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Abstract
Pseudomonas syringae pv. tomato DC3000 is a gram negative bacterial pathogen that causes bacterial speck disease on Arabidopsis and tomato. The pathogenicity of P. syringae relies on effector proteins that are injected into plant cells by the type III secretion system (TTSS). hrpJ is one of the genes in the hrp/hrc gene cluster, which encodes the TTSS apparatus. I demonstrated that HrpJ was secreted and injected into plant cells via the P. syringae TTSS. A DC3000 hrpJ mutant, UNL140, was greatly reduced in its ability to cause disease. UNL140 was defective in the injection of AvrB1, AvrRpt2, AvrPto1, HopB1, and AvrPtoB effectors into plant cells. Moreover, UNL140 secreted AvrB1, AvrPto1, HrpA1, but not HrpZ1. This defect in the secretion of HrpZ1 and possibly other putative translocators in the hrpJ mutant probably results in disabled injection of effector proteins into plant cells. Genome investigation in DC3000 identified three effectors, including HopU1, that share similarity with mono-ADPribosyltransferases (ADP-RTs). HopU1 inhibited outputs of plant innate immunity including the hypersensitive responses and callose deposition dependent on the ADPRT active site. We identified three chloroplast and two glycine rich RNA binding proteins in Arabidopsis extracts that are ADP-ribosylated by HopU1. T-DNA knockout mutants of one of these glycine rich RNA binding proteins, AtGRP7, exhibited enhanced susceptibility to DC3000 infection indicating that this protein has a role in the plant innate immunity. When co-expressed in plants, HopU1 was capable of ADP-ribosylating AtGRP7 indicating that HopU1 could modify AtGRP7 in planta. Two arginine residues within AtGRP7’s RNA-binding domain were required for it to be ADP-ribosylated by HopU1-His, suggesting that ADP-ribosylation interferes with AtGRP7’s RNA binding ability. Our results suggest a novel strategy employed by a bacterial pathogen where ADP-ribosylation of plant RNA-binding proteins results in posttranscriptional inhibition of host innate immunity. Using a bioinformatic approach, we identified seven putative ARTs from plant bacterial pathogens. At least one of them, XAC3230 from Xanthomonas axonopodis pv. citri strain 306 has the characteristics of being a type III effector.
