Expression of a CO₂-permeable aquaporin enhances mesophyll conductance in the C₄speciesSetaria viridis

A fundamental limitation of photosynthetic carbon fixation is the availability of CO₄. In C₄plants, primary carboxylation occurs in mesophyll cytosol, and little is known about the role of CO₂diffusion in facilitating C₄photosynthesis. We have examined the expression, localization, and functional role of selected plasma membrane intrinsic aquaporins (PIPs) fromSetaria italica(foxtail millet) and discovered thatSiPIP2;7 isCO₂-permeable. When ectopically expressed in mesophyll cells ofS. viridis(green foxtail), SiPIP2;7 was localized to the plasma membrane and caused no marked changes in leaf biochemistry. Gas-exchange and C¹⁸O¹⁶O discrimination measurements revealed that targeted expression of SiPIP2;7 enhanced the conductance to CO₂diffusion from the intercellular airspace to the mesophyll cytosol. Our results demonstrate that mesophyll conductance limits C₄photosynthesis at lowpCO₂and that SiPIP2;7 is a functional CO₂permeable aquaporin that can improve CO₂diffusion at the airspace/mesophyll interface and enhance C₄photosynthesis.