Pollen: Structure and Development

Abstract Pollen grains contain the highly reduced haploid microgametophyte generation in seed plants that establishes the male germline during sexual reproduction. Pollen protects the germline from environmental injury, promotes gamete dispersal and ensures delivery of a pair of sperm cells to the female gametes via the pollen tube. Microgametogenesis is initiated from haploid microspores that undergo one or more asymmetric mitotic divisions followed by differentiation to form a tube cell and a pair of sperm cells. Knowledge of the molecular mechanisms governing microgametogenesis has advanced rapidly through genetic analysis of mutants that disturb cellular patterning and gamete specification in flowering plants and through the application of genome‐wide transcriptomic studies. Recent evidence also supports a diverse and significant role for functional small RNA pathways and epigenetic regulation during microgametogenesis. Together these advances are helping to establish models of the gene regulatory networks governing gametophyte development and gamete specification. Key Concepts: Pollen grains harbour the haploid microgametophytes of seed plants (spermatophytes) that deliver twin sperm cells to the female gametophyte via the pollen tube. Pollen contains high levels of stored transcripts and protein for use during germination and pollen tube growth. The complexity of haploid gene expression is reduced during pollen development and is accompanied by a relative increase in the expression of pollen‐specific transcripts. Sperm cells possess a reduced but extensive transcriptome that has a relatively high number of sperm cell‐specific transcripts. Formation of the male germline results from microtubule‐dependent asymmetric division of a progenitor microspore. Ubiquitin‐mediated proteolysis is essential for germline cell cycle progression. Male germline specification involves the coordination of cell cycle and differentiation by the germline‐specific transcription factor DUO1. Multiple small RNA pathways function in the microgametophyte to control gene expression, transposon activity and fertilisation.