Plants capture the energy of sunlight to turn carbon dioxide and water initially into carbohydrates, and then eventually into food and fiber that feeds and clothes the world's population. A major problem with this process is that the protein that carries out the incorporation (or fixation) of carbon dioxide into carbohydrate, ribulose bisphosphate carboxylase/oxygenase (Rubisco), is inefficient. This is because this enzyme also fixes atmospheric oxygen into products that the plant must break down and recycle. These processes of fixation by Rubisco of carbon dioxide and oxygen are in competition, and the fixation of oxygen is so large that over 30% of the light energy captured by a plant is wasted in this process. This project seeks to maximize the amount of carbon dioxide (and minimize the amount of oxygen) fixed by Rubisco by increasing the concentration of carbon dioxide inside the cell, where Rubisco is located. This will be accomplished by introducing a light-driven carbon-dioxide pump into the plant cell membrane, to move carbon dioxide from the atmosphere into the cell. This pump will use infrared light, which is not required for photosynthesis, as its source of energy. In this project halorhodopsin will first move chloride ions into the cell, using the energy of light. The chloride ions will then move out of the cell and (through the action of a membrane protein called an antiporter) be exchanged for bicarbonate ions that move into the cell. The bicarbonate will then be decomposed (by an enzyme called anhydrase) into carbon dioxide and water, with the net result that carbon dioxide has been pumped into the cell by a light-driven process. In a second approach it is planned to re-engineer halorhodopsin to pump bicarbonate directly into the cell without using chloride ions as an intermediate.
Broader Impacts: Large increases in crop yields can be expected from the more productive fixation of carbon dioxide that is the goal of this project. The new technologies derived from this project will provide tools and knowledge to boost photosynthetic capacity that should lead rapidly to applications in agricultural and industrial systems. The project will also include intensive research training for undergraduate and graduate students and for postdoctoral fellows.
