Organization of the nucleus, nucleolus, and protein-synthesizing apparatus in relation to cell development in roots of Pisum sativum

Abstract

For analysis, roots were divided into 10 1-mm segments starting at the tip. Root marking and cell-length measurements revealed three main developmental zones in the cortex: segment 1, cell division with compensating cell growth; segments 3–5, rapid cell elongation; segments 7–10, cell maturation without growth. Nuclear volume increased steadily to segment 5 and remained high. Chromatin was moderately condensed in segment 1, became quite decondensed in many nuclei of segment 3, and showed strongly condensed regions in about half of the maturing cells. Karyosomes, chromatin 'puffs,' and perichromatin granules were present in segments 1 and 3 but became much reduced as development proceeded. The nucleolus was large in segments 1 and 3 but was markedly smaller in segment 5 and beyond. As nucleolar size decreased the granular zone was reduced and chromatin within lacunae was withdrawn. Nucleoli of maturing cells were small compact fibrillar bodies. Nucleolar vacuoles were present in segment 1 but varied in size and frequency from root to root. The cytoplasm of segment 1 was packed with ribosomes and contained only isolated elements of endoplasmic reticulum (ER). By segment 3 extensive rough ER (RER) was present; this persisted through to segment 9, although becoming reduced in extent. Segments 3–7 showed the highest density of ribosomes on the ER. Ribosome number was decreased in the fully elongated cells. Incorporation of ³H-cytidine into ribosomal RNA was highest in segment 1 and declined steadily with development. The functional role of the described chromatin organizations is unresolved; the nucleolar and ribosomal changes seem characteristic of plant cell development and their significance is discussed.