A lowly weed uses one of the same communications systems as does the human brain, scientists have found.
In the November 12 issue of the journal Nature, Dr. Gloria Coruzzi and co-workers at New York University report that they turned up the sequence for proteins called glutamate receptors in a gene hunt through the DNA of Arabidopsis, a plant that is frequently used in laboratory research.
In the human brain, the amino acid glutamate acts as a chemical messenger and carries out a host of important functions, playing a role in everything from acquiring and storing memories to possibly contributing to certain mental health ailments. Past research has detected signs of glutamate overload in the post-mortem brains of people with schizophrenia, and faulty glutamate signaling has also been linked to Alzheimer's disease.
Glutamate and other neurotransmitters are squirted out by nerve cells and exert their effects through protein molecules called receptors that are nestled within the outer layers of adjacent nerve cells. These receptors serve as sentries that permit the passage of only certain molecules.
"This opens up a new connection between plants and animals," said Coruzzi, whose research on plant glutamate receptors was funded by NIH's National Institute of General Medical Sciences.
Glutamate is not the only substance produced by plants that functions in the brain. Other plant-derived molecules that act in the brain include caffeine and cocaine. Until now, it was widely believed that the receptors for these molecules--which are essential for transmitting messages into the cell--were found only in animals. But Coruzzi's previous research with Arabidopsis led her to believe otherwise.
Several years ago, while studying the weed's metabolism, Coruzzi noted that the incorporation of nitrogen from the environment into glutamate and other related molecules was regulated by light. Acting on a hunch that the plant cells might have a light-activated glutamate "sensor," she embarked upon a search for such a receptor. Coruzzi and her collaborator at the Chinese University of Hong Kong, Dr. Hon-Ming Lam, initially fished fragments of the glutamate receptor genes out of the Arabidopsis genome by searching for gene sequences in the plant similar to the human glutamate receptor sequence, which was already known in part through efforts of the Human Genome Project.
Her team then showed that the plant receptors work similarly in Arabidopsis cells as do the human receptors in brain cells. When plants were grown in the presence of a compound called DNQX, which blocks human glutamate receptors, they grew noticeably longer stems and made less than half the normal amount of chlorophyll, the pigment that gives green plants their characteristic color. This discovery suggested to Coruzzi that inactivating glutamate receptors blocked the ability of a plant to respond to light.
Such effects in plants are interesting in their own right, but they're important for another reason, Coruzzi explained. What the results illustrate, she said, are that plants possess a signaling system for "brain" chemicals, which could enable researchers to use the plants as a model system to study how glutamate and its chemical cousins work inside the cell. What's more, the findings provide an interesting clue to evolutionary biologists about plant-produced neurotoxins: Might such compounds be part of a larger group of many such molecules that "naturally" communicate through the glutamate receptor, or through similar receptors?
The usual answer to the question of why plants make such classes of chemicals at all has been, "to defend themselves against herbivores," Coruzzi said. But the new data led her to offer yet another, more intriguing possibility. It may be, she suggested, that the glutamate receptor and other similar signaling systems are actually ancestral methods of simple communication, common to plants and animals alike. Selective environmental pressures--such as the hearty appetites of leaf-devouring forest dwellers--may have led to a higher production of these neurotoxins for defense against predators more recently in evolutionary time, she added. For example, cycads, which are the most primitive living seed plants on Earth, produce a potent glutamate receptor neurotoxin that has been linked to dementia in animals.
A yeast geneticist by training, Coruzzi appreciates the extraordinary value of using model systems such as Arabidopsis to study scientific topics that are extremely complex in animal models, such as fruit flies and worms. And inserting genes or removing them is easier in plants than in many laboratory organisms, she added.
"We might be able to use [these plants] as a screen for new drugs," Coruzzi said, suggesting that growing Arabidopsis in the presence of candidate drugs and simply keeping an eye out for longer stem growth may be a useful and cost-effective first pass at sifting through thousands of potentially therapeutic compounds.
Please mention support for this work from the National Institute of General Medical Sciences (NIGMS), a component of the National Institutes of Health that supports basic biomedical research. Please fax clips to
(301) 402-0224. After the embargo date, this release will be available online at http://www.nih.gov/nigms/news/releases/coruzzi/ .
Lam H-M, Chiu J, Hsieh M-H, Meisel L, Oliveira IC, Shin M, Coruzzi G. Glutamate Receptor Genes in Plants. Nature 1998; 396:125-26.
Dr. Gloria Coruzzi
Carroll and Milton Petrie Professor of Biology
Department of Biology
New York University
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For scientific perspective on this research, call the NIGMS Office of Communications and Public Liaison at (301) 496-7301 to interview Dr. James Anderson, program director, Division of Genetics and Developmental Biology.