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Plant and Cell Physiology Advance Access originally published online on April 17, 2008
Plant and Cell Physiology 2008 49(6):891-900; doi:10.1093/pcp/pcn063
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© The Author 2008. Published by Oxford University Press on behalf of Japanese Society of Plant Physiologists. All rights reserved. For permissions, please email: journals.permissions@oxfordjournals.org

The Gravity-Regulated Growth of Axillary Buds is Mediated by a Mechanism Different from Decapitation-Induced Release

Daisuke Kitazawa1, Yutaka Miyazawa1, Nobuharu Fujii1, Atsushi Hoshino2, Shigeru Iida2, Eiji Nitasaka3 and Hideyuki Takahashi1,*

1Graduate School of Life Sciences, Tohoku University, 2-1-1 Katahira, Aoba-ku, Sendai, 980-8577 Japan
2National Institute for Basic Biology, Nishigonaka 38, Myodaiji, Okazaki, 444-8585 Japan
3Department of Biological Science, Graduate School of Science, Kyushu University, Hakozaki, Fukuoka, 812-8581 Japan

*Corresponding author: E-mail, hideyuki{at}ige.tohoku.ac.jp; Fax, +81-22-723-8218.


  Abstract

When the upper part of the main shoot of the Japanese morning glory (Pharbitis nil or Ipomoea nil) is bent down, the axillary bud situated on the uppermost node of the bending region is released from apical dominance and elongates. Here, we demonstrate that this release of axillary buds from apical dominance is gravity regulated. We utilized two agravitropic mutants of morning glory defective in gravisensing cell differentiation, weeping (we) and weeping2 (we2). Bending the main shoots of either we or we2 plants resulted in minimal elongation of their axillary buds. This aberration was genetically linked to the agravitropism phenotype of the mutants, which implied that shoot bending-induced release from apical dominance required gravisensing cells. Previous studies have shown that basipetal translocation of auxin from the apical bud inhibits axillary bud growth, whereas cytokinin promotes axillary bud outgrowth. We therefore compared the roles of auxin and cytokinin in bending- or decapitation-induced axillary bud growth. In the wild-type and we plants, decapitation increased cytokinin levels and reduced auxin response. In contrast, shoot bending did not cause significant changes in either cytokinin level or auxin response, suggesting that the mechanisms underlying gravity- and decapitation-regulated release from apical dominance are distinct and unique.

Keywords: Apical dominance - Auxin - Gravitropism - Cytokinin - Morning glory - weeping

Abbreviations: ELISA, enzyme-linked immunosorbent assay; EST, expressed sequence tag; IPT, isopentenyltransferase; ORF, open reading frame; RT–PCR, reverse transcription–PCR; t-ZR, trans-zeatin riboside; we, weeping; WT, wild type

The nucleotide sequence reported in this paper has been submitted to GenBank data libraries under accession numbers: PnIAA1 (AB371299), PnIPT1 (AB371300) and PnIPT2 (AB371301).

(Received January 14, 2008; Accepted April 14, 2008)
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