Re-creation of a Key Step in the Evolutionary Switch from C3 to C4 Leaf Anatomy

The C-4 photosynthetic pathway accounts for similar to 25% of primary productivity on the planet despite being used by only 3% of species. Because C-4 plants are higher yielding than C-3 plants, efforts are under-way to introduce the C-4 pathway into the C-3 crop rice. 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. 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. The transition from C-3 to so-called proto-Kranz'' anatomy requires an increase in organelle volume in sheath cells surrounding leaf veins. Here we induced chloroplast and mitochondrial development in rice vascular sheath cells through constitutive expression of maize GOLDEN2-LIKE genes. Increased organelle volume was accompanied by the accumulation of photosynthetic enzymes and by increased intercellular connections. 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.