Overexpression of C4-cycle enzymes in transgenic C3 plants: A biotechnological approach to improve C3-photosynthesis

Abstract

The process of photorespiration diminishes the efficiency of CO2 assimilation and yield of C3-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 O2 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, C4 plants, such as maize, sugar cane and Sorghum, possess a CO2 concentrating mechanism, by which atmospheric CO2 is bound to C4-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 C4-like CO2 concentrating mechanism in only one cell type when CO2 becomes limiting. A single cell C4-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 C4-cycle in C3 plants by a transgenic approach. In the meantime, there has been substantial progress in overexpressing the key enzymes of the C4 cycle in rice, potato, and tobacco. In this review there will be a focus on biochemical and physiological consequences of the overexpression of C4-cycle genes in C3 plants. Bearing in mind that C4-cycle enzymes are also present in C3 plants, the pitfalls encountered when C3 metabolism is perturbed by the overexpression of individual C4 genes will also be discussed.