Can scientists give rice a turbocharged photosynthesis?

Rice is naturally a C3 plant: it uses the older kind of photosynthesis, which loses energy to photorespiration in heat and strong light. Some of its cousins, such as maize, use the C4 pathway, which concentrates CO2 around the enzyme RuBisCO. The C4 Rice Project is a group of scientists from institutions around the world trying to give rice the C4 pathway, using maize and Brachypodium as guides.

The stakes are large. Rice is the world's most important human food, the staple food for more than half the planet. The team claims C4 rice could produce up to 50% more grain, with less water and nutrients. According to the project, the researchers have identified genes needed for C4 photosynthesis in rice and are working towards a prototype plant. Funding has followed: in 2012 the UK government and the Gates Foundation provided US$14 million, and in 2019 the Gates Foundation granted another US$15 million to the Oxford-led project, hoping for a prototype C4 rice strain and field plots in Taiwan by 2024. Our sources do not say whether those milestones were met, so C4 rice should be read as a goal still in progress, not a finished crop.

Why is it hard? C4 is not one gene but a whole piece of leaf engineering: most C4 plants have a special anatomy with rings of cells around the veins, and CO2 must be kept from leaking back out. That is why some scientists favour a simpler halfway route. C2 photosynthesis needs fewer steps of genetic engineering and performs better than C3 at all temperatures and light levels, though it is less optimized for high light and heat than C4.