Dynamics of the endoplasmic reticulum in living onion epidermal cells in relation to microtubules, microfilaments, and intracellular particle movement

Abstract The endoplasmic reticulum (ER) and associated organelle and particle movements in onion ( Allium cepa ) bulb scale epidermal cells were observed, recorded, and analyzed using computer‐assisted video (AVEC‐DIC, AVEC‐POL and fluorescence) microscopy. The ER is composed of two interconnected sets of filamentous membrane tubules with diameters ranging from 0.1 to 0.5 μm. The first form a more stable, stationary network of intersecting polygonal membrane tubules lying closely appressed to the plasma membrane and continuous with a second very dynamic set of longer membrane tubules that often are located parallel to each other, shifting rapidly around the cytoplasm and forming dynamic knots or organization centers. The ER, mitochondria, and spherosomes fluoresced upon chlortetracycline treatment and are therefore presumed to sequester calcium. ER and mitochrondria also stain with the fluorescent dye, rhodamine 123. Mitochrondria and spherosomes are seen to move in the cytoplasm only along paths parallel to the axis of the ER tubules. Smaller particles (0.5 μm) tend to follow these same paths but may occasionally move independently. Particles and organelles move in close, but not in direct, association with the ER tubules. In optically favored cells, actin filaments were occasionally recorded located in parallel with the ER tubules and directly associated with moving particles. Streaming ceased promptly and reversibly upon treatment with cytochalasin B, which did not visibly disrupt the ER. Short‐term treatment with colchicine did not inhibit streaming or disrupt the ER network, whereas long‐term (hours) colchicine treatments caused the disappearance of the stationary, cortical polygonal networks and an aggregation of still slowly moving organelles and particles onto now visible actin filaments. This suggests that microtubule breakdown disrupts the three‐dimensional distribution of the ER and rearranges actin filaments in the cell's cytoplasm. Actin filaments must be directly involved in generation of movement of the particles and organelles. A three‐dimensional model, based on optical sectioning of the epidermal cells, is proposed to illustrate the distribution of the endoplasmic reticulum in onion epidermal cell cytoplasm.