Journal Article

<i>Arabidopsis</i> ABCB21 is a Facultative Auxin Importer/Exporter Regulated by Cytoplasmic Auxin Concentration

Yoshihisa Kamimoto, Kazuyoshi Terasaka, Masafumi Hamamoto, Kojiro Takanashi, Shoju Fukuda, Nobukazu Shitan, Akifumi Sugiyama, Hideyuki Suzuki, Daisuke Shibata, Bangjun Wang, Stephan Pollmann, Markus Geisler and Kazufumi Yazaki

in Plant and Cell Physiology

Published on behalf of Japanese Society of Plant Physiologists

Volume 53, issue 12, pages 2090-2100
Published in print December 2012 | ISSN: 0032-0781
Published online November 2012 | e-ISSN: 1471-9053 | DOI: http://dx.doi.org/10.1093/pcp/pcs149
Arabidopsis ABCB21 is a Facultative Auxin Importer/Exporter Regulated by Cytoplasmic Auxin Concentration

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  • Biochemistry
  • Molecular and Cell Biology
  • Plant Sciences and Forestry

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The phytohormone auxin is critical for plant growth and many developmental processes. Members of the P-glycoprotein (PGP/ABCB) subfamily of ATP-binding cassette (ABC) transporters have been shown to function in the polar movement of auxin by transporting auxin over the plasma membrane in both monocots and dicots. Here, we characterize a new Arabidopsis member of the ABCB subfamily, ABCB21/PGP21, a close homolog of ABCB4, for which conflicting transport directionalities have been reported. ABCB21 is strongly expressed in the abaxial side of cotyledons and in junctions of lateral organs in the aerial part, whereas in roots it is specifically expressed in pericycle cells. Membrane fractionation by sucrose density gradient centrifugation followed by Western blot showed that ABCB21 is a plasma membrane-localized ABC transporter. A transport assay with Arabidopsis protoplasts suggested that ABCB21 was involved in IAA transport in an outward direction, while naphthalene acetic acid (NAA) was a less preferable substrate for ABCB21. Further functional analysis of ABCB21 using yeast import and export assays showed that ABCB21 mediates the 1-N-naphthylphthalamic acid (NPA)-sensitive translocation of auxin in an inward direction when the cytoplasmic IAA concentration is low, whereas this transporter mediates outward transport under high internal IAA. An increase in the cytoplasmic IAA concentration by pre-loading of IAA into yeast cells abolished the IAA uptake activity by ABCB21 as well as ABCB4. These findings suggest that ABCB21 functions as a facultative importer/exporter controlling auxin concentrations in plant cells.

Keywords: ABC protein; ABCB21; Arabidopsis thaliana; Auxin transport; GUS expression; Pericycle cell; Transport direction

Journal Article.  5687 words.  Illustrated.

Subjects: Biochemistry ; Molecular and Cell Biology ; Plant Sciences and Forestry

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