Angle-dependence and auxin asymmetry: towards the formulation of a universal theory of root gravitropism

Gravitropism has been conceptualized through three main theories: the starch-statolith model for sensing, the Cholodny-Went model of growth control via auxin asymmetry, and the law of angle-dependence for behavior, in which the magnitude of gravitropic response increases with displacement from the vertical. While experimental data show that the generation of both Cholodny-Went-type auxin gradients and angle-dependent behavior requires statolith sedimentation, a link between auxin asymmetry and angle-dependence has not been demonstrated. Here, we use large scale reorientation assays of Arabidopsis roots, epidermal length measurements, and confocal microscopy to quantify auxin distribution and PIN localization during graviresponse. We show angle-dependence in auxin asymmetry and growth response, even at low stimulation angles. As such, our work integrates the three theories sensing, signal transduction and behavior into a single unified model of gravitropism and provides an important framework for exploring major outstanding questions in the field.