Gibberellin (GA) is an essential hormone that regulates diverse aspects of plant growth and development. Molecular genetic analyses have identified several key factors in the upstream signaling pathway of GA; however, the downstream signaling pathway of GA is still unclear.Identifying and characterizing new targets of DELLA proteins is an effective approach to reveal the complicated GA signaling networks.
Prof. YU Diqiu and his team of Xishuangbanna Tropical Botanical Garden (XTBG) identified two DELLA interacting proteins, bHLH48 and bHLH60, from Arabidopsis thaliana ecotype Columbia (Col-0).
The researchers employed the yeast-two-hybrid system to screen for DELLA interacting proteins, in order to identify novel components involved in GA signaling pathways. They also generated overexpressing transgenic plants to explore the functions of bHLH48 and bHLH60.
To determine the potential causes of the early flowering phenotype of the overexpressors, the expression levels of the photoperiodic pathway genes the FLOWERING LOCUS T (FT), CONSTANS (CO) and GIBBERELLIN INSENSITIVE (GAI) were examined. The expression levels of bHLH48 and bHLH60 were examined after exogenous GA application.
They found that overexpression of bHLH48 and bHLH60 caused early flowering under long day conditions by directly activating the transcription of FT. REPRESSOR OF GA1 (RGL1) interacted with bHLH48 and bHLH60 and this interaction could reverse the activation ability of bHLH48 and bHLH60 in regards to FT.
Gain-of-function of bHLH48 and bHLH60 caused an early flowering phenotype, whereas their functional repression resulted in a late-flowering phenotype, suggesting that bHLH48 and bHLH60 are positive regulators of the flowering transition.
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.
The study revealed that bHLH48 and bHLH60 are two novel transcription factors involved in GA-mediated LD flowering. They positively regulate GA-mediated flowering.