Hi-C map of the three-dimensional genome organization

The spatial arrangement of interphase chromosomes in the nucleus is important for gene expression and genome function in animals and in plants. The recently developed Hi-C technology is an efficacious method to investigate genome packing. Here we present a detailed Hi-C map of the three-dimensional genome organization of the plant Arabidopsis thaliana. We find that local chromatin packing differs from the patterns seen in animals, with kilobasepair-sized segments that have much higher intra-chromosome interaction rates than neighboring regions and which represent a dominant local structural feature of genome conformation in A. thaliana. These regions appear as positive strips on two-dimensional representations of chromatin interaction and they are enriched in epigenetic marks H3K27me3, H3.1 and H3.3. We also identify over 400 insulator-like regions. Furthermore, although topologically associating domains (TADs), which are prominent in animals, are not the dominant feature of A. thaliana genome packing, we found over 1,000 regions that have properties of TAD boundaries, and a similar number of regions similar to the interior of TADs. These insulator-like, TAD-boundary-like, and TAD-interior-like regions show strong enrichment for distinct epigenetic marks, and correlate with gene transcription levels. We conclude that epigenetic modifications, gene density, and transcriptional activity all contribute to shaping the local structure of the A. thaliana nuclear genome.