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Abscisic acid (ABA) is a ubiquitousplant horone that regulates arious

aspects of plant groth and deelop-ent, including seed aturation anddorancy, and has a central role in

the adaptation of egetatie tissuesto enironental stresses, notably drought. Although any coponents

of the ABA signalling pathay areknon, the identity and signalling

echaniss of ABA receptors haereained elusie. No, six researchgroups report the echanis by 

hich the pyrabactin resistance(PYR)/PYR-like (PYL)/regulatory coponent of ABA receptors (RCAR)

faily of START proteins, preiously identified as ABA receptors, bindABA and positiely regulate ABA

signalling.Preious findings hae reported

that the Arabidopsis thaliana PYR/

PYL/RCAR proteins function as ABAreceptors that, folloing ABA binding,inhibit the actiity of knon negatie

regulators of ABA signalling — thetype 2C protein phosphatases (PP2Cs)

ABI1, ABI2, HAB1, HAB2 andPP2CA — in both seeds and egeta-

tie tissues. The suggested odelpredicts that, in the presence of ABA,PYR/PYL/RCARs bind to PP2Cs and

induce the release of SNRK2 Ser/Thrkinases fro PP2Cs, hich ouldotherise keep SNRK2s in an inactie

state. SNRK2s can then phosphorylatedonstrea substrates, including

ABA responsie eleent-bindingprotein (AREB) and ABA responsieeleent-binding factor (ABF)

bZIP transcription factors, hichactiate ABA-responsie genes andABA-related responses.

In vitro studies by Fuii et al. sho that cobining PYR1, ABI1,SNRK2.6 (also knon as SRK2E) and ABF2 is sufficient for ABA-triggered ABF2 phosphorylation.This indicates that these four proteins

are the core coponents of the ABAsignalling pathay.

The other fie groups carried out

structural analyses of PYL1, PYL2,PYR1 and PYR2 alone, bound to

ABA or in coplex ith both ABAand PP2Cs. The data reeal ho ABA

binding induces a conforationalchange that allos the receptors to sta-bly bind to PP2Cs. PYR and PYL hae

a seen-stranded cured β-sheet thatfors a central caity resebling thatof a folded hand. The surface of the

caity also coprises α-helices, hich,together ith the inner side of theβ-sheet, constitute the ABA-binding

site. To highly consered β-loops actas caity ‘lids’, hich, in the absence

of ABA, are in an open conforationthat allos ABA to access the caity.On ABA binding, the β-loops adopt aclosed-lid conforation. Iportantly,

this conforational change odifiesthe lid to create a binding site forPP2C. The receptor–PP2C interaction

results in the β-loops coering thePP2C actie site, thereby inhibiting itsactiity. This suggests that ABA recep-

tors function as PP2C inhibitors in anABA-dependent anner.

Understanding the structural

basis of ABA receptor interactionshas iportant iplications as it paes

the ay for the design of agonistolecules that could increase theresistance of crops to ater stress.

Kim Baumann

ORIGINAL RESEARCH PAPERS Fujii, H. et al.

In vitro reconstitution of an abscisic acid

signalling pathway. Nature 462, 660–664 (2009) |

Nishimura, N. et al. Structural mechanism of 

abscisic acid binding and signaling by dimeric

PYR1. Science326, 1373–1379 (2009) |

Miyazono, K. et al. Structural basis of abscisic acid

signalling. Nature 462, 609–614 (2009) | Melcher, K.

et al. A gate-latch-lock mechanism for hormone

signalling by abscisic acid receptors. Nature 462,

602–608 (2009) | Santiago, J. et al. The abscisic acidreceptor PYR1 in complex with abscisic acid.

Nature 462, 665–668 (2009) | Yin, P. et al. Structural

insights into the mechanism of abscisic acid

signaling by PYL proteins. Nature Struct. Mol. Biol.

16, 1230–1236 (2009)

S I G N A L L I N G

ABA’s atst hts

 ABA receptors

function

as PP2C

inhibitors

in an ABA-dependent

manner.

R e s e a R c h h i g h l i g h t s

NATURE REvIEwS | MoleculAr cell Biology  vOLUmE 11 | jANUARY 2010

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