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The Xanthomonas oryzae pv. oryzae PhoPQ Two-Component System Is Required for AvrXA21 Activity, hrpG Expression, and Virulence ^,

Sang-Won Lee,^1, Kyu-Sik Jeong,^1,2, Sang-Wook Han,¹ Seung-Eun Lee,^1,2 Bong-Kwan Phee,³ Tae-Ryong Hahn,³ and Pamela Ronald^1*

Department of Plant Pathology, University of California, One Shields Ave., Davis, California 95616,¹ Department of Plant Medicine, Chungbuk National University, Cheongju, South Korea,² Graduate School of Biotechnology and Plant Metabolism Research Center, Kyung Hee University, Suwon, South Korea³

Received 29 August 2007/ Accepted 3 January 2008

The rice pathogen recognition receptor, XA21, confers resistance to Xanthomonas oryzae pv. oryzae strains producing the type one system-secreted molecule, AvrXA21. X. oryzae pv. oryzae requires a regulatory two-component system (TCS) called RaxRH 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 knockout strain. This survey led to the identification of the phoP gene encoding a response regulator that is up-regulated in the raxR knockout strain. Next we generated a phoP knockout strain and found it to be impaired in X. oryzae pv. oryzae 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 knockout strain can be partially complemented by constitutive expression of hrpG, indicating that PhoP controls a key aspect of X. oryzae pv. oryzae 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 knockout strain as reflected by enhanced growth of this strain in rice lines carrying XA21. These results suggest that the X. oryzae pv. oryzae PhoPQ TCS functions in virulence and in the production of AvrXA21 in partnership with RaxRH.

Published ahead of print on 18 January 2008.

Supplemental material for this article may be found at http://jb.asm.org/.

The first two authors contributed equally to this work.

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