The most prevalent protein on Earth, one that comprises as much as 50% of the nitrogen in plants, is Rubisco. This enzyme catalyzes the reaction of a sugar with either CO2 or O2 and thereby initiates, respectively, the photosynthetic CO2 assimilatory or photorespiratory pathways. The balance between the two reactions depends on the relative concentrations of CO2 and O2 in the atmosphere. At current atmospheric levels, photorespiration in C3 plants dissipates over 25% of the sugars synthesized during CO2 assimilation and, thus, has been viewed as a wasteful process. Under high CO2 conditions, photorespiration is inhibited and photosynthetic CO2 assimilation dominates.
Nitrate and ammonium are the major sources of nitrogen for plants. Previous work showed that plants require photorespiration to convert nitrate into proteins. Thus high CO2 concentrations, by inhibiting photorespiration, may also inhibit nitrate assimilation. This provides a new interpretation for plant responses to rising atmospheric CO2 concentrations. For example, sugar maple is one of the few hardwoods that is highly dependent on nitrate as a nitrogen source. The decline of sugar maples in the Northeast U.S. may derive from protein deprivation of these trees under rising CO2. Therefore, the balance between nitrate and ammonium usage will influence the primary productivity and distribution of many plant species.
The proposed research would examine the physiological mechanisms that link nitrate assimilation to photorespiration. It would determine if Rubisco serves another function: facilitating heretofore unexplained energy transfers in plant cells. It would quantify to what degree plants in nature use nitrate and ammonium as nitrogen sources. It would check the validity of current methods for following the flow of nitrogen through organisms. Such information will determine future research in the plant sciences as well as guide public policy on global climate change issues. The project will involve training of graduate students and a post-doctoral associate and results will be incorporated into the P.I.'s undergraduate teaching.
