Radial endoplasmic reticulum arrays co-localize with radial F-actin in polarizing cells of brown algae
The organisation of the endoplasmic reticulum (ER) was examined in branching thallus cells and germinating cells derived from regenerated protoplasts (protoplast derived cells; PDCs) of Macrocystis pyrifera gametophytes by both immunofluorescent labelling of the HDEL sequence and vital staining with DiOC6(3), and in branching thallus cells of Splachnidium rugosum and Choristocarpus tenellus with DiOC6 (3) staining. In M. pyrifera and S. rugosum cells, distinctive cortical ER configurations with a clear radial arrangement were found at sites where a protrusion will emerge, while branching C. tenellus cells had a cortical ER patch from which a few linear ER elements expanded towards the rest of the cell. Examination of the organization of actin filaments (AFs) revealed similar radial AF structures at the site of the future protrusion, while the microtubule (Mt) organisation did not show any similar characteristic structure at the protrusion site. Latrunculin B inhibited germination in M. pyrifera PDCs, and the treated cells lacked radial ER or AF configurations. Treatment with taxol did not affect the germination process or the radial configurations. The structural similarity and the temporal and spatial coincidence of the ER and AF configurations combined with the inhibitor experiments lead to the hypothesis that ER, in conjunction with AFs, is implicated in the fixation of the polar axis in brown algal cells. The existence of characteristic ER and AF configurations in polarising cells of the three species suggests that they are a common characteristic of the brown algae.
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