Arabidopsis thaliana BBX14 is a target of GLK1 and involved in high-light acclimation, photomorphogenesis and GUN-type retrograde signaling

Development of photosynthetically competent seedlings requires both light and retrograde biogenic signaling pathways. The transcription factor GLK1 functions at the interface between these pathways, and receives input from the biogenic-signaling integrator GUN1. BBX14 was previously identified, together with GLK1, in a core module that mediates the response to high light levels and biogenic signaling. To gain insight into the function of BBX14, we generated BBX14 overexpressors and CRISPR/Cas-mediated bbx14 mutant plants, conducted high-light, RT-qPCR and ChIP-Seq experiments, measured photosynthetic parameters, chlorophyll contents and growth rates, and analyzed alterations in transcriptomics. We found that, although overexpression of BBX14 is deleterious under normal growth conditions, BBX14 is needed to acclimate plants to high light stress. BBX14 is a direct target of GLK1, and RNA-Seq analysis suggests that BBX14 is involved in the circadian clock. Knockout of BBX14 results in a long-hypocotyl phenotype that depends on a retrograde signal, and BBX14 expression during biogenic signaling requires GUN1. Finally, we clarify the role of BBX14 in GUN-type biogenic signaling. We conclude that BBX14 is an integrator of photomorphogenetic and biogenic signals, and suggest that BBX14 is a nuclear target of retrograde signals downstream of the GUN1/GLK1 module.