Root cap size and shape influence responses to the physical strength of the growth medium in Arabidopsis thaliana primary roots.

During the progression of root in soil, root cap cells are the first to encounter obstacles and are known to sense environmental cues, thus making the root cap a potential mechanosensing site. In this study, a two-layered growth medium system was developed in order to study root responses to variations in the physical strength of the medium and the importance of the root cap in the establishment of these responses. Root growth and trajectory of primary roots of Arabidopsis seedlings were investigated using in vivo image analysis. After contact with the harder layer of the medium, the root either penetrated it or underwent rapid curvature, thus enabling reorientation of growth. We initially hypothesized that the root-cap structure would affect 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 lines of the fez-2 mutant, 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. These results, which contradict our original hypothesis, reveal a role for resistance to buckling in determining root penetration abilities.

A new scenario for gravity detection in plants: the position sensor hypothesis.

The detection of gravity plays a fundamental role during the growth and evolution of plants. Although progress has been made in our understanding of the molecular, cellular and physical mechanisms involved in the gravity detection, a coherent scenario consistent with all the observations is still lacking. In this special issue article, we discuss recent experiments showing that the response to inclination of shoots is independent of the gravity intensity, meaning that the gravity sensor detects an inclination and not a force. This result questions some of the commonly accepted hypotheses and leads to propose a new 'position sensor hypothesis'. The implications of this new scenario are discussed in light of the different observations available in the literature.

[Circadian rhythms of cortisol in burns (author's transl)]. / Variations nycthémérales du cortisol chez le brûlé. Etude de dix cas.

Circadian rhythms of cortisol were studied in 10 patients. This study was made shortly after the burn and for a short period of one or two days maximum in 9 patients. Blood level measurements of cortisone were made in 4 patients. 7 patients died. The analysis of circadian rhythm was made with a computer (méthode de régression harmonique combinée à une analyse de variance). The results are deceiving: there are qualitative and quantitative disturbances but we could not give a significance to them, each patient reacting differently. In one case, a drop of cortisol to 0 was observed and the patient died, this observation shows the development of corticosurrenal failure. So, one or several measurements do not allow the study of corticosurrenal function which differs in each case. Theoretical appreciations in handbooks on the subject must be tempered.