Two B-box domain proteins, BBX28 and BBX29, regulate flowering time at low ambient temperature in Arabidopsis.

KEY MESSAGE: This paper demonstrates that BBX28 and BBX29 proteins in Arabidopsis promote flowering in association with the CO-FT regulatory module at low ambient temperature under LD conditions. Flowering plants integrate internal developmental signals with external environmental stimuli for precise flowering time control. The expression of BBX29 is up-regulated by low temperature treatment, but the biological function of BBX29 in low temperature response is unknown. In the current study, we examined the biological role of BBX29 and its close-related protein BBX28 in flowering time control under long-day conditions. Although neither BBX28 single mutant nor BBX29 single mutant has a flowering-associated phenotype, the bbx28 bbx29 double mutant plants have an obvious delayed flowering phenotype grown at low ambient temperature (16°C) compared to the wild-type (WT) plants. The expression of FT and TSF was lower in bbx28 bbx29 double mutant plants than in wild-type plants at 16°C. Both BBX28 and BBX29 interact with CONSTANS (CO), an important flowering integrator that directly binds to the FLOWERING LOCUS T (FT) promoter. In the effector-reporter assays, transcriptional activation activity of CO on the FT promoter was reduced in bbx28 bbx29 double mutant plants compared to that in WT plants. Taken together, our results reveal that BBX28 and BBX29 are promoters of flowering in Arabidopsis, especially at low ambient temperature.

Two B-Box Domain Proteins, BBX18 and BBX23, Interact with ELF3 and Regulate Thermomorphogenesis in Arabidopsis.

Plants coordinate their growth and developmental programs with various endogenous signals and environmental challenges. Phytochrome interacting factor 4 (PIF4) plays a critical positive role in thermoresponsive gene expression and hypocotyl growth in Arabidopsis, whereas early flowering 3 (ELF3) negatively regulates the activity of PIF4 at elevated temperatures. However, it is unknown how ELF3 activity is regulated at warm temperatures. Here, we report the identification of B-box 18 (BBX18) and BBX23 as important thermomorphogenesis regulators in Arabidopsis. BBX18 and BBX23 mutations result in reduced thermoresponsive hypocotyl elongation. In contrast, BBX18 overexpression promotes hypocotyl growth at elevated temperatures, which depends on either PIF4 or constitutive photomorphogenic 1 (COP1). BBX18 and BBX23 interact with ELF3 or COP1. Knocking out BBX18 and BBX23 increases ELF3 abundance under normal and warm temperature conditions. The expression of multiple thermoresponsive genes is impaired in both a PIF4 mutant and a BBX18/BBX23 double mutant. Thus, our findings reveal an important role of B-box proteins during thermomorphogenesis and provide insights into our understanding of how warm temperature signals regulate ELF3 activity and PIF4-dependent genes.