Gametophytic Gene Expression

Abstract

The male gametophyte of flowering plants is a microscopic structure which during its early development is enclosed within the sporophyte tissue in the anther of a flower bud. Dramatic advances have been made recently in our understanding of gene expression in the different tissues of the anther including the tapetum (Koltunow et al, 1990: Mariani et al, 1990). Following meiosis there is a long interphase period during which the microspores enlarge greatly in size. Microspore mitosis results in an unequal partitioning of cytoplasm into the vegetative and generative cells. Following anthesis the pollen grain is deposited on the stigma of the pistil where it begins another phase of its development by germinating and extruding a tube within which the sperm cells are transported to the embryo sac. In many species studied the pollen grain at maturity contains all the proteins that are required for germination and early tube growth, since protein synthesis inhibitors do not block these events. Moreover a large number of enzymes have been reported to be present in pollen grains of various species (Mascarenhas, 1975). Protein synthesis begins early in pollen germination. Most of the studies in the literature indicate that the mature pollen grain contains a store of stable mRNAs that are translated early during germination and that play a greater or lesser role in pollen germination and tube growth depending on the plant species (reviewed in Mascarenhas, 1975, 1990a,b).