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SPECIAL SESSION: Host-pathogen interactions at the plant surface

Bacterial pathogen targets blue light receptors to suppress plant immunity
Guangyong Li - University of Nebraska. James Alfano- University of Nebraska

The bacterial plant pathogen Pseudomonas syringae injects effector proteins into plant cells via a type III protein secretion system. We previously showed that the HopK1 effector uses a cleavable transit peptide to localize to chloroplasts and suppress plant immunity. Here we show that HopK1 targets an Arabidopsis phototropin, a blue light receptor. Arabidopsis phototropin1 (phot1) and phototropin2 (phot2) were identified by mass spectrometry to co-immunoprecipitate with HopK1. These interactions were confirmed with direct immunoprecipitation and bimolecular-fluorescence complementation experiments. Arabidopsis plants expressing HopK1 phenocopied many of the phenotypes of an Arabidopsis phot1phot2 double mutant. Moreover, phot2 and phot1phot2 mutants were more susceptible to P. syringae and exhibited attenuation of plant immunity whereas the phenotypes of the phot1 mutant resembled wild type plants suggesting that phot2 is the physiological target of HopK1. The P. syringae hopK1 mutant, which has a growth reduction phenotype on wild type plants, exhibited no reduction of growth when inoculated on phot2 mutants further indicating that phot2 is the primary HopK1 plant target. Finally, we found that plants exposed to strong blue light were more resistant to P. syringae and produced increased immune responses in a phot2-dependent manner. Thus, our results suggest that blue light plays an important role in plant immunity and phytopathogenic bacteria have evolved a strategy to block the ability of the plant to perceive this environmental cue to promote infection.