1.3.5S
INTERACTIONS BETWEEN PLANT CELLS AND BACTERIA: EVIDENCE FOR SUPPRESSION OF LOCALIZED RESISTANCE REACTIONS

JW MANSFIELD, C BESTWCK, I BROWN, M KESHAVARZI and S SOYLU

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

Background and objectives
Electron microscopy including histochemistry and immunocytochemistry has been used to study responses of plants to challenge by bacteria. Responses of Arabidopsis, lettuce and pepper to pathogenic strains and Hrp mutants of Xanthomonas campestris pv. vesicatoria (Xcv) and pathovars of Pseudomonas syringae were examined. The interactions studied allowed comparison of hypersensitive reactions (HR) activated by different resistance genes and also the HR to non-pathogens, e.g. the response of lettuce to the bean pathogen P.s. pv. phaseolicola. Activation of the oxidative burst was detected by localization of H2O2 accumulation by formation of cerium perhydroxides. Antisera were used to detect callose, hydroxyproline-rich glycoproteins and peroxidase at reaction sites, and the protein product of the avrBs3 avirulence gene in Xcv [1, 2].

Results and discussion
Resistance in pepper and Arabidopsis determined by Bs1 and Bs3, and RPM1 and RPS5, respectively, were differentiated by the speed of cell collapse. Major alterations to ultrastructure followed accumulation of H2O2 in the plant cell wall adjacent to bacteria. No nuclear changes characteristic of apoptosis were observed but a final 'corpse morphology' was apparent and distinct from the structure of cells which died during compatible interactions. In all plants, inoculation with Hrp mutants caused striking localized alterations to plant cell walls, and the deposition of papillae adjacent to bacteria. Accumulation of callose, HRGPs and peroxidase in addition to some H2O2 was observed at reaction sites. If pepper leaves were inoculated with mixtures of wild-type and Hrp mutants of Xcv, no papillae developed. In the presence of the wild-type strain, the Hrp mutant (tagged by its production of avrBs3) was able to multiply and form colonies. The wild-type appeared to suppress local reactions to Hrp mutants. We suggest that cell wall alterations and papilla deposition represent a primitive defence mechanism which is suppressed by pathogenicity factors delivered to plant cells by the Hrp-dependent, type III secretion system.

References
1. Bestwick CS, Bennett MH, Mansfield JW, 1995. Plant Physiology 108, 503-516.
2. Brown I, Mansfield J, Bonas U, 1995. Molecular Plant-Microbe Interactions 8, 825-836.