Project description:Background: There is a growing interest in the role of chromatin in acquiring and maintaining cell identity. Despite the ever growing availability of genome-wide gene expression data, understanding how transcription programs are established and regulated to define cell identity remains a puzzle. An important mechanism of gene regulation is the binding of transcription factors to specific DNA sequence motifs across the genome. However, these sequences are hindered by the packaging of DNA to chromatin. Thus the accessibility of these loci for TF binding is highly regulated and determines where and when TF bind. We present a workflow for measuring chromatin accessibility in Arabidopsis thaliana, and define organ-specific regulatory sites and binding motifs of transcription factors at these sites. Results: We coupled the recently described INTACT (Isolation of Nuclei TAgged in specific Cell Types) and ATAC-seq (Assay for Transposase-Accessible Chromatin with highthroughput sequencing) as a genome-wide strategy to uncover accessible regulatory sites in Arabidopsis, based on their accessibility to nuclease digestion. By applying this pipeline in Arabidopsis roots, we revealed 41,419 accessible sites, of which approximately half are found in gene promoters and contain the H3K4me3 active histone mark. The root-unique accessible sites from this group are enriched for root processes. Interestingly, most of the root-unique accessible sites are found in nongenic regions, but are correlated with root-specific expression of distant genes. Importantly these gene-distant sites are enriched for binding motifs of TFs important for root development, as well as motifs for TFs that may play a role as novel transcriptional regulators in roots, suggesting that these accessible loci are functional novel genedistant regulatory elements. Conclusion: By coupling the INTACT with ATAC-seq methods, we present a feasible pipeline to profile accessible chromatin in plants. We also introduce a rapid measure of the experiment quality. We find that chromatin accessibility at promoter regions is strongly associated with transcription and active histone marks. However, root specific chromatin accessibility is primarily found at intergenic regions, suggesting their predominance in defining organ identity, possibly via long-range chromatin interactions. This workflow can be rapidly applied to study the regulatory landscape in other cell types, plant species and conditions.

Project description:X chromosome reactivation (XCR) represents a paradigm to study epigenetic regulation and the reversal of chromatin silencing, although how it is linked to the pluripotency gene regulatory network, pluripotency transcription factors (TFs) and chromatin remodelling processes remains largely unexplained. Several pluripotency TFs have been linked to XCR through binding to the long non-coding RNA gene Xist, but whether other regulatory elements are involved is unclear. Here, we show that reprogramming to iPSCs induces gradual acquisition of chromatin accessibility at specific sites on the inactive X chromosome (Xi) which is subsequently propagated to other regulatory elements including enhancers and promoters. Regions that are genomically closer to day 0 accessible regions become biallelically accessible earlier, than the more distant ones. For distinct categories of chromatin regions that become biallelically accessible at different times, different sets of transcription factor (TF) motifs are enriched, including the pluripotency TFs KLF4 and cMYC. In addition, regions that become bialleliacally accessible earlier also show increased TF pluripotency binding, supporting the link between pluripotency TFs in XCR. To further explore the relationship between pluripotency TFs and XCR, we construct the active Gene Regulatory Networks (GRNs) during iPSC reprogramming using scRNA-seq and computational analyses. We reveal gene targets of 311 TFs expressed across reprogramming to iPSCs. Using this single-cell regulatory atlas, we show that X-linked genes are preferentially targeted by several pluripotency TFs. Our results suggest a new model where XCR may be coordinated by direct targeting of regulatory elements on the X chromosome by pluripotency TFs, concomitant with step-wise acquisition of chromatin accessibility. Altogether, our results demonstrate how gradual acquisition of a new GRN during reprogramming of cellular identity is linked with dynamic induction of chromatin accessibility and overcomes stable chromatin silencing on the Xi.

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RNA-seq from roots of 14 day old col-0 Arabidopsis thaliana carrying INTACT

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