The three-dimensional folding of chromatin contributes to the control of genome functions in eukaryotes, including transcription, replication, chromosome segregation, and DNA repair. In recent decades, many cytological and molecular methods have provided profound structural insights into the hierarchical organization of plant chromatin. With the Hi-C (high-throughput chromosome conformation capture) technique, analyses of global chromatin organization in plants indicate considerable differences across species. However, our knowledge of how chromatin organization at a local level is connected to tissue-specific gene expression is rather limited. This problem can be tackled by performing fluorescence-activated sorting of fixed nuclei followed by Hi-C, which is tailored for a limited number of input nuclei. Here, we describe an approach of isolating Arabidopsis thaliana nuclei with defined endopolyploidy level and subsequent in situ Hi-C library preparation for low-input plant materials. In principle, this method can be applied to any types of fluorescence-labeled nuclei, offering researchers a useful tool to unveil temporal and spatial chromatin dynamics in 3D in a tissue-specific context.

The three-dimensional (3D) organization of the genome is important for chromatin regulation. This organization is nonrandom and appears to be tightly correlated with or regulated by chromatin state and scaffolding proteins. To understand how specific DNA and chromatin elements contribute to the functional organization of the genome, we developed a new tool-the tagged chromosomal insertion site (TCIS) system-to identify and study minimal DNA sequences that drive nuclear compartmentalization and applied this system to specifically study the role of cis elements in targeting DNA to the nuclear lamina. The TCIS system allows Cre-recombinase-mediated site-directed integration of any DNA fragment into a locus tagged with lacO arrays, thus enabling both functional molecular studies and positional analysis of the altered locus. This system can be used to study the minimal DNA sequences that target the nuclear periphery (or other nuclear compartments), allowing researchers to understand how genome-wide results obtained, for example, by DNA adenine methyltransferase identification, chromosome conformation capture (HiC), or related methods, connect to the actual organization of DNA and chromosomes at the single-cell level. Finally, TCIS allows one to test roles for specific proteins in chromatin reorganization and to determine how changes in nuclear environment affect chromatin state and gene regulation at a single locus.