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Plant and Cell Physiology Advance Access originally published online on July 30, 2008
Plant and Cell Physiology 2008 49(9):1342-1349; doi:10.1093/pcp/pcn109
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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

Thermospermine is Required for Stem Elongation in Arabidopsis thaliana

Jun-ichi Kakehi1, Yoshitaka Kuwashiro1, Masaru Niitsu2 and Taku Takahashi1,*

1 Division of Bioscience, Graduate School of Natural Science and Technology, Okayama University, Okayama, 700-8530 Japan
2 Faculty of Pharmaceutical Sciences, Josai University, Sakado, Saitama, 370-0290 Japan

*Corresponding author: E-mail, perfect{at}cc.okayama-u.ac.jp; Fax, +81-86-251-7876.


  Abstract

Loss-of-function mutants of the ACAULIS5 (ACL5) gene in Arabidopsis thaliana have severe defects in stem elongation. ACL5 was previously reported as encoding a spermine synthase. A more recent study, however, showed that the bacterial expressed recombinant ACL5 protein catalyzes the conversion of spermidine to thermospermine, a structural isomer of spermine, rather than to spermine. In the present study, we found that thermospermine was detected in wild-type seedlings but was not detectable in the acl5-1 mutant. We further examined the effect of exogenous application of these isomers on the growth of acl5-1. Daily application of 0.1 mM thermospermine onto the shoot apex partially rescued the dwarf phenotype of acl5-1, while that of spermine had no effects on the morphology of the mutant. The acl5-1 transcript level in acl5-1 seedlings, which is much higher than the ACL5 transcript level in wild-type seedlings, was reduced by exogenous thermospermine. Thus we conclude that thermospermine is indeed produced through the action of ACL5 and required for stem elongation in Arabidopsis.

Keywords: acaulis5 - Arabidopsis thaliana - Polyamine - Spermine - Stem elongation - Thermospermine

Abbreviations: ACL5, ACAULIS5; SPDS, SPERMIDINE SYNTHASE; SPMS, SPERMINE SYNTHASE; SAC51, SUPPRESSOR OF ACAULIS5-1; MS, Murashige-Skoog; AMV, Avian myeloblastosis virus; GUS, β-glucuronidase; PAO, polyamine oxidase; uORF, upstream open reading frame; UTR, untranslated region

(Received June 6, 2008; Accepted July 29, 2008)
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