Strategies for engineering a two-celled C4 photosynthetic pathway into rice

Every day almost one billion people suffer from chronic hunger, and the situation is expected to deteriorate with a projected population growth to 9 billion worldwide by 2050. In order to provide adequate nutrition into the future, rice yields in Asia need to increase by 60%, a change that may be achieved by introduction of the C-4 photosynthetic cycle into rice. The international C-4 Rice Consortium was founded in order to test the feasibility of installing the C-4 engine into rice. This review provides an update on two of the many approaches employed by the C-4 Rice Consortium: namely, metabolic C-4 engineering and identification of determinants of leaf anatomy by mutant screens. The aim of the metabolic C-4 engineering approach is to generate a two-celled C-4 shuttle in rice by expressing the classical enzymes of the NADP-ME C-4 cycle in a cell-appropriate manner. The aim is also to restrict RuBisCO and glycine decarboxylase expression to the bundle sheath (BS) cells of rice in a C-4-like fashion by specifically down-regulating their expression in rice mesophyll (M) cells. In addition to the changes in biochemistry, two-celled C-4 species show a convergence in leaf anatomy that include increased vein density and reduced numbers of M cells between veins. By screening rice activation-tagged lines and loss-of-function sorghum mutants we endeavour to identify genes controlling these key traits.