Control of Male Gametophyte Development

Abstract

In a previous review of male gametophyte development (McCormick, 1993), it was noted that the two areas posed by Mascarenhas (1975)as fruitful areas for future research were the following. What are the differences in the two cytoplasms that determine the different cell fates of the generative and vegetative cells? And what are the functions of pollen-specific proteins? Now, 10 years later, the genome sequences of Arabidopsis and rice have been completed. There are extensive EST databases for many plants and several data sets from microarray hybridizations. There are extensive resources for disrupting the functions of genes. The pollen research community has made significant progress toward a deeper understanding of pollen development using community resources as well as novel techniques developed specifically for the analysis of pollen. This review will provide an overview of these advances and prospects for the future, focusing on male gametophyte development, strictly defined as postmeiosis development, after the formation of the haploid microspores. Meiosis will not be discussed in detail. Anther development and the role of the tapetum in pollen development are discussed by Dickinson and Scott in this issue. Scheme of Microsporogenesis. Tetrads of Arabidopsis. Male meiosis is essentially synchronous within an anther. A common way to dissect a developmental pathway is to isolate mutants that disrupt the pathway. This approach has been used extensively to dissect male meiosis and subsequent stages of pollen development. Comprehensive reviews of meiotic stages and cytology in Arabidopsis (Ross et al., 1996; Armstrong and Jones, 2003) as well as a time course for meiotic stages (Armstrong et al., 2003) now provide the necessary foundation for the interpretation of phenotypes in mutants that disrupt particular phases of meiotic development. Caryl et al. (2003)comprehensively reviewed the mutants known to affect meiosis in Arabidopsis. Some mutants that disrupt pollen development affect the function of the tapetum. Because the tapetum is sporophytic, such male-sterile mutants generally are recessive and all of the pollen grains within an anther are affected. Mutants are termed gametophytic if they disrupt genes that act after meiosis, in the haploid phase of pollen development, and thus only the pollen grains carrying the mutant allele are affected. Male gametophytic mutants often exhibit segregation distortion (i.e., reduced transmission through the male) if the gene that is mutated is important for pollen development or function. Indeed, this feature was used successfully as a first-pass method in screens designed to identify gametophytic mutants (Feldmann et al., 1997; Bonhomme et al., 1998; Howden et al., 1998; Grini et al., 1999; Oh et al., 2003). The developmental stage affected in each mutant was determined later. In some cases, the phenotype was demonstrated to be attributable to a single gene disruption. For example, several of the male transmission–defective mutants described by Bonhomme et al. (1998)had similar phenotypes, and upon further mapping and cloning they were found to be alleles. The disrupted gene was renamed kinky pollen because pollen grains harboring a mutant allele exhibited aberrant tube growth (Procissi et al., 2003). However, frequently, mutations identified from T-DNA insertion screens were unlinked to the T-DNA insertion or exhibited complex molecular lesions. In such cases, it was difficult to identify the molecular basis for the transmission defect. For example, the T-DNA insertion mutant named halfman (Oh et al., 2003) was demonstrated to have a gametophytic defect with low penetrance. Molecular analysis showed that the T-DNA insertion in halfman had induced an adjacent deletion of 150 kb that encompassed 38 genes; the loss of 1 or more of these genes might have been responsible for the observed phenotype, in which ∼30% of the pollen aborted at approximately the bicellular stage of development. Assiduous screening for mutants based on morphology or staining characteristics of pollen from individual M1 or M2 plants also has yielded very interesting phenotypes (Chen and McCormick, 1996; Johnson and McCormick, 2001; Lalanne and Twell, 2002). As illustrated in Figure 1, pollen development undergoes stereotypical cell divisions to yield the three-celled male gametophyte. To test whether the asymmetric division that resulted in the formation of the vegetative cell and the generative cell could be disrupted mutationally, or if the number of cell divisions typical of pollen development could be disrupted mutationally, pollen of mutagenized populations was examined after 4′,6-diamidino-2-phenylindole staining. Such screens yielded, for example, the mutants sidecar pollen (Chen and McCormick, 1996) and gemini pollen (Park et al., 1998). At the mature pollen stage in sidecar pollen heterozygotes, ∼50% of the pollen was normal, ∼43% was aborted, and ∼7% showed the sidecar phenotype, namel