Restoration of pathogenicity of avirulent Xanthomonas oryzae pv. oryzae and X. campestris pathovars by reciprocal complementation with the hrpXo and hrpXc genes and identification of HrpX function by sequence analyses

Abstract

The molecular basis of pathogenesis by Xanthomonas oryzae pv. oryzae has been partly elucidated by the identification of a gene, hrpXo, required for bacterial blight on rice. A mutation in hrpXo results in the loss of pathogenicity on rice and the loss of hypersensitivity on nonhosts such as Datura stramonium and radishes. Pathogenicity and its ability to cause Xanthomonas oryzae pv. oryzae (30), a yellow-pigmented gram-negative member of the family Pseudomonadaceae, causes bacterial blight on rice, a disease of considerable importance in the lowland rice-growing regions of the world (21). X. oryzae pv. oryzae infects through natural openings by first multiplying at that site and then invading primarily the vascular tissues by propagating in the xylem (22, 31). Severe necrosis along the interveinal regions, known as leaf blight, occurs as a result of pathogen ingression and multiplication.X. campestris pv. campestris causes the black rot disease of cruciferous plants. Like X. oryzae pv. oryzae, X. campestnis pv. campestris enters through natural openings, mainly hydathodes located along the leaf edge. This organism propagates in the xylem, which results in the necrosis of interveinal tissues and blackening along the veins.For both bacteria, the mechanism of pathogenesis is poorly understood. this gene, designated hrpXo (for hypersensitive reaction pathogenicity), has been examined by protein sequence analysis. Unlike the so-called avirulence (avr) genes, whose products catalyze the formation of metabolites that differentially elicit hypersensitivity responses on incompatible cultivars of a particular host (recently reviewed in references 14, 17a, 27, and 38), the hrpX gene product prevents a vascular hypersensitivity response (VHR), which is a rapid tissue necrosis in the vascular system, in the respective compatible host (17).The clustered hrp genes that are likely involved in secretion (7) and that are unrelated to hrpX have been studied with X. campestris pv. campestris (1)

HrpXv, an AraC-type regulator, activates expression of five of the six loci in the hrp cluster of Xanthomonas campestris pv. vesicatoria

“…The hrpXv gene is located at the extremity of the insert of pXV751 on a 2.5-kb EcoRI-HindIII (vector site) fragment that was subcloned in pBluescript-KS II (pBX10) and partially sequenced. Comparison of the obtained sequence with the published sequences of hrpXo (31) and hrpXc (40) indicated that the hrpXv gene was only partially present in the cosmid clones. We therefore cloned the genomic 9.4-kb EcoRI fragment described above from X. campestris pv.…”

“…Sequences homologous to the hrpX gene of X. campestris pv. campestris are present in several Xanthomonas species (31). To find out whether X. campestris pv.…”

“…campestris and X. oryzae pv. oryzae (31). We also analyzed whether the hrpXv mutant could be complemented by the related hrp regulatory gene hrpB of B. solanacearum (Fig.…”

Molecular determinants of disease and resistance in interactions of Xanthomonas oryzae pv. oryzae and rice

“…armoraciae. This result indicates that hrpXo and hrpXc are functionally equivalent [30]. PCR amplification of hrpXo in 16 X. campestris pathovars and 12 X. oryzae pv.…”

“…PCR amplification of hrpXo in 16 X. campestris pathovars and 12 X. oryzae pv. oryzae strains indicates that the hrpX gene is highly conserved in Xanthomonas and suggests that the functional equivalence may extend into other pathovars of X. campestris [30,31]. By using a portion of the X. campestris pv.…”