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The phytohormone abscisic acid (ABA) coordinates plant responses to stressors such as drought, extreme temperature and high salinity, as well regulates non-stress responses including seed formation and maturation, seed and bud dormancy, root growth, leaf senescence, and transition between vegetative and reproductive growth. ABA, similarly like the others plant hormones, functions through a complex network of signaling pathway, where ABA signal perception by ABA receptors is the primary event that triggers downstream signaling cascades to induce the final physiological responses. Since 2006, several proteins have been identified as possible ABA receptors. These are the nuclear flowering-time protein FCA, the plastid-associated Mg-protoporphyrin IX chelatase H subunit (CHLH/GUN5), a membrane-bound GCR2, and two novel G-protein coupled receptors GTG1 and GTG2. However, the results obtained in these studies are contested or questioned by several researchers, and in one case lately retracted. The situation improved with the publication of the recent studies indicating that receptors for this hormone are a group of small soluble proteins referred to as Pyrabactin Resistance 1 (PYR1), or PYR- like (PYL), encoded in Arabidopsis thaliana by fourteen genes. Proteins of this family were found to bind ABA and inhibit the activity of specific protein phosphatase enzymes, the type 2C plant PP2Cs, which were previously implicated in the ABA responses. In the absence of ABA, the PP2Cs act as constitutive negative regulators of a family of protein kinases SnRK2s whose autophosphorylation is required for kinase activity towards downstream targets. When abscisic acid enters a plant cell, PYR1/PYL receptor forms a binary complex with hormone and then immediately binds to, and inhibits the PP2C. The inhibition of the negatively acting PP2Cs leads to the successful activation of SnRK2.2, SnRK2.3 and SnRK2.6, which phosphorylate the basic leucine zipper (bZIP) transcription factors called ABFs/AREBs inducing the expression of ABA-responsive genes. By determining the crystal structures of members of the receptor family, with and without bound abscisic acid and protein phosphatase, high-definition structural images of the receptor complex are now available. In the hormone-free form, the PYR1/PYL protein presents an open and accessible cavity with two flexible surface loops that guard the ABA-binding pocket. Binding of ABA to the receptor-protein initiates an allosteric transition of the gating loops that sequester ABA in the pocked. The protein phosphatase PP2C binds to the hydrophobic site on the gating loops, which interact closely with the active site of the phosphatase, blocking its ability to bind and dephosphorylate its substrate. A conserved tryptophan residue of the phosphatase inserts its side chain to the gating loops. In this respect, PP2C acts as a potent co-receptor to enhance the affinity of the hormone for its receptor.
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The Editorial Board
Andrzej Łukaszyk - przewodniczący, Zofia Bielańska-Osuchowska, Szczepan Biliński, Mieczysław Chorąży, Aleksander Koj, Włodzimierz Korochoda, Leszek Kuźnicki, Aleksandra Stojałowska, Lech Wojtczak

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