On the role of microtubules, cell end markers, and septal microtubule organizing centres on site selection for polar growth in Aspergillus nidulans

Tip growth of filamentous fungi depends on continuous polarized growth and requires the actin and microtubule (MT) cytoskeleton. Cortical proteins at polarity sites, also known as cell end markers, play important roles in polarity maintenance. Deletion of the cell end marker teaA results in zigzag hyphal morphologies, which is contrary to the normal rectilinear growth pattern. Here we studied the role of TeaA and MTs in the establishment of polarity during tip growth of Aspergillus nidulans, including conidia germination, second germtube formation, hyphal branching and conidiophore development. TeaA is delivered to the cortex by growing MTs. In conidia TeaA appeared at the germination site prior to germtube formation, and deletion of teaA resulted in germination at multiple sites, increased branching and abnormal conidiophores. The formation of a second germtube opposite the first conidial germtube depended on the presence of a septum at the base of the first germtube. An MT-organizing centre, associated to the septum, produced microtubules, which delivered TeaA towards the opposite side of the conidium. These results suggest a new function for TeaA in polarity establishment. It can be a positive function, but TeaA could also suppress polarity sites in the vicinity of the first germtube.