title: Site- and kinase-specific phosphorylation-mediated activation of SLAC1, a guard cell anion channel stimulated by abscisic acid.
abstract: Under drought stress, abscisic acid (ABA) triggers closure of leaf cell pores called stomata, which are formed by two specialized cells called guard cells in plant epidermis. Two pathways downstream of ABA stimulate phosphorylation of the S-type anion channels SLAC1 (slow anion channel associated 1) and SLAH3 (SLAC1 homolog 3), which causes these channels to open, reducing guard cell volume and triggering stomatal closure. One branch involves OST1 (open stomata 1), a calcium-independent SnRK2-type kinase, and the other branch involves calcium-dependent protein kinases of the CPK (calcium-dependent protein kinase) family. We used coexpression analyses in Xenopus oocytes to show that the calcineurin B-like (CBL) calcium sensors CBL1 and CBL9 and their interacting protein kinase CIPK23 also triggered SLAC1 and SLAH3 opening. We analyzed whether regulation of SLAC1 opening by these different families of kinases involved the same or different sites on SLAC1 by measuring channel conductance of SLAC1 with mutations in the putative phosphorylation sites in the amino or carboxyl termini coexpressed with specific kinases in Xenopus oocytes. SLAC1 mutants lacking the OST1-phosphorylated site were still activated by CPK or by CBL/CIPK complexes. Phosphorylation and activation of SLAC1 by any of the kinases were inhibited by the phosphatase ABI1 (ABA insensitive 1), which is inactivated in response to ABA signaling. These findings identified CBL/CIPK complexes as potential regulators of stomatal aperture through S-type anion channels and indicated that phosphorylation at distinct sites enables SLAC1 activation by both calcium-dependent and calcium-independent pathways downstream of ABA.
authors.affiliation: Department of Molecular Plant Physiology and Biophysics, University Würzburg, Julius-von-Sachs Platz 2, 97082 Würzburg, Germany.
authors.forename: Tobias
authors.initials: T
authors.lastname: Maierhofer
authors.affiliation: Department of Molecular Plant Physiology and Biophysics, University Würzburg, Julius-von-Sachs Platz 2, 97082 Würzburg, Germany.
authors.forename: Marion
authors.initials: M
authors.lastname: Diekmann
authors.affiliation: Institute of Plant Biology and Biotechnology, University of Münster, Schlossplatz 4, 48149 Münster, Germany.
authors.forename: Jan Niklas
authors.initials: JN
authors.lastname: Offenborn
authors.affiliation: Department of Molecular Plant Physiology and Biophysics, University Würzburg, Julius-von-Sachs Platz 2, 97082 Würzburg, Germany.
authors.forename: Christof
authors.initials: C
authors.lastname: Lind
authors.affiliation: Department of Molecular Plant Physiology and Biophysics, University Würzburg, Julius-von-Sachs Platz 2, 97082 Würzburg, Germany.
authors.forename: Hubert
authors.initials: H
authors.lastname: Bauer
authors.affiliation: Institute of Plant Biology and Biotechnology, University of Münster, Schlossplatz 4, 48149 Münster, Germany.
authors.forename: Kenji
authors.initials: K
authors.lastname: Hashimoto
authors.affiliation: Zoology Department, College of Science, King Saud University, P.O. Box 2455, Riyadh 11451, Saudi Arabia.
authors.forename: Khaled A
authors.initials: KA
authors.lastname: S Al-Rasheid
authors.affiliation: Department of Plant and Microbial Biology, University of California, Berkeley, CA 94720, USA.
authors.forename: Sheng
authors.initials: S
authors.lastname: Luan
authors.affiliation: Institute of Plant Biology and Biotechnology, University of Münster, Schlossplatz 4, 48149 Münster, Germany. King Saud University, Riyadh 11451, Saudi Arabia.
authors.forename: Jörg
authors.initials: J
authors.lastname: Kudla
authors.affiliation: Department of Molecular Plant Physiology and Biophysics, University Würzburg, Julius-von-Sachs Platz 2, 97082 Würzburg, Germany. geiger@botanik.uni-wuerzburg.de.
authors.forename: Dietmar
authors.initials: D
authors.lastname: Geiger
authors.affiliation: Department of Molecular Plant Physiology and Biophysics, University Würzburg, Julius-von-Sachs Platz 2, 97082 Würzburg, Germany. King Saud University, Riyadh 11451, Saudi Arabia.
authors.forename: Rainer
authors.initials: R
authors.lastname: Hedrich
mesh_terms: D000040:Abscisic Acid / Q000494:pharmacology*; D029681:Arabidopsis Proteins / Q000187:drug effects / Q000378:metabolism*; D008565:Membrane Proteins / Q000187:drug effects / Q000378:metabolism*; D010766:Phosphorylation; D011494:Protein Kinases / Q000378:metabolism*
chemical_list: D029681:Arabidopsis Proteins; D008565:Membrane Proteins; C528438:SLAC1 protein, Arabidopsis; D000040:Abscisic Acid; D011494:Protein Kinases