The arrangement of F-actin and microtubules during germination of Mucor rouxii sporangiospores.

The distribution of F-actin microfilaments and microtubules was analyzed in germinating sporangiospores of Mucor rouxii by labeling with rhodamine-tagged phalloidin and by immunofluorescence microscopy. The transition from isodiametrical to apical growth was accompanied by a switch from uniform distribution of F-actin patches to a polarized accumulation of F-actin material at the germ tube tips. Immunoblotting of cell-free extracts of M. rouxii with a monoclonal anti-porcine alpha-tubulin antibody (TU-01) disclosed two discrete bands of alpha-tubulin suggesting the existence of two alpha-tubulin genes in this fungus. Immunofluorescence microscopy of germinating cells stained with the same antibody revealed an elaborate network of cytoplasmic microtubules that persisted during the entire germination process and extended into the apex of the germ tube. Although their precise roles remain undetermined, the observed arrangement of cytoskeletal elements during germination is consistent with their presumed involvement in cell wall morphogenesis: the long axial microtubules serving as long-distance conveyors of wall-building vesicles to the apical region while the concentrated F-actin patches mark the participation of microfilaments in the zone of intense vesicle exocytosis at the hyphal apex.