Adapting C4photosynthesis to atmospheric change and increasing productivity by elevating Rubisco content in Sorghum and Sugarcane

Meta-analyses and theory show that with rising atmospheric [CO 2 ], Rubisco has become the greatest limitation to light-saturated leaf CO 2 assimilation rates ( A sat ) in C 4 crops. So would transgenically increasing Rubisco increase A sat and result in increased productivity in the field? Here, we successfully overexpressed the Rubisco small subunit ( RbcS ) with Rubisco accumulation factor 1 ( Raf1 ) in both sorghum and sugarcane, resulting in significant increases in Rubisco content of 13-25% and up to 90% respectively. A sat increased 12-15% and Rubisco enzyme activity ∼40% in three independent transgenic events of both species. Sorghum plants also showed increased speeds of photosynthetic induction and decreased bundle sheath leakiness. These improvements translated into average increases of 15.5% in biomass in field-grown sorghum and a 37-81% increase in greenhouse-grown sugarcane. This suggests a potential opportunity to achieve substantial increases in productivity of this key economically important clade of C 4 crops, future proofing their value under global atmospheric change. Significance Statement The world is projected to need a 60% increase in food supply by 2050 (UN), and this must be achieved under conditions of global change without expanding onto yet more land. C 4 crops, while few in number, account for a large proportion of agricultural productivity. We reason that rising atmospheric [CO 2 ] has very recently made Rubisco, the enzyme used for all carbon fixation in plants, the greatest limitation to light saturated photosynthesis in C 4 crops. We demonstrate that transgenically increasing Rubisco content in sorghum and sugarcane, increases their photosynthetic efficiency and productivity, including in a field trial of sorghum. This shows a means to sustainably increase the productivity of this key group of crops.