Localization of components of the oxidative cross-linking of glycoproteins and of callose synthesis in papillae formed during the interaction between non-pathogenic strains of Xanthomonas campestris and French bean mesophyll cells

Immunogold labelling was used to probe the responses of mesophyll cells in French bean ( Phaseolus vulgaris L.) to an hrpA mutant of Xanthomonas campestris pv. vesicatoria and a saprophytic strain of X.c. The non-pathogenic strains both caused localized alterations to the plant cell wall and formation of large papillae in adjacent cells. Immunocytochemistry showed the co-localization, in the cell wall and paramural deposits, of an M _r 42 000 proline-rich glycoprotein with chitin-binding activity (CBPRP) and the enzyme responsible for its immobilization, an M _r 46 000 peroxidase. The CBPRP appeared to lose antigenicity after cross-linking, and, unlike the peroxidase, was not detected consistently in the extracellular matrix that encapsulated bacteria onto the plant cell wall. The peroxidase may have a dual function in both the generation and utilization of H₂O₂ for cross-linking of proteins and phenolics during the construction of papillae. A burst of H₂O₂ was detected 1–5 h after inoculation at reaction sites by histochemical staining with cerium chloride. Progressive expansion of papillae and cell-wall alterations was, however, not associated with the maintenance of high levels of H₂O₂. Co-localization of callose and an M _r 65 000 polypeptide component of callose synthase was also demonstrated. Synthesis of callose appeared so rapid that the enzyme became embedded in the polysaccharide so that both were detected as integral to the developing papilla. Localized alterations to the cell wall and deposition of papillae were found to involve co-ordinated synthetic and oxidative activities at microsites within responding cells, without activation of the hypersensitive reaction.