Overexpression of the Barley Aquaporin HvPIP2;1 Increases Internal CO2 Conductance and CO2 Assimilation in the Leaves of Transgenic Rice Plants

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Abstract
The internal conductance for CO2 diffusion (gi) and CO2 assimilation rate were measured and the related anatomical characteristics were investigated in transgenic rice leaves that overexpressed barley aquaporin HvPIP2;1. This study was performed to test the hypothesis that aquaporin facilitates CO2 diffusion within leaves. The gi value was estimated for intact leaves by concurrent measurements of gas exchange and carbon isotope ratio. The leaves of the transgenic rice plants that expressed the highest levels of Aq-anti-HvPIP2;1 showed a 40% increase in gi as compared to gi in the leaves of wild-type rice plants. The increase in gi was accompanied by a 14% increase in CO2 assimilation rate and a 27% increase in stomatal conductance (gs). The transgenic plants that had low levels of Aq-anti-HvPIP2;1 showed decreases in gi and CO2 assimilation rate. In the plants with high levels of Aq-anti-HvPIP2;1, mesophyll cell size decreased and the cell walls of the epidermis and mesophyll cells thickened, indicating that the leaves had become xeromorphic. Although such anatomical changes could partially offset the increase in gi by the aquaporin, the increase in aquaporin content overcame such adverse effects.