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

Abstract

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 twocomponent 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 Ca 2؉ . 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.

Secretion, modification, and regulation of Ax21

“…The role of divalent cations as signals for PhoQ is also supported by the crystal structures of the PhoQ periplasmic sensor domains from S. typhimurium and E. coli [48]. Similarly, we have shown that the Xoo PhoPQ system is required for sensing low extracellular Mg 2+ and Ca 2+ concentrations, conditions that the pathogen likely is confronted with upon entry into the xylem of the rice plant [33]. In addition, we have shown that Ax21 activity is impaired in a phoQ knockout strain (PXO99DphoQ) as reflected by enhanced growth of this strain in rice lines carrying XA21 [33].…”

“…RaxR is a substrate for RaxH trans-phosphorylation [32]. We demonstrated that the Xoo phoP gene, which encodes a response regulator, is up-regulated in the raxR knockout (PXO99DraxR) strain [33].…”

“…We hypothesize that RaxST catalyzes the transfer of the sulfuryl-group from PAPS to the tyrosine residue(s) of Ax21 (Figure 1). Furthermore, two different two-component regulatory systems (TCSs) are required for Ax21 activity: the RaxR/H system [32] and the PhoP/Q system [33]. RaxR shows 44% amino acid similarity to response regulators (RRs) of the well-studied E. coli OmpR subfamily.…”

Characterization of Xylella fastidiosa popP Gene Required for Pathogenicitya

“…PhoPPhoQ of X. oryzae pv. oryzae (Xoo), E. carotovora, and E. chrysanthemi appears to be cell density-dependent in plant host [4][5][6][7][8][9][10]. In this study, while both wild type and popP mutant X. fastidiosa strains showed similar growth curves in vitro, grapevines infected with popP deletion strain have significantly decreased bacterial populations in comparison to the bacterial populations of grapevines inoculated with wild type.…”

“…In phytopathogenic Xanthomonas oryzae pv. oryzae, TCS PhoP-PhoQ was recently shown to be a requirement for the induction of density-dependent gene expression, including genes related with biofilm formation [4][5][6][7]. PhoP-PhoQ controls virulence functions in the plant pathogen Erwinia carotovora and E. chrysanthemi [8][9][10][11].…”