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(2). 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 Setaria 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 pCO(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.