Arabidopsis leaf hydraulic conductance is regulated by xylem sap pH, controlled, in turn, by a P-type H+-ATPase of vascular bundle sheath cells

The leaf vascular bundle sheath cells (BSCs) that tightly envelop the leaf veins, are a selective and dynamic barrier to xylem sap water and solutes radially entering the mesophyll cells. Under normal conditions, xylem sap pH below 6 is presumably important for driving and regulating the transmembranal solute transport. Having discovered recently a differentially high expression of a BSC proton pump, AHA2, we now test the hypothesis that it regulates the xylem sap pH and leaf radial water fluxes. We monitored the xylem sap pH in the veins of detached leaves of wild-type Arabidopsis, AHA mutants and aha2 mutants complemented with AHA2 gene solely in BSCs. We tested an AHA inhibitor (vanadate) and stimulator (fusicoccin), and different pH buffers. We monitored their impact on the xylem sap pH and the leaf hydraulic conductance (K-leaf), and the effect of pH on the water osmotic permeability (P-f) of isolated BSCs protoplasts. We found that AHA2 is necessary for xylem sap acidification, and in turn, for elevating K-leaf. Conversely, AHA2 knockdown, which alkalinized the xylem sap, or, buffering its pH to 7.5, reduced K-leaf, and elevating external pH to 7.5 decreased the BSCs P-f. All these showed a causative link between AHA2 activity in BSCs and leaf radial hydraulic water conductance.

Automated summary

The study identified the crucial role of AHA2 protein in regulating xylem sap pH and leaf radial water fluxes within the leaf vascular bundle sheath cells. Manipulation of AHA2 activity significantly affected the acid-base balance of xylem sap and leaf hydraulic conductance, demonstrating a causal relationship between AHA2 activity and leaf radial hydraulic water conductance.