Spatial determinants in morphogenesis: recovery from plasmolysis in the diatom Ditylum

Ditylum cells are enclosed in a rigid wall consisting of two "valves" (end walls) connected by "girdle bands." A hollow spine, the Labiate Process (LP), extends from each valve and a stable cytoplasmic strand connects its base with the nucleus. We investigated whether cells might possess "spatial determinants" for controlling their internal organization and wall morphogenesis. Upon plasmolysis, cells contracted into a spherical protoplast detached from the wall. Recovery was initiated by growing filopodia that "searched" the inside of the wall. Some attached to the inside corners, generating tension that could temporarily displace the protoplast. Others consolidated into the strand connecting nucleus with the LP. The protoplasts soon expanded and cells recovered: some divided immediately, the rest within 24 h. When recently divided cells were plasmolysed, their nascent valves were exocytosed. These were ignored by the filopodia during recovery. Later, protoplasts secreted a new valve, while the nascent valves were discarded. The interphase microtubule (MT) cytoskeleton radiates from a central Microtubule Center. A thicker bundle connects the nucleus to each LP. Plasmolysis destroyed the MT cytoskeleton; its re-establishment matched growth of the filopodia. The anti-MT drug oryzalin prevented filopodial extension while existing filopodia retracted, except those stabilized by attachment to the corners of the cell and the LP. Several anti-actin agents had relatively little effect. However, one, mycalolide B, caused the nucleus to be extruded from the protoplast by a bundle of MTs. We conclude that the geometry of the wall could provide spatial information to which the MT-cytoskeleton/filopodia respond.