Brassinosteroid induction of AtACS4 encoding an auxin-responsive 1-aminocyclopropane-1-carboxylate synthase 4 in Arabidopsis seedlings

Brassinosteroid (BR), an endogenous steroid growth regulator of higher plants, enhances expansion and division of the cell in a number of plant species. It has been recently reported that a shared auxin-BR signalling pathway is involved in the seedling growth in Arabidopsis. Here, we show that BR specifically enhanced the expression of AtACS4, which encodes an auxin-responsive ACC synthase 4, by a distinct temporal induction mechanism compared with that of IAA in etiolated Arabidopsis seedlings. This BR induction of AtACS4 was undetectable in the light-grown seedlings. In addition, BR failed to activate the AtACS4 gene in auxin-resistant1 (axr1-3) and auxin-resistant2 (axr2-1), both of which are auxin-resistant mutants. Thus, it appears that there is a possible regulatory link between light, auxin and BR to control ethylene synthesis in Arabidopsis young seedlings. Analysis of transgenic Arabidopsis plants harbouring AtACS4::GUS fusion revealed the AtACS4 promoter-driven GUS activity in the highly elongating zone of the hypocotyls in response to BR treatment. Furthermore, Arabidopsis plants homozygous for the T-DNA insertion in the AtACS4 gene exhibited longer hypocotyls and roots than those of control seedlings. Taken together, these results suggest that the BR-induced ethylene production may participate in the elongation growth response in early seedling development of Arabidopsis.