Development of reproductive tissue and control of cell division are common challenges to all sexually reproducing eukaryotes. The Arabidopsis thaliana TSO1 gene is involved in both these processes. Mild tso1 mutant alleles influence only ovule development, whereas strong alleles have an effect on all floral tissues and cause cell division defects. The tso1 mutants described so far carry point mutations in a conserved cysteine-rich domain, the CRC domain, but the reason for the range of phenotypes observed is poorly understood. In the present study, the tesmin/TSO1-like CXC (TCX) proteins are characterized at the biochemical, genomic, transcriptomic, and functional level to address this question. It is shown that the CRC domain binds zinc, offering an explanation for the severity of tso1 alleles where cysteine residues are affected. In addition, the phylogenetic and expression analysis of the TCX genes suggested an overlap in function between AtTSO1 and the related gene AtTCX2. Their expression ratios indicated that pollen, in addition to ovules, would be sensitive to loss of TSO1 function. This was confirmed by analysis of novel tso1 T-DNA insertion alleles where the development of both pollen and ovules was affected.

The haploid gametophyte generation occupies a short but vital phase in the life cycle of flowering plants. The male gametophyte consists of just two or three cells when shed from the anthers as pollen grains. It is this functional specialization that is thought to be a key factor in the evolutionary success of flowering plants. Moreover, pollen development offers an excellent model system to study many fundamentally important biological processes such as polarity, cell fate determination, cell cycle regulation, cell signaling and mechanisms of gene regulation.In the first part of this chapter we review the progress achieved through genetic analysis in identifying gametophytic mutants and genes required for key aspects of male gametogenesis. In the second part we discuss recent advances in genome-wide transcriptomic studies of haploid gene expression and a critical evaluation of data treatment methods. Finally we provide a perspective of the impact of these data on future strategies for understanding the gametophytic control of pollen development.

Summary Pollen germination may occur via the so‐called germination pores or directly through the pollen wall at the site of contact with the stigma. In this study, we addressed what processes take place during pollen hydration (i.e. before tube emergence), in a species with extra‐poral pollen germination, Arabidopsis thaliana. A T‐DNA mutant population was screened by segregation distortion analysis. Histological and electron microscopy techniques were applied to examine the wild‐type and mutant phenotypes. Within 1 h of the start of pollen hydration, an intine‐like structure consisting of cellulose, callose and at least partly de‐esterified pectin was formed at the pollen wall. Subsequently, this ‘germination plaque’ gradually extended and opened up to provide passage for the cytoplasm into the emerging pollen tube. BURSTING POLLEN (BUP) was identified as a gene essential for the correct organization of this plaque and the tip of the pollen tube. BUP encodes a novel Golgi‐located glycosyltransferase related to the glycosyltransferase 4 (GT4) subfamily which is conserved throughout the plant kingdom. Extra‐poral pollen germination involves the development of a germination plaque and BUP defines the correct plastic‐elastic properties of this plaque and the pollen tube tip by affecting pectin synthesis or delivery.

Asymmetric cell division at pollen mitosis I (PMI) is required to specify the differential fate of the daughter vegetative and generative cells. Cytokinesis at PMI displays specialized features, and it has been suggested that there might be distinct molecular pathways underpinning different modes of cytokinesis in plants. Activation of the NACK-PQR MAP kinase signaling pathway, which is essential for somatic cell cytokinesis in tobacco, depends upon the NACK1 and NACK2 kinesin-related proteins. Their Arabidopsis orthologs, HINKEL (HIK) and TETRASPORE (TES), were reported to be essential for cytokinesis in somatic cells and in microsporocytes, respectively. More recently, HIK and TES were shown to have a functionally redundant role in female gametophytic cytokinesis. We report here that HIK and TES are co-expressed in microspores and developing pollen, and, through analysis of microspore and pollen development in double heterozygote mutants, the occurrence of cell plate expansion defects during cytokinesis at PMI. The data demonstrate a functionally redundant role for HIK and TES in cell plate expansion during male gametophytic cytokinesis, extending the concept that different modes of cytokinesis are executed by a common signaling pathway, but reinforcing the individuality of gametophytic cytokinesis in its requirement for either TES or HIK.