Plant Physiology Preview
Published on May 27, 2005; 10.1104/pp.104.059055
Received January 12, 2005
Returned for revision February 10, 2005
Accepted February 15, 2005
The Involvement of Gibberellin 20-Oxidase Genes in Phytochrome-Regulated Petiole Elongation of Arabidopsis
Tamotsu Hisamatsu , Rod W. King *, Chris A. Helliwell , and Masaji Koshioka
National Institute of Floricultural Science, Tsukuba 305-8519, Japan; Commonwealth Scientific and Industrial Research Organization, Plant Industry, Canberra 2601, Australian Capital Territory, Australia
Commonwealth Scientific and Industrial Research Organization, Plant Industry, Canberra 2601, Australian Capital Territory, Australia
National Institute of Floricultural Science, Tsukuba 305-8519, Japan
* Corresponding author; email: rod.king{at}csiro.au.
Long day (LD) exposure of rosette plants causes rapid stem/petiole elongation, a more vertical growth habit, and flowering; all changes are suggestive of a role for the gibberellin (GA) plant growth regulators. For Arabidopsis (Arabidopsis thaliana) L. (Heynh), we show that enhancement of petiole elongation by a far-red (FR)-rich LD is mimicked by a brief (10 min) end-of-day (EOD) FR exposure in short day (SD). The EOD response shows red (R)/FR photoreversibility and is not affected in a phytochrome (PHY) A mutant so it is mediated by PHYB and related PHYs. FR photoconversion of PHYB to an inactive form activates a signaling pathway, leading to increased GA biosynthesis. Of 10 GA biosynthetic genes, expression of the 20-oxidase, AtGA20ox2, responded most to FR (up to a 40-fold increase within 3 h). AtGA20ox1 also responded but to a lesser extent. Stimulation of petiole elongation by EOD FR is reduced in a transgenic AtGA20ox2 hairpin gene silencing line. By contrast, it was only in SD that a T-DNA insertional mutant of AtGA20ox1 (ga5-3) showed reduced response. Circadian entrainment to a daytime pattern provides an explanation for the SD expression of AtGA20ox1. Conversely, the strong EOD/LD FR responses of AtGA20ox2 may reflect its independence of circadian regulation. While FR acting via PHYB increases expression of AtGA20ox2, other GA biosynthetic genes are known to respond to R rather than FR light and/or to other PHYs. Thus, there must be different signal transduction pathways, one at least showing a positive response to active PHYB and another showing a negative response.
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