PLANTS, unlike animals, have no nerves, so scientists have been jolted by the discovery this month that the tomato plant uses an electric signal to alert its defense system against grazing caterpillars.
A team of researchers from England and New Zealand reported in the Nov. 5 issue of Nature that when a leaf on a tomato seedling is chewed by insects it sends out electrical warning signals, alerting the rest of the plant to the danger. As undamaged leaves receive the signal, they begin producing defensive chemicals that make them difficult to digest.
"It's a very exciting result," said Dr. Keith Roberts, head of the cell biology department at the John Innes Institute in England and author of the commentary published along with the article. "It's really a conceptual breakthrough to think plants use electrical signals in the same way animals use them in nerve cells. It brings plants a little bit closer to animals."
Dr. David Wildon, a plant physiologist at the University of East Anglia, England, and one of the authors of the new study, said: "Everyone can see now that if you find a system in animals and you look for it in plants, it turns up. The more we look, the more similarities there are."
But the newly discovered signals are not entirely identical to nerve impulses in animals. Unlike animals, plants don't require the very fast nerve signals that allow an animal to escape from its predators or to catch its prey.
"They have all the time in the world," said Dr. Roberts. "And these signals are appropriately slow when compared with a nerve response. At about the same rate it would take half an hour to register that I'd stubbed my toe."
The new finding promises to shake up the field of plant cell communication, where the study of electrical signaling has long been shunned.
Dr. Barbara G. Pickard, professor of biology at Washington University and one of the few biologists studying electrical signaling in plants, said: "There's a great abyss in our understanding of these signals. But I believe this is all going to open up now. The electrical stuff is undoubtedly going to break and break big and lots of plants are going to be found to use electrical signaling."
Researchers say that most serious scientists turned away from the study of electrical signaling in plants because they found results could be difficult or impossible to reproduce. And to make matters worse, a spate of popular writing began tearing down the respectability of the field by associating electrical signaling with extrasensory perception and emotions in plants.
According to Dr. Wildon, "The field procured a sort of bad odor, and so it became a backwater."
Things got so bad that even by suggesting that an electrical message could be an important signal in plant cell communication a scientist could open himself to ridicule.
"Plant electrophysiology was squelched," said Dr. Pickard. "It was reduced to the level of spoon bending." According to Dr. Eric Davies, professor of biology at the University of Nebraska, electrical signaling in plants "became to botany what astrology is to astronomy."
Plant researchers became enamored instead with chemical messengers, or hormones. And though many of the hormones assumed to run the lives of plants continued to elude the best efforts of scientists trying to isolate them, researchers continued to ignore the possibility of electrical signaling. Some of these postulated hormones, including florigen, a chemical hypothesized to induce flowering, have been sought in vain for decades.
The tomato's defensive warning signal has proved equally elusive.
Twenty years ago Dr. Clarence Ryan, a molecular biologist at Washington State University and a leader in the study of plant defenses, showed, along with colleagues, that a tomato's defensive molecules, known as proteinase inhibitors, could be turned on by an insect chewing on a distant leaf. Researchers have since shown that when an insect eats these proteinase inhibitors, the defensive chemicals act by gumming up the insect's digestive enzymes, making it difficult or impossible for it to get the nutrition that it needs. Mysterious Warning Signal
While researchers learned a great deal about the many molecules associated with the production of a leaf's proteinase inhibitors, the nature of the initial warning signal that was traveling from leaf to leaf remained mysterious.
In this newest study, researchers showed that in every case that an electrical signal was allowed to pass out from a damaged leaf, leaves in other parts of the plant would begin producing defense molecules. When the electrical signal was blocked, no defense molecules were produced.
When the movement of hormones was blocked, however, the plant's defense mechanisms were unaffected.
Dr. Davies said the new study "is incredibly exciting work, as it opens up whole new possible explanations for many things which people have had no explanation for at all."
"People have been trying to force a hormones explanation on everything," he added, "but other people have begun finding as I have that you simply cannot use hormones to explain everything."
Curiously, carnivorous plants like venus flytraps and touch-sensitive plants have long been known to use electrical signals to quickly close or drop their leaves.
According to Dr. Davies, these dramatic behaviors should have suggested to botanists that plants were likely to be hotbeds of electrical activity.
"You either have to say electrical signal mechanisms arose out of nothing in 10 totally different kinds of plants," said Dr. Davies, "or that these are just obvious outward manifestations of a signaling system that all plants have. It's much more likely all plants have these systems and it's only rarely that they tie them to obviously massive responses like shifting their leaves." The Hydra Example
Researchers still don't know exactly which plant cells are propagating the electrical signal, also known as an action potential, or how the wounding actually turns the electrical messaging on. But they suggest that the signaling in tomatoes is probably most akin to the non-nerve signaling found in lower animals, like jellyfish and hydra.
These animals send electrical signals through non-nerve tissues, propagating the message from cell to cell using channels in their membranes known as gap junctions. Researchers suggest that plant cells may be making use of similar channels known as plasmodesmata, which connect their membranes. While structurally the two kinds of channels are quite different, plasmodesmata are now being found to function similarly to gap junctions, with electrical signaling just the latest in the list of possible shared functions.
According to Dr. David Spray, professor of neuroscience at Albert Einstein College of Medicine, the importance of electrical signaling through non-nerve cells has been ignored in animals, as it has in plants. But because most animal and plant cells are connected to one another via these channels, he suggested that this sort of signaling could turn out to be very important.
Researchers interested in electrical signaling say they hope that the remarkable new finding may encourage other scientists to explore the field that has now been opened.
"They have all the time in the world," said Dr. Roberts. "And these signals are appropriately slow when compared with a nerve response. At about the same rate it would take half an hour to register that I'd stubbed my toe."
The new finding promises to shake up the field of plant cell communication, where the study of electrical signaling has long been shunned.
Dr. Barbara G. Pickard, professor of biology at Washington University and one of the few biologists studying electrical signaling in plants, said: "There's a great abyss in our understanding of these signals. But I believe this is all going to open up now. The electrical stuff is undoubtedly going to break and break big and lots of plants are going to be found to use electrical signaling."
Researchers say that most serious scientists turned away from the study of electrical signaling in plants because they found results could be difficult or impossible to reproduce. And to make matters worse, a spate of popular writing began tearing down the respectability of the field by associating electrical signaling with extrasensory perception and emotions in plants.
According to Dr. Wildon, "The field procured a sort of bad odor, and so it became a backwater."
Things got so bad that even by suggesting that an electrical message could be an important signal in plant cell communication a scientist could open himself to ridicule.
"Plant electrophysiology was squelched," said Dr. Pickard. "It was reduced to the level of spoon bending." According to Dr. Eric Davies, professor of biology at the University of Nebraska, electrical signaling in plants "became to botany what astrology is to astronomy."
Plant researchers became enamored instead with chemical messengers, or hormones. And though many of the hormones assumed to run the lives of plants continued to elude the best efforts of scientists trying to isolate them, researchers continued to ignore the possibility of electrical signaling. Some of these postulated hormones, including florigen, a chemical hypothesized to induce flowering, have been sought in vain for decades.
The tomato's defensive warning signal has proved equally elusive.
Twenty years ago Dr. Clarence Ryan, a molecular biologist at Washington State University and a leader in the study of plant defenses, showed, along with colleagues, that a tomato's defensive molecules, known as proteinase inhibitors, could be turned on by an insect chewing on a distant leaf. Researchers have since shown that when an insect eats these proteinase inhibitors, the defensive chemicals act by gumming up the insect's digestive enzymes, making it difficult or impossible for it to get the nutrition that it needs. Mysterious Warning Signal
While researchers learned a great deal about the many molecules associated with the production of a leaf's proteinase inhibitors, the nature of the initial warning signal that was traveling from leaf to leaf remained mysterious.
In this newest study, researchers showed that in every case that an electrical signal was allowed to pass out from a damaged leaf, leaves in other parts of the plant would begin producing defense molecules. When the electrical signal was blocked, no defense molecules were produced.
When the movement of hormones was blocked, however, the plant's defense mechanisms were unaffected.
Dr. Davies said the new study "is incredibly exciting work, as it opens up whole new possible explanations for many things which people have had no explanation for at all."
"People have been trying to force a hormones explanation on everything," he added, "but other people have begun finding as I have that you simply cannot use hormones to explain everything."
Curiously, carnivorous plants like venus flytraps and touch-sensitive plants have long been known to use electrical signals to quickly close or drop their leaves.
According to Dr. Davies, these dramatic behaviors should have suggested to botanists that plants were likely to be hotbeds of electrical activity.
"You either have to say electrical signal mechanisms arose out of nothing in 10 totally different kinds of plants," said Dr. Davies, "or that these are just obvious outward manifestations of a signaling system that all plants have. It's much more likely all plants have these systems and it's only rarely that they tie them to obviously massive responses like shifting their leaves." The Hydra Example
Researchers still don't know exactly which plant cells are propagating the electrical signal, also known as an action potential, or how the wounding actually turns the electrical messaging on. But they suggest that the signaling in tomatoes is probably most akin to the non-nerve signaling found in lower animals, like jellyfish and hydra.
These animals send electrical signals through non-nerve tissues, propagating the message from cell to cell using channels in their membranes known as gap junctions. Researchers suggest that plant cells may be making use of similar channels known as plasmodesmata, which connect their membranes. While structurally the two kinds of channels are quite different, plasmodesmata are now being found to function similarly to gap junctions, with electrical signaling just the latest in the list of possible shared functions.
According to Dr. David Spray, professor of neuroscience at Albert Einstein College of Medicine, the importance of electrical signaling through non-nerve cells has been ignored in animals, as it has in plants. But because most animal and plant cells are connected to one another via these channels, he suggested that this sort of signaling could turn out to be very important.
Researchers interested in electrical signaling say they hope that the remarkable new finding may encourage other scientists to explore the field that has now been opened.
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