Auxin transport is sufficient to generate a maximum and gradient guiding root growth

The plant growth regulator auxin controls cell identity, cell division and cell expansion. Auxin efflux facilitators (PINs) are associated with auxin maxima in distal regions of both shoots and roots. Here we model diffusion and PIN-facilitated auxin transport in and across cells within a structured root layout. In our model, the stable accumulation of auxin in a distal maximum emerges from the auxin flux pattern. We have experimentally tested model predictions of robustness and self-organization. Our model explains pattern formation and morphogenesis at timescales from seconds to weeks, and can be understood by conceptualizing the root as an ‘auxin capacitor’. A robust auxin gradient associated with the maximum, in combination with separable roles of auxin in cell division and cell expansion, is able to explain the formation, maintenance and growth of sharply bounded meristematic and elongation zones. Directional permeability and diffusion can fully account for stable auxin maxima and gradients that can instruct morphogenesis. Polar auxin transport is a factor in almost all developmental processes in plants but there is still much to be learned about how it operates. In cells, directional auxin transport is controlled by the PIN protein carrier complex. A new computational model has been developed to describe diffusion and PIN-facilitated auxin transport in and across the cells of the root system. The model addresses two major issues in plant development. First it supports the concept of an auxin maximum and gradient to guide root growth. Second, it shows how a single molecule can generate a field of information: a concentration gradient across the organ growth zone is closely associated with auxin maxima. A related paper shows that four PLETHORA genes are vital for root development and are candidate output factors for the auxin gradient. A computational model that spans molecular and cellular levels describing diffusion and PIN-facilitated auxin transport in and across cells within the root system is experimentally tested. The model strongly supports that plant roots are able to generate an auxin maximum and a highly robust auxin gradient with morphogenic properties.