Types of Chromatin Organization in Plant Nuclei

Abstract

The nuclear ultrastructure of 15 angiospermal plants was studied with respect to the structural type, the proportion of condensed chromatin, the diameter of the chromatin fibers, and the DNA content. It was found that (1) at least three species-specific ultrastructural types can be distinguished among plants: the diffuse, the chromomeric and the chromonematic type: the diffuse nuclei exhibit always chromocenters, but evenly distributes, often hardly visible euchromatin, the two other types occur in sub-types, characterized by the presence or absence of chromocenters, and exhibit euchromatin of the same electron density as heterochromatin;—(2) the species-specific nuclear type is mainly determined by the nuclear DNA content (2 C value), high values favoring the chromonematic type, low values the other three types and the appearance of chromocenters;—(3) the diameters of the chromatin fibers are similar, although significantly different between certain species, in all structural types, falling into distinct classes of about 50, 100, 150, 200, 300 and 400 A. The proportion of the various fibers varies between species, and one or the other class may be absent in a given species;—(4) The proportion of chromatin that is condensed into an electron-dense state is positively correlated with the DNA content; while in chromocentric nuclei the condensed chromatin corresponds to heterochromatin at the light microscope level, it corresponds to both eu- and heterochromatin in the other types of nuclei.