Organization and dynamics of cortical endoplasmic reticulum in inner epidermal cells of onion bulb scales

Abstract

The cortical endoplasmic reticulum (ER) of living onion inner epidermal cells has been studied by video-microscopy. We observed local movements of individual ER membranes, which cause transformations of the polygonal net. Membrane tubules glide along one another, causing transfiguration, reduction and decomposition of polygons. Membrane tubules and lamellae also extend from the existing net and thus increase the amount of ER. These movements occur in close correlation with organelle movements, suggesting a structural coalignment of the net with actin microfilaments (MFs). The membranes in the cortical cytoplasm are not distributed randomly but are tethered to certain domains; even when dislocated, they return to such anchoring points. This was not observed with ER reaching deeper into the cytoplasm. We therefore propose that close associations of ER and the plasma membrane (PM) stabilize the cortical ER and may stabilize coaligning MFs as well.