Expression of a CO ₂ -permeable aquaporin enhances mesophyll conductance in the C ₄ species Setaria viridis

A fundamental limitation of photosynthetic carbon fixation is the availability of CO ₄ . In C ₄ plants, primary carboxylation occurs in mesophyll cytosol, and little is known about the role of CO ₂ diffusion in facilitating C ₄ photosynthesis. We have examined the expression, localization, and functional role of selected plasma membrane intrinsic aquaporins (PIPs) from Setaria italica (foxtail millet) and discovered that SiPIP2;7 is CO ₂ -permeable. When ectopically expressed in mesophyll cells of S. viridis (green foxtail), SiPIP2;7 was localized to the plasma membrane and caused no marked changes in leaf biochemistry. Gas-exchange and C ¹⁸ O ¹⁶ O discrimination measurements revealed that targeted expression of SiPIP2;7 enhanced the conductance to CO ₂ diffusion from the intercellular airspace to the mesophyll cytosol. Our results demonstrate that mesophyll conductance limits C ₄ photosynthesis at low p CO ₂ and that SiPIP2;7 is a functional CO ₂ permeable aquaporin that can improve CO ₂ diffusion at the airspace/mesophyll interface and enhance C ₄ photosynthesis.