Overexpression of the barley aquaporin HvPIP2;1 increases internal CO ₂ conductance and CO₂ assimilation in the leaves of transgenic rice plants

The internal conductance for CO₂ diffusion (g_i) 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 g_i 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 g _i as compared to g_i in the leaves of wild-type rice plants. The increase in g_i was accompanied by a 14% increase in CO2 assimilation rate and a 27% increase in stomatal conductance (g_s). The transgenic plants that had low levels of Aq-anti-HvPIP2;1 showed decreases in g_i 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 g_i by the aquaporin, the increase in aquaporin content overcame such adverse effects.