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Plant pathogens usually promote pathogenesis by secreting effector proteins into host plant cells. One of the secreted effectors of P seudomonas syringae pv. phaseolicola, the causative agent of halo-blight disease in common bean ( P haseolus vulgaris), HopF1, activates effector-triggered immunity ( ETI) in a bean cultivar containing R1 resistance gene, but displays virulence function in a bean cultivar without the R1 gene. The virulence mechanism of the effector remained unknown, although it was identified more than a decade ago. Here we demonstrated that HopF1 can inhibit pathogen-associated molecular pattern-triggered immunity ( PTI) in a susceptible bean cultivar Tendergreen. HopF1 directly interacted with two RPM1-interacting protein 4 ( RIN4) orthologs of bean, PvRIN4a and PvRIN4b. Like RIN4 in A rabidopsis, both PvRIN4 orthologs negatively regulated the PTI responses in bean. However, the virulence function of HopF1 was enhanced in Tendergreen silencing PvRIN 4. Furthermore, silencing PvRIN 4a compromised the avrβ1-induced hypersensitive response ( HR), which previously was reported to be suppressed by HopF1. Together, these results demonstrated that PvRIN4 orthologs were not the virulence target of HopF1 for inhibiting PTI, but probably for interfering with ETI. [ABSTRACT FROM AUTHOR]

The successful recognition of pathogen-associated molecular patterns (PAMPs) as a danger signal is crucial for plants to fend off numerous potential pathogenic microbes. The signal is relayed through mitogen-activated protein kinase (MPK) cascades to activate defenses. Here, we show that the Pseudomonas syringae type III effector HopF2 can interact with Arabidopsis thaliana MAP KINASE KINASE5 (MKK5) and likely other MKKs to inhibit MPKs and PAMP-triggered immunity. Inhibition of PAMP-induced MPK phosphorylation was observed when HopF2 was delivered naturally by the bacterial type III secretion system. In addition, HopF2 Arg-71 and Asp-175 residues that are required for the interaction with MKK5 are also necessary for blocking MAP kinase activation, PAMP-triggered defenses, and virulence function in plants. HopF2 can inactivate MKK5 and ADP-ribosylate the C terminus of MKK5 in vitro. Arg-313 of MKK5 is required for ADP-ribosylation by HopF2 and MKK5 function in the plant cell. Together, these results indicate that MKKs are important targets of HopF2. [ABSTRACT FROM AUTHOR]