Department of Biological Sciences, Wye College, University of London, Wye, Ashford, Kent TN25 5AH, UK

Background and objectives
Electron microscopy was used to examine interactions between Xanthomonas campestris pv. vesicatoria (Xcv) strains and pepper (Capsicum annuum) leaves. Reactions to hrp mutants, avirulent, virulent and mixtures of virulent and hrp mutant strains were compared. In each interaction, responses of plant cells including cell wall alterations, papilla deposition and H2O2 generation were investigated [1]. lmmunocytochemistry was used to determine and compare constituents of papillae formed adjacent to wild-type and hrp mutants [2].

Results and conclusion
Inoculation of leaves with hrp mutants caused localized modifications of the cell walls adjacent to bacterial cells and formation of large papillae. By contrast, only minor cell wall alterations were observed in response to inoculation with wild-type strains. The papillae formed adjacent to hrp mutant were rich in callose, whereas cellulose was a more common component of the papillae close to wild-type strains. Inoculation with virulent, wild-type bacteria prior to the hrp mutant and with a mixture of virulent and hrp mutant strains induced a susceptible state in the treated area, during which only minor wall alterations were observed. The hrp mutant cells, which were normally agglutinated onto the modified cell wall, succeeded in producing large intercellular colonies. Generation of hydrogen peroxide (H2O2) was investigated cytochemically by its reaction with cerium chloride to produce electron-dense deposits of cerium perhydroxides [1]. Papilla deposition in the hrp mutant interaction was associated with localized generation of H2O2 which was often most intense in the interior layers of papillae. No staining was detected during compatible interactions caused with either wild-type or a mixture of wild-type and hrp mutant strains. Our results suggest that cell wall alterations occuring in response to the Xcv hrp mutant strain are mediated by H2O2 and that the wild-type strain suppresses papilla deposition by the delivery of pathogenicity factors which are dependent upon an intact hrp gene cluster.

1. Bestwick CS, Brown IR, Bennett MH, Mansfield JW, 1997. Plant Cell 9, 209-221.
2. Brown I, Mansfield J, Bonas U, 1995. Molecular Plant-Microbe Interactions 8, 825-836.