Conformation and movement of the vegetative nucleus of the angiosperm pollen tube: association with the actin cytoskeleton

Abstract

Actin is present in the cytoplasm of the vegetative cell of angiosperm pollens in numerous fusiform, spiculate or toroidal bodies, and also as a sheath enveloping the vegetative nucleus. During activation following hydration, the compact cytoplasmic bodies are translated into skeins of extended fibrils, and circulatory movements begin in the cytoplasm. Throughout this period the vegetative nucleus, with fibrillar actin now associated with the surface, undergoes a continuous change of shape. In the extending tube following germination the actin cytoskeleton consists of numerous mainly longitudinally oriented fibrils. After entry into the tube the vegetative nucleus remains associated with the fibrils, usually extending greatly in length and developing attenuated, often pointed extensions. The observed conformations, which change continuously, suggest that varying local tensions are applied to the vegetative nucleus during passage through the tube. Cytochalasin D breaks up the actin fibril system and brings about a rapid contraction of the nucleus, at the same time eliminating the elastic extensions of the nuclear envelope. Nuclei isolated physically from unfixed tubes also contract in length as the fibrillar components of the cytoskeleton are detached. These findings indicate that the movement of the vegetative nucleus depends on local associations of the nuclear envelope with the actin cytoskeleton of the vegetative cell.