A predator plant's chemical radar.

A predator plant's chemical radar

One of the world's most devastating grain-crop parasites is Striga asiatica, a red-flowered plant that siphons nutrients from the roots of corn, sorghum and other plants. It is the second leading cause of cereal famine in Africa. In hopes of learning how to discourage the parasite, David Lynn, Gwendolyn Fate and Christopher Smith at the University of Chicago have unearthed a chemical system that enables Striga to seek out and attack its hosts.

Biologists have known that Striga seeds will germinate only if they are within 5 millimeters of a sorghum root. How does the seed "know' when it's at the proper distance? Lynn's group isolated a hydroquinone compound that is exuded by the sorghum root and that makes Striga seeds germinate. This compound reacts with oxygen to form a another quinone, which does not induce Striga germination. The researchers believe that the rate at which hydroquinones diffuse into the soil away from the sorghum root, along with the rate at which the oxygen-hydroquinone reaction takes place, defines a specific zone--around 5 mm from the sorghum root--that contains sufficient hydroquinone levels to germinate Striga.

Lynn's group also discovered that the newly germinated seedling roots use a kind of chemical radar to determine when they should launch their balloon-like haustoria, the specialized organs that attach to the host root. The researchers report that a Striga root tip releases an enzyme that they think diffuses through the soil and reacts with a sorghum root to form another quinone compound, which in turn diffuses back toward the Striga root, where it triggers haustoria formation.