BES1 directly binds to the promoter of the ACC oxidase 1 gene to regulate gravitropic response in the roots of<i>Arabidopsis thaliana</i>

Abstract

Brassinosteroids (BRs) are known to be endogenous regulators of ethylene production, suggesting that some BR activity in plant growth and development is associated with ethylene. Here, we demonstrated that ethylene production in <i>Arabidopsis thaliana</i> roots is increased by BR signaling via the ethylene biosynthetic gene for ACC oxidase 1 (<i>ACO1</i>). Electrophoretic mobility shift and chromatin immune-precipitation assays showed that the BR transcription factor BES1 directly binds to two E-box sequences located in the intergenic region of <i>ACO1</i>. GUS expression using site mutations of the E-box sequences verified that <i>ACO1</i> is normally expressed only when BES1 binds to the E-boxes in the putative promoter of <i>ACO1</i>, indicating that this binding is essential for <i>ACO1</i> expression and the subsequent production of ethylene in <i>A. thaliana</i> roots. BR exogenously applied to <i>A. thaliana</i> roots enhanced the gravitropic response. Additionally, <i>bes1-D</i> exhibited a greater gravitropic response than did the wild-type specimens, proving that BR is a positive regulator of the gravitropic response in <i>A. thaliana</i> roots. The knock-down mutant <i>aco1-1</i> showed a slightly lower gravitropic response than did the wild-type specimens, while <i>bes1-D X aco1-1</i> exhibited a lower gravitropic response than did bes1-D. Therefore, <i>ACO1</i> is a direct downstream target for BR transcription factor BES1, which controls ethylene production for gravitropism in <i>A. thaliana</i> roots.