Molecular Mechanisms of Pollen Tube Growth and Differentiation.

INTRODUCTION ' The extremely reduced, three-celled, haploid male plant (male gametophyte) of flowering plants has a number of specialized functions to perform. The primary functions are the production of two sperm cells and their transport within the pollen tube through the tissues of the style and ovary into the embryo sac in the ovule, where they participate in double fertilization. The early part of the life of the male gametophyte, following meiosis, occurs within the sporophytic tissues of the anther. During this period, a number of critical developmental events occur that are preceded or accompanied by the induction of activity of a large number of genes (for reviews, see The molecular events that take place during pollen maturation from meiosis until the time the pollen is released make up the topic of an accompanying paper (McCormick, 1993, this issue) and will not be discussed here. At the time of anther dehiscence, the pollen grains of most flowering plants are bicellular: they contain a vegetative cell and a generative cell. Among the 265 families studied by Brewbaker (1967), 179 produced only bicellular pollen and 54 produced only tricellular pollen, which contains the products of generative cell division, the two sperm cells. Thirty-two families included both bi-and tricellular pollen-producing species. Seventy percent of the .u2000 species studied released pollen in the bicellular stage, and 30% shed their pollen as tricellular grains following sperm production. Only 10 genera were found to include both bicellular and tricellular species. In addition, all taxa considered to be phylogenetically primitive were bicellular (Brewbaker, 1967). When the pollen grain is released from the anther, it exists as a free organism until it is carried by wind, insects, or other agents to the stigma of the pistil in a flower, where it then begins another phase of its life. If the pistil is a compatible one, the pollen grain germinates and extrudes a tube, the pollen tube, within which the sperm cells are transported to the embryo sac. During this phase of development, there is an intimate interaction of the pollen tube, a part of the vegetative cell of the male gametophyte, with the cells and tissues of the pistil. Many of the biochemical events that occur in the vegetative cell of the pollen grain prior to anthesis are related to the preparedness of the gametophyte for the demands of germination and growth of the tube in the pistil. …