Sites of Origin of the Peripheral Microtubule System of the Vegetative Cell of the Angiosperm Pollen Tube

Abstract

From earlier published work it is known that microtubules in the vegetative cell of angiosperm pollen tubes mainly occur in the form of longitudinally disposed strands closely associated with the plasmalemma. This peripheral cytoskeleton is extended apically at a speed matching the growth rate of the tube. Immunofluorescence localization shows that in the actively elongating tube it originates in the sub-apical zone in bands or ribbons up to 2 μm wide, interpreted here as comprising aggregates of apposed, axially oriented microtubules. These appear first in the cortical cytoplasm in close association with the wall in the part of the tube where the callose inner lining can first be detected. The bands do not extend apically into the region of the pectic sheath of the extreme tip. In the course of normal growth, the peripheral microtubule investment remains in the older parts of the tube from which the bulk of the cytoplasm has been withdrawn, indicating that tubulin is probably not recycled. If the growth of the tube is retarded, the inner callosic layer extends apically. The acropetal movement of callose is accompanied by a migration of the limit of detectable tubulin towards the extreme tip, and the axially oriented bands are replaced by a confused mass of granules and short spicules. It is suggested that the bands represent nucleation zones associated with the stabilizing plasmalemma in the sub-apical stretch of the tube where the insertion of wall-precursor material is diminishing, and that it is from these zones that the microtubule cytoskeleton of the pollen tube originates. Since during growth the nucleation zones progress rapidly forward into association with new membrane, it is considered unlikely that their sites are determined by local differentiations of the plasmalemma. An alternative possibility is that the distribution of the zones is related to the calcium ion gradient known to be present in the apical stretch of the extending pollen tube.