A tale of two pumps: Blue light and ABA alter Arabidopsis leaf hydraulics via bundle sheath cells H+-pumps and channels

This study focuses on the cellular mechanism underlying the co-regulation of the leaf hydraulic conductance (Kleaf) by blue light (BL) and the stress hormone ABA in Arabidopsis thaliana. Our previous work has demonstrated that (1) Kleaf increased by BL signaling within the leaf bundle sheath cells (BSCs), which activated their plasmalemma (PM) H+-ATPase (AHA2), acidifying the xylem sap; (2) external acidification enhanced the BSCs Kleaf and their osmotic water permeability (Pf); (3) ABA decreased both Kleaf and Pf by reducing the BSCs PM aquaporins activity. We now show, using pH and EM (membrane potential) probes combined with H+-pumps inhibitors and manipulations of cytosolic and external Ca2+ concentrations ([Ca2+]CYT, [Ca2+]EXT, respectively), that, in the BSCs: (a) under BL, ABA inhibits AHA2, depolarizing the BSCs and alkalinizing the xylem sap, (b) ABA stimulates the BSCs vacuolar H+-ATPase (VHA), alkalinizing their cytosol; (c) each pump stimulation, AHA2 by BL and VHA by ABA, requires [Ca2+]CYT elevation. ABA-effect-mimicking conditions in patch-clamp experiments activate the BSCs K+-release channels (SKOR and/or GORK). ABA decreased the Kleaf of skor mutants less than WTs, while during water deprivation stress, skor plants transpired more and their leaves lost relatively less K+ than WT. This suggests a role for SKOR in water conservation under drought.