Silent S-Type Anion Channel Subunit SLAH1 Gates SLAH3 Open for Chloride Root-to-Shoot Translocation

8 páginas.-- 4 figuras.-- 40 referencias.-- Supplemental Information includes Supplemental Experimental Procedures, four figures, and one table and can be found with this article online at http:// dx.doi.org/10.1016/j.cub.2016.06.045 .-- Article in press as: Cubero-Font et al., Silent S-Type Anion Channel Subunit SLAH1 Gates SLAH3 Open for Chloride Root-to-Shoot Translocation, Current Biology (2016), http://dx.doi.org/10.1016/j.cub.2016.06.045

We appreciate J.D. Franco-Navarro, Inmaculada Flores, and F.J. Durán for the technical assistance provided to this work. Investigations of plant ion channels in Xenopus oocytes adhere to the regulations and provisions of the Animal Protection Act and the Experimental Animals Ordinance. Permission for keeping African clawfrog Xenopus laevis and using Xenopus oocytes exists at the Julius-von-Sachs Institute, University Würzburg and is registered and oversight at/from the district government of Unterfranken, Germany.

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, 2, 3, 4, 5 and 6]. The fact that salt-tolerant genotypes possess lower xylem-sap Cl− contents compared to salt-sensitive genotypes [7, 8, 9 and 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, 12 and 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.

Supported by the Spanish Ministry of Science and Innovation FEDER grants AGL2009-08339/AGR and AGL2015-71386-R. R.H. and D.G. were supported by the German Research Foundation (DFG) within the SFB/TR166 “ReceptorLight” project B8. P.C.-F. had fellowship support from the Spanish National Research Council (CSIC) and the German Academic Exchange Service (DAAD). R.H. and K.A.S.A.-R. were further supported by the International Research Group Program (project IRG14-08) of the Deanship of Scientific Research, King Saud University.

Peer reviewed