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J. Bacteriol. doi:10.1128/JB.01406-07
Copyright (c) 2008, American Society for Microbiology and/or the Listed Authors/Institutions. All Rights Reserved.

The Xanthomonas oryzae pv. oryzae PhoP/Q Two-Component System Is Required for AvrXA21 Activity, hrpG Expression, and Virulence

Department of Plant Pathology, University of California, One Shields Ave., Davis, CA., 95616, USA; Department of Plant Medicine, Chungbuk National University, Cheongju, Korea; Graduate School of Biotechnology and Plant Metabolism Research Center, Kyung Hee University, Suwon, Korea

^* To whom correspondence should be addressed. Email: pcronald{at}ucdavis.edu.

	Abstract

The rice pathogen recognition receptor, XA21, confers resistance to Xanthomonas oryzae pv. oryzae (Xoo) strains producing the Type One System-secreted molecule, AvrXA21. Xoo requires a two-component regulatory system (TCS) called RaxR/H to regulate expression of eight rax (required for AvrXA21 activity) genes and to sense population cell density. To identify other key components in this critical regulatory circuit, we assayed proteins expressed in a raxR gene knock-out strain. This survey led to the identification of the phoP gene encoding a response regulator that is up-regulated in the raxR knock-out strain. Next we generated a phoP knock-out strain and found it to be impaired in Xoo virulence and no longer able to activate the response regulator HrpG (hypersensitive reaction and pathogenicity G) in response to low levels of Ca²⁺. The impaired virulence of the phoP knock-out strain can be partially complemented by constitutive expression of hrpG indicating that PhoP controls a key aspect of Xoo virulence through regulation of hrpG. A gene encoding the cognate putative histidine protein kinase, phoQ, was also isolated. Growth curve analysis revealed that AvrXA21 activity is impaired in a phoQ knock-out strain as reflected by enhanced growth of this strain in rice lines carrying XA21. These results suggest that the Xoo PhoP/Q TCS functions in virulence as well as in the production of AvrXA21 in partnership with RaxR/H.

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