The dynamic behaviour of microtubules and their contributions to hyphal tip growth in Aspergillus nidulans

Creating and maintaining cell polarity are complex processes that are not fully understood. Fungal hyphal tip growth is a highly polarized and dynamic process involving both F-actin and microtubules (MTs), but the behaviour and roles of the latter are unclear. To address this issue, MT dynamics and subunit distribution were analysed in a strain of Aspergillus nidulans expressing GFP-alpha-tubulin. Apical MTs are the most dynamic, the bulk of which move tipwards from multiple subapical spindle pole bodies, the only clear region of microtubule nucleation detected. MTs populate the apex predominantly by elongation at rates about three times faster than tip extension. This polymerization was facilitated by the tipward migration of MT subunits, which generated a tip-high gradient. Subapical regions of apical cells showed variable tubulin subunit distributions, without tipward flow, while subapical cells showed even tubulin subunit distribution and low MT dynamics. Short MTs, of a similar size to those reported in axons, also occasionally slid into the apex. During mitosis in apical cells, MT populations at the tip varied. Cells with less distance between the tip and the first nucleus were more likely to loose normal MT populations and dynamics. Reduced MTs in the tip, during mitosis or after exposure to the MT inhibitor carbendazim (MBC), generally correlated with reduced, but continuing growth and near-normal tip morphology. In contrast, the actin-disrupting agent latrunculin B reduced growth rates much more severely and dramatically distorted tip morphology. These results suggest substantial independence between MTs and hyphal tip growth and a more essential role for F-actin. Among MT-dependent processes possibly contributing to tip growth is the transportation of vesicles. However, preliminary ultrastructural data indicated a lack of direct MT-organelle interactions. It is suggested that the population of dynamic apical MTs enhance migration of the 'cytomatrix', thus ensuring that organelles and proteins maintain proximity to the constantly elongating tip.