C4 Photosynthesis CO₂ Capture — C Combinator / International Rice Research Institute

Biological model: Maize / Sugarcane

Company: C Combinator / International Rice Research Institute

Engineering C4 carbon-fixation pathways into C3 crops and industrial reactors

The challenge

Rice and other C3 crops lose up to 25 percent of photosynthetic output to photorespiration, reducing yields and increasing water and nitrogen demand — a vulnerability that worsens as climate change raises growing temperatures.

Nature's strategy

C4 plants such as maize pre-concentrate CO₂ into bundle-sheath cells using a biochemical pump, suppressing photorespiration and achieving 50 percent greater carbon fixation efficiency under high-temperature conditions.

The innovation

The C4 Rice Project and C Combinator are engineering C4 photosynthetic architecture into rice and industrial bioreactors, targeting 50 percent yield increases and significant water and nitrogen efficiency gains for climate-resilient crop production.

Full case study

Rice feeds more than 3.5 billion people and is the primary calorie source for the world's poorest populations. It is also a C3 crop — it uses the ancestral three-carbon photosynthetic pathway that loses up to 25 percent of fixed carbon to photorespiration, a wasteful side reaction of Rubisco that increases at higher temperatures. As climate change raises average growing temperatures, C3 crop yields are projected to fall precisely as demand rises. C4 plants evolved a solution 60 million years ago. By pre-concentrating CO₂ into specialised bundle-sheath cells before delivering it to Rubisco, C4 plants suppress photorespiration almost entirely. The result is 50 percent higher water-use efficiency, 20 percent less nitrogen required per unit of biomass, and stable yields at temperatures where C3 crops decline. Maize, sugarcane, and sorghum are all C4 plants — all domesticated from tropical environments where the C4 advantage is most pronounced. The C4 Rice Project at the International Rice Research Institute and the synthetic biology startup C Combinator are pursuing the transfer of C4 photosynthetic architecture into rice. The project involves introducing the anatomical bundle-sheath cell structure, the biochemical carbon-concentrating mechanism, and the regulatory gene networks that orchestrate C4 carbon fixation — a multi-gene, multi-tissue engineering challenge that represents one of the most ambitious crop improvement programmes in history. Parallel work at C Combinator applies C4-inspired carbon concentrating mechanisms to industrial bioreactors, using engineered algae with enhanced Rubisco activity to capture atmospheric CO₂ at rates approaching the theoretical C4 optimum. If successful, C4-enhanced photosynthesis offers a pathway to food security under climate change while simultaneously providing the most energy-efficient known route to biological carbon capture.