Regulation of vectorial supply of vesicles to the hyphal tip determines thigmotropism in Neurospora crassa.

Thigmotropism is the ability of an organism to respond to a topographical stimulus by altering its axis of growth. The thigmotropic response of the model fungus Neurospora crassa was quantified using microfabricated glass slides with ridges of defined height. We show that the polarity machinery at the hyphal tip plays a role in the thigmotropic response of N. crassa. Deletion of N. crassa genes encoding the formin, BNI-1, and the Rho-GTPase, CDC-42, an activator of BNI-1 in yeast, CDC-24, its guanine nucleotide exchange factor (GEF), and BEM-1, a scaffold protein in the same pathway, were all shown to significantly decrease the thigmotropic response. In contrast, deletion of genes encoding the cell end-marker protein, TEA-1, and KIP-1, the kinesin responsible for the localisation of TEA-1, significantly increased the thigmotropic response. These results suggest a mechanism of thigmotropism involving vesicle delivery to the hyphal tip via the actin cytoskeleton and microtubules. Neurospora crassa thigmotropic response differed subtly from that of Candida albicans where the stretch-activated calcium channel, Mid1, has been linked with thigmotropic behaviour. The MID-1 deficient mutant of N. crassa (Δmid-1) and the effects of calcium depletion were examined here but no change in the thigmotropic response was observed. However, SPRAY, a putative calcium channel protein, was shown to be required for N. crassa thigmotropism. We propose that the thigmotropic response is a result of changes in the polarity machinery at the hyphal tip which are thought to be downstream effects of calcium signalling pathways triggered by mechanical stress at the tip.