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Increasing salinity of agricultural soils may ultimately limit the sustainability of food production in some areas of the world. Work from our laboratory and the labs of others demonstrates that mannitol, a six-carbon sugar alcohol, is important as a stress-related metabolite in some plants. Mannitol helps plants resist the damaging effects of stressful growth environments, such as drought, high soil salinity, and perhaps attack by microorganisms that cause plant diseases. In the long run, we hope to genetically engineer plants to produce and use mannitol for increased productivity and tolerance to environmental stresses. Basic information about how plants regulate those genes important to mannitol metabolism is of critical importance to this long-term goal. Our laboratory discovered an enzyme, mannitol dehydrogenase, that is the first critical biochemical step in mannitol use in vascular plants. Later, we cloned the gene for this enzyme. We discovered that hexose sugars “turn off” the expression of this gene. So, as long as adequate sugars are available for energy, maintenance, and growth, the production of the mannitolusing enzyme is repressed. After the sugars are gone, mannitol dehydrogenase is produced very rapidly, and this allows mannitol to be used metabolically. This type of gene regulation is ideally designed to help plants cells conserve mannitol as long as possible, which in turn allows the cells to retain stress tolerance as long as possible.