User:Alice Harmon/Sandbox 5: Difference between revisions

Abscisic acid (ABA,)[[Image:Abscisic acid.svg.png|thumb| Abscisic Acid]] is a plant hormone that regulates seed 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 an ABA receptor (PYR1, PYR2, PYL1, PYL2, and RCAR), a protein phosphatase 2C in Clade C (ABI1, ABI2, and HAB1), and a protein kinase in subclass III of SnRK2s ([[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. When the ABA level in the cytosol is low, the ABA receptor (R) is an unliganded dimer<ref>PMID:19933100</ref><ref>PMID:23265948</ref> (monomer shown), 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 (R)<ref>PMID:19893533</ref><ref>PMID:19407143</ref><ref>PMID:19407142</ref>. The activated receptor (R^.ABA) binds to a protein phosphatase 2C<ref>PMID:19624469</ref> (R^.ABA-P), and 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.