Harpin proteins secreted by phytopathogenic bacteria have been shown to activate the plant defense pathway, which involves transduction of a hydrogen peroxide (H2O2) signal generated in the apoplast. However, the way in which harpins are recognized in the pathway and what role the apoplastic H2O2 plays in plant defenses are unclear. Here, we examine whether the cellular localization of Hpa1Xoo, a harpin protein produced by the rice bacterial leaf blight pathogen, impacts H2O2 production and pathogen resistance in Arabidopsis thaliana. Transformation with the hpa1 Xoo gene and hpa1 Xoo fused to an apoplastic localization signal (shpa1 Xoo ) generated h pa1 Xoo - and sh pa1 Xoo -expressing transgenic A . t haliana (HETAt and SHETAt) plants, respectively. Hpa1Xoo was associated with the apoplast in SHETAt plants but localized inside the cell in HETAt plants. In addition, Hpa1Xoo localization accompanied H2O2 accumulation in both the apoplast and cytoplasm of SHETAt plants but only in the cytoplasm of HETAt plants. Apoplastic H2O2 production via nicotinamide adenine dinucleotide phosphate (NADPH) oxidase (NOX) located in the plasma membrane is a common feature of plant defenses. In SHETAt plants, H2O2 was generated in apoplasts in a NOX-dependent manner but accumulated to a greater extent in the cytoplasm than in the apoplast. After being applied to the wild-type plant, Hpa1Xoo localized to apoplasts and stimulated H2O2 production as in SHETAt plants. In both plants, inhibiting apoplastic H2O2 generation abrogated both cytoplasmic H2O2 accumulation and plant resistance to bacterial pathogens. These results suggest the possibility that the apoplastic H2O2 is subject to a cytoplasmic translocation for participation in the pathogen defense.
Plant Molecular Biology – Springer Journals
Published: May 13, 2012
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