Engineering for drought avoidance: expression of maize NADP-malic enzyme in tobacco results in altered stomatal function

Water is a principal limitation to agricultural production during drought and in arid regions of the world. Mechanisms that plants use to cope with drought can be grouped into two different strategies: drought tolerance and drought avoidance. Previous efforts toward engineering plants for improved performance during drought have focused on drought tolerance, the ability to adjust to dry conditions. This report addresses the engineering of a drought-avoidance phenotype, which allows for the conservation of water during plant growth. The majority of water lost from plants occurs through stomata. When stomata are open, potassium, chloride and/or malate are present at high concentrations in guard cells. The accumulation of large numbers of ions during stomatal opening increases the turgor pressure of the guard cells, which results in increased pore size. Expression of a single gene from maize, NADP-malic enzyme (ME), which converts malate and NADP to pyruvate, NADPH, and CO2, resulted in altered stomatal behaviour and water relations in tobacco. The ME-transformed plants had decreased stomatal conductance and gained more fresh mass per unit water consumed than did the wild type, but they were similar to the wild type in their growth and rate of development. Providing chloride via the transpiration stream partially reversed the effects of ME expression on stomatal aperture size, which is consistent with the interpretation that expression of ME altered malate metabolism in guard cells. These results suggest a role for malic enzyme in the mechanism of stomatal closure, as well as a potential mechanism for genetically altering plant water use.