Mechanics of Phycomyces growth and rotation

The growth of Phycomyces Blakesleeanus has been reported in the literature as a means to understand the mechanisms of plant growth. This giant unicellular fungus that rises as a cylindrical tube (the sporangiophore) has been observed to exhibit fluctuating rotation patterns during growth. It is hypothesized that this is a result of the orientation and stresses of cellulose microfibrils that constitute the cell wall. The biophysical processes leading to this peculiar behavior are however poorly understood. We aim here to shine a light on the molecular events leading to both growth and rotation. For this, we present a statistical framework in which the cell wall is modeled as a dynamic network of microfibrils cross-linked by tethers (hemicellulose molecules). The tethers can periodically associate and dissociate with microfibrils over time, giving the cell wall the ability to tune the elastic and rheological response with the applied stress. Combining this approach with a continuum model, we will present simulation results of the flow and reorientation of microfibrils. We will then discuss how tether dynamics and elasticity affect microfibril rheology during growth and provide new hypotheses regarding the role of molecular events on sporangiophore rotation.