Root responses to mechanical impedance: model experiments and implication of root cap

Root growth and development are highly modulated by soil conditions which influence its mechanical impedance i.e. the soil resistance to root penetration. Root cap cells surrounded by the mucilage they secrete are the firsts to encounter obstacles during soil exploration, and are known to sense and integrate environmental cues, making the root cap a relevant candidate as mechanoperception site. We thus investigated in this study a putative implication of the root cap in the sensing and in the response to mechanical impedance. We developed two-Layer-Phytagel media in which an increase in mechanical impedance was generated by a lower harder layer covered by an upper softer layer. Root growth and trajectory of primary roots of Arabidopsis thaliana seedlings were investigated using image analysis. In response to the contact with the harder layer, the roots rapidly bent allowing the reorientation of root primary growth. The root bending was associated with an inhibition of root growth rate, suggesting both mechanical and active root responses to increase in mechanical impedance. To investigate the role of root cap integrity, we examined in the two-layer media the behavior of the primary root of mutants affected either in root cap establishment, stem cell differentiation or border cell sloughing. Interestingly, the primary roots of mutant seedlings showed impaired mechanical responses, suggesting a function of the root cap in the mecanosensing.