ABA receptors: the START of a new paradigm in phytohormone signalling - PubMed (original) (raw)

Review

. 2010 Jul;61(12):3199-210.

doi: 10.1093/jxb/erq151. Epub 2010 Jun 3.

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Review

ABA receptors: the START of a new paradigm in phytohormone signalling

John P Klingler et al. J Exp Bot. 2010 Jul.

Abstract

The phytohormone abscisic acid (ABA) plays a central role in plant development and in plant adaptation to both biotic and abiotic stressors. In recent years, knowledge of ABA metabolism and signal transduction has advanced rapidly to provide detailed glimpses of the hormone's activities at the molecular level. Despite this progress, many gaps in understanding have remained, particularly at the early stages of ABA perception by the plant cell. The search for an ABA receptor protein has produced multiple candidates, including GCR2, GTG1, and GTG2, and CHLH. In addition to these candidates, in 2009 several research groups converged on a novel family of Arabidopsis proteins that bind ABA, and thereby interact directly with a class of protein phosphatases that are well known as critical players in ABA signal transduction. The PYR/PYL/RCAR receptor family is homologous to the Bet v 1-fold and START domain proteins. It consists of 14 members, nearly all of which appear capable of participating in an ABA receptor-signal complex that responds to the hormone by activating the transcription of ABA-responsive genes. Evidence is provided here that PYR/PYL/RCAR receptors can also drive the phosphorylation of the slow anion channel SLAC1 to provide a fast and timely response to the ABA signal. Crystallographic studies have vividly shown the mechanics of ABA binding to PYR/PYL/RCAR receptors, presenting a model that bears some resemblance to the binding of gibberellins to GID1 receptors. Since this ABA receptor family is highly conserved in crop species, its discovery is likely to usher a new wave of progress in the elucidation and manipulation of plant stress responses in agricultural settings.

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Figures

Fig. 1.

Fig. 1.

Schematic view of the ABA signal–receptor complex, including a PYR/PYL/RCAR homodimer (A) and hormone-bound protomer (B); a PP2C phosphatase, with the active site indicated in dark red; a SnRK2 kinase; and the ABF2 transcription factor. In the absence of ABA (A), the receptor forms a homodimer, while the PP2C inhibits both autophosphorylation of the SnRK2 and phosphorylation of ABF2. In the presence of ABA (B), a receptor protomer engulfs the hormone within a pocket, allowing the receptor to bind the PP2C and cover the phosphatase active site. This permits the autophosphorylation of the SnRK2 and phosphorylation of its ABF2 substrate. In its phosphorylated, active state (C), ABF2 binds to an ABA-responsive element (ABRE) in the promoter of ABA-responsive genes, activating transcription.

Fig. 2.

Fig. 2.

Regulation of SLAC1 phosphorylation status by the ABA-dependent PYR1, ABI1, SnRK2.6 signalling cascade. GST-SLAC1 N-terminal fragment (SLAC N, Met1 to Phe188) was incubated in the presence of [γ-32P]ATP with MBP-SnRK2.6 pretreated without (–) or with (+) GST-ABI1, His-PYR1, and 100 μM (±)-ABA. Bands of SLAC N fragment and MBP–SnRK2.6 are indicated by an arrow and an arrowhead, respectively. Coomassie-stained SDS-PAGE (bottom) and autoradiogram of the gel (top) are shown. Recombinant proteins and reaction conditions were as described previously (Fujii et al., 2009; Park et al., 2009). When SLAC N was incubated with SnRK2.6 not treated with GST-ABI1, both SnRK2.6 autophosphorylation and SLAC N phosphorylation bands were visualized (first lane). SnRK2.6 pre-treated with GST-ABI1 was unable to phosphorylate SLAC N (second lane). In the absence of ABA, the addition of His-PYR1 to the pretreatment of SnRK2.6 with GST-ABI could not restore SnRK2.6 phosphorylation activity on SLAC N (third lane). However, when ABA was added to the pretreatment reaction, SLAC N phosphorylation was recovered (fourth lane).

Fig. 3.

Fig. 3.

Amino acid sequence alignment of the Arabidopsis PYR1 protein with the most similar homologues in five cultivated species. The alignment was performed with the CLUSTALW2 program (

http://www.ebi.ac.uk/Tools/clustalw2/index.html

) using the default settings. Asterisks indicate residues in contact with ABA hormone, according to the PYL2 crystal structure of Melcher et al. (2009). The locations of the gate and latch domains are indicated.

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