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.