Dynamic aspects of apical zonation in the angiosperm pollen tube

Summary In the apical 10–20 m of actively extending pollen tubes of Epilobium angustifolium, in a zone where the polysaccharide-containing wall precursor bodies (P-particles) dominate and where their movements on superficial observation seem to be random, there is in fact a concerted flux, acropetal movement taking place along the flanks of the tip zone, with a basipetal return flow along the centre. Detailed tracking of individuals shows that lipid globuli (diameters up to 1.5 m) and amyloplasts (dimensions up to 5.5 × 2.5 m) follow similar patterns of movement, but are sorted out in the sub-apical region, the smaller bodies penetrating further towards the apex. The findings are interpreted as indicating that the well-documented apical zonation of the pollen tube is maintained in the fluid circumstances of the growing tube by the filtering of cytoplasmic inclusions through the actin cytoskeleton, which, in conformity with recent fine-structural and other observations, is envisaged as consisting of a network of cross-linked microfilaments and microfilament aggregates at the tube tip giving place progressively to a system of more ordered, longitudinally oriented fibrils in the older parts of the tube. The implications for the operation of the actomyosin motility system and the tip growth mechanism are discussed.