Microtubules regulate the generation of polarity in zoospores of Phytophthora cinnamomi.
Zoospores of Phytophthora cinnamomi are formed by cleavage of a multinucleate sporangium and contain nine different components that are distributed or oriented about a well-defined axis running through a pair of basal bodies near the nucleus. In this study, the importance of the cytoskeleton in establishing and maintaining cellular polarity was examined by using the anti-microtubule drug oryzalin and the anti-microfilament drug cytochalasin D (CD). The effects of the drugs on uncleaved and cleaving sporangia were determined, using fluorescence microscopy, for six of the components that are polarized in untreated cleaved cells: an astral microtubule (MT) array, the nucleus, mitochondria and three different types of vesicles, two of which are involved in directed exocytosis. CD had no effect upon the MT arrays, the positioning of nuclei or the polarized redistribution of mitochondria and vesicles to the cortical cytoplasm, although it did cause abnormal cleavage. The effects of oryzalin, however, indicate that the asymmetric disposition of the MT array is fundamental to zoospore polarities: when the array is itself eliminated with this drug, none of the other five elements show any signs of polar positioning within the cleaved sporangium. Oryzalin also caused abnormal cleavage similar to that seen in CD-treated cells. Most intriguing, however, was the finding that although the three vesicle types in cleaved, oryzalin-treated sporangia did not exhibit the polarized distribution seen in control and CD-treated cells, in many cases the vesicles had, nevertheless, lost their initially random distributions and had become concentrated in the cytoplasm adjacent to the abnormal cleavage planes. Thus although an intact MT array is required for segregation of the vesicles within the cortex, their redistribution to the cortex can somehow occur in the absence of MTs and actin microfilaments.