Immunogold localization of callose and other plant cell wall components in soybean roots infected with the oomycete Phytophthora sojae

Immunolabeling and transmission electron microscopic techniques were used to investigate the chemical nature of wall appositions in roots of susceptible and resistant soybean plants inoculated with Phytophthora sojae race 2. The extrahaustorial matrix associated with the haustorium of Phytophthora sojae also was examined. Antibodies against (1-->3)-beta-glucan, a terminal alpha-fucosyl-containing epitope present in xyloglucan and rhamnogalacturonan I, and an arabinosylated (1--6)-beta-galactan epitope present in arabinogalactan proteins were used. (1-->3)-beta-Glucan (callose), xyloglucan, and arabinogalactan proteins were found to be localized in all wall appositions regardless of how long after inoculation the appositions developed or whether plants were susceptible or resistant to Phytophthora sojae. (1-->3)-beta-Glucan also was found in fungal walls and at host cell plasmodesmata. None of the four antibodies labeled the extrahaustorial matrix. The antibody against arabinogalactan protein recognized the host plasma membrane, but not the invaginated host plasma membrane associated with the extrahaustorial matrix. This result indicates that the properties or the composition of the host plasma membrane may change locally once it becomes an extrahaustorial membrane.