ABA Signaling Pathway

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Abscisic acid (ABA,)
Abscisic Acid
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.[1][2][3] In Arabidopsis, this pathway involves families of ABA receptors (PYR1/RCAR11, PYL1/RCAR12, PYL2/RCAR14, PYL3/RCAR13, PYL8/RCAR3, PYL9/RCAR1)[4][5][6][7][8], protein phosphatase 2Cs (ABI1, ABI2, HAB1, HAB2, PP2CA/AHG3)[4][6][9][10], and SNRK2 protein kinases (SnRK2.6/OST1/SRK2E, SNRK2.2/SRK2D and SnRK2.3/SRK2IA)[11][12][13][14].

The core ABA signaling pathway is shown in the figure to the left. In unstimulated cells, the ABA receptor (R) is an unliganded dimer[5][15][16][7] (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[4][6][17], and this creates a binding surface for PP2C. The activated receptor (R.ABA) dissociates to monomers which bind to protein phosphatase 2C[9] (R.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[18] and KAT1[19] in guard cells and stomatal closure; 2) transcription factor ABI5[13] in seeds/seedlings and dormancy/growth arrest; or 3) phosphorylation of transcription factor AREB/ABF [20][14] in vegetative tissue and stress tolerance and growth regulation.

Structures in the figure are: R, apo PYL2, 3kdh; R.ABA, PYL2.ABA, 3kdi; K-P; SnRK2.6-HAB1, 3ujg; R.ABA-P, PYL2.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.


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See Also

[1] Abscisic Acid in Wikipedia

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