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Fucus zygotes polarise and germinate a rhizoid before their first asymmetrical division. The role of microtubules (MTs) in orienting the first division plane has been extensively studied by immunofluorescence approaches. In the present study, the re-organisation of MT arrays during the development of Fucus zygotes and embryos was followed in vivo after microinjection of fluorescent tubulin. A dynamic cortical MT array that shows dramatic reorganization during zygote polarization was detected for the first time. Randomly distributed cortical MTs were redistributed to the presumptive rhizoid site by the time of polarisation and well before rhizoid germination. The cortical MT re-organisation occurs independently of centrosome separation and nucleation. By the time of mitosis the cortical array depolymerised to cortical foci in regions from which it also reformed following mitosis, suggesting that it is nucleated from cortical sites. We confirm previous indications from immunodetection studies that centrosomal alignment and nuclear rotation occur via MT connexions to stabilised cortical sites and that definitive alignment is post-metaphasic. Finally, we show that cortical MTs align parallel to the growth axis during rhizoid tip growth and our results suggest that they may be involved in regulating rhizoid growth by shaping the rhizoid and containing turgor pressure.

We studied the effects of auxin (indolyl-3 acetic acid) on formation of the primary polarity axis in zygotes of the brown algae Fucus vesiculosus. Within the first 2.5 h after fertilization, the zygotes release this phytohormone in the environment. The treatment of developing zygotes with the inhibitor of indolyl-3-acetic acid transport from the cell triiodobenzoic acid at 5 mg/l arrests the auxin secretion and leads to its accumulation in the cells. This causes a significant delay in zygote polarization. The treatment of zygotes with the exogenous indolyl-3-acetic acid at 1 mg/l stimulates cell polarization and formation of a rhizoid process. When auxin was added to the medium with triiodobenzoic acid, the inhibitory effect of the latter was fully relieved. It has been proposed that the content of indolyl-3-acetic acid in the environment is a key factor in the induction of polarity of the F. vesiculosus zygotes.