Phytochromes and cryptochromes control morning photomorphogenesis pathways via BBX family proteins and HY5

A regulatory network of BBX transcription factors, HY5 and the related bZIP transcription factor HY5 HOMOLOGUE (HYH) has been postulated to control a large portion of the light-regulated transcriptome \cite{Xu2019}. We observe that multiple members of this network feature strongly during the dawn burst: not only HY5 and BBX31, but also HYHBBX16BBX24/SALT TOLERANCE (STO)BBX25/SALT TOLERANCE HOMOLOG (STH)BBX30 and BBX32 transcript levels peak within an hour after dawn (Fig. 4). These peaks are completely abolished in the phyA phyB cry1 cry2 mutant background, but only partially reduced in phyA phyB and cry1 cry2 double mutants (Supplemental Fig. X). This implies that both sets of photoreceptors act in parallel to induce HY5HYH and the BBX genes (hereafter referred to as BBX Set A). Another set of BBX genes (BBX Set B), comprised of BBX20 and BBX21, does not exhibit a dawn burst in expression, but their transcript levels are strongly upregulated in the photoreceptor mutant (Fig. 4; Supplemental Figure X). Interestingly, apart from BBX16 all transcription factors included in Set B have been shown to negatively regulate photomorphogenesis \cite{Indorf2007,Holtan2011,Gangappa2013,Gangappa2013a,Job2018,Heng2019}, while HY5, HYH as well as BBX20 and BBX21 represent photomorphogenesis-promoting transcription factors \cite{Chattopadhyay1998,Holm2002,Fan2012,Job2018}. Our results suggest that these factors constitute a highly orchestrated transcriptional network that controls photomorphogenic responses at dawn downstream of phytochrome and cryptochrome photoreceptors (Fig. 4B?)