Rice (Oryza sativa L.) is the most important human food crop worldwide, yet the yield potential of rice needs to be increased by at least 50% by 2050 to support the burgeoning human population. This can only be achieved by improvements in rates of biomass production. Rice productivity could be radically improved by the introduction of C4 photosynthetic properties. C4 photosynthesis is characterized by a CO2 concentrating mechanism involving the coordination of metabolism in two cell types, the mesophyll and bundle sheath. It results in the elimination or substantial reduction in photorespiration and consequently an enhancement in the capacity and quantum yield of photosynthesis at high temperatures.

One of the key C4 properties is a high leaf vein density, considered to be a prerequisite to the evolution of the complete suite of C4 traits in plants. High leaf vein density is needed to ensure the optimal ratio of mesophyll and bundle sheath cells with close contact permitting the rapid exchange of photosynthates. This is achieved via ‘Kranz’ anatomy which typically shows a single or double layer of mesophyll cells enclosing bundle sheath cells in a concentric fashion. Bundle sheath cells in turn enclose the vascular tissue. This arrangement permits bundle sheath cells and mesophyll cells to occupy similar volumes within the C4 leaf, whereas the total mesophyll cell volume is greater in the C3 leaf.

So how do you allow rice to carry out C4 photosynthesis? By utilizing the plasticity of rice plants to cram more veins into the leaves:

Increasing Leaf Vein Density by Mutagenesis: Laying the Foundations for C4 Rice. (2014) PLoS ONE 9(4): e94947. doi:10.1371/journal.pone.0094947

A high leaf vein density is both an essential feature of C4 photosynthesis and a foundation trait to C4 evolution, ensuring the optimal proportion and proximity of mesophyll and bundle sheath cells for permitting the rapid exchange of photosynthates. Two rice mutant populations, a deletion mutant library with a cv. IR64 background (12,470 lines) and a T-DNA insertion mutant library with a cv. Tainung 67 background (10,830 lines), were screened for increases in vein density. A high throughput method with handheld microscopes was developed and its accuracy was supported by more rigorous microscopy analysis. Eight lines with significantly increased leaf vein densities were identified to be used as genetic stock for the global C4 Rice Consortium. The candidate population was shown to include both shared and independent mutations and so more than one gene controlled the high vein density phenotype. The high vein density trait was found to be linked to a narrow leaf width trait but the linkage was incomplete. The more genetically robust narrow leaf width trait was proposed to be used as a reliable phenotypic marker for finding high vein density variants in rice in future screens.