Summary-Analysis of a synthetic ABA agonist uncovers a new family of ABA binding proteins that control signal transduction by directly regulating the activity of type 2C protein phosphatases.-PP2Cs are vital phosphatases that play important roles in abscisic acid (ABA) signaling. Using chemical genetics, we previously identified a synthetic growth inhibitor called pyrabactin. Here we show that pyrabactin is a selective ABA agonist that acts through PYR1, the founding member of a family of START proteins called PYR/PYLs, which are necessary for both pyrabactin and ABA signaling in vivo. We show that ABA binds to PYR1, which in turn binds to and inhibits PP2Cs. We therefore suggest that PYR/PYLs are ABA-receptors that function at the apex of a negative regulatory pathway that controls ABA signaling by inhibiting PP2Cs. Our results

The phytohormone abscisic acid (ABA) regulates the expression of many genes in plants and plays critical roles in stress resistance, and growth and development1-7. Several proteins have been reported to function as ABA receptors8-13 and many more are known to be involved in ABA signaling3,4,14. However, the identities of ABA receptors remain controversial and the mechanism of signaling from perception to downstream gene expression is unclear15,16. Here we show that by combining the recently identified ABA receptor PYR1, with the protein phosphatase 2C ABI1, the serine/threonine protein kinase SnRK2.6/OST1, and the transcription factor ABF2/AREB1, we can reconstitute ABA-triggered phosphorylation of the transcription factor in vitro. Introduction of these four components into plant protoplasts results in ABA-responsive gene expression. The protoplast and test tube reconstitution assays were used to test the function of various members of the receptor, protein phosphatase, and kinase families. Our results suggest that the default state of the SnRK2 kinases is an autophosphorylated, active state and that the SnRK2 kinases are kept inactive by the PP2Cs through physical interaction and dephosphorylation. We found that in the presence of ABA, the PYR/PYL receptor proteins can disrupt the interaction between the SnRK2s and PP2Cs, thus preventing the PP2Cs-mediated dephosphorylation of the SnRK2s and resulting in the activation of the SnRK2 kinases. Our results reveal new insights into ABA signaling mechanisms and define a minimal set of core components of a complete major ABA signaling pathway.

Abscisic acid (ABA) is a ubiquitous hormone that regulates plant growth, development, and responses to environmental stresses. Its action is mediated by the PYR/PYL/RCAR family of START proteins, but it remains unclear how these receptors bind ABA and in turn, how hormone binding leads to inhibition of the downstream type 2C protein phosphatase (PP2C) effectors. Here we report crystal structures of apo and ABA-bound receptors as well as a ternary PYL2-ABA-PP2C complex. The apo receptors contain an open ligand-binding pocket flanked by a gate that closes in response to ABA via conformational changes in two highly conserved β-loops that serve as a gate and latch. Moreover, ABA-induced closure of the gate creates a surface that enables the receptor to dock into and competitively inhibit the PP2C active site. A conserved tryptophan in the PP2C inserts directly between the gate and latch, which functions to further lock the receptor in a closed conformation. Together, our results identify a conserved gate-latch-lock mechanism underlying ABA signaling.