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.