The interaction between plant roots and soils is an interdisciplinary issue that concerns many communities, from biophysics and agronomy to civil engineering and geophysics. The presence of zones of high mechanical resistance, due for example to stones, soil compaction or grain aggregation is one of the most common physical limitations to soil exploration by roots.  These mechanical heterogeneities have a direct impact on the healthy development of the whole plant. At root level, the root apex must exert a growth pressure to overcome the resistance to deformation of the surrounding soil or reorient its growth to bypass obstacles. The mechanisms by which this is achieved are not well understood. 

In this talk, I will present some model experimental systems for studying how the root modifies its growth when it encounters a single or a collection of obstacles mimicking the mechanical heterogeneities of a soil. In particular we investigated the growth response of a root pushing against a single obstacle such as a force sensor or growing within arrays of obstacles of varying rigidities. In a first part I will present how, by coupling force and kinematic measurements under infra-red lighting, we probed the force-growth relationship of a primary root contacting a stiff resisting obstacle. In a second part, I will show how a primary root growing within a 3D-printed array of stiff obstacles switches from a vertical to an oblique trajectory due to the competition between gravitropism and thigmotropism.