Distinct clades of TELOMERE REPEAT BINDING transcriptional regulators interplay to regulate plant development

TELOMERE REPEAT BINDING proteins (TRBs) are plant-specific transcriptional regulators that combine two DNA-binding domains, the GH1 domain shared with H1 histones that binds to linker DNA and the Myb/SANT domain that specifically recognizes the telobox DNA binding site motif. TRB1, TRB2 and TRB3 proteins recruit the Polycomb group complex 2 (PRC2) to deposit H3K27me3 and JMJ14 to remove H3K4me3 at target genes containing telobox motifs in their promoters to repress transcription. Here, we characterize the function of TRB4 and TRB5, which belong to a separate TRB clade conserved in spermatophytes. TRB4 and TRB5 affect the transcriptional control of several hundred genes involved in developmental responses to environmental cues, the majority of which differ from differentially regulated genes in trb1 trb2 trb3, suggesting distinct modes of action at the chromatin level. Indeed, TRB4 binds to several thousand sites in the genome, mainly at TSS and promoter regions of transcriptionally active and H3K4me3-marked genes but is not enriched at H3K27me3-marked gene bodies. TRB4 physically interacts with the PRC2 component CURLY LEAF (CLF), but, unexpectedly, loss of TRB4 and TRB5 partially suppresses the developmental defects of clf mutant plants, by acting as transcriptional activators of the key flowering genes SOC1 and FT. We further show that TRB4 and TRB1 share multiple target genes and reveal physical and genetic interactions between TRBs of the two distinct clades, collectively unveiling that TRB proteins engage in both positive and negative interactions with other members of the family to regulate plant development through PRC2-dependent and independent mechanisms.