Summary

The angiosperm microgametophyte displays a complex genetic program during its development. There appear to be two main classes of genes expressed during pollen development, with the transition occurring roughly at microspore mitosis. Although the earlier class of genes is presumed to be associated with immature microspore formation, the later genes probably represent those associated with pollen maturation, germination, and tube growth. This is supported by the observation that several pollen-specific clones of the late class have been identified whose sequences are similar to enzymes associated with cell wall metabolism, as well as many cy to skeleton genes, the products of which are clearly necessary for pollen tube growth.

The genetic promoter elements required for a pollen response continue to elude our understanding, but the seemingly unrestricted interchangeability of pollen promoters among a wide range of host plants argues for some kind of universal pollen element within them. Numerous pollen-specific genes have been isolated recently, and when their promoters are studied in detail, it might be easier to identify common elements in the promoters. Eventually, characterization of genes and promoters involved in key regulatory processes in pollen should provide insights into the nature of haploid gene expression and its relation to that of the diploid plant. Findings of this nature should point the way toward practical applications of pollen molecular biology in the areas of plant breeding, biotechnology, and basic science.

Introduction

A pollen grain represents the male portion of the gametophytic stage in the angiosperm life cycle. Following the completion of microsporogenesis and microgametogenesis, the pollen grain is only a three-celled organism.