Microtubule-dependent processes in Funaria hygrometrica spore mother cells (SMCs) were analysed by monitoring the effects of colchicine and oryzalin on pre-meiotic and meiotic events. The techniques used were electron microscopy, immunofluorescence microscopy of microtubules (MTs)and continuous video recording of events in treated and recovering live cells sampled at various stages of sporogenesis. Inferences drawn from previous work that the SMC plastids serve as MT-organizing centres were confirmed in so far as MT recovery in MT-depleted cells starts at the tips of the plastids. The MTs that emanate from these regions are required for positioning the plastids in the tetrahedral conformation, which defines the meiotic poles, for positioning lipid droplets in clusters at these poles and for positioning and holding the nucleus in the tetrahedral cage. If released, the nucleus can be moved by a non-MT system. Other phenomena not controlled by MTs are plastid elongation, maintenance of the tetrahedral conformation-when the MTs are absent (during divisions or as a result of drug treatment) and (probably) development of the organelle band that spans the cell between divisions I and II.
In cells treated during division, when there is nonuclear envelope, the pattern of MT recovery is different: the plastids are inactive as microtubule-organizing centres (MTOCs) but MTs reappear among the chromosomes. Spindles capable of transporting chromosomes regenerate. However, the importance of interactions between nucleus and plastids is highlighted by cases in which treatmenthas resulted in: (1) movement of the nucleus out of the quadripolar plastid cage; and (2) loss of the MTs at plastid tips that normally contribute to the spindle poles; in such cases quadripolarity is lost even though functional spindles return. Plastid MTOC activity returns when the nuclear envelope isin place, i.e. in interkinesis and after telophase II.