Auxin and self-organisation

Abstract

This essay develops a conceptual framework to understand the role of auxin for the genesis of plant organisms. This framework has to consider the specificities of the plant lifestyle and underscores the fact that plant organisation is highly modular. The assembly of these modules is controlled through robust self-organisation driven by autocatalytic loops linked to lateral inhibition which can be formally described as reaction-diffusion system in sensu Turing. Instead of actual inhibitory molecules as in the original Turing model, they achieve lateral inhibition by mutual competition for an activator (auxin). This can be demonstrated for phyllotaxis, but also for vascular differentiation. We study self-organisation in cell strains from tobacco and find that individual cell divisions within a file are synchronised through weak coupling based on a directional flow of auxin. We use this system as a simple minimal organism we have identified an oscillatory circuit as central element of self-organisation. This self-referring circuit connects auxin-dependent remodelling of the actin cytoskeleton with actin-dependent remodelling of auxin flux. The essay concludes with the working hypothesis that the contiguity of plant organisms is manifest in time (rhythm) rather than in space (body) and describes an experimental model where the induction of cell axis and polarity as base for self-organisation can be studied de novo in regenerating protoplasts.