Microtubule Arrangement during the Formation of a New Growing Tip in the Coenocytic Green Alga Bryopsis plumosa (Caulerpales, Chlorophyta)

The coenocyte of Bryopsis gametophytes is composed of a main axis and side branches lateral to the main axis, both of which grow by cell growth at a dome-like tip of cylindrical axis or branches. Regular development of side branches along the main axis formed pinnate thallus of Bryopsis gametophyte and, therefore, the formation of new growing tip is a key event in the thallus morphogenesis of the alga. Previous inhibitory experiments showed that the microtubule cytoskeleton is involved in cell growth and regular development of side branches. To clarify the role of microtubules in the formation of new growing tip further, we observed the microtubule arrangement during 2 different modes of growing tip formation—formation of side branches and regeneration of protoplasm aggregates in B. plumosa by immunofluorescence microscopy. A longitudinal, dense arrangement of cortical microtubules was observed in the cytoplasm of the main axes. These microtubules appeared to be interrupted in the region of side branch formation during its earliest stage. In the spherical cells regenerated from protoplast aggregates, a radial array of cortical microtubules was observed in the cell protrusion, which later became the growing tip of the regenerating cell. Thus, microtubules showed specific morphological changes in the region of growing tip formation in B. plumosa.