Confocal microscopy of microtubule arrays in cryosectioned sporangia of Phytophthora cinnamomi

Immunofluorescence microscopy has been used to study the distribution of microtubules within the uncleaved and cleaving sporangium of the Oomycete Phytophthora cinnamomi. Prior to the induction of cleavage, extensive astral arrays of microtubules were seen within the sporangium. Each array emanated from a narrow, basal body-associated pole of the regularly spaced nuclei. The extent of these arrays was much greater than that suggested by previous ultrastructural studies of nucleusassociated microtubules in the sporangia of lower fungi. After induction, microtubule arrays of similar form to those evident prior to induction were consistently seen throughout the 60-min course of cleavage. During cleavage, flagella developed from the paired basal bodies associated with each narrow nuclear pole. The flagella were first evident as short linear initials at 20 min after induction. They proceeded to extend linearly until 40 min, by which time their form appeared more sinuous. The flagellar rootlet system, which had been previously described in released zoospores, was also evident in fully cleaved sporangia. A number of models are proposed to explain how the astral microtubule arrays may regulate the regular spacing of nuclei and the orderly development of the cleavage planes.