Down the Tubes: Amino acid proves key to plant reproduction

Scientists have discovered that one of the myriad signals that human brain cells use to communicate also enables flowering plants to have sex. This versatile substance, an amino acid known as amino butyric acid or GABA, appears to help pollen grains form the sperm-carrying tubes that snake their way to a flowering plant’s eggs.

GABA’s new role came to light as Daphne Preuss, a Howard Hughes Medical Institute investigator at the University of Chicago, and her colleagues studied a sterile mutant strain of the mustard weed Arabidopsis thaliana, a small flowering plant that serves as a model of plant biology for many scientists.

In normal strains of the plant, pollen grains settle on a flower’s stigma and germinate, sending out tubes that burrow through various tissues before each tube makes its way to an ovule–one of many egg-bearing regions in a flower’s ovary. The tubes are “very precise and orderly. It’s a beautiful process,” says Preuss.

In the sterile strain, pollen tubes either stall in their journey or go astray. “They’re just meandering everywhere,” says Preuss.

In the July 11 Cell, she and her colleagues chronicle how they traced this oddity to a mutation in the gene for a protein belonging to a family of enzymes that modify amino acids. Curious about the enzyme’s normal target, they next analyzed the amino acid content of flowers from the mutant strain and found that GABA’s concentration was 100 times normal. The enzyme apparently speeds the degradation of GABA under normal conditions, so GABA builds up inside mutant plants that lack the enzyme.

While nerve cells in the human brain secrete GABA to convey signals to each other, the amino acid has many other roles in plants and animals. Preuss’ group, for example, showed that GABA spurs the growth of pollen tubes in petri dishes.

Subsequently, the investigators discovered that in normal plants, GABA concentrations increase along a pollen tube’s path to the ovule. In the mutant plants, however, pollen tubes are overstimulated by GABA signals and stop growing or head off in wrong directions, Preuss and her colleagues conclude.

“To find GABA cuing pollen tubes in plants is very interesting,” says Animesh Ray of the University of California, San Diego. He wonders how the leading edge of a tube senses differences in GABA concentrations and whether GABA itself is the guiding signal. A modified form of the amino acid or a metabolite of it might be the true signal, he says.

Preuss’ team was unable to show that pollen tubes growing in petri dishes are actually directed by GABA. “How it may be acting in guidance is still a mystery,” says Elizabeth Lord of the University of California, Riverside.

At a meeting later this month, Lord plans to present her research group’s discovery of a small protein that attracts lily pollen tubes growing in petri dishes. “It’s an exciting time for plant reproductive biology,” she says.

For pollen tubes, says Preuss, “it’s a long journey. I think it’s likely that they respond to multiple signals along the way.”

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