Two DELLA-interacting proteins bHLH48 and bHLH60 regulate flowering under long-day conditions in Arabidopsis thaliana

Gibberellin (GA) regulates many developmental transitions in the plant life cycle. Although great progress has been made, the GA signaling pathways have not been fully elucidated. Identifying and characterizing new targets of DELLA proteins is an effective approach to reveal the complicated GA signaling networks. In this study, two novel DELLA-interacting transcription factors, bHLH48 and bHLH60, were identified. Their overexpression caused plants to flower early under long-day conditions, whereas their functional repression resulted in the opposite result. The constitutive expression of bHLH48 and bHLH60 upregulated the transcription of the FLOWERING LOCUS T (FT) gene. Chromatin immunoprecipitation experiments confirmed that bHLH48 bound to the promoter of FT and that GA promoted the DNA-binding activity of bHLH48. Genetic analyses indicated that the early flowering phenotype of plants overexpressing bHLH48 and bHLH60 depended on FT and that the overexpression of bHLH48 and bHLH60 could rescue the late-flowering phenotypes of RGL1 overexpressing plants. Transient expression assays suggested that RGL1 inhibited the transcription activation ability of bHLH48 and bHLH60. Taken together, this study confirmed that bHLH48 and bHLH60 positively regulate GA-mediated flowering.