LAZZARO, MARK D. Department of Biology, College of Charleston, Charleston SC 29424. - Conifer pollen tubes as a model system for the cytoskeleton's role in polarized growth.

Conifer pollen tubes elongate through female tissue to deliver sperm for fertilization. Since the spermatogenous cell initially remains within the pollen grain, these tubes lack callose plugs. Elongating tubes of Picea abies (Norway spruce-Pinaceae) are filled with amyloplasts and a vegetative nucleus. The pollen tube tip lacks plastids but contains numerous mitochondria and a dense endomembrane system. Elongation in conifer pollen tubes is fundamentally distinct from angiosperms and microtubules have a unique functional role. Conifer pollen tubes have a network of microtubules and microfilaments that are both required for elongation, extending from the grain and down the tube in a longitudinal array. At the tip, microtubules form a radial network beneath the plasma membrane and a longitudinal array within the tip center while microfilaments extend into the growing region but are absent from the tip. In the tip, the radial microtubules maintain cell shape and interact with microfibril deposition in the cell wall. The specific disruption of these microtubules causes tip swelling. Cellulose synthesis occurs within the tip in conifer pollen tubes and the disruption of synthesis also causes tip swelling and the disorganization of radial microtubules. The longitudinal microtubule array leading into and within the tip controls cytoplasmic streaming together with microfilaments. Organelles move in a fountain pattern, streaming towards the tip within the center of the tube, and back away from the tip along the edges. This pattern is the opposite of that seen in angiosperm pollen tubes. Disruption of microfilaments stops streaming and causes the accumulation of membranous tubules within the tip. Disruption of microtubules also stops streaming but collapses cytoplasmic organization from the tip back into the tube. The importance of microtubules in conifer pollen tube elongation parallels that seen in elongating protonema of ferns and mosses.

Key words: conifer, cytoskeleton, microtubule, pollen tube, reproduction