Microtubules in the fungal pathogen Ustilago maydis are highly dynamic anddetermine cell polarity

Many fungal pathogens undergo a yeast-hyphal transition during their pathog enic development that requires rearrangement of the cytoskeleton, followed by directed membrane traffic towards the growth region. The role of microtu bules and their dynamic behavior during this process is not well understood . Here we set out to elucidate the organization, cellular role and in vivo dynamics of microtubules in the dimorphic phytopathogen Ustilago maydis. Hy phae and unbudded yeast-like cells of U, maydis contain bundles of spindle pole body-independent microtubules, At the onset of bud formation two spher ical tubulin structures focus microtubules towards the growth region, sugge sting that they support polar growth in G(2), while spindle pole body-nucle ated astral microtubules participate in nuclear migration in M and early G( 1), Conditional mutants of an essential alpha -tubulin gene from U, maydis, tub1, confirmed a role for interphase microtubules in determination of cel l polarity and growth. Observation of GFP-Tub1 fusion protein revealed that spindle pole body-independent and astral microtubules are dynamic, with el ongation and shrinkage rates comparable to those found in vertebrate system s. In addition, very fast depolymerization was measured within microtubule bundles. Unexpectedly, interphase microtubules underwent bending and rapid translocations within the cell, suggesting that unknown motor activities pa rticipate in microtubule organization in U, maydis.