Towards a Comprehensive Understanding of Molecular Mechanisms of Sexual Reproduction in Higher Plants

Abstract

Sexual reproduction in higher plants includes a key phase for producing male and female gametes (i.e. microsporogenesis, microgametogenesis, megasporogenesis and megagametogenesis), in addition to the ensuing pollination and fertilization. Development of the gametes is regulated by gametophytic and sporophytic gene expression ( McCormick 2004 ); for example, gametophytic microspores are infl uenced by gene expression in sporophytic tapetum cells of anthers. Such networks of gene expression during gamete development are quite important for the understanding of sexual reproduction, because failure of gene regulation often results in a defect of fertilization, which is known as male and/or female sterility. To date, many reproductive tissuespecifi c genes have been identifi ed and characterized ( Taylor and Hepler 1997 , Endo et al. 2004 , Kagi and Gross-Hardt 2007 ). However, the total picture of gene regulation of events during sexual reproduction is still not clear. This special issue consists of four original articles about transcriptome analysis focused on male gamete development, two original articles about mutant analysis of male or female gamete development, and one review article about male sterility. All the papers take up a common theme about ‘gene regulation in plant reproduction’, and should help towards a comprehensive understanding of sexual reproduction in higher plants. Tapetum cells, which are the innermost cell layer of the anther wall, are important for feeding nutrients to microspores, and pollen maturation is accompanied by the characteristic gene expression of microspore/pollen and tapetum. Therefore, analyses of separated transcriptomes of microspore/pollen and tapetum are necessary to gain an understanding of male gametophyte development. Laser microdissection (LM) is a powerful tool for isolating specifi c cells from complicated plant tissues ( Nakazono et al. 2003 ), and can be applied to these separated transcriptomes. In this issue, authors of four papers have referred to the LM 44K-microarray (44K-LMM) of microspore/pollen and tapetum in japonica rice. First, Suwabe and colleagues established the 44K-LMM with amplifi ed labeled RNAs isolated from microspore/pollen and tapetum cells, which were physically separated from a cross-section of rice anthers. In order to validate the 44K-LMM data, they compared the 44K-LMM data with that of 156 previously characterized anther-specifi c genes, which were identifi ed by Endo et al. (2004) . From this validation, it was demonstrated that the 44K-LMM is highly reliable, and can be used for further comprehensive analysis of gene networks in the development of the male gametophyte (pp. 1407–1416). Secondly, Hobo and colleagues characterized 44K-LMM data by using in silico analyses (cluster analysis, gene ontology, and searching gene regulatory cis -elements and mutant phenotypes). Interestingly, synchronous gene expression between the microspore and tapetum was observed as a novel characteristic in these transcriptomes. Further detailed analyses will reveal the signifi cance of this synchronous expression in the development of the male gamete. In addition, about half of the Tos17 insertion lines in microspore/pollenor tapetumspecifi c genes showed sterile or low fertility phenotypes, indicating that these genes were necessary for the development of anther tissues (pp. 1417–1428). These two fundamental papers clearly indicate that these precise transcriptome data sets of male gametophyte development are useful for understanding various phenomena in the anther, and will shed light on new research fi elds of plant reproduction. The relationship between phytohormones and microspore/ pollen and tapetum transcriptomes is one of the most interesting fi ndings using this 44K-LMM. The phytohormones are important for plant growth, development, differentiation, etc. However, only a few functions in microspore/pollen and tapetum development are known. To date, many genes encoding the regulation of biosynthesis, perception and signal transduction of several phytohormones have been identifi ed and characterized ( Guo and Ecker 2004 , Guilfoyle and