In most C 4 species, the bundle sheath cells surrounding the vein are the site of refixation, but in some grasses an inner layer of mestome sheath cells with thick suberized cell walls play this role (either instead of or in addition to the bundle sheath cells). This cellular arrangement, which is referred to as Kranz anatomy, facilitates initial fixation of carbon dioxide in the outer mesophyll cells followed by decarboxylation and refixation by ribulose bisphosphate carboxylase/oxygenase (RuBisCo) in the CO 2-enriched environment of the inner sheath cells (reviewed in ). In most C 4 plants, photosynthetic reactions are compartmentalized between two cell types that are arranged in concentric wreaths around closely spaced veins.

The C 4 photosynthetic pathway evolved over 60 times independently in a diverse range of flowering plant species, with trajectories from C 3 to C 4 apparent in a number of lineages. This suite of traits reflects that seen in “proto-Kranz” species, and, as such, a key step toward engineering C 4 rice has been achieved. Increased organelle volume was accompanied by the accumulation of photosynthetic enzymes and by increased intercellular connections. Here we induced chloroplast and mitochondrial development in rice vascular sheath cells through constitutive expression of maize GOLDEN2-LIKE genes. The transition from C 3 to so-called “proto-Kranz” anatomy requires an increase in organelle volume in sheath cells surrounding leaf veins. One approach toward engineering C 4 rice is to recapitulate this trajectory, one of the first steps of which was a change in leaf anatomy. This is an ambitious endeavor however, the C 4 pathway evolved from C 3 on multiple independent occasions over the last 30 million years, and steps along the trajectory are evident in extant species. Because C 4 plants are higher yielding than C 3 plants, efforts are underway to introduce the C 4 pathway into the C 3 crop rice. The C 4 photosynthetic pathway accounts for ∼25% of primary productivity on the planet despite being used by only 3% of species.

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