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)