T3Es plant targets, their enzymatic activity and the mechanism of suppression of plant immunity are not known. P. syringae pv. tomato (Pto) DC3000 injects about 35 T3Es into plant cells.
In this study I characterized two T3Es from Pto DC3000. Firstly, I focused on the T3E HopD1. HopD1 suppresses plant immunity associated with ETI but not PTI, suggesting that HopD1 was acquired later in the co-evolution of the pathogen and plant. HopD1 is targeted to the endoplasmic reticulum of plant cells where it interacts with the Arabidopsis NAC transcription factor NTL9. HopD1’s function in virulence involves the inhibition of NTL9-regulated genes during ETI. Secondly, I focused on the T3E HopA1. This T3E exists in two classes, which I found are recognized differently in plants. HopA1 suppresses PTI and its structure resembles phosphothreonine lyases form animal pathogens. The putative active site of HopA1 was identified and I found that site-directed mutations in the active site abrogated HopA1-dependent phenotypes. HopA1 localizes mainly to plasma membrane of plant cells where it interacts with the Arabidopsis type 2C phosphatases PLL4 and PLL5. These phosphatases play roles in plant immunity as negative regulators and HopA1 likely prevents their deregulation preventing induction of the plant immune system.
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