The Organization of the Nucleolus in Meristematic Plant Cells

Abstract

The architecture of the nucleolus in Allium porum and Triticum vulgare meristematic cells has been investigated by means of digestions with various enzymes. After staining with azure B at pH4, plant nucleoli exhibit lighter regions which, under electron microscopy, correspond to the fibrillar zones characterizing these organelles. Evidence is presented indicating that these latter zones contain coarse convoluted filaments quite similar to the loops first demonstrated by La Cour (24) and which are assumed to originate from the nucleolar-organizing chromosomes. These coarse, 0.2p wide filaments are remarkably resistant to the action of deoxyribonuclease, ribonuclease, pepsin, trypsin, or of various combinations of these enzymes and, moreover, they show insignificant incorporation of labeled thymidine even after long exposure to this DNA precursor. The clearing action of pepsin on different regions of the nucleolus lends support to the hypothesis that an amorphous material or matrix pervades the mass of this organelle. This effect is particularly striking within the particulate nucleolar zones themselves. Both ribonuclease and trypsin disorganize the RNP (ribonucleoprotein) nucleolar particles. The effect of the latter enzyme on the RNP particles is taken to indicate that they contain proteins particularly susceptible to trypsin which are essential for maintenance of their morphological integrity. Trypsin also interferes with azure B-staining of the nucleolar mass as a whole and, according to radioautographic data, extracts RNA throughout this organelle. Accordingly, the hypothesis is considered that RNA is complexed with proteins not only within the particulate nucleolar portions, as is already well known, but also in the fibrillar zones.Recent electron microscopic studies have shown that the organizational pattern of the nucleolus varies to a certain extent with species. This diversity in nucleolar architecture is especially striking in animals (see references 3 and 19 for recent reviews). However, in plants, except for numerous vacuoles and lacunae, the nucleolar mass is gener--ally composed of two main types of zones (22, 23, 25 28, 41, 47) There is now good evidence that these latter zones are intimately associated with intranucleolar chromatin (22,23,(25)(26)(27)(28)41).As judged from available information on the biochemical composition of the nucleolus, there 633 on May 9, 2018 jcb.rupress.org Downloaded from

Nucleolonema as a fundamental substructure of the nucleolus

“…The authors argue that the increment of the FC reflects activation of the rDNA replication activity. The FCs of plant cells such as Vicia faba and tobacco BY-2 cells take on a homogeneous texture (Lord and Lafontaine 1969;Sato and Myoraku 1994;Saito et al 2001), but they sometimes involve condensed chromatin in Allium cepa and Triticum aestivum (Jordan et al 1982;Risueño et al 1982: Motte et al 1988Sato and Fujie 1997). The former is termed a homogeneous FC and the latter a heterogeneous FC (Risueño et al 1982).…”

Silver staining of nucleolar organizers and nucleoli during mitosis and interphase of mammalian cells

“…That nucleoti are constituted of nucleolonemata has been also revealed after treatment with tween 80 (e.g. LaCour, 1966;Lord & Lafontaine, 1969). The chain-like beaded appearance of the Ag-positive materials in interphase nucleoli could be a functional expression of the redundant ribosomal genes.…”

Conditions aseptiques pour l'endurcissement au froid de plantules de ble d'automne et de luzerne.

“…3). Comme c'est le cas des autres organismes (Lord et Lafontaine 1969), le nucleole du noyau interphasique possede des zones fibrillaires entremelees de zones granulaires. On retrouve des granules de petite taille au sein du nucieoplasme.…”