Impact of mechanically-regulated auxin transport dynamics on plant morphogenesis

Auxin is a major plant growth regulator whose interaction with cell wall properties is central to its role. Efflux carriers of this phytohormone of the PIN-FORMED family (PIN) respond to the mechanical state of the tissue. As patterns of auxin and of its transport are at the onset of plant morphogenesis, the work herein focuses on how mechanical cues contribute to the auxin flows underlying developmental patterning. We approach this task by developing a vertex model mathematical description of an epithelial tissue and build biophysical models of plant tissue upon it. The mechanical state is perturbed by mechanical property changes induced by auxin, which in turn feeds back onto auxin through the binding of its carriers induced by mechanical stresses within the tissue. In an abstract setting the model predicts sharper auxin spot patterns and higher PIN polarity, a phenomenon mediated by stress patterns arising from auxin-dependent stiffness gradients. Moreover, we find a more robust distinction of auxin-dependent cell-fate. We show, under this hypothesis, that auxin maxima can exist in cell turgor minima, revealing developmental history to be just as important for predicting auxin maxima. Form these findings we highlight how plant mechanical responses have the potential to strengthen already existing signals. We then move to lateral root formation as a model system for studying the interactions of auxin flows and mechanics under this hypothesis. We show how cell wall remodelling of endodermal cells alone can potentiate founder cell swelling by implementing a cell wall growth model. By predicting mechanical perturbation effects on auxin of founder cells prior to swelling we found longitudinal stress of walls parallel to the root surface to be crucial for auxin accumulation, highlighting the role of endodermal auxin reflux. By coupling growth and auxin transport, we argue what mechanical state founder cells have to exhibit in order for growth to induce a PIN polarity shift towards the endodermis. With this work, not only have we highlighted how mechanical auxin transport regula- tion can positively impact different developmental patterning processes, but we also have made predictions on the lateral root formation system that inspire new approaches, both theoretical and empirical, to test this hypothesis.2021-10-2