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Distinct Enzymatic Activities and Molecular Mechanisms of Two Pseudomonas syringae Type III Effectors
In order to cause disease, plant pathogens like the gram-negative bacterium Pseudomonas syringae must subvert plant immunity. The plant immune system detects conserved pathogen-associated molecular patterns (PAMPs) through the use of cell-surface pattern-recognition receptors (PRRs) and detects pathogenic virulence factors called effectors through the use of intracellular NOD-like receptors (NLRs). PRR-triggered immunity (PTI) is a baseline response to microbes that can restrict non-adapted pathogens while NLR-triggered immunity (NTI), also called effector-triggered immunity (ETI), amplifies PTI and confers robust, qualitative immunity. P. syringae and other pathogens secrete effectors to suppress plant immunity and promote virulence. P. syringae secretes over forty type III effectors through the type III secretion system (T3SS) but, while the majority of these effectors are known to suppress plant immunity, the enzymatic activity and host virulence targets of most of these effectors remain to be demonstrated. In this study, I demonstrate the enzymatic activities of two P. syringae effectors: HopAW1 of P. syringae pv. glycinea (Pgy) LN10 and HopAM1 of P. syringae pv. tomato (Pto) DC3000. HopAW1, which is predicted to be a cysteine protease, suppresses plant immunity and contributes to Pgy LN10 virulence on soybean. HopAW1 self-cleaves at a TIG recognition site in planta but not in bacteria dependent on its putative catalytic site. Self-cleavage of HopAW1 exposes an internal myristoylation / palmitoylation site which is required for HopAW1 to localize to the plant plasma membrane. HopAM1 contains a noncanonical Toll/interleukin-1 receptor (TIR)-domain, a domain found in plant TIR-NLRs that hydrolyze the essential metabolite NAD+ into nicotinamide, ADPR, and v-cADPR to activate plant immunity. HopAM1 also hydrolyzes NAD+ in vitro but generates v2-cADPR, a cyclic product distinctive from cADPR and v-cADPR. HopAM1 activity and immune suppression is TIR-domain dependent and detectable in planta via agrobacterium-mediated transient expression and Pto T3SS delivery. HopAM1 mimics the enzymatic activity of plant TIR-NLRs and manipulates NAD+ to produce v2-cADPR, allowing HopAM1 to suppress plant immunity and contribute to virulence.
Eastman, Samuel Bradford, "Distinct Enzymatic Activities and Molecular Mechanisms of Two Pseudomonas syringae Type III Effectors" (2022). ETD collection for University of Nebraska - Lincoln. AAI29257539.