Cross talk. Mycorrhizal fungi surrounding a plant root need a molecular signal from the plant to grow.

Fungi's Little Helper

John is a Science contributing correspondent.

Our leafy world would be impossible without the help of nutrient-producing soil fungi that live on plant roots. But just how the fungi find the roots has been a mystery. Now a long-sought molecule integral to the plant-fungi dialogue has been found, and it turns out others are listening in.

The majority of land plants rely on friendly fungi to extract the key nutrients nitrogen and phosphorous from the soil. These so-called micorrhizal fungi form a network of fibers surrounding the plants' roots where they receive carbohydrates in return for their service. Scientists can only grow the fungi in the lab if they add a pinch of ground-up plant root, so researchers have speculated that plants must make a compound that prompts the fungi to grow.

To find this needle in a haystack, a team led by Kohki Akiyama, a biochemist at Osaka University in Japan, started with an extract of the roots of lotus plants. Like players in a game of Clue, the researchers narrowed down the list of suspects. They chemically filtered the extract and tested whether it could still cause the fungi to branch, ruling out whole categories of molecules as they went. Finally, the team was able to finger a single molecule, known as strigolactone.

Interestingly, scientists originally identified the compound as a promoter of weed growth. This connection is "completely unexpected" says Akiyama, whose team reports its findings online 9 June in Nature. He speculates that during the course of evolution, parasitic weeds have tapped into the private "dialogue" between symbiotic fungi and their hosts to find the best places to steal nutrients from roots.

"This is a very exciting finding," says Maria Harrison, a plant pathologist at Cornell University in Ithaca, New York. She points out that the study "paves the way" for finding other molecules that play a role in the plant-fungus dialogue, allowing scientists to engineer plant-fungi partnerships that don't let the weeds in.

Related sites
The study
Harrison's site
All about mycorrhizal fungi

Posted in Biology