Brassinosteroids signaling via BZR1 down-regulates expression of<i>ACC oxidase 4</i>to control growth of<i>Arabidopsis thaliana</i>seedlings

Abstract

<i>ProACO4-GUS</i> expression and RT-PCR analysis revealed that <i>ACO4</i> is predominantly expressed in shoots of Arabidopsis seedlings under light conditions. <i>ACO4</i>-overexpressed mutant <i>35S-ACO4</i> produced more ethylene relative to the wild-type, which resulted in reduced growth of Arabidopsis seedlings. The abnormal growth of seedlings recurred after the application of Co²⁺ ions, suggesting that <i>ACO4</i> is a functional <i>ACO</i> necessary to regulate the growth and development of Arabidopsis seedlings. Exogenously-applied brassinosteroids (BRs) inhibited the expression of <i>ACO4</i>, and an enhanced <i>ACO4</i> expression was found in <i>det2</i>, a BR-deficient mutant. Additionally, expression of <i>ACO4</i> was decreased in <i>bzr1-D</i> (a <i>BZR1</i>-dominant mutant), implying that BR signaling negatively regulates <i>ACO4</i> expression via <i>BZR1</i> in Arabidopsis. In the intergenic region of <i>ACO4</i>, four E-boxes and a BR regulatory element (BRRE) are found. Electrophoretic mobility shift and chromatin immunoprecipitation assays showed that BZR1 binds directly to the BRRE in the putative promoter region of <i>ACO4</i>. By binding of BZR1 to BRRE, less ethylene was produced, which seems to regulate the growth and development of Arabidopsis seedlings.