Pollen Tube Primer

Origin of the pollen tube

The pollen tube is a cellular protuberance formed by a germinating pollen grain upon contact with a receptive stigma. It is therefore part of the male gametophyte and carries within its cytoplasm the generative cell, or, after its division, the sperm cells.

Biological function

The biological function of pollen tube elongation is building a tunnel transport system through the stylar transmitting tissue. Through this tunnel the sperm cells are delivered to the ovule to achieve fertilization. The speed of growth is a direct factor for selection as the law "first come first served" applies. The speed of pollen tube growth is therefore impressively fast with rates up to 1 cm/h.

Tip growth

The invasive nature pollen tube growth is reflected in the mechanism of cellular expansion. Unlikely diffusely growing cells that expand over their entire surface or large portions of it, cell wall expansion in pollen tubes is confined to the apex of the cell. This highly polarized mechanism is called "tip growth". Confining cell wall expansion to the tip minimizes friction between the tube and the invaded matrix. This way of life is shared by other invasive cell types such as root hairs, fungal hyphae, and, to a certain extent, neurons (Geitmann et al. 2001, Heath and Geitmann 2000).

Tropism

To find their target, pollen tubes must have the ability to find their way through the stigma and stylar transmitting tissue and into the ovary. Once close to an ovule the tube must turn to enter the micropyle and reach the embryo sac. This targeted growth requires communication between the pollen tube and the pistil and the ability of the pollen tube to respond to chemical signals and mechanical triggers by changing its growth direction (Geitmann and Palanivelu 2007).

In vitro culture

Very conveniently for the scientist, pollen are able to germinate in vitro and the morphology of the tube that is formed under these conditions is virtually indistinguishable from tubes growing in planta.
Because of its rapid growth rate and easy in vitro culture, the pollen tube is an excellent model system to study plant cell growth. We use various plant species for our experiments: lily, tobacco, camellia, poppy and Arabidopsis, the model species for plant molecular biologists (Bou Daher et al. 2009).

References

Bou Daher F, Chebli Y, Geitmann A. 2009. Optimization of conditions for germination of cold-stored Arabidopsis thaliana pollen. Plant Cell Reports 28: 347-457

Geitmann A, Palanivelu R. 2007. Fertilization requires communication: Signal generation and perception during pollen tube guidance, Floriculture and Ornamental Biotechnology 1:77-89

Geitmann A, Cresti M, Heath IB (eds). 2001. NATO Science Series: Cell biology of plant and fungal tip growth. IOS Press, Amsterdam

Heath IB, Geitmann A. 2000. Cell biology of plant and fungal tip growth - getting to the point. Plant Cell 12: 1513-1517

 

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