Pollen Development

Pathways of pollen development 

Pollen is produced within the anthers (microsporangia or pollen sacs) of the flower. During its developmentfrom an undifferentiated mound of cells (anther primordium) the antherforms two general groups of cells. The reproductive or sporogenous cellsgive rise to the microspores and are formed from cells locatedcentrally within the developing anther. The non-reproductive cells formdiscrete anther tissues layers and include the epidermal, cortical andtapetal cell layers surrounding the sporogenous cells. The tapetum whichis the innermost layer of the pollensac plays a dominant role particularly during the microspore stage. Forexample, many male sterile mutations affect tapetal cell functions and development is often arrested during the microspore stage.

Microsporogenesis and microgametogenesis

Two distinct and successive developmental phases, microsporogenesis and microgametogenesis, lead to the production of the mature microgametophytes.

Microsporogenesis comprises the events which lead tothe formation of the haploid unicellular microspores. Duringmicrosporogenesis the diploid sporogenous cells differentiate asmicrosporocytes (pollenmother cells or meiocytes) which divide by meiosis to form four haploidmicrospores. Each diploid meiocyte gives rise to a tetrad of fourhaploid microspores and microsporogenesis is complete with the formationof distinct single-celled haploid microspores.

Microgametogenesis comprises events which lead to the progressive developmentof the unicellular microspores into mature microgametophytes containingthe gametes. This phase begins with the expansion of the microsporewhich is commonly associated with the formation of a single largevacuole. Vacuolation is accompanied by the displacement of themicrospore nucleus to an eccentric position against the microspore wall.In this position the nucleus undergoes first pollen mitosis (pollenmitosis I) which results in the formation of two unequal cells, a largevegetative cell and a small generative cell each containing a haploidnucleus. The generative cell subsequently detaches from the pollengrain wall and is engulfed by the vegetative cell forming a unique'cell within a cell' structure. The engulfed generative cell dividesonce more by mitosis (pollen mitosis II) to form the two sperm cells completely enclosed within the vegetative cell cytoplasm either before pollen is shed (tricellular pollen) or within the pollen tube (bicellular pollen).