C4 rice: early endeavours and models tested

Abstract

Plans to make a C4 rice plant date back to a document in 1987 and the first patent application for C4 rice submitted in 1991. In addition, an attempt to make a C4 rice plant was made in collaboration with Japan Tobacco Inc. during the 1990s. This collaboration recognized the importance of two compartments in C4 photosynthesis, normally provided by mesophyll and bundle sheath cells. However, a single-cell system was devised in which the endogenous compartments of the cytosol and the chloroplast of C3 plants were used to mimic the two C4 compartments. Phosphoenolpyruvate carboxykinase (PEPCK) was used as the C4 acid decarboxylating enzyme and was synthesized with a transit peptide to ensure location in the chloroplasts. The PEPCK gene from Urochloa panicoides was transferred to rice and was expressed successfully: carbon flow was altered toward a C4 pathway but without appreciable increases in photosynthesis or growth. The properties and location of enzymes postulated to be required to convert a C3 plant to a C4 plant (carbonic anhydrase, phosphoenolpyruvate carboxylase, PEPCK, and pyruvate, orthophosphate dikinase) are reviewed. Further modifications to maximize the efficiency of a C4 pathway in C3 plants are discussed.