Physical obstacles in the substrate cause maize root growth trajectories to switch from vertical to oblique

Hard pans, soil compaction, soil aggregation, and stones create physical barriers that can affect the development of a root system. Roots are known to exploit paths of least resistance to avoid such obstacles, but the mechanism through which this is achieved is not well understood. Here, we used a combination of 3D-printed substrates with a high-throughput live-imaging platform to study the responses of maize roots to a range of physical barriers. Using image analysis algorithms, we determined the properties of growth trajectories and identified how the presence of rigid circular obstacles affects the ability of a primary root to maintain its vertical trajectory. The results showed that the types of growth responses were limited, with both vertical and oblique trajectories being found to be stable and influenced by the size of the obstacles. When obstacles were of intermediate sizes, trajectories were unstable and changed in nature through time. We formalized the conditions required for root trajectory to change from vertical to oblique, linking the angle at which the root detaches from the obstacle to the root curvature due to gravitropism. Exploitation of paths of least resistance by a root might therefore be constrained by the ability of the root to curve and respond to gravitropic signals.