One of the foremost challenges of the 21st century is to ensure food security for the world's population of over 7 billion people. Plants use the process of photosynthesis to harness the energy of sunlight to convert carbon dioxide into food and fiber that helps feed and clothe the world's population. The enzyme responsible for this process (RuBisCO) incorporates carbon dioxide into sugars, but it also incorporates oxygen into a product (glycolate) that cannot be used by the plant and therefore must be broken down and recycled; it is estimated that up to 30% of the light energy from the sun that is captured by a plant is thereby wasted. Carbon dioxide and oxygen compete for the binding site on the enzyme RuBisCO thus one way to increase plant productivity would be to raise the concentration of carbon dioxide relative to that of oxygen in the plant cell. In this collaborative research program engaging teams of researchers from the US and the United Kingdom, this goal will be addressed by engineering a biological pump, driven by sunlight, that will transport atmospheric carbon dioxide into the plant cell, concentrating and storing it there for use in photosynthesis. This project engages a large interdisciplinary team of plant biologists, biochemists, biophysicists, protein designers, chemical engineers, and mathematical modelers. This project will provide educational and training opportunities for undergraduate students, graduate students and postdoctoral researchers, all working in a highly cooperative, international scientific context. There are also provisions for public outreach, including publications, public talks, and websites geared toward the general public.
Increasing the amount of carbon dioxide available for fixation in photosynthesis will be achieved with a light-driven bicarbonate pump and a scaffold to retain the carbon dioxide until it can be fixed by ribulosebisphosphate carboxylase/oxygenase (RuBisCo). This will be accomplished by introducing into chloroplasts and cyanobacteria the proteins halorhodopsin which uses light to pump chloride ions, and AE1, a chloride/bicarbonate exchanger, to achieve net light-driven bicarbonate transport. In a parallel strategy, halorhodopsin will be modified to transport bicarbonate directly as well as to utilize light outside the visible spectrum. The carbon dioxide released in the chloroplast by carbonic anhydrase needs to be retained long enough to react with RuBisCO. To accomplish this, molecular scaffolds will be designed for the delivery of carbon dioxide to RuBisCO, including a carbon dioxide sponge and an engineered reverse C4 pathway. Mathematical modeling will link theory with experiment in both the transport and scaffolding efforts. The approach of using a light-driven bicarbonate pump and a carbon dioxide scaffold/sponge has the potential to raise the partial pressure of carbon dioxide by up to 60% inside the chloroplast, thereby allowing a large increase in the ratio of carbon dioxide to oxygen fixed by RuBisCO. The consequence to the plant should be a significantly higher photosynthetic productivity in the laboratory as well as in the field.
This award is supported jointly by the Cellular Dynamics and Function Cluster in the Division of Molecular and Cellular Biosciences and by the Biotechnology, Biochemical and Biomass Engineering Program in the Division of Chemical, Bioengineering, Environmental and Transport Systems.
