The contribution of PIP2-type aquaporins to photosynthesis in response to increased vapour pressure deficit

Roles of three different plasma membrane aquaporins (PIPs) in leaf-level gas exchange of Arabidopsis thaliana were examined using single, double and triple knockout mutants and compared to the Columbia-0 wild type (WT) plants. Since multiple Arabidopsis PIPs are implicated in conducting carbon dioxide across membranes, we focused on identifying whether the examined isoforms affect photosynthesis, either mediated through the control of stomatal conductance to water vapour (gs) or mesophyll conductance of CO2 (gm) or a combination of both. In two separate studies, we grew Arabidopsis plants in a low humidity environment and under high humidity conditions. We found that the contribution of functional PIPs to gs was larger under conditions of low air humidity when the evaporative demand was high, whereas any effect of lacking PIP function was minimal under higher humidity conditions. The pip2;4 knockout mutants had 44% higher gs than the WT under low humidity conditions, which in turn resulted in an increased photosynthetic rate (Anet). AtPIP2;4 is thus likely to be involved in maintaining a positive water balance and high water use efficiency through mediation of transmembrane water flow. The lack of functional AtPIP2;5 on the other hand did not affect gs, but reduced gm indicating a possible role in regulating CO2 membrane permeability. This potential regulatory function was indeed confirmed by subsequent stopped flow measurements of yeast expressing AtPIP2;5.