Pollen tubes: getting the sperm to the egg

Scanning electon micrograph of pollen grains. Image by Carsten Pietzsch distributed under a CC SA-BY 3.0 licence

Scanning electon micrograph of pollen grains. Image by Carsten Pietzsch distributed under a CC SA-BY 3.0 licence

Pollen. Enemy of hayfever sufferers everywhere, including myself. However, as a plant scientist I can (almost) forgive pollen for making me feel unwell because I find the role it plays in plant sexual reproduction fascinating.

A typical pollen grain from a flowering plant (angiosperm) contains a vegetative cell and a reproductive cell containing two nuclei. One of the nuclei from the reproductive cell splits to form two sperm cells. The group of cells is surrounded by a cellulose rich cell wall and a chemically resistant outer cell wall made mostly of sporopollenin. Within the pollen grain the sperm cells are protected from the environment (e.g. UV radiation or drying out) during the transfer from the anther (where pollen is produced) to the stigma, the entry point to the female part of the flower.

423px-Mature_Flower_Diagram_svg

Image credit: Lady of Hats.

Transfer of pollen between flowers is usually mediated by the wind or insect pollinators (e.g. bees, beetles). However, once pollen reaches the stigma there is a problem. The female gametes (ovules or egg cells) are found below the style of the flower in the ovary, surrounded by plant tissue and there is no obvious route for the sperm cells to reach the egg.

Solution: the pollen grain swells by absorbing water and produces a pollen tube to transport the sperm cells down the style to ovary. Below is a video of pollen tube formation in vitro (in a petri dish).

Pollen tubes grow rapidly with lily (Lilium longiflorum) pollen tubes able to grow at an impressive 200–300 nm/sec in vitro  (1). The growing pollen tube has to be able to respond to environmental cues from the maternal flower tissue to ensure it is growing in the right direction to reach an egg cell.

To grow the pollen tube needs to produce more cell membrane components and transport them to the tip of the pollen tube. It’s very easy to think of cell membranes as static but in fact they are very dynamic.  Vesicles (membrane-bound organelles) fuse to the membrane at the tip of the cell to deposit new membrane  (exocytosis) but at the same time existing membrane is being removed by endocytosis (vesicles made from the membrane) for recycling. This process ensures that the various membrane proteins are regularly replaced with freshly produced ones to maintain good function. During periods of tip growth the rate of exocytosis at the tip of the pollen tube exceeds that of endocytosis so there is a net deposition of membrane.

The video below shows the streaming of vesicles in the cytoplasm of the pollen tube towards the tip along the edges of the tube and fusing at the tip, and the formation of vesicles that then travel away from the tip along the centre of the tube.

Imaging of pollen tubes using calcium reporters (fluorescent dyes or proteins such as Cameleon) has revealed that there are oscillations of calcium ions at the tip of growing pollen tubes.

These oscillations are required for growth and are due to influxes of calcium ions into the tip of the pollen tube and then the action of calcium pumps returning the calcium ions to the outside. They are accompanied by fluctuations in other ions including protons and chloride ions. The ion fluxes are regulated by a family of Rho of Plants (ROP) GTPases (similar to the mammalian Rho GTPases), the master regulators of tip growth.

Once the pollen tube reaches the ovary the sperm cells are released to fertilise the egg cell and a second cell that contains two nuclei. The fertilised egg begins to develop into an embryo and the other cell remains as a triploid cell that becomes the endosperm (food source for the embryo). This is known as double fertilisation and is unique to flowering plants.

Video credits:

Pollen tube development: massalvaje

Petunia pollen tube growth with cytoplasmic streaming: Mortrek

Calcium oscillations in the Arabidopsis thalian pollen tube: Jose Feijo / Instituto Gulbenkian de Ciencia

References:

1) Cheung and Wu (2008) Structural and signaling networks for the polar cell growth machinery in pollen tubes. Annual Review of Plant Biology

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