title: Vacuolar CAX1 and CAX3 influence auxin transport in guard cells via regulation of apoplastic pH.
abstract: CATION EXCHANGERs CAX1 and CAX3 are vacuolar ion transporters involved in ion homeostasis in plants. Widely expressed in the plant, they mediate calcium transport from the cytosol to the vacuole lumen using the proton gradient across the tonoplast. Here, we report an unexpected role of CAX1 and CAX3 in regulating apoplastic pH and describe how they contribute to auxin transport using the guard cell's response as readout of hormone signaling and cross talk. We show that indole-3-acetic acid (IAA) inhibition of abscisic acid (ABA)-induced stomatal closure is impaired in cax1, cax3, and cax1/cax3. These mutants exhibited constitutive hypopolarization of the plasma membrane, and time-course analyses of membrane potential revealed that IAA-induced hyperpolarization of the plasma membrane is also altered in these mutants. Both ethylene and 1-naphthalene acetic acid inhibited ABA-triggered stomatal closure in cax1, cax3, and cax1/cax3, suggesting that auxin signaling cascades were functional and that a defect in IAA transport caused the phenotype of the cax mutants. Consistent with this finding, chemical inhibition of AUX1 in wild-type plants phenocopied the cax mutants. We also found that cax1/cax3 mutants have a higher apoplastic pH than the wild type, further supporting the hypothesis that there is a defect in IAA import in the cax mutants. Accordingly, we were able to fully restore IAA inhibition of ABA-induced stomatal closure in cax1, cax3, and cax1/cax3 when stomatal movement assays were carried out at a lower extracellular pH. Our results suggest a network linking the vacuolar cation exchangers to apoplastic pH maintenance that plays a crucial role in cellular processes.
authors.affiliation: Department of Cell Biology and Molecular Genetics, University of Maryland, College Park, Maryland 20742, USA.
authors.forename: Daeshik
authors.initials: D
authors.lastname: Cho
authors.forename: Florent
authors.initials: F
authors.lastname: Villiers
authors.forename: Laetitia
authors.initials: L
authors.lastname: Kroniewicz
authors.forename: Sangmee
authors.initials: S
authors.lastname: Lee
authors.forename: You Jin
authors.initials: YJ
authors.lastname: Seo
authors.forename: Kendal D
authors.initials: KD
authors.lastname: Hirschi
authors.forename: Nathalie
authors.initials: N
authors.lastname: Leonhardt
authors.forename: June M
authors.initials: JM
authors.lastname: Kwak
mesh_terms: D000040:Abscisic Acid / Q000494:pharmacology; D017920:Antiporters / Q000235:genetics / Q000378:metabolism*; D017360:Arabidopsis / Q000166:cytology / Q000187:drug effects / Q000235:genetics / Q000378:metabolism* / Q000528:radiation effects; D029681:Arabidopsis Proteins / Q000235:genetics / Q000378:metabolism*; D001692:Biological Transport / Q000187:drug effects / Q000528:radiation effects; D027682:Cation Transport Proteins / Q000235:genetics / Q000378:metabolism*; D002462:Cell Membrane / Q000187:drug effects / Q000378:metabolism / Q000528:radiation effects; D018506:Gene Expression Regulation, Plant / Q000187:drug effects / Q000528:radiation effects; D006863:Hydrogen-Ion Concentration / Q000187:drug effects / Q000528:radiation effects; D007210:Indoleacetic Acids / Q000378:metabolism* / Q000494:pharmacology; D008027:Light; D008954:Models, Biological; D009154:Mutation / Q000235:genetics; D009280:Naphthaleneacetic Acids / Q000494:pharmacology; D054046:Plant Stomata / Q000166:cytology* / Q000187:drug effects / Q000528:radiation effects; D006180:Proton-Translocating ATPases / Q000378:metabolism; D014617:Vacuoles / Q000187:drug effects / Q000378:metabolism* / Q000528:radiation effects
chemical_list: D017920:Antiporters; D029681:Arabidopsis Proteins; C506406:CAX3 protein, Arabidopsis; D027682:Cation Transport Proteins; D007210:Indoleacetic Acids; D009280:Naphthaleneacetic Acids; C027614:calcium-hydrogen antiporters; C034182:1-naphthaleneacetic acid; C030737:indoleacetic acid; D000040:Abscisic Acid; D006180:Proton-Translocating ATPases