Summary

The highly specialised angiosperm male gametophyte is both the site of production and the vehicle by which the male gametes are transported to the embryo sac to participate in fertilisation. Because of its relative simplicity compared with the sporophyte and accessibility for cytological and molecular analysis, the male gametophyte represents an excellent system in which to unravel the molecular basis of gene regulation and cellular differentiation in plants. The intent of this chapter is to review evidence for haploid gene expression in the developing male gametophyte, emphasising in particular the contribution of molecular cloning and transgenic approaches. cDNA cloning and RNA analysis has led to the characterisation of more than 30 microspore or pollen-expressed genes from 12 different plant species which are preferentially activated at particular stages of development. Two broadly defined groups of pollen genes are recognised; the ‘early genes’, which are activated prior to pollen mitosis I (PMI), and the ‘late genes’ which are first activated after PMI. Promoter studies have demonstrated differential gene expression between the vegetative and generative cells and insight into the significance of PMI for late gene activation. Detailed analyses of the promoters of several late pollen genes have led to the identification of several distinct cis-regulatory elements which control both the level and specificity of haploid gene expression. Some of these regulatory elements are functionally conserved among homologous promoters from diverse plant species and among different promoters within a single species.