A SAM oligomerization domain shapes the genomic binding landscape of the LEAFY transcription factor

Multicellular organisms develop new structures by initiating specific gene regulatory programs through coordinated changes at the transcriptional and chromatin levels. Master transcription factors orchestrate such global changes but the underlying molecular mechanisms remain unclear. Here we focus on LEAFY (LFY), a key regulator of flower development in angiosperms. The resolution of the crystallographic structure of its N-terminal domain shows that it forms an unanticipated Sterile Alpha Motif (SAM) domain. This domain drives LFY oligomerization, which we establish to be essential for LFY floral switch function. Combining in vitro and in vivo experiments, we demonstrate that oligomerization facilitates LFY binding to DNA regions with multiple binding sites. Moreover, genome-wide analyses reveal an additional prominent role for the oligomerization domain: it confers upon LFY the capacity to bind to closed chromatin regions. This novel property, combined with its previously demonstrated capacity to recruit chromatin remodelers, indicates that LFY controls floral fate by acting as a plant pioneer transcription factor. Overall design: A. thaliana LEAFY oligomerization ChIP-seq vs. control, 2 replicates.