Abstract: The bacterial pathogen Pseudomonas syringae is dependent on a type III secretion system and the type III effector proteins (T3Es) it injects into plant cells to cause disease. The enzymatic activities of T3Es and their plant targets are not well understood. I will discuss the progress that we have made on T3E activities and plant targets. One T3E that will be discussed is HopE1, which strongly suppresses both effector-triggered and PAMP-triggered immunity. We found that HopE1 requires a eukaryotic co-factor that is needed for HopE1 to target a plant protein that functions in the microtubule network.
Arabidopsis plants expressing HopE1 dissociate this protein from the microtubule network. Arabidopsis mutants lacking this target are more susceptible to P. syringae and exhibit reduced immune responses. Thus, HopE1 disables the microtubule network and one effect is that HopE1 inhibits protein secretion from plant cells, which is predicted to be beneficial to the pathogen because it inhibits the delivery of immunity-associated products to the apoplast.
I will also discuss the progress that we have made on other P. syringae T3Es including an update on HopU1, which is a mono-ADP-ribosyltransferase that suppresses plant immunity by modifying several Arabidopsis RNA-binding proteins including GRP7. We have found that over-expression of GRP7 protects plants to multiple types of pathogens. This benefit is greatly reduced in salicylic acid signaling and biosynthesis mutants. GRP7 binds to several different immunity-associated RNAs and enhances their translation. Thus, HopU1 benefits pathogenesis by inactivating an RNA-binding protein that helps translate immunity-associated RNAs into components of the plant immune response.
Block, A. and J. R. Alfano (2011). "Plant targets for Pseudomonas syringae type III effectors: virulence targets or guarded decoys?"Curr. Opin. Microbiol. 14: 39-46.
Nicaise, V., et al. (2013). "Pseudomonas HopU1 modulates plant immune receptor levels by blocking the interaction of their mRNAs with GRP7." Embo J 32(5): 701-712.
Li, G., et al. (2014). "Distinct Pseudomonas type-III effectors use a cleavable transit peptide to target chloroplasts." Plant journal 77(2): 310-321.