The cap size and shape of Arabidopsis thaliana primary roots condition responses to an increase in medium strength.

During root progression in soil, root cap cells are the first to encounter obstacles, and are known to sense environmental cues, making the root cap a relevant candidate for a mechanosensing site. A two-layered medium system was adopted to study root responses to variations in growth medium strength and the importance of the root cap in the establishment of these responses. Root growth and trajectory of primary roots of Arabidopsis thaliana seedlings were investigated using in vivo image analysis. After contact with the harder layer, the root either penetrated it or underwent rapid curvature, enabling reorientation of growth. We initially hypothesized that the root cap structure impacted apex penetration and reorientation, with pointed caps facilitating and domed caps impeding root penetration. This hypothesis was investigated by analysing the responses of Arabidopsis mutants with altered root caps. The primary root of fez-2 mutant lines, which has fewer root cap cell layers and a more pointed root cap than wild-type roots, showed impaired penetration ability. Conversely, smb-3 roots, which display a rectangular-shaped cap, showed enhanced penetration abilities. Our results, which contradict our original hypothesis, uncover a role for resistance to root buckling in determining penetration abilities.