How does Root Cap Integrity Modulate Root Responses to Medium Strength?

Plant root system development is highly modulated by the physical properties of the soil and especially by its mechanical resistance to penetration i.e its mechanical impedance. During root progression in soil, root cap cells are the firsts to encounter obstacles, and are known to sense and integrate environmental cues, making the root cap a relevant mechanosensing site. In this study we used an experimental approach to investigate the implication of root cap in the sensing and in the response to a mechanical impedance variation. A Phytagel medium was developed in which an increase in mechanical impedance was generated by a lower harder layer covered by a softer one. In this two-layer medium, root growth and trajectory of primary roots of Arabidopsis thaliana seedlings were investigated using in vivo image analysis. In response to contact with the harder layer, roots underwent rapid curvature, allowing the reorientation of root primary growth. Root reorientation was associated with an inhibition of root growth rate, which suggests an active root response occurring simultaneously to root deformation following an increase in mechanical impedance. The role of root cap integrity was investigated by analyzing the root mechanical responses of Arabidopsis mutants with altered root caps (Willemsen et al., 2008). Interestingly, the primary roots of fez-2, which exhibit a reduction of root cap cell layers, showed impaired penetration and reduced root growth rate in harder media. Conversely, smb-3 roots, which exhibit an increased number of root cap cells, showed enhanced penetration abilities. Our results suggest an overriding function of root cap in mechanoresponse.

Willemsen et al., 2008. The NAC domain transcription factors FEZ and SOMBRERO control the orientation of cell division plane in Arabidopsis root stem cells. Developmental Cell, 15, 913-922.