Photorespiration is an ineluctable process in today’s plants, which dissipates some of the energy produced by photosynthesis and releases CO2. It begins when the enzyme RuBisCO acts on oxygen instead of carbon dioxide and creates toxic side-products (2-phosphoglycolate – 2PG) that require a set of costly recycling reactions. Photorespiration uses up fixed carbon and wastes energy thus reducing CO2 assimilation efficiency and biomass yield by ~30%. As such, it represents a prime target for improving agricultural productivity. Generally, hot, dry conditions tend to cause more photorespiration—unless plants have special features to minimize the problem.
RuBP oxygenase-carboxylase is a key enzyme in photosynthesis. In the process of carbon fixation, RuBisCO incorporates CO2 into an organic molecule during the first stage of the Calvin cycle. Hence, almost every carbon in the food we consumed and most of the fossil energy resources were ultimately derived from RuBisCO’s activity. RuBisCO is so important to plants that is one of the most abundant protein on Earth . But RuBisCO also has a major flaw: the enzyme cannot fully distinguish between CO2 and molecular oxygen. Thus, oxygen competes with CO2 as starting material for RuBisCO ‘s activity, causing a large fraction of the energy produced by photosynthesis to be wasted during the Photorespiration.
Carbon fixation or сarbon assimilation is the process by which inorganic carbon (particularly in the form of CO2) is converted to organic compounds by living organisms. The organic compounds are then used to store energy (e.g. sugars) and as building blocks for other important biomolecules. The most prominent example of carbon fixation is photosynthesis, in particular thanks to a set of reactions called Calvin Benson cycle that convert CO2 and other compounds into glucose in the chloroplasts of plants and algae, and in the cyanobacteria by energy harnessed in advance from sun light.
The majority of plants (85%), including rice, wheat, soybeans and all trees, are C3 plants, which have no special features to combat photorespiration. They only use the Calvin Benson cycle for fixing the CO2 from the atmosphere. They have the disadvantage that in warm and dry conditions their photosynthetic efficiency suffers because of photorespiration.
C4 plants minimize photorespiration by separating initial CO2 fixation and the Calvin cycle in space, performing these steps in different cell types. This solution implies that the leaf anatomy is organized in specific cell compartments: the light-dependent reactions occur in the mesophyll cells (spongy tissue in the middle of the leaf) while the Calvin cycle occurs in special cells around the leaf veins, called bundle-sheath cells. In the latter, thanks to the active transfer of CO2, the environment has 10-120x more CO2 available, thus reducing RuBisCO’s activity on oxygen and the resulting photorespiration. The drawback to C4 photosynthesis is the extra energy needed to transfer molecules back and forth from different cell types. The C4 pathway is used in about 3%, percent of all vascular plants; some examples are crabgrass, sugarcane and corn. C4 plants are common in habitats that are hot but are less abundant in areas that are cooler. In hot conditions, the benefits of reduced photorespiration likely exceed the associated costs.
The design of alternative photorespiration routes to date is restricted to pathways that release CO2, leaving space of considerable further improvement. Another opportunity to reduce the inefficiencies of photorespiration is to take inspiration from the naturally occurring C4 metabolism, which serves as a carbon-pump that increases the CO2 concentrations near RuBisCO. Yet, attempts to include C4 metabolism into C3 crops have only met with limited success, primarily due to difficulties in changing leaf anatomy and the complex cellular regulatory networks. In G4C these shortcomings are overcome by focusing on the sunflower, which belongs to a tribe of plants with an innate capacity to evolve towards the C4 metabolism (C3-C4 intermediacy).
These plants have intermediate leaf anatomies that contain bundle sheath cells that are less distinct and developed than the C4 plants. These intermediates are characterized by their resistance to photorespiration so that they can operate in higher temperatures and dryer environments than C3 plants.
Often metabolism that minimizes the deleterious effects of photorespiration still leads to the release of CO2, thus decrease carbon fixation rate and yield, just like in natural photorespiration. In G4C we will develop a CO2-positive pump, that avoid releasing back the CO2, thus keeping it available for the plant metabolism.
