The B-box transcription factor 4 regulates seedling photomorphogenesis and flowering in tomato

• SlBBX4 positively regulates seedling photomorphogenesis in red light. • SlBBX4 promotes flowering in both long-day and short-day photoperiod conditions. • SlBBX4 interacts with tomato COP1 in yeasts and in planta. Tomato ( Solanum lycopersicum L. ) is an important solanaceous crop worldwide. Research in the model plant species Arabidopsis revealed that B-box family transcription factors (BBXs) play key roles in plant development, however, only very few tomato BBXs have been functionally characterized. In this work, we report that the transcription factor SlBBX4 (Solyc08g006530) plays critical roles in tomato seedling photomorphogenesis. The slbbx4 frame-shift mutants showed hyposensitive response to red light, while maintained normal responses to blue, far-red light and UV-B in hypocotyl length assays. The red light-specific function in seedling photomorphogenesis is analogous to Arabidopsis BBX4 (AtBBX4). Whereas atbbx4 mutant flowered earlier than wildtype, the slbbx4 mutants showed delayed flowering, under both long-day and short-day conditions. Thus, SlBBX4 and AtBBX4 play both shared and distinct functions in plant development. The E3 ligase CONSTITUTIVELY PHOTOMORPHOGENIC1 (COP1) is known to play important functions in the regulation of seedling photomorphogenesis and flowering in Arabidopsis. SlBBX4 interacts with tomato COP1 (SlCOP1) in yeast and in planta, suggesting that SlBBX4 is likely regulated by SlCOP1. Taken together, our data suggest that SlBBX4 plays promotive roles in red light-induced photomorphogenesis and flowering, both of which are potentially beneficial in agriculture.