Pollen Cell Fate

The asymmetric cell division of the microspore at pollen mitosis I plays a critical role in the determination and subsequent fate of the two unequal daughter cells, the vegetative and generative cells. As a result of this highly asymmetric division the larger vegetative cell does not divide further, accumulates a dense cytoplasm rich in protein, lipid and carbohydrates and possesses a large diffusely staining nucleus. Storage compounds synthesised during vegetative cell maturation are utilised for intense metabolic activity during pollen tube growth by tip extension. In contrast, the smaller and metabolically repressed generative cell does not exit the cell cycle and continues through one more round of mitosis to produce two sperms cells with highly condensed chromatin. This asymmetric division may be described as determinative in that daughter cells are immediately different (in size, gene expression and chromatin structure) after division and their fate is determined as a result of division asymmetry.

For example, microspores which have been induced to divide symmetrically as a result of mitotic spindle displacement produce daughters which adopt equal 'vegetative-like' cell fate with both expressing a vegetative cell-specific marker. Therefore division asymmetry at pollen mitosis I is required to establish germ cell fate and is an essential component of male gametogenesis.

We adopted a genetic approach to investigate the control of asymmetric division in the model plant species Arabidopsis thaliana and isolated a class of mutants which fail to establish correct polarity of the microspore. These gemini pollen mutants often divide symmetrically at pollen mitosis I and fail to produce functional sperm cells [Park et al., 1998;Twell et al., 1998]. GEMINI POLLEN genes encode proteins which play vital roles in the establishment or execution of cell polarity required for asymmetric division at pollen mitosis I [Twell et al., 2002].

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