An hrcU-Homologous Gene Mutant of Xanthomonas campestris pv. glycines 8ra That Lost Pathogenicity on the Host Plant but Was Able to Elicit the Hypersensitive Response on Nonhosts

Transposon mutagenesis was used to isolate nonpathogenic mutants of Xanthomonas campestris pv. glycines 8ra, which causes bacterial pustule disease in soybean. A 6.1-kb DNA region in which a mutation gave loss of pathogenicity was isolated and found to carry six open reading frames (ORFs). Four ORFs had homology with hrcU, hrcV, hrcR, and hrcS genes of Ralstonia solanacearum and X. campestris pv. vesicatoria. One nonpathogenic mutant, X. campestris pv. glycines H80, lost pathogenicity on soybean but was able to elicit the hypersensitive response (HR) on nonhost pepper and tomato plants. This mutant still multiplied as well as the wild type in the leaves or cotyledons of soybean. Although the DNA and amino acid sequences showed high homology with known hrp genes, the hrcU-homolog ORF is not required for HR induction on nonhost plants, pepper and tomato, or for the multiplication of bacteria in the host plant. This gene was only required for the pathogenic symptoms of X. campestris pv. glycines 8ra on soybean.