Self-organisation of auxin transport in plant cells

The phytohormone auxin plays a key role in many plant developmental processes. Its polar cell-to-cell transport is linked to and dependent on auxin efflux transporters and their polar localisation in cell membranes. This relies on feedback loops between auxin and its transport on many levels. Hypotheses brought forward in auxin biology, trying to elucidate the nature of these feedbacks, such as the canalisation hypothesis, depend on mechanisms by which auxin transport is established and maintained on specific routes through tissues. Auxin transport canalisation is based on a proposed feedback between auxin flux and auxin transport polarisation, with the result that auxin transport is directed by the strength of auxin fluxes. Despite this phenomenon being well described in biology, its underlying mechanisms are largely unknown. Many of them are occurring at the cell level, justifying a focus on cells in elucidating the nature of this feedback.
In this thesis, computational modelling of self-organising mechanisms potentially leading to such phenomena at a cell level has been accomplished. While many auxin transport models are already available at tissue and whole plant scales, such a single cell model is a novel contribution. With the main focus on auxin/proton interactions grounded on the results of biological experiments, a feedback by which auxin influences its own transport by the activation of plasma membrane-bound proton pumps is described. In simulations, it is shown to lead to increased allocation of auxin in cells as well as to enhancement of all auxin transport fluxes over the membrane, and in due course to the establishment of canalisation-type polarisation patterns, without polarised transporter localisation. The results point towards a functional redundancy of polarisation in auxin transport and lead to hypotheses on differential energisation of auxin transporters, which may play a role in auxin transport polarisation events.