Brassinosteroids signaling via BZR1 down-regulates expression of ACC oxidase 4to control growth of Arabidopsis thalianaseedlings

ABSTRACTProACO4–GUSexpression and RT-PCR analysis revealed that ACO4is predominantly expressed in shoots of Arabidopsis seedlings under light conditions. ACO4-overexpressed mutant 35S–ACO4produced 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 ACO4is a functional ACOnecessary to regulate the growth and development of Arabidopsis seedlings. Exogenously-applied brassinosteroids (BRs) inhibited the expression of ACO4, and an enhanced ACO4expression was found in det2, a BR-deficient mutant. Additionally, expression of ACO4was decreased in bzr1-D(a BZR1-dominant mutant), implying that BR signaling negatively regulates ACO4expression via BZR1in Arabidopsis. In the intergenic region of ACO4, 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 ACO4. By binding of BZR1 to BRRE, less ethylene was produced, which seems to regulate the growth and development of Arabidopsis seedlings.