Overexpression of C-4-Cycle Enzymes in Transgenic C-3 Plants: A Biotechnological Approach to Improve C-3-Photosynthesis

The process of photorespiration diminishes the efficiency of CO2 assimilation and yield of C-3-crops such as wheat, rice, soybean or potato, which are important for feeding the growing world population. Photorespiration starts with the competitive inhibition of CO2 fixation by O-2 at the active site of ribulose-1,5-bisphosphate carboxylase/oxygenase (Rubisco) and can result in a loss of up to 50% of the CO2 fixed in ambient air. By contrast, C-4 plants, such as maize, sugar cane and Sorghum, possess a CO2 concentrating mechanism, by which atmospheric CO2 is bound to C-4-carbon compounds and shuttled from the mesophyll cells where the prefixation of bicarbonate occurs via phosphoenolpyruvate carboxylase (PEPC) into the gas-tight bundle-sheath cells, where the bound carbon is released again as CO2 and enters the Calvin cycle. However, the anatomical division into mesophyll and bundle-sheaths cells ('Kranz'-anatomy) appears not to be a prerequisite for the operation of a CO2 concentrating mechanism. Submerged aquatic macrophytes, for instance, can induce a C-4-like CO2 concentrating mechanism in only one cell type when CO2 becomes limiting. A single cell C-4-mechanism has also been reported recently for a terrestrial chenopod. For over 10 years researchers in laboratories around the world have attempted to improve photosynthesis and crop yield by introducing a single cell C-4-cycle in C-3 plants by a transgenic approach. In the meantime, there has been substantial progress in overexpressing the key enzymes of the C-4 cycle in rice, potato, and tobacco. In this review there will be a focus on biochemical and physiological consequences of the overexpression of C-4-cycle genes in C-3 plants. Bearing in mind that C-4-cycle enzymes are also present in C-3 plants, the pitfalls encountered when C-3 metabolism is perturbed by the overexpression of individual C-4 genes will also be discussed.