View source for ABA Signaling Pathway

[[Image:ABA model 2 med.jpg|left]]

Abscisic acid (ABA,)[[Image:Abscisic acid.svg.png|thumb| Abscisic Acid]] is a plant hormone that regulates seed development, dormancy and germination, stomatal closure, and responses to drought stress, and its core signaling pathway has recently been identified.<ref>PMID:15012233</ref><ref>PMID:12045268</ref><ref>PMID:20192755</ref> In Arabidopsis, this pathway involves families of ABA receptors (PYR1/RCAR11, PYL1/RCAR12, PYL2/RCAR14, PYL3/RCAR13, PYL8/RCAR3, PYL9/RCAR1)<ref name = "Ma2009">PMID:19407143</ref><ref name = "Melcher2009">PMID:19898420</ref><ref name = "Park2009" >PMID:19407142</ref><ref name = Zhang2012>PMID:22579247</ref><ref>PMID:23370718</ref>, protein phosphatase 2Cs (ABI1, ABI2, HAB1, HAB2, PP2CA/AHG3)<ref name = "Ma2009"/><ref name = "Park2009"/><ref name = Santiago 2009">PMID:19624469</ref><ref>PMID:16339800</ref>, and SNRK2 protein kinases (SnRK2.6/OST1/SRK2E, SNRK2.2/SRK2D and SnRK2.3/SRK2IA)<ref name = "Mustilli2002">PMID:12468729</ref><ref>PMID:12514244</ref><ref name = "Nakashima2009">PMID:19541597</ref><ref name = "Fujii2007">PMID:17307925</ref>. 

The core ABA signaling pathway is shown in the figure to the left. In unstimulated cells, the ABA receptor (R) is an unliganded dimer<ref name = "Melcher2009"/><ref>PMID:19933100</ref><ref>PMID:23265948</ref><ref name = "Zhang2012"/> (monomer shown) in the cytosol and nucleus, and the SNRK2 protein kinase (K) is bound to a protein phosphatase 2C (P) in a complex (K-P) in which the kinase is dephosphorylated and inactivated by the phosphatase.  When the ABA level rises, ABA binds to the ABA receptor<ref name = "Ma2009"/><ref name = "Park2009"/><ref>PMID:19893533</ref>, and this creates a binding surface for PP2C. The activated receptor (R<sup>.</sup>ABA) dissociates to monomers which bind to protein phosphatase 2C<ref name = Santiago 2009"/> (R<sup>.</sup>ABA-P). The sequestration of the protein phosphatase frees the protein kinase to be activated by autophosphorylation or phosphorylation by another protein kinase. Activation of SnRK2.6/OST1/SRK2E leads to phosphorylation of: 1) ion channels SLAC1<ref>PMID: 19955405</ref> and KAT1<ref>PMID:19785574</ref> in guard cells and stomatal closure; 2)  transcription factor ABI5<ref name = "Nakashima2009"/> in seeds/seedlings and dormancy/growth arrest; or 3) phosphorylation of transcription factor AREB/ABF <ref>PMID: 16446457</ref><ref name = "Fujii2007"/> in vegetative tissue and stress tolerance and growth regulation. 

Structures in the figure are: R, apo PYL2, [[3kdh]];  R<sup>.</sup>ABA, PYL2<sup>.</sup>ABA, [[3kdi]]; K-P; SnRK2.6-HAB1, [[3ujg]]; R<sup>.</sup>ABA-P, PYL2<sup>.</sup>ABA-ABI2, [[3ujl]]; K, SnRK2.6,[[3uc4]].

==Structures of Proteins==
For details about the structures of proteins involved in this pathway see [[PYR/PYL/RCAR family of ABA receptors]], [[ABA-regulated Protein Phosphatase 2C]], [[ABA-regulated SNRK2 Protein Kinase]].  

==References==


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==See Also==
[http://en.wikipedia.org/wiki/Abscisic_acid] Abscisic Acid in Wikipedia