June 14, 2002 Scientists at the University of California, Riverside, studying plant tolerance of low oxygen stress (a condition that can be caused by flooding or poor soil irrigation and that can result in significant crop losses), report in the June 14th, 2002, issue of the journal Science that plants use a rheostat-like mechanism at the cellular level to balance the production of an enzyme (i.e., a kind of protein) with the consumption of stored carbohydrates. The enzyme, called "ADH" (alcohol dehydrogenase), is needed for plant survival when little oxygen is available. The research is likely to interest biotechnologists and has vast implications, particularly for agriculture.
"Plants vary in their ability to tolerate low oxygen conditions," said Julia Bailey-Serres, associate professor of genetics at UC Riverside and a coauthor of the paper. "In our study we focused on the thale cress weed and detected a molecular switch in many of the root cells. This switch, called 'Rop,' needs to be turned up and then down to initiate the proper physiological response to this stress of low oxygen."
"This is breakthrough research," said Airica Baxter-Burrell, graduate student in plant genetics at UC Riverside and lead author of the paper. "We have new information on the low-oxygen pathway in plant cells, affecting the way we study these cells. It is likely that the molecular switch we found in the thale cress weed is present in most plants. But is the switch working the same way in all of them? We are looking into that now. We feel confident that the same switching mechanism operates in at least rice and corn."
Bailey-Serres, whose laboratory has been studying the response of plants to flooding for 12 years, explained that in a flooding situation oxygen is limited; so the production of energy gets limited, too. In other words, a cellular energy crisis kicks in. When this happens, an organism can still produce a limited amount of energy through "glycolysis" (i.e., the metabolic breakdown of sugar molecules to generate energy). Plant cells that lack oxygen may increase the amount of stored carbohydrates used in order to produce energy via glycolysis, the end product of this process being ethanol. The enzyme ADH, which Bailey Serres's group has studied, catalyzes this final step in the ethanol production process.
"When a plant cell has oxygen, there is little ADH made by the cell," said Bailey-Serres. "But when the oxygen supply decreases, the cell needs to make this enzyme. Our group has long puzzled over why there isn't a direct correlation between the production of this beneficial enzyme and tolerance to flooding. Our research identified the molecular Rop switch, studied by coauthor Zhenbiao Yang's laboratory at UC Riverside, as being directly responsible for the production of the enzyme ADH. Plants that cannot turn up and then down the Rop switch in response to flooding die quickly. It is possible, too, that the Rop switch is involved in the response of plants to other stresses as well, such as a drought and pathogens."
In their paper, the Bailey-Serres group demonstrates that the Rop switch is activated in thale cress weed when oxygen levels are low. But simply turning the switch on is not enough, they report; it also needs to be controlled, much like a dimmer switch. When the switch is turned on, the cell makes hydrogen peroxide, which is not necessarily beneficial to the plant's overall health but which is needed, nonetheless, for the cell to trigger the production of the enzyme ADH.
Bailey-Serres explained that once the hydrogen peroxide is made and, presumably, when it reaches a certain level, it triggers a negative feedback loop in the form of a negative regulator, which then turns down the switch, ultimately decreasing the production of hydrogen peroxide. "This dimmer or rheostat-like mechanism exists in all plants," she said, "but it is likely that different plants differ in the speed with which the switching mechanism operates." As the figure below shows, the beneficial enzyme ADH is produced first, before the negative regulator.
"We can use biotechnology and traditional breeding more effectively to produce stress resistant plants once we understand the actual molecular mechanisms that govern stress tolerance," said Bailey-Serres. "So far we've found the molecular switch that has to be turned on so that the enzyme ADH is produced. But what exactly is the sensor that causes the switch to turn on in the first place? Our research is now focused on identifying this sensor."
The paper by Bailey-Serres and colleagues is the second paper in Science to emerge in less than a month from the newly founded Center for Plant Cell Biology at UC Riverside.
The Center for Plant Cell Biology attempts to answer significant outstanding questions in plant biology by integrating genomic, bioinformatic, cellular, molecular, biochemical, and genetic approaches. The center synergizes UC Riverside's existing strengths in botany and plant sciences, in part by providing an infrastructure that promotes interdisciplinary research and interaction among researchers. For more information, please visit http://www.cepceb.ucr.edu/.
The University of California, Riverside, founded in 1954, offers undergraduate and graduate education to nearly 15,000 students. It is a member of the 10-campus UC system, which is recognized as the largest public research university system in the world. The picturesque 1,200 acre UC Riverside campus is located at the foot of the Box Springs Mountains near downtown Riverside, California. For more information about UC Riverside, visit http://www.ucr.edu.
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