Cell Wall Dynamics in a Filamentous Fungus

Hyphal tip growth allows filamentous fungi to colonize space, reproduce or infect. It features remarkable morphogenetic plasticity including unusually fast elongation rates, tip turning, branching or bulging. These shape changes are all driven from the expansion of a protective cell wall (CW) secreted from apical pools of exocytic vesicles. How CW secretion, remodeling and deformation are modulated in concert to support rapid tip growth and morphogenesis while ensuring surface integrity remains poorly understood. We implemented sub-resolution imaging to map the dynamics of CW thickness and secretory vesicles in Aspergillus nidulans. We found that tip growth is associated with balanced rates of CW secretion and expansion, which limit temporal fluctuations in CW thickness, elongation speed and vesicle amount, to less than 10-15%. Affecting this balance through modulations of growth or trafficking yield to near-immediate changes in CW thickness, mechanics and shape. We developed a model with mechanical feedback which accounts for steady states of hyphal growth as well as rapid adaptation of CW mechanics and vesicle recruitment to different perturbations. These data provide unprecedented details on how CW dynamics emerges from material secretion and expansion, to stabilize fungal tip growth as well as promote its morphogenetic plasticity.