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GA₄ Is the Active Gibberellin in the Regulation of LEAFY Transcription and Arabidopsis Floral Initiation^[W]

Umeå Plant Science Centre, Department of Forest Genetics and Plant Physiology, Swedish University of Agricultural Sciences, S-90183 Umeå, Sweden

¹ To whom correspondence should be addressed. E-mail ove.nilsson{at}genfys.slu.se; fax 46-90-786-8165.

Flower initiation in Arabidopsis thaliana under noninductive short-day conditions is dependent on the biosynthesis of the plant hormone gibberellin (GA). This dependency can be explained, at least partly, by GA regulation of the flower meristem identity gene LEAFY (LFY) and the flowering time gene SUPPRESSOR OF CONSTANS1. Although it is well established that GA₄ is the active GA in the regulation of Arabidopsis shoot elongation, the identity of the GA responsible for the regulation of Arabidopsis flowering has not been established. Through a combination of GA quantifications and sensitivity assays, we show that GA₄ is the active GA in the regulation of LFY transcription and Arabidopsis flowering time under short-day conditions. The levels of GA₄ and sucrose increase dramatically in the shoot apex shortly before floral initiation, and the regulation of genes involved in GA metabolism suggests that this increase is possibly due to transport of GAs and sucrose from outside sources to the shoot apex. Our results demonstrate that in the dicot Arabidopsis, in contrast with the monocot Lolium temulentum, GA₄ is the active GA in the regulation of both shoot elongation and flower initiation.

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