Cell Type-Specific Profiling of Chromatin Modifications and Associated Proteins

Probing the 3D architecture of the plant nucleus with microscopy approaches: challenges and solutions

The eukaryotic cell nucleus is a central organelle whose architecture determines genome function at multiple levels. Deciphering nuclear organizing principles influencing cellular responses and identity is a timely challenge. Despite many similarities between plant and animal nuclei, plant nuclei present intriguing specificities. Complementary to molecular and biochemical approaches, 3D microscopy is indispensable for resolving nuclear architecture. However, novel solutions are required for capturing cell-specific, sub-nuclear and dynamic processes. We provide a pointer for utilising high-to-super-resolution microscopy and image processing to probe plant nuclear architecture in 3D at the best possible spatial and temporal resolution and at quantitative and cell-specific levels. High-end imaging and image-processing solutions allow the community now to transcend conventional practices and benefit from continuously improving approaches. These promise to deliver a comprehensive, 3D view of plant nuclear architecture and to capture spatial dynamics of the nuclear compartment in relation to cellular states and responses. Abbreviations: 3D and 4D: Three and Four dimensional; AI: Artificial Intelligence; ant: antipodal nuclei (ant); CLSM: Confocal Laser Scanning Microscopy; CTs: Chromosome Territories; DL: Deep Learning; DLIm: Dynamic Live Imaging; ecn: egg nucleus; FACS: Fluorescence-Activated Cell Sorting; FISH: Fluorescent In Situ Hybridization; FP: Fluorescent Proteins (GFP, RFP, CFP, YFP, mCherry); FRAP: Fluorescence Recovery After Photobleaching; GPU: Graphics Processing Unit; KEEs: KNOT Engaged Elements; INTACT: Isolation of Nuclei TAgged in specific Cell Types; LADs: Lamin-Associated Domains; ML: Machine Learning; NA: Numerical Aperture; NADs: Nucleolar Associated Domains; PALM: Photo-Activated Localization Microscopy; Pixel: Picture element; pn: polar nuclei; PSF: Point Spread Function; RHF: Relative Heterochromatin Fraction; SIM: Structured Illumination Microscopy; SLIm: Static Live Imaging; SMC: Spore Mother Cell; SNR: Signal to Noise Ratio; SRM: Super-Resolution Microscopy; STED: STimulated Emission Depletion; STORM: STochastic Optical Reconstruction Microscopy; syn: synergid nuclei; TADs: Topologically Associating Domains; Voxel: Volumetric pixel.

Genome-wide identification of RETINOBLASTOMA RELATED 1 binding sites in Arabidopsis reveals novel DNA damage regulators

Retinoblastoma (pRb) is a multifunctional regulator, which was likely present in the last common ancestor of all eukaryotes. The Arabidopsis pRb homolog RETINOBLASTOMA RELATED 1 (RBR1), similar to its animal counterparts, controls not only cell proliferation but is also implicated in developmental decisions, stress responses and maintenance of genome integrity. Although most functions of pRb-type proteins involve chromatin association, a genome-wide understanding of RBR1 binding sites in Arabidopsis is still missing. Here, we present a plant chromatin immunoprecipitation protocol optimized for genome-wide studies of indirectly DNA-bound proteins like RBR1. Our analysis revealed binding of Arabidopsis RBR1 to approximately 1000 genes and roughly 500 transposable elements, preferentially MITES. The RBR1-decorated genes broadly overlap with previously identified targets of two major transcription factors controlling the cell cycle, i.e. E2F and MYB3R3 and represent a robust inventory of RBR1-targets in dividing cells. Consistently, enriched motifs in the RBR1-marked domains include sequences related to the E2F consensus site and the MSA-core element bound by MYB3R transcription factors. Following up a key role of RBR1 in DNA damage response, we performed a meta-analysis combining the information about the RBR1-binding sites with genome-wide expression studies under DNA stress. As a result, we present the identification and mutant characterization of three novel genes required for growth upon genotoxic stress.

The Retinoblastoma (pRb) tumor suppressor is a master regulator of the cell cycle and its inactivation is associated with many types of cancer. Since pRb’s first description as a transcriptional repressor of genes important for cell cycle progression, many more functions have been elucidated, e.g. in developmental decisions and genome integrity. Homologs of human pRb have been identified in most eukaryotes, including plants, indicating an ancient evolutionary origin of pRb-type proteins. We describe here the first genome-wide DNA-binding study for a plant pRb protein, i.e. RBR1, the only pRb homolog in Arabidopsis thaliana. We see prominent binding of RBR1 to the 5’ region of genes involved in cell cycle regulation, chromatin organization and DNA repair. Moreover, we also reveal extensive binding of RBR1 to specific classes of DNA transposons. Since RBR1 is involved in a plethora of processes, our dataset provides a valuable resource for researches from different fields. As an example, we used our dataset to successfully identify new genes necessary for growth upon DNA damage exerted by drugs such as cisplatin or the environmentally prevalent metal aluminum.

Localization of Chromatin Marks in Arabidopsis Early Embryos

During early embryo development, profound changes in chromatin structure and regulation take place. It is difficult to study these changes in plant embryos however, largely because of their relative inaccessibility, which impedes the application of current epigenomic and biochemistry protocols. To circumvent this issue and to analyze the epigenetic status of the embryo at both the cellular and subcellular level, we describe here a simple method to immunolocalize chromatin marks in whole mount early Arabidopsis embryos, either within maternal tissues or isolated from seeds. We show that this protocol can be combined with fluorescent protein markers, allowing for the simultaneous detection of several chromatin components and/or cell fate markers. This new protocol will facilitate deciphering the epigenetic circuits controlling early embryogenesis in plants.

A Compendium of Methods to Analyze the Spatial Organization of Plant Chromatin

The long linear chromosomes of eukaryotic organisms are tightly packed into the nucleus of the cell. Beyond a first organization into nucleosomes and higher-order chromatin fibers, the positioning of nuclear DNA within the three-dimensional space of the nucleus plays a critical role in genome function and gene expression. Different techniques have been developed to assess nanoscale chromatin organization, nuclear position of genomic regions or specific chromatin features and binding proteins as well as higher-order chromatin organization. Here, I present an overview of imaging and molecular techniques applied to study nuclear architecture in plants, with special attention to the related protocols published in the "Plant Chromatin Dynamics" edition from Methods in Molecular Biology.

Unraveling the Complex Epigenetic Mechanisms that Regulate Gene Activity

Our understanding of the epigenetic mechanisms that regulate gene expression has been largely increased in recent years by the development and refinement of different techniques. This has revealed that gene transcription is highly influenced by epigenetic mechanisms, i.e., those that do not involve changes in the genome sequence, but rather in nuclear architecture, chromosome conformation and histone and DNA modifications. Our understanding of how these different levels of epigenetic regulation interact with each other and with classical transcription-factor based gene regulation to influence gene transcription has just started to emerge. This review discusses the latest advances in unraveling the complex interactions between different types of epigenetic regulation and transcription factor activity, with special attention to the approaches that can be used to study these interactions.

Profiling Developmentally and Environmentally Controlled Chromatin Reprogramming

Dynamic reshuffling of the chromatin landscape is a recurrent theme orchestrated in many, if not all, plant developmental transitions and adaptive responses. Spatiotemporal variations of the chromatin properties on regulatory genes and on structural genomic elements trigger the establishment of distinct transcriptional contexts, which in some instances can epigenetically be inherited. Studies on plant cell plasticity during the differentiation of stem cells, including gametogenesis, or the specialization of vegetative cells in various organs, as well as the investigation of allele-specific gene regulation have long been impaired by technical challenges in generating specific chromatin profiles in complex or hardly accessible cell populations. Recent advances in increasing the sensitivity of genome-enabled technologies and in the isolation of specific cell types have allowed for overcoming such limitations. These developments hint at multilevel regulatory events ranging from nucleosome accessibility and composition to higher order chromatin organization and genome topology. Uncovering the large extent to which chromatin dynamics and epigenetic processes influence gene expression is therefore not surprisingly revolutionizing current views on plant molecular genetics and (epi)genomics as well as their perspectives in eco-evolutionary biology. Here, we introduce current methodologies to probe genome-wide chromatin variations for which protocols are detailed in this book chapter, with an emphasis on the plant model species Arabidopsis.