TheXanthomonas oryzaepv. oryzae PhoPQ Two-Component System Is Required for AvrXA21 Activity,hrpGExpression, and Virulence

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Abstract
The rice pathogen recognition receptor, XA21, confers resistance toXanthomonas oryzaepv. oryzae strains producing the type one system-secreted molecule, AvrXA21.X. oryzaepv. oryzae requires a regulatory two-component system (TCS) called RaxRH to regulate expression of eightrax(required forAvrXA21 activity) genes and to sense population cell density. To identify other key components in this critical regulatory circuit, we assayed proteins expressed in araxRgene knockout strain. This survey led to the identification of thephoPgene encoding a response regulator that is up-regulated in theraxRknockout strain. Next we generated aphoPknockout strain and found it to be impaired inX. oryzaepv. 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 thephoPknockout strain can be partially complemented by constitutive expression ofhrpG, indicating that PhoP controls a key aspect ofX. oryzaepv. oryzae virulence through regulation ofhrpG. A gene encoding the cognate putative histidine protein kinase,phoQ, was also isolated. Growth curve analysis revealed that AvrXA21 activity is impaired in aphoQknockout strain as reflected by enhanced growth of this strain in rice lines carrying XA21. These results suggest that theX. oryzaepv. oryzae PhoPQ TCS functions in virulence and in the production of AvrXA21 in partnership with RaxRH.