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Plant Sex: Pollen Tubes Caught in the Act

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Courtship in flowering plants begins when a pollen grain lands on the stigma of a flower. Only the pollen from the same species sticks, while foreign pollen grains fall off. Sitting on the stigma, the pollen grain grows a tube, which carries the sperm all the way to where the eggs are.

Working with the mustard plant Arabidopsis thaliana, researchers have now identified a key molecular signal that lures the pollen tube toward the egg cells. The molecule—called gamma-aminobutyric acid, or GABA—also plays a role in the mammalian nervous system.

The finding could provide clues about similar biological processes, such as how nerve cells find each other, and could even shed light on how to repair spinal cord injuries.

The images below helped scientists at the University in Chicago, Illinois, identify how plants mate with their own species.

This image is taken shortly after pollen (red) was dusted onto a stigma (green). The pollen tubes, carrying the sperm nuclei, exit the pollen grains and invade the female tissue, en route to the ovary chambers and ovules (where the eggs are).
Inside the Arabidopsis ovary chamber, mutant pollen tubes (the red tubules) do not grow up to an ovule (upper right) because of excess GABA (gamma-aminobutyric acid).
Mutant pollen tubes fail to find the micropyle, which is the door into the ovule and egg cells.

Birgit Reinert

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Palanivelu, R., L. Brass, A.F. Edlund, and D. Preuss. Pollen tube growth and guidance is regulated by POP2, an Arabidopsis gene that controls GABA levels. Cell 114, 47-59 (July 11, 2003).

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