Poster: Hormones

Abs # 589: Auxin/gibberellin interactions in the folowering stem of Arabidopsis

Presenter:	O'Neill, Damian P., doneill@postman.riken.go.jp
Authors	O'Neill, Damian P. (A)   Kamiya, Yuji  (A)   Yamaguchi, Shinjiro  (A)
Affiliations:	(A): Plant Science Center, RIKEN

Recent studies have shown that the auxin indole-3-acetic acid (IAA), synthesised in the apical bud of pea, regulates gibberellin (GA) biosynthesis in elongating internodes of pea shoots. The application of IAA to decapitated (IAA-deficient) peas rapidly increased the transcript level of PsGA3ox1 (Mendel's LE), which encodes a GA 3-oxidase that converts GA₂₀ to the bioactive GA₁, and decreased the transcript level of PsGA2ox1 (SLN), which encodes a GA 2-oxidase shown to convert GA₂₀ to the inactive GA₂₉. These changes were associated with an increase in the endogenous level of GA₁, a decrease in the level of GA₂₉ and an increase in the rate of shoot elongation. IAA-induced up-regulation of PsGA3ox1, and down-regulation of PsGA2ox1 has also been demonstrated in excised pea internodes cultured on liquid Murashige and Skoog (MS) medium. In the current study, early expanding internodes were excised from the flowering stems of Arabidopsis ecotypes Colombia (Col-0) and Landsberg erecta (Ler), and cultured on liquid MS medium with or without IAA for 6 and 24 hours. The transcript levels of AtGA20ox, AtGA3ox and AtGA2ox genes were measured using quantitative RT-PCR in control (IAA deficient) and IAA-treated segments. In both ecotypes, and at both time-points, IAA treatment up-regulated AtGA20ox1, AtGA20ox2, AtGA3ox1 and AtGA2ox1 transcript levels, and strongly down-regulated the transcript level of AtGA2ox2. These results suggest that IAA regulates GA biosynthesis in internodes of the Arabidopsis thaliana floral stem. The discovery of this interaction in Arabidopsis will enable us to study the molecular mechanisms that underlie the regulation of GA biosynthesis by IAA, using the numerous auxin response mutants available in this species.