Morphological responses of plant roots to mechanical stress

Abstract

Background: Roots are continuously exposed to mechanical pressure and this often results in their morphological modification. Most obvious are changes in the overall form of the root system as well as in the shapes of particular roots. These changes are often accompanied by modifications of the cell pattern and cell morphology.

Scope: This review focuses on the morphological responses of roots to mechanical stress. Results of early and recent experiments in which roots have been exposed to mechanical pressure are assembled, analysed and discussed. Research applying different experimental sets, obstacles, media of various compactness and structure are reviewed. An effect of the combination of mechanical stresses with other abiotic stresses on roots, and results of estimating the force exerted by the roots are briefly discussed. Possible consequences of the cell pattern rearrangements are considered.

Conclusions: Several modifications in root morphology are commonly reported: (1) decreased root size, (2) radial swelling accompanied by increased radial dimension of the cortex cell layers and (3) enhanced cap cell sloughing. Nevertheless, because of differences between species and individual plants, a universal scenario for root morphological changes resulting from externally applied pressures is not possible. Thus, knowledge of the root response to mechanical impedance remains incomplete. Studies on the mechanical properties of the root as well as on possible modifications in cell wall structure and composition as the elements responsible for the mechanical properties of the plant tissue are required to understand the response of root tissue as a biomaterial.

Fig. 1.

Morphology of maize root growing through tight tubes with circular (A) or oval (B) cross sections. Arrows indicate the narrowest region of the tubes. Root tip in A is covered with mucilage (asterisk); schematic drawing in the left lower corner shows the real shape of the apex (root proper + root cap). In B the front (left) and the side (right) view of the same root are shown, and root buckling is indicated by asterisks. (C) Axial section of the maize root. The root-cap boundary is broken by the RAM cells growing onto the cap side (arrow). Scale bars: 0.5 mm (A), 1 mm (B), 50 μm (C).