C4 photosynthesis is a system that uses resources present in C3 plants. C4 photosynthesis has evolved numerous times, in widely separated phylogenetic groups. All existing species that are able to fix carbon dioxide through one of the variety of C4 schemes appear to rely on enzymatic activities and other factors present in most or all plant species, but regulated to exhibit an extreme intercellular or intracellular compartmentalization that supports the delivery of CO2-derived carbon to Rubisco in an environment that disfavors competition from oxygen. C4 species are particularly numerous in certain subfamilies of grasses, suggesting that the resources required for C4 physiology are present and predisposed to this re-regulation. Rice lacks the dense leaf venation, bundle sheath (BS) differentiation, high BS plasmodesmatal density, and compartmentalization of photosynthetic activities that characterize nearly all C4 grass species. To what extent are these C4 resources already networked together in C3 grasses such as rice and how might we find the targets and means for engineering the re-regulation of this network? A systems biology approach that compares the development of cell types in rice leaves to those in C4 grasses could provide these targets and means. Emerging techniques such as laser microdissection of cell types and microarray profiling can provide the comprehensive data needed for a systems approach.
