Expression of a CO-permeable aquaporin enhances mesophyll conductance in the C species .

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