The open stomata 1-like protein-coding sequence encodes a sucrose non-fermenting 1-related protein kinase 2 (SnRK2)-type serine/threonine protein kinase. In Arabidopsis thaliana, OST1/SnRK2.6 is a central component of abscisic acid (ABA) signalling that regulates stomatal closure. In barley, the OST1-like kinases HvSnRK2.7 and HvSnRK2.9 contribute to stomatal regulation and guard-cell responses to ABA, changes in vapour-pressure deficit and water loss. In guard cells, OST1/SnRK2-type kinases phosphorylate downstream signalling proteins and ion channels, promoting anion efflux, membrane depolarization, reduced guard-cell turgor and stomatal closure. These kinases therefore participate in molecular pathways regulating stomatal aperture, plant water loss and responses to drought and other environmental stresses.