Stress is basic: ABA alkalinizes both the xylem sap and the cytosol of Arabidopsis vascular bundle sheath cellsby inhibiting their P-type H+-ATPaseand stimulating their V-type H+-ATPase

BACKGROUND AND HYPOTHESIS* Under water deprivation, in many perennial species, the stress hormone, ABA, appears in the xylem sap in the shoot (including leaf) veins and the xylem sap pH (pHEXT) increases. This study aimed to test the hypothesis that ABA is the signal for an altered proton balance of the leaf-vein-enwrapping bundle sheath cells (BSCs). METHODS* Plant Material. We used a few Arabidopsis thaliana (L.) Heynh. genotypes: wildtype (WT) of two accessions, Landsberg erecta (Ler) and Columbia (Col), and a few mutants and transformants in these backgrounds. * H+-Pumps activities. We monitored ABA effects on the H+-pump activities in the BSCs cytosol-delimiting membranes (plasma membrane and tonoplast) by monitoring the cytosol and the xylem pH, and the membrane potential (EM), by imaging the fluorescence of pH- and membrane potential (EM)-reporting probes: (a) the BSCs pHEXT - with the ratiometric fluorescent dye FITC-dextran petiole-fed into detached leaves in unbuffered xylem perfusion solution (XPS), (b) the BSCs pHCYT - with the ratiometric dye SNARF1 loaded into BSCs isolated protoplasts, and (c) the BSCs EM - with the ratiometric dye di- 8-ANEPPS. RESULTS* ABA increased the pHEXT; this response was abolished in an abi1-1 mutant with impaired signaling via a PP2C (ABI1) and in an aha2-4 mutant with knocked-down AHA2; * ABA depolarized the WT BSCs; * ABA increased pHCYT irrespective of AHA2 activity (i.e., whether or not AHA was inhibited by vanadate, or in the aha2-4 mutant); * The ABA-induced cytosol alkalinization was abolished in the absence of VHA activity (i.e., when VHA was inhibited by bafilomycin A1, or in the vha-a2 vha-a3 double mutant with inactive VHA); * All these results resemble the ABA effect on GCs; * In contrast to GCs, AHA2 and not AHA1 is the ABA major target in BSCs; * Blue light (BL) enabled the response of the BSCs VHA to ABA; * The ABA- and BL-signaling pathways acting on both BSCs pumps, AHA2 and VHA, are likely to be BSCs autonomous, based on (a) the presence in the BSCs of many genes of the ABA- and BL-signaling pathways and (b) ABA responses (depolarization and pHCYT elevation) demonstrated under BL in isolated protoplasts. SIGNIFICANCE STATEMENTWe reveal here an alkalinizing effect of the plant drought-stress hormone ABA on the pH on both sides of the plasmalemma of the vein-enwrapping bundle sheath cells (BSCs), due to ABA inhibition of the BSCs AHA2, the plasmalemma H+- ATPase and stimulation of VHA, their vacuolar H+-ATPase. Since pH affects the BSCs selective regulation of solute and water fluxes into the leaf, these H+- pumps may be attractive targets for manipulations aiming to improve plant drought response.