LOCALIZATION OF COMPONENTS OF THE OXIDATIVE CROSS-LINKING OF GLYCOPROTEINS AND OF CALLOSE SYNTHESIS IN PAPILLAE FORMED DURING THE INTERACTION BETWEEN NONPATHOGENIC STRAINS OF XANTHOMONAS-CAMPESTRIS AND FRENCH BEAN MESOPHYLL-CELLS

Immunogold labelling was used to probe the responses of mesophyll cell s in French bean (Phaseolus vulgaris L.) to an hrpA mutant of Xanthomo nas campestris pv. vesicatoria and a saprophytic strain of X.c. The no n-pathogenic strains both caused localized alterations to the plant ce ll wall and formation of large papillae in adjacent cells. Immunocytoc hemistry showed the co-localization, in the cell wall and paramural de posits, of an M(r)42 000 proline-rich glycoprotein with chitin-binding activity (CBPRP) and the enzyme responsible for its immobilization, a n M(r)46 000 peroxidase. The CBPRP appeared to lose antigenicity after cross-linking, and, unlike the peroxidase, was not detected consisten tly in the extracellular matrix that encapsulated bacteria onto the pl ant cell wall. The peroxidase may have a dual function in both the gen eration and utilization of H2O2 for cross-linking of proteins and phen olics during the construction of papillae. A burst of H2O2 was detecte d 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 Go-localization of callose and an M(r)65 000 polypeptid e component of callose synthase was also demonstrated. Synthesis of ca llose appeared so rapid that the enzyme became embedded in the polysac charide so that both were detected as integral to the developing papil la. Localized alterations to the cell wall and deposition of papillae were found to involve co-ordinated synthetic a nd oxidative activities at microsites within responding cells, without activation of the hype rsensitive reaction.