The regulation of genes in C3 plants that have been co-opted into C4 photosynthesis, and implications for making a C4 rice

Placing the efficient C4 pathway into rice appears ambitious because it would involve modifications to biochemistry, leaf development, and cell biology. The biochemical modifications need to include high expression of genes encoding carbonic anhydrase, phosphoenolpyruvate carboxylase, malate dehydrogenase, and pyruvate orthophosphate dikinase in the mesophyll, while a decarboxylase and Rubisco are specifically needed in the bundle sheath. Alterations in leaf development required are increased venation, larger bundle sheath cells, and fewer mesophyll cells. Changes in cell biology include chloroplast proliferation and expansion in the bundle sheath, and increased plasmodesmatal connectivity between mesophyll and bundle sheath cells. Although these modifications appear complex, C3 species have the ability to accumulate proteins needed for C4 photosynthesis in defined cell types, and it also appears that they possess trans-factors needed for the expression of genes specifically in mesophyll or bundle sheath cells. When intact genes from a C4 species are placed in a closely related C3 plant, they are expressed in the correct cell type places in a closely related C3 plant, they are expressed in the correct cell type for C4 photosynthesis, but in more distantly related species this is less likely. It should therefore be possible to integrate enzymes needed for C4 photosynthesis into rice if genes are sourced from a closely related C4 plant. I propose a dual-track approach to the challenge of integrating C4 traits into rice. Fist, studies of rice leaf development are needed. Second, fundamental work is needed on C4 photosynthesis itself, and the species used should depend on the particular question being asked. The hypothesis that introducing the biochemistry of C4 photosynthesis into a C3 plant leads to leaf development associated with the C4 pathway should be tested. It would be fastest to do this by placing genes from Cleome gynandra into Arabidopsis thaliana. If this hypothesis is supported, a shortcut to the whole process of generating a C4 rice could be found. If the hypothesis is not supported, many phynotypes of C4 plants are shared, and so loci controlling these could be identified in systems other than rice and maize. For example, bundle sheath enlargement and increased plasmodesmatal connectivity should be investigated with A. thaliana and Cleome because resources and generation times are favorable.