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Pseudomonas syringae mono-ADP-ribosyltransferase type III effectors: Their plant targets and effects on plant immunity
The Gram-negative bacterial plant pathogen Pseudomonas syringae requires a type III secretion system (T3SS) to cause disease. The T3SS is a molecular syringe that injects type III effector (T3E) proteins into eukaryotic cells. The primary function of T3Es is the suppression of plant immunity to favor pathogenicity. In P. syringae pv. tomato DC3000, there are three effectors that share similarity with mono-ADP-ribosyltransferases (ADP-RTs). Among these, HopU1 has been shown to ADP-ribosylate Arabidopsis RNA-binding proteins including glycine-rich RNA-binding protein 7 (GRP7). Here, we show a function for GRP7 in PAMP-triggered immunity (PTI) that is inhibited by HopU1. The molecular mechanism underlying this is that GRP7 associates with translational components as well as with the pattern recognition receptor (PRR) FLS2 and EFR transcripts. The interaction GRP7 has with PRR transcripts requires its RNA recognition motif, which is inhibited by HopU1 ADP-ribosylation. This inhibition caused reduced FLS protein levels in planta upon DC3000 infection in a HopU1- dependent manner. We also searched for a broad role of GRP7 in plant immunity and found plants over-expressing GRP7 are more resistant to other pathogens, especially those which induce salicylic acid (SA)-dependent immunity. Surprisingly, important SA-related SID2 and NPR1 mRNAs also bind GRP7 to facilitate their translation. Therefore, plants over-expressing GRP7 produced more SA and NPR1 protein, resulting in increased SA-dependent immunity. The interaction that GRP7 has with these RNAs is inhibited by HopU1 ADPribosylation in vivo and in vitro. Two other putative ADP-RTs are HopO1-1 and HopO1-2. DC3000 mutants lacking hopO1-1 or hopO1-2 were reduced in their growth in planta suggesting that DC3000 requires these T3Es for full virulence. HopO1-1 and HopO1-2 derivatives with mutations in their ADP-RT catalytic site were unable to contribute to virulence and were no longer capable of suppressing PTI and effector-triggered immunity (ETI). Both of these T3Es localized to the plasma membrane of plant cells. These results indicate that HopO1-1 and HopO1-2 are likely active ADP-RTs and that their target are in the plant plasma membrane,
Joe, Anna, "Pseudomonas syringae mono-ADP-ribosyltransferase type III effectors: Their plant targets and effects on plant immunity" (2014). ETD collection for University of Nebraska - Lincoln. AAI3613778.