MICROTUBULES REGULATE THE GENERATION OF POLARITY IN ZOOSPORES OF PHYTOPHTHORA-CINNAMOMI

Zoospores of Phytophthora cinnamomi are formed by cleavage of a multin ucleate sporangium and contain nine different components that are dist ributed or oriented about a well-defined axis running through a pair o f basal bodies near the nucleus. In this study, the importance of the cytoskeleton in establishing and maintaining cellular polarity was exa mined by using the anti-microtubule drug oryzalin and the anti-microfi lament 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 ce lls: an astral microtubule (MT) array, the nucleus, mitochondria and t hree different types of vesicles, two of which are involved in directe d 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. Ory zalin also caused abnormal cleavage similar to that seen in CD-treated cells. Most intriguing, however, was the finding that although the th ree vesicle types in cleaved, oryzalin-treated sporangia did not exhib it the polarized distribution seen in control and CD-treated cells, in many cases the vesicles had, nevertheless, lost their initially rando m distributions and had become concentrated in the cytoplasm adjacent to the abnormal cleavage planes. Thus although an intact MT array is r equired for segregation of the vesicles within the cortex, their redis tribution to the cortex can somehow occur in the absence of MTs and ac tin microfilaments.