Pollen: Structure and Development

Abstract Pollen is the highly reduced male gametophyte generation of seed plants that protects the male germline while promoting gamete dispersal. Pollen grains produce the male gametes required for sexual reproduction and are adapted for interaction with pollinators and with female tissues upon pollination. While pollen of gymnosperms produces motile sperm or delivers nonmotile sperm to the egg via a slow‐growing pollen tube, angiosperm pollen typically forms a fast‐growing pollen tube to transport a pair of nonmotile sperm cells. Pollen develops after meiosis from haploid microspores that complete one or more asymmetric cell divisions before differentiating to form a tube cell and a pair of sperm cells. The analysis of mutants and the use of genome‐wide transcriptome studies has advanced knowledge of the underlying complexity, molecular mechanisms and evolution of pathways governing pollen development. Epigenetic pathways also play a crucial role in the regulation of pollen gene expression and its contribution to seed development. Active research and technological advances continue to enhance understanding of key regulatory pathways governing pollen structure and development in a variety of angiosperms. 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. Distinct epigenetic reprogramming events play a key role in rewiring transcription to facilitate the specification of each pollen cell type. Chromatin states in the sperm cells forecast the earliest phases of gene expression during seed development.