Fixing Photosynthetic Inefficiencies

Plants convert sunlight into energy through photosynthesis. Photosynthesis uses the enzyme Rubisco and sunlight to turn carbon dioxide and water into sugars that fuel plant growth. Rubisco cannot reliably distinguish between CO₂ and O₂. It grabs O₂ instead of CO₂ about 20 % of the time, resulting in the production of plant-toxic by-products such as glycolate. These must be recycled through the process of photorespiration which happens at the cost of energy. To meet growing food demands, increasing the yield potential of plants has long been of major interest. Recently, improved photosynthetic efficiency gains the interest of scientists.

Helen W. Liu, University of Illinois, Urbana, IL, USA, and colleagues have engineered and tested three alternative photorespiratory pathways in field-grown tobacco. Using different sets of promoters and genes, they have engineered a more efficient photorespiratory pathway into tobacco as a model plant for crops while inhibiting the native pathway. The pathway increased both photosynthetic efficiency and vegetative biomass.

The most successful pathway used plant malate synthase and a green algal glycolate dehydrogenase. All enzymes were directed to the chloroplast. In addition, RNA interference (RNAi) was used to down-regulate a native chloroplast glycolate transporter in the photorespiratory pathway to limit the metabolite flux through the native pathway.

In greenhouse screens, the pathway increased biomass by 18 % without RNAi and 24 % with RNAi, which were consistent with changes in photorespiratory metabolism and higher photosynthetic rates. Field testing across two different growing seasons showed >25 % increase in biomass of the plants compared to wild type plants, and with RNAi productivity increased by >40 %. In addition, this pathway increased the light-use efficiency of photosynthesis by 17 % in the field. Currently, the team is translating their findings to soybean, cowpea, rice, potato, tomato, and eggplant.