Journal Article

Differential Downward Stream of Auxin Synthesized at the Tip Has a Key Role in Gravitropic Curvature via TIR1/AFBs-Mediated Auxin Signaling Pathways

Takeshi Nishimura, Hitomi Nakano, Ken-ichiro Hayashi, Chiharu Niwa and Tomokazu Koshiba

in Plant and Cell Physiology

Published on behalf of Japanese Society of Plant Physiologists

Volume 50, issue 11, pages 1874-1885
Published in print November 2009 | ISSN: 0032-0781
Published online November 2009 | e-ISSN: 1471-9053 | DOI: http://dx.doi.org/10.1093/pcp/pcp129
Differential Downward Stream of Auxin Synthesized at the Tip Has a Key Role in Gravitropic Curvature via TIR1/AFBs-Mediated Auxin Signaling Pathways

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

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Since the early days of Darwin, monocot coleoptiles have been used to investigate indole-3-acetic acid (IAA) production, polar transport and tropisms. Here, using maize coleoptiles, we first showed that polar transport of IAA synthesized at the tip region is regulated by ZmPIN(s). Then, the TIR/AFBs-mediated auxin signaling pathway corresponds to the asymmetric IAA flow after gravi-stimulus, which results in tropic curvature. When [13C1115N2]Trp was applied to coleoptile tips, substantial amounts of the stable isotope were incorporated into IAA at the tip region, and the labeled IAA was transported in a polar manner at approximately 7 mm h−1. Immunohistochemical analyses revealed that ZmPIN1(s) was present in almost all cells. ZmPIN1(s) showed a relatively non-polar distribution at the tip, but a basal cellular localization at lower regions. Application of the IAA transport inhibitors 1-N-naphthylphthalamic acid (NPA) and brefeldin A (BFA) at the very tip region almost completely inhibited IAA movement from the tip. These inhibitors also severely suppressed gravitropic bending. PEO-IAA, an auxin antagonist that binds to TIR1/AFBs, suppressed not only the expression of an auxin-responsive ZmSAUR2 gene, but also gravitropic curvature. Expression of ZmSAUR2 was up-regulated on the lower side and down-regulated on the upper side of the coleoptile elongation zone, corresponding to the asymmetric IAA distribution. These results indicate that the asymmetric downward streams of IAA control the differential growth rate of the cells by attenuating TIR1/AFBs-mediated auxin response genes, including ZmSAUR2, and therefore result in tropic curvature.

Keywords: Coleoptiles; IAA; Maize (Zea mays); PEO-IAA; ZmPIN1; ZmSAUR2

Journal Article.  7442 words.  Illustrated.

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

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