OST1

OST1, also known as OPEN STOMATA 1 and functionally identified as SnRK2.6, is a serine/threonine protein kinase in the model plant Arabidopsis thaliana. It belongs to the SNF1-related protein kinase 2 (SnRK2) family and acts as a central positive regulator of abscisic acid (ABA) signaling in guard cells, coordinating drought responses by promoting stomatal closure.

In guard cells, ABA accumulation during drought triggers signaling through PYR/PYL/RCAR receptors, which inhibit clade A PP2C phosphatases such as ABI1 and ABI2. This inhibition relieves PP2C-mediated suppression of OST1, allowing OST1 to become active and phosphorylate downstream targets. A key substrate is the S-type anion channel SLAC1, whose phosphorylation promotes anion efflux, membrane depolarization, and subsequent potassium efflux, culminating in guard cell turgor loss and stomatal closure. OST1 is also thought to influence other ion channels and components of reactive oxygen species signaling, integrating multiple inputs to regulate stomatal movements.

Genetic analyses show ost1 loss-of-function mutants exhibit impaired ABA-induced stomatal closure, greater transpirational water loss, and increased sensitivity to drought, illustrating OST1’s essential role in ABA-mediated stress responses. Regulation of OST1 involves phosphorylation status and interactions within the ABA signaling network, with activity suppressed by PP2Cs in the absence of ABA and activated when ABA signaling inhibits these phosphatases.

OST1 orthologs are conserved across many plant species and are similarly implicated in ABA signaling and stress responses, making OST1 a focus for strategies aimed at improving crop drought tolerance.