Effect of mechanical stress on Zea root apex. I. Mechanical stress leads to the switch from closed to open meristem organization

Abstract

The effect of mechanical stress on the root apical meristem (RAM) organization of Zea mays was investigated. In the experiment performed, root apices were grown through a narrowing of either circular (variant I) or elliptical (variant II) shape. This caused a mechanical impedance distributed circumferentially or from the opposite sides in variant I and II, respectively. The maximal force exerted by the growing root in response to the impedance reached the value of 0.15 N for variant I and 0.08 N for variant II. Significant morphological and anatomical changes were observed. The changes in morphology depended on the variant and concerned diminishing and/or deformation of the cross-section of the root apex, and buckling and swelling of the root. Anatomical changes, similar in both variants, concerned transformation of the meristem from closed to open, an increase in the number of the cell layers at the pole of the root proper, and atypical oblique divisions of the root cap cells. After leaving the narrowing, a return to both typical cellular organization and morphology of the apex was observed. The results are discussed in terms of three aspects: the morphological response, the RAM reorganization, and mechanical factors. Assuming that the orientation of division walls is affected by directional cues of a tensor nature, the changes mentioned may indicate that a pattern of such cues is modified when the root apex passes through the narrowing, but its primary mode is finally restored.

Physical root–soil interactions

“…When the nominal diameter of the root is not known, it must be estimated from the diameter in the elongation zone of a hypothetical identical root which would not have experienced the constriction. At least, this requires measuring the diameter in situ for avoiding relaxation effect due to stress retrieval [121] when the root is extracted from the embedding soil or substrate. A further complication comes from a possible artefactual swelling due to uncontrolled axial force.…”

Morphomechanics of Plants

“…A similar response to imposed lateral compression was observed for the apical meristem of the root (Potocka et al 2011). In contrast to the SAM, the root apical meristem (RAM) is not a strictly terminal structure, but is situated at a distance of a few millimeters from the apex surmounted by a root cap that protects the stem cell niche from the harsh soil environment.…”

“…In both cases, the experimental manipulations led to an increase in lateral-to-radial stress ratio in the second cell layer. In the study of Reinhardt et al (2003b), this shift was achieved by decreasing the radial pressure, while in the study of Potocka et al (2011), this ratio alteration was due to lateral compression. These common points imply that the switch from the closed to open RAM organization can be explained in a way similar to that suggested above in the discussion of the ablation experiments of Reinhardt et al (2003b) and constraint experiments of Green (Hernández and Green 1993;Green 1999), that is, as a tendency for stress restoration.…”

“…A fairly straightforward demonstration that mechanical forces can profoundly influence the pattern of meristem morphogenesis in the manner contemplated by the concept of stress hyperrestoration comes from constraint experiments (Hernández and Green 1993;Green 1999;Potocka et al 2011). Hernández and Green (1993) found that lateral compression of a developing sunflower capitulum between two parallel clamps altered both phyllotaxy and organ identity.…”

“…In this line of inquiry, Potocka et al (2011) investigated the effect of mechanical stress on RAM organization of Zea mays. This plant under normal conditions has the closed RAM with three clearly defined cell tires exhibiting a neat pattern of cell divisions (Fig.…”