Genes for a receptor that helps transmit nerve signals in animals have been found in, of all things, plants. While this doesn't mean you should talk to your broccoli, it does suggest that the receptor evolved long before plants and animals diverged. The finding, described in this week's Nature, may help neurobiologists screen for drugs that bind to the human version of this brain receptor. Such compounds might prevent overstimulation of the glutamate pathway, which appears to play a role in Alzheimer's and other brain diseases.
The glutamate receptor helps nerve impulses move from one neuron to the next. Glutamate released at the end of one cell binds to receptors on the adjacent cell, starting up a signal inside that cell. Once the message has been passed on, a neuron normally turns off its own response to glutamate. Some poisons, however, can bind to glutamate receptors and lead to toxic overstimulation of nerve cells. Plants contain several such toxins, which they may use to ward off insects.
Gloria Coruzzi and her colleagues at New York University and the Chinese University of Hong Kong suspected that these toxins may also be part of plant signaling system, and so they set out to see if plants had proteins like glutamate receptors. They surveyed the genome of the mustard plant Arabidopsis, finding two genes resembling the human glutamate receptor. Next they found similar genes in tobacco, peas, rice, and corn. To test the function of proteins from these genes, they grew mustard plants in a broth spiked with DNQX, a chemical that blocks the glutamate receptor. When exposed to light after a period of darkness, treated plants produced half as much chlorophyll as untreated plants. Also, the stems of DNQX-treated plants grew twice as fast as those of untreated plants--like seedlings pushing through the soil. Together, the results suggest that the receptor helps the plant respond to light, Coruzzi says.
Coruzzi hopes that these plant receptors could help identify new inhibitors of human glutamate receptors. If a plant treated with a potential drug exhibits low chlorophyll levels and fast stem growth, the compound might also inhibit glutamate overstimulation in the brain, she says. But other researchers, including Jim Buettner of Washington University in St. Louis, are waiting to see more biochemical proof of the plant receptors. "We need to know whether these genes form functional [receptors] and where these proteins are expressed," he says.