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ABA signaling in guard cells entails a dynamic protein-protein interaction relay from RCAR2 ABA receptor to SLAC1 ion channel
The plant hormone abscisic acid (ABA) serves as an integrator of environmental stress such as drought, to trigger stomatal closure by regulating specific ion channels in guard cells. We previously reported that SLAC1, an outward anion channel required for stomtal closure, was regulated via reversible protein phosphorylation events involving ABA signaling components including protein phosphatase 2C members and a SnRK2-type kinase (OST1). In this study, we reconstituted the ABA signaling pathway as a protein-protein interaction relay from the PYL/RCAR type receptors, to the PP2C-SnRK2 phosphatase-kinase pairs, to the ion channel SLAC1. The ABA receptors interact with and inhibit PP2C phosphatase activity against the SnRK2-type kinase, releasing active SnRK2 kinase to phosphorylate and activate the SLAC1 channel, leading to reduced guard cell turgor and stomatal closure. Both yeast-two hybrid and bi-molecular fluorescence complementation assays were used to verify the interactions among the components in the pathway. The biochemical assays demonstrated the activity modifications of phosphatases and kinases by their interaction partners. The SLAC1 channel activity was used as a readout for the strength of the signaling pathway depending on the presence of different combinations of signaling components.
