Abscisic Acid Signal off the STARTing Block Joshi-Saha Archana , Valon Christiane , Leung Jeffrey   Molecular Plant  Volume 4, Issue 4, Pages 562-580 (July 2011) DOI: 10.1093/mp/ssr055 Copyright © 2011 The Authors. All rights reserved. Terms and Conditions

Figure 1 Cartoon Depiction of the Receptor Highlighting the Movements of the Proline Gate, Leucine Latch, and the Recoil Region during ABA Reception. Top left: The ABA molecule (green ball), with its carboxyl and hydroxyl groups complexed with water molecules (blue balls), is nestled deep within the cavity of the receptor. When the gate and lock are closed onto the ABA molecule, S112 of the lock region of PYL1 becomes exposed to the solvent and docks with ABI1 through E148 and G180 of the PP2C. G180 is mutated into D in the abi1-1 dominant allele, giving rise to the ABA-insensitivity phenotype of the mutant; presumably, it escapes binding from the receptors. The binding by the receptor blocks access to the catalytic site of the PP2C. The carboxyl group of the ABA molecule is oriented towards K86 of PYL1 (top right) and the hexane ring of ABA secures the closed gate by hydrophobic interaction. W300 is the key residue in ABI1 because it interacts with both the ABA molecule and the receptor simultaneously. Bottom: (1) In unstressed conditions, the signaling pathway is muted by the negative regulation of the SnRK2s (with OST1 as the prototype example) by clade A PP2C (with ABI1 as the founding member). (2) The pathway is triggered by the binding of ABA to the receptor, which then sequesters the PP2C, allowing the downstream SnRK2s to activate or inactivate targets by phosphorylation, which now include SLAC1, KAT1, the NADPH oxidase AtrbohF and several b-ZIP transcription factors. Molecular Plant 2011 4, 562-580DOI: (10.1093/mp/ssr055) Copyright © 2011 The Authors. All rights reserved. Terms and Conditions

Figure 2 Phylogenetic Relationship among the 10 SnRK2 in Arabidopsis thaliana Noted by Both of their Frequently Used Nomenclatures. They are either numbered from 2.1 to 2.10 or designated from A to J. These kinases are also divided into three subclasses. The AAPK and PKABA are homologs from Vicia faba and from wheat, respectively, and were the first kinases identified to have roles in ABA signaling. In Arabidopsis, the Class III SnRK2.2/D, SnRK2.3/I, and SnRK2.6/OST1/SRK2E are ABA-activated kinases and represent key positive regulators of ABA signaling. All three kinases, shown on the right by a cartoon representation, are characterized by a small motif called Domain II in the C-terminus, which is the binding site for HAB1, ABI1, and ABI2. Domain I is an ABA-independent activation domain in these kinases. Molecular Plant 2011 4, 562-580DOI: (10.1093/mp/ssr055) Copyright © 2011 The Authors. All rights reserved. Terms and Conditions