A correlative microscopy approach relates microtubule behaviour, local organ geometry, and cell growth at the Arabidopsis shoot apical meristem

Abstract

Cortical microtubules (CMTs) are often aligned in a particular direction in individual cells or even in groups of cells and play a central role in the definition of growth anisotropy. How the CMTs themselves are aligned is not well known, but two hypotheses have been proposed. According to the first hypothesis, CMTs align perpendicular to the maximal growth direction, and, according to the second, CMTs align parallel to the maximal stress direction. Since both hypotheses were formulated on the basis of mainly qualitative assessments, the link between CMT organization, organ geometry, and cell growth is revisited using a quantitative approach. For this purpose, CMT orientation, local curvature, and growth parameters for each cell were measured in the growing shoot apical meristem (SAM) of Arabidopsis thaliana. Using this approach, it has been shown that stable CMTs tend to be perpendicular to the direction of maximal growth in cells at the SAM periphery, but parallel in the cells at the boundary domain. When examining the local curvature of the SAM surface, no strict correlation between curvature and CMT arrangement was found, which implies that SAM geometry, and presumed geometry-derived stress distribution, is not sufficient to prescribe the CMT orientation. However, a better match between stress and CMTs was found when mechanical stress derived from differential growth was also considered.

Differential Growth in Periclinal and Anticlinal Walls during Lobe Formation in Arabidopsis Cotyledon Pavement Cells

“…A recent study showed that physical stresses in the outer periclinal wall are in a fan-like arrangement at the convex side of lobes, similar to the pattern of cortical microtubules (Sampathkumar et al, 2014). Microtubules are known to align to the direction of maximal stress and their orientation alters to match new directions of stress, for example, during development (Hamant et al, 2008;Burian et al, 2013) or after the application of pressure or wounding (Hush et al, 1990;Hush and Overall, 1991;Sampathkumar et al, 2014). Since microtubules were persistent at the same lobes during their development, it suggests the direction of stresses was maintained in the periclinal wall at the convex side of lobes from day 1 to 3.…”

Update: Plant Cortical Microtubule Arrays

“…By changing cell geometries and the parameters governing the microtubules, simulation studies have shown the plausibility of self-organization to create coaligned patterns in the absence of other information to the cell. Other investigators, most focused on apical meristems, have shown correlations between microtubule array orientation and proposed strain fields encompassing multiple cells (Heisler et al, 2010;Burian et al, 2013). These observations have led to the hypothesis that the microtubules are organized by tissue-level tensions and their resulting stress gradients across cells.…”

Coordinating cell polarity: heading in the right direction?

“…2A,B) (Hamant et al, 2008;Sampathkumar et al, 2014). Two specific manifestations of these principles were described in the emergence of lateral roots by Amaya Vilches Barro (Maizel laboratory, University of Heidelberg, Germany) and in the generation of new organs in the shoot by Jan Traas (INRA, Lyon, France), who also integrated auxin and PIN protein localization, noting that both microtubules and PIN protein localization responded to mechanical force, but that the disassembly of highly aligned microtubule arrays was actually a requirement for symmetry breaking and the auxin-driven outgrowth of organs (Heisler et al, 2010;Burian et al, 2013).…”