Biologists Discover Protein's Impact On Plant-Water Balance

In response to drought, sunlight, and other stimuli, guard cells control the opening and closing of microscopic stomatal pores on leaves of plants through which the plant gives off water vapor and oxygen to the atmosphere and takes in carbon dioxide for photosynthesis. Guard cells thereby moderate the amount of water and carbon dioxide in the plants. Researchers Sarah M. Assmann, professor of biology at Penn State, and Xi-Qing Wang, a postdoctoral scholar at Penn State, along with collaborators at the University of North Carolina, discovered that by altering a specific protein in the guard cells those cells had less control over the amount of water lost by the plants through their stomatal pores. The research team's results will be published in the 15 June 2001 edition of Science.

"The potential agricultural significance is being able to regulate stomatal apertures," Assmann says. "From a farmer's perspective, finding a way to maximize photosynthesis and yield, and a way to minimize irrigation, which can be expensive, would be important."

In Arabidopsis thaliana, the researchers' model organism, also known as mouse-ear cress, the natural hormone abscisic acid (ABA) usually promotes water conservation under drought conditions by signaling guard cells to change their volume, thereby decreasing the size of the stomatal pores and limiting the amount of water lost by plants. The North Carolina researchers identified Arabidopsis plants in which a naturally occurring G-protein component within the guard cells, the G-protein alpha subunit, had been eliminated. The Penn State group analyzed those plants and found that they had lost a typical response to ABA regulation of the guard cells. As a result, opening of the stomatal pores was no longer inhibited by ABA and water loss from the plants was greater than in unaltered plants.

Animals, including humans, have approximately two dozen G proteins that fulfill fundamentally important roles by relaying signals for smell, taste, and vision. Plants have a much smaller family of G proteins that perform similar duties regarding response and signaling about their environment, and those proteins are not as well understood as their counterparts in animals and humans. With their findings, the researchers gained a better appreciation of the proteins' role in plants. Because the plant system has fewer G proteins, and therefore less "crosstalk" between G-protein pathways, its simplicity also may allow researchers to unravel universal questions regarding G-protein mechanisms more easily than in animal systems.

"These proteins have been of interest to me for a long time because of their fundamental importance for environmental responses," Assmann says. "Until recently, though, we did not have the genetic tools that allowed us to completely prevent the plants from expressing just one particular type of response. With that capability, we have been able to learn a great deal about what happens to the plants."

With the G-protein information as a starting point, researchers should be able to learn even more about environmental response and signaling in plants. In addition, because Hong Ma, associate professor of biology at Penn State and the first to identify plant G-protein genes, already has identified G-protein components in crop species such as corn and tomatoes, the research results present some interesting possibilities.

"We cannot state that the G proteins will do the same thing in agronomic species that they do in Arabidopsis, but it's a testable hypothesis and it's a reasonable hypothesis," Assmann says. "There are also many potential applications, such as controlling stomates to allow plants like feed corn to dry faster in the field before harvest. Conversely, if we could control the guard cells to increase plant water retention during the growing season and reduce the need for expensive irrigation, we also would improve the agroenviroment because irrigation increases the amount of salt left behind in the soil and saline soils are toxic to most crop plants."

Collaborators from North Carolina were: Alan Jones, professor of biology, and Hemayet Ullah, graduate student. This research was supported by the National Science Foundation and the U.S. Department of Agriculture.