Bundle-sheath cells are internal leaf "water valves" controlled via xylem acidification by H+-ATPase

Bundle-sheath cells (BSCs), parenchymatous cells tightly enwrapping the leaf veins, constitute selective and dynamic barrier to solutes and water. The over-three-fold-higher abundance of the AHA2 (Arabidopsis H+-ATPase 2) transcript we reported recently in the Arabidopsis BSCs (compared to mesophyll cells) suggested AHA2 participation in this barrier function. Manipulating AHA2 activity in BSCs pharmacologically and genetically while monitoring the xylem sap pH by fluorescence imaging of FITC-dextran fed into veins of detached leaves demonstrated AHA2 necessity for low sap pH. AHA2 activity or sap acidification by buffers enhanced the hydraulic leaf conductance, while alkaline sap pH buffered or in AHA2 knockouts reduced it. These results resolve the decades-lasting enigma about the molecular origin of xylem sap acidification and demonstrate AHA2 importance for controlling leaf water influx.