Surface ultrastructure and elasticity in growing tips and mature regions of Aspergillus hyphae describe wall maturation

This study reports the first direct, high-resolution physical and structural evidence of wall changes during hyphal tip growth, visualized by atomic force microscopy (AFM) in Aspergillus nidulans. Images from AFM and cryo-scanning electron microscopy provided comparable information, but AFM was also able to image and physically probe living cells. AFM images showed changes in the surface ultrastructure of A. nidulans hyphae, from newly deposited walls at hyphal tips to fully mature walls, as well as additional changes at young branches arising from mature walls. Surface architecture during wall maturation correlated with changes in the relative viscoelasticity (compliance per unit applied force) of walls measured by force spectroscopy (FS) in growing A. nidulans hyphae. Growing tips showed greater viscoelasticity than mature walls, despite equal support from turgor. Branch tips had comparable viscoelasticity to hyphal tips, unlike the mature wall from which they grew. FS also revealed differences in surface hydrophilicity between newly deposited and mature walls, with the tips being more hydrophilic. The hydrophilicity of young branch tips was similar to that of hyphal tips, and different from that of mature walls. Taken together, AFM images and FS data suggest that the A. nidulans wall matures following deposition at the hyphal tip.