Leaf hydraulic maze; Differential effect of ABA on vascular bundle-sheath, palisade, and spongy mesophyll controlling hydraulic conductance

Leaf hydraulic conductance (Kleaf) facilitates water for transpiration, enabling the continuation of CO2 upsurge while maintaining water status. Nevertheless, radial water flow out-of-the-xylem and its regulation under well and stress conditions remain vague. We hypothesized that bundle-sheath and mesophyll cells play key roles in the radial flow of water out of the xylem and the regulation of that flow under optimal and stress conditions. We generated transgenic Arabidopsis plants that were insensitive to abscisic acid (ABA) exclusively in their bundle sheath (BSabi) or mesophyll (MCabi). BSabi plants showed greater Kleaf and transpiration under optimal conditions, with stomatal apertures and index and vein density like those of the wild type. MCabi exhibited even higher Kleaf and transpiration, as well as larger stomatal apertures, a higher stomatal index and greater vascular diameter and biomass, relative to the wild type. In response to xylem-fed ABA, both transgenic lines exhibited reduced Kleaf and transpiration, while their leaf water potentials remained unchanged. The greater osmotic water permeability (Pf) of the wild types’ spongy mesophyll was reduced by ABA; whereas the lower Pf of the palisade mesophyll was not affected by ABA. Genetically modified, ABA-insensitive spongy mesophyll cells had a low Pf; whereas ABA-insensitive bundle-sheath cells had a higher Pf. The flow over the symplastic water pathway was increased by ABA, but its contribution to Kleaf was negligible. High levels of ABA appear to regulate the Pf of bundle-sheath and spongy mesophyll cells, to control Kleaf. Under optimal conditions, ABA limits transpiration and Kleaf via physiological, morphological, and developmental modifications of bundle-sheath and mesophyll cells. The bundle sheath–spongy mesophyll pathway controls Kleaf and leaf water balance under both optimal and drought conditions.