Laboratory of Todd Vision
Department of Biology
University of North Carolina at Chapel Hill

Genomic Analysis of Water Use Efficiency

Water use efficiency (WUE) is a fundamental aspect of the physiology of terrestrial plants. When defined at the level of the leaf, it corresponds to the ratio of energy captured by photosynthesis per unit of water transpired. As the world's fresh water reserves decline, WUE has become a major focus of crop improvement efforts, particularly in the arid regions of the world, where up to 80% of fresh water supplies are already being diverted for human use. Unfortunately, it has proven to be very difficult to improve WUE in many crops without reducing yield at the same time. One way plants can exert control over WUE is by modulating the behavior of the stomates on the undersides of leaves. Open stomates allow CO₂ to diffuse in, which helps drive photosynthesis, but at the same time allow water vapor to escape. Thus, plants have to adjust their stomatal behavior depending on both internal and external conditions, such as gas pressures, temperature, light availability, and so on, but little is know about how this is achieved. By gaining a better understanding of the molecular and physiological control of this process, it may be possible to engineer improvements to WUE that do not have undesirable side-effects. As part of a multi-institutional collaboration involving plant physiologists and genomics researchers, we have been investigating the genetic basis for naturally-occuring variation in WUE tomato and rice. Our approach has been to map loci affecting the relative concentration of the two most common stable carbon isotopes in plant tissue. The relative concentrations provide a reliable proxy for WUE in plants with C3 metabolism, because on of the key enzymes in photosynthesis, Rubisco, discriminates against the heavier of the carbon isotopes. Having this relatively inexpensive and high-throughput proxy for WUE is the key that has allowed us to screen hundreds of plants and precisely map the genes underlying this trait. The genes thus discovered will serve as the key to unlock the control pathways that plants use to balance energy acquisition with water loss.