Immunogold localization of pectin and callose in pollen grains and pollen tubes of Brugmansia suaveolens - Implications for the self-incompatibility reaction

Pollen of Brugmansia suaveolens was grown in vitro or in vivo after compatible or incompatible pollination. Ultrathinsections of fixed and embedded specimens were treated with immunogold label for ultrastructural localization of acidic pectins (monoclonal antibody JIM 5), methyl-esterified pectins (monoclonal antibody JIM 7) and callose (anti-(1-3)-beta-glucan antibody) in order to elucidate the distribution and intracellular pathway of the cell wall constituents. Pectins, present in the intine of the pollen grain and in the outer layer of the bilayered pollen tube wall, seem to be synthesized in the dictyosomes activated upon germination. Their intracellular transport from the dictyosomes to the pollen tube tip, the site of fusion with the cell wall, occurs in fibrillar or bipartite particles that contain pectins in a highly methyl-esterified form, but no callose. In both intine and pollen tube cell wall a radial gradient of pectin esterification is observed. Pectins are less esterified with increasing distance from the plasma membrane. Considering the fact that pectins are secreted in a highly esterified form, the gradient indicates the presence of enzymatic activity, causing pectin de-esterification during cell wall development. Upon incompatible pollination both layers of the pollen tube wall undergo thickening. Pectinaceous particles are deposited on the inside of the cell wall or seemingly get stuck during migration through the inner callosic layer. Whereas pectinaceous secretory vesicles in compatible pollen tubes show a high grade of esterification, during the incompatibility reaction pectic aggregates of low esterification grade appear in the cytoplasm. This might indicate the presence of de-esterifying enzymes in the secretory vesicles.