Ninety-five percent of the water plants lose exits through the stomata¿pores located on the leaves and stems. So knowing what controls a stomata's opening and closing could help scientists to engineer plants better able to deal with drought conditions. In today¿s issue of Nature, scientists at the University of California at San Diego explain how plants "tune in" to a chemical signal that allows them to close their stomata and thus hold on to more water.

Plant scientists have long suspected calcium as a key player in controlling the action of the stomata. The UCSD researchers pinpointed how it's done: specialized cells, called guard cells, surround each pore and govern the opening and closing of the stomata. "Plants are smarter than we thought they were," lead author Gethyn J. Allen says. "They can tune in to appropriate stimuli and tune out inappropriate stimuli."

The scientists compared normal laboratory plants with a mutant strain that is immune to hormones triggered by drought conditions. In the normal plants, high calcium concentrations in the guard cells caused the stomata to close for less than an hour. But when the oscillations of the calcium concentrations reach a specific frequency, the stomata closed for longer time periods. When the oscillations deviated from the target frequency, the stomata reopened within an hour. Under regular conditions, calcium fluctuations in the mutant plants were too fast to force the guard cells closed. But when the researchers artificially slowed the mutants¿ calcium concentration changes to the same frequency that closed the pores of the normal plants, the stomata of the mutant plants closed as well.

Julian I. Schroeder, a UCSD biologist who worked on the project, cautions that the findings are far from producing practical applications. "We don¿t know how to genetically engineer a plant to hit the right frequency to close its stomata in response to a drought," he says. "But understanding this calcium code means we can now learn more about the mechanisms that control a plant's resistance to drought conditions."