Improving Photosynthesis

Increasing the efficiency of converting absorbed light energy into biomass energy is critical for meeting future needs for food and fuel. Energy losses occur at several steps in photosynthetic energy conversion, resulting in an overall, theoretical maximum efficiency of converting total sunlight into biomass of ~5-6%. However, the actual efficiencies of crop plants are substantially less than the theoretical efficiencies, leaving considerable room for improvement.

Several strategies for improving photosynthesis have recently been proposed, such as reducing light-harvesting antenna size, introducing components of algal CO2-concentrating mechanisms, engineering of photorespiratory bypasses, and accelerating recovery from photoprotection (NPQ). We are participating in the Realizing Increased Photosynthetic Efficiency (RIPE) project (http://ripe.illinois.edu), which aims to increase the productivity of staple food crops in sub-Saharan Africa and southeast Asia.

We have successfully improved photosynthesis in a model crop by altering NPQ. Upon a transition from sun to shade, the relatively slow recovery from NPQ transiently depresses the quantum yield of CO2 assimilation. Thus, increasing the relaxation rate of NPQ has been highlighted as a potential strategy to improve crop photosynthetic efficiency and yield. In collaboration with the lab of Steve Long (University of Illinois), we showed that overexpression of three genes involved in NPQ resulted in faster kinetics of NPQ relaxation, higher quantum yield, higher gross CO2 assimilation in fluctuating light, and ~10-20% increased biomass productivity in the greenhouse and in replicated field trials, while maintaining wild-type NPQ amplitude in high light. This work clearly shows that genetic modification of photosynthesis, and specifically NPQ, can increase crop yield, and we hope to see similar yield increases in food crops such as rice, cowpea, and cassava.