Global nuclear reorganization during heterochromatin replication in the giant-genome plant Nigella damascena L

Among land plants, genome sizes vary remarkably by > 2,200-fold. This variation depends on the loss and gain of non-coding DNA sequences, forming different heterochromatin complexes during interphase. In plants with a giant genome, the major part of chromatin stays condensed in interphase, forming a dense meshwork of heterochromatin threads, called interphase chromonemata. Using super-resolution light and electron microscopy, we studied the ultrastructure of chromonemata during and after replication in root meristem nuclei of Nigella damascena L. During S-phase, heterochromatin undergoes transient decompaction locally at the active sites of DNA synthesis, and due to the heterochromatin abundance, chromonema replication is accompanied by a robust chromonema meshwork disassembly, which led to the general reorganization of the nucleus morphology visible even by conventional light microscopy. After replication, the heterochromatin condenses again, restoring the chromonema structure. Thus, we showed that heterochromatin replication in interphase nuclei of giant-genome plants induces a global chromonema decondensation and reorganization. ### Competing Interest Statement The authors have declared no competing interest. * EdU : 5-ethynyl-2′-deoxyuridine 3D-SIM : Three-dimensional structured illumination microscopy CLEM : Correlative light and electron microscopy TEM : Transmission electron microscopy