Researchers at The Rockefeller University have discovered that an experimental plant may harbor an additional line of defense against drought, once it has left the safety of its seed. The work suggests that a well-known plant hormone delays the growth of newly sprouted plants in order to give them one last chance to monitor their environment for signs of dryness before initiating growth. Furthermore, they have identified a specific protein as a key player in the process.
The findings, reported in the April 3 issue of the Proceedings of the National Academy of Sciences (Early Edition #14), are of immediate interest to agricultural and biotechnology industries, because crops could potentially be genetically modified to be more resistant to drought. Dry, salty lands in developing countries tend to dampen food productivity, hence tougher crops that are less sensitive to arid conditions might prove beneficial.
"Our work reveals a novel level of complexity in the early growth process and suggests that it may be possible to manipulate plants so that they can better cope with stressful conditions, such as dry or high salt soils," says Luis Lopez-Molina, Ph.D., a Rockefeller postdoctoral fellow and one of two lead authors of the paper.
Previously, it was believed that once a developing plant made the decision to break seed dormancy and germinate, it would continue to blossom, unchecked. This new research proposes a second checkpoint, so that, for example in the case of the weed Arabidopsis, the plant has an opportunity to hold-off on growth in case it is accidentally triggered to germinate by a cold summer’s night.
"You have a seed that’s asleep, but when it wakes up it looks around and asks: do I have enough water?"says Nam-Hai Chua, Ph.D., head of the Laboratory of Plant Molecular Biology at Rockefeller and co-author of the paper.
Arabidopsis, a well-studied weed in the mustard family, is a model system for the study of plant development because of a number of factors, including its small size and rapid generation time.
Lopez-Molina and Sebastien Mongrand, Ph.D., both postdoctoral fellows at Rockefeller show that ABA––a plant hormone known to inhibit germination––also arrests growth of newly germinated Arabidopsis plants for up to 30 days. Moreover, they provide evidence that ABA activates a recently isolated Arabidopsis protein called ABI5 and demonstrate that this protein is essential to the newborn plant’s ability to protect itself against drought during this developmental delay.
ABA plays a role in both germination in young plants and stress responses in adult plants. Its levels rise during germination, and it has been shown to have an inhibitory effect on growth. Furthermore, when adult plants are under environmental stress, such as drought, this hormone will induce the stomata-––a plant’s pores––to close. In essence, it prevents the plant from sweating so that it doesn’t lose precious water.
ABA’s ability to delay both germination and early growth was discovered when Lopez-Molina and Mongrand realized that seeds would in fact germinate after a certain period of time when grown in the presence of the hormone. They noticed, however, that the germinated plants did not green right away, and they later demonstrated that ABA could effectively block growth for up to 30 days. "One of the messages of this paper is that ABA delays germination, but is more efficient at keeping germinated embryos in a resting, protective state," says Lopez-Molina.
Because previous studies demonstrated that mutant Arabidopsis plants lacking the ABI5 protein grew without interruption after germination, the researchers wondered how this protein was linked to ABA’s ability to maintain arrested germinated embryos. To study more precisely the role of ABI5, they genetically-engineered strains of Arabidopsis to produce an excess of the protein and then observed the plants’ behavior. Newborn plants overproducing ABI5 were found to exhibit a second growth checkpoint only when ABA was present. Therefore, the researchers concluded, ABA must activate ABI5.
Next, Lopez-Molina and Mongrand showed that mutant strains lacking the ABI5 protein, when grown in the presence of ABA, had lower survival rates than their normal counterparts if faced with drought conditions. Whereas normal plants survived, on average, after 36 hours of drought treatment, mutants survived after only 12.
But perhaps the most intriguing finding of all was that adult Arabidopsis plants overproducing the ABI5 protein lost less water than average, implying that they were more resistant to drought. "A normal plant will lose water. A transgenic line overproducing ABI5 loses water less rapidly, probably because it is oversensitive to ABA," says Lopez-Molina.
Chua says that ABI5, in addition to being used to produce more durable plants, might also have applications in the seed priming business. A common problem in agriculture is that seeds sometimes germinate too early, before they are taken out of the bag. One solution, called "seed priming," is to induce the seeds to germinate just a little bit, then cut off their water supply, forcing them back into a state of rest. This way, when the time comes for the seeds to germinate, they will do so in a uniform fashion. ABI5 could offer a way to regulate this process more tightly.
Lopez-Molina was supported by the Swiss National Science Foundation and the Human Frontier Science Program Organization. Chua is the Andrew W. Mellon Professor.
John D. Rockefeller founded Rockefeller University in 1901 as The Rockefeller Institute for Medical Research. Rockefeller scientists have made significant achievements, including the discovery that DNA is the carrier of genetic information. The University has ties to 21 Nobel laureates, six of whom are on campus. Rockefeller University scientists have received the award in two consecutive years: neurobiologist Paul Greengard, Ph.D., in 2000 and cell biologist Günter Blobel, M.D., Ph.D., in 1999, both in physiology or medicine.
At present, 32 faculty are elected members of the U.S. National Academy of Sciences, including the president, Arnold J. Levine, Ph.D. Celebrating its centennial anniversary in 2001, Rockefeller — the nation’s first biomedical research center—continues to lead the field in both scientific inquiry and the development of tomorrow’s scientists.
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