title: The endoplasmic reticulum localized PIN8 is a pollen-specific auxin carrier involved in intracellular auxin homeostasis.
abstract: The plant hormone auxin is a mobile signal which affects nuclear transcription by regulating the stability of auxin/indole-3-acetic acid (IAA) repressor proteins. Auxin is transported polarly from cell to cell by auxin efflux proteins of the PIN family, but it is not as yet clear how auxin levels are regulated within cells and how access of auxin to the nucleus may be controlled. The Arabidopsis genome contains eight PINs, encoding proteins with a similar membrane topology. While five of the PINs are typically targeted polarly to the plasma membranes, the smallest members of the family, PIN5 and PIN8, seem to be located not at the plasma membrane but in endomembranes. Here we demonstrate by electron microscopy analysis that PIN8, which is specifically expressed in pollen, resides in the endoplasmic reticulum and that it remains internally localized during pollen tube growth. Transgenic Arabidopsis and tobacco plants were generated overexpressing or ectopically expressing functional PIN8, and its role in control of auxin homeostasis was studied. PIN8 ectopic expression resulted in strong auxin-related phenotypes. The severity of phenotypes depended on PIN8 protein levels, suggesting a rate-limiting activity for PIN8. The observed phenotypes correlated with elevated levels of free IAA and ester-conjugated IAA. Activation of the auxin-regulated synthetic DR5 promoter and of auxin response genes was strongly repressed in seedlings overexpressing PIN8 when exposed to 1-naphthalene acetic acid. Thus, our data show a functional role for endoplasmic reticulum-localized PIN8 and suggest a mechanism whereby PIN8 controls auxin thresholds and access of auxin to the nucleus, thereby regulating auxin-dependent transcriptional activity.
authors.affiliation: Institute of Biology II/Molecular Plant Physiology, Faculty of Biology, Albert-Ludwigs-University of Freiburg, Schänzlestrasse 1, D-79104 Freiburg, Germany. cristina.dal.bosco@biologie-uni-freiburg.de
authors.forename: Cristina
authors.initials: C
authors.lastname: Dal Bosco
authors.forename: Alexander
authors.initials: A
authors.lastname: Dovzhenko
authors.forename: Xing
authors.initials: X
authors.lastname: Liu
authors.forename: Nina
authors.initials: N
authors.lastname: Woerner
authors.forename: Tatiana
authors.initials: T
authors.lastname: Rensch
authors.forename: Margitta
authors.initials: M
authors.lastname: Eismann
authors.forename: Stefan
authors.initials: S
authors.lastname: Eimer
authors.forename: Jan
authors.initials: J
authors.lastname: Hegermann
authors.forename: Ivan A
authors.initials: IA
authors.lastname: Paponov
authors.forename: Benedetto
authors.initials: B
authors.lastname: Ruperti
authors.forename: Erwin
authors.initials: E
authors.lastname: Heberle-Bors
authors.forename: Alisher
authors.initials: A
authors.lastname: Touraev
authors.forename: Jerry D
authors.initials: JD
authors.lastname: Cohen
authors.forename: Klaus
authors.initials: K
authors.lastname: Palme
mesh_terms: D017360:Arabidopsis / Q000378:metabolism*; D029681:Arabidopsis Proteins / Q000378:metabolism*; D004721:Endoplasmic Reticulum / Q000378:metabolism*; D017930:Genes, Reporter; D006706:Homeostasis; D007210:Indoleacetic Acids / Q000378:metabolism*; D026901:Membrane Transport Proteins / Q000378:metabolism*; D010641:Phenotype; D011058:Pollen / Q000378:metabolism*; D036226:Seedlings / Q000378:metabolism; D015854:Up-Regulation
chemical_list: D029681:Arabidopsis Proteins; D007210:Indoleacetic Acids; D026901:Membrane Transport Proteins; C581622:PIN8 protein, Arabidopsis