Silent S-Type Anion Channel Subunit SLAH1 Gates SLAH3 Open for Chloride Root-to-Shoot Translocation
2016
Cubero-Font, Paloma | Maierhofer, Tobias | Jaslan, Justyna | Rosales Villegas, Miguel Angel | Espartero, Joaquin | Diaz-Rueda, Pablo | Mueller, Heike M. | Huerter, Anna Lena | Al-Rasheid, Khaled A. S. | Marten, Irene | Hedrich, Rainer | Colmenero-Flores, Jose | Geiger, Dietmar | Institute of Natural Resources and Agrobiology of Seville (IRNAS) | Consejo Superior de Investigaciones Cientificas [España] = Spanish National Research Council [Spain] (CSIC) | Institute for Molecular Plant Physiology and Biophysics ; Julius-Maximilians-Universität Würzburg = University of Würzburg [Würsburg, Germany] (JMU) | Biochimie et Physiologie Moléculaire des Plantes (BPMP) ; Institut National de la Recherche Agronomique (INRA)-Centre international d'études supérieures en sciences agronomiques (Montpellier SupAgro)-Université de Montpellier (UM)-Centre National de la Recherche Scientifique (CNRS)-Institut national d’études supérieures agronomiques de Montpellier (Montpellier SupAgro) | Zoology Department, College of Science ; King Saud University [Riyadh] (KSU) | Spanish Ministry of Science and Innovation FEDER [AGL2009-08339/AGR, AGL2015-71386-R]; German Research Foundation (DFG) [SFB/TR166]; Spanish National Research Council (CSIC); German Academic Exchange Service (DAAD); Deanship of Scientific Research, King Saud University [IRG14-08] | European Project: 267196,EC:FP7:PEOPLE,FP7-PEOPLE-2010-COFUND,AGREENSKILLS(2012)
Higher plants take up nutrients via the roots and load them into xylem vessels for translocation to the shoot. After uptake, anions have to be channeled toward the root xylem vessels. Thereby, xylem parenchyma and pericycle cells control the anion composition of the root-shoot xylem sap [1-6]. The fact that salt-tolerant genotypes possess lower xylem-sap Cl- contents compared to salt-sensitive genotypes [7-10] indicates that membrane transport proteins at the sites of xylem loading contribute to plant salinity tolerance via selective chloride exclusion. However, the molecular mechanism of xylem loading that lies behind the balance between NO3- and Cl- loading remains largely unknown. Here we identify two root anion channels in Arabidopsis, SLAH1 and SLAH3, that control the shoot NO3-/Cl- ratio. The AtSLAH1 gene is expressed in the root xylem-pole pericycle, where it co-localizes with AtSLAH3. Under high soil salinity, AtSLAH1 expression markedly declined and the chloride content of the xylem sap in AtSLAH1 loss-of-function mutants was half of the wild-type level only. SLAH3 anion channels are not active per se but require extracellular nitrate and phosphorylation by calcium-dependent kinases (CPKs) [11-13]. When co-expressed in Xenopus oocytes, however, the electrically silent SLAH1 subunit gates SLAH3 open even in the absence of nitrate- and calcium-dependent kinases. Apparently, SLAH1/SLAH3 heteromerization facilitates SLAH3-mediated chloride efflux from pericycle cells into the root xylem vessels. Our results indicate that under salt stress, plants adjust the distribution of NO3- and Cl- between root and shoot via differential expression and assembly of SLAH1/SLAH3 anion channel subunits.
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