Poster: Hormones

Abs # 588: Autoregulatory loop between brassinosteroid, auxin and gibberellin actions

Presenter:	Nomura, Takahito , nomura@nasu.bio.teikyo-u.ac.jp
Authors	Nomura, Takahito  (A)   Bishop, Gerard J. (B)   Kaneta, Tsuyoshi  (C)   Reid, James B. (D)   Chory, Joanne  (E)   Yokota, Takao  (A)
Affiliations:	(A): Department of Biosciences, Teikyo University (B): Institute of Biological Sciences, University of Wales Aberystwyth (C): Department of Biology and Earth Sciences, Ehime University (D): School of Plant Science, University of Tasmania (E): Plant Biology Laboratory, The Salk Institute for Biological Studies

We found that gibberellin (GA) mutants le, ls, lh and na of pea (Pisum sativum L.) have a reduced response to brassinolide. Conversely brassinosteroid (BR) mutants lka, lkb and lk of pea have a reduced response to GA. Physiological and molecular evidence obtained in this work indicated that GA mutants would respond to BR in the presence of GA and BR mutants would respond to GA in the presence of BR. It was concluded that coexistence of GA and BR is necessary to elicit cell elongation and, in essence, GA cannot substitute for BR and vice versa. However, we further found that there is a close linkage between GA, BR and auxin actions. The transcript level of LKA, pea BRI1 ortholog, was increased after GA1 treatment and, in support of this, was elevated in the overgrowth mutant sln that have excessive GA content. Furthermore, it was found that the synthesis of BRs is promoted by GA action as the levels of castasterone and 6-deoxocastasterone were 50% higher in sln plants than in WT. These findings indicate that both the synthesis and signaling of BR are up-regulated by GA. Furthermore, earlier findings that the level of IAA in the internodes of lka and lkb is decreased to half as compared with WT controls, indicate that BR signaling increases IAA levels. Recently, Ross et al. (2001) demonstrated that IAA was found to up-regulate GA synthesis in pea and tobacco. We found, in this study, that IAA increases the LKA transcript level of pea. This IAA action, therefore, may be considered to be mediated by GA. Altogether, we postulate a hypothesis that an autoregulatory loop between GA, BR and auxin actions is operative in planta.