Ultrastructural changes in cells of pea root cortical explants culturedIn vitro

Abstract

Root cortical explants from seedlings ofPisum sativum L., cv. Little Marvel were cultured on a sterile nutrient medium in the presence of auxins or auxins and cytokinin. Explants were fixed (and subsequently processed for electron microscopic observation) at the outset and after 30, 60, and 72 hours of culture under the two hormonal conditions. In the presence of auxin alone, the cell walls of the cortical parenchyma showed distinctive structural changes involving the deposition of a new, diffusely fibrillar primary wall. A considerable increase of rough ER in the adjacent cytoplasm was associated with the new wall synthesis. These wall changes are interpreted as auxin-induced and prelude to cell enlargement and later cell separation. No dramatic changes occurred in other cytoplasmic organelles or in the nucleus. In the presence of cytokinin and auxin, the striking cytological events observed included marked nuclear changes and greater cytoplasmic density due to increased organelles associated with the onset of DNA synthesis, mitosis and cytokinesis. New cell walls formed from the developed phragmoplasts, cleaving the original parenchyma cells into smaller cellular compartments with no accompanying cell enlargement. No marked changes in the original primary cell walls were observed in cytokinin-auxin-treated explants. By 72 hours some cells already had completed two successive cell divisions. No ultrastructural evidence was obtained suggesting that these cells were committed to their known fate of differentiating into mature tracheary elements in the subsequent 2–4 days. At 72 hours each explant represented a population of actively dividing, still considerably vacuolated meristematic cells.