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.