Cell wall-driven mechanisms underlying emergent growth in phycomyces.

Expansive growth and morphogenesis of plant and fungal cells are controlled by the mechanical properties of their cell wall. Despite extensive research, how the cell wall translates biochemical and environmental cues into anisotropic growth remains poorly understood. The sporangiophore (aerial hypha) of the filamentous fungus, Phycomyces blakesleeanus, exhibits remarkable behaviors, including rapid helical tip growth, light-stimulated growth responses, and handedness inversion during development. We present a mechanical model, informed by cell wall structure and experimental data, to account for these phenomena. The model incorporates anisotropic elasticity, viscoelastic creep from reversible bond dynamics, and tip extension via material deposition. Using this framework, we predict sporangiophore behavior in passive mechanical tests, including uniaxial stress relaxation and loading-unloading, and attribute features such as rotational inversion and light-stimulated growth responses to the interplay between fibril-tether network stiffness and bond kinetics. We further predict how changes in helical growth elements under altered turgor pressure correlate with the cell wall dynamics. Overall, our results show that fungal morphogenesis emerges from the mechanical properties of the cell wall rooted in its molecular organization.