Abstract

We are studying the role of physical forces during plant development. Pioneering work on plants, in particular by Green and colleagues, has suggested that physical stress patterns created by differential tissue expansion are locally interpreted in terms of particular cell growth characteristics and patterns of differentiation. So far, however, it has been impossible to go beyond the general idea that physical forces are important in morphogenesis. We are re-examining this issue exploiting new imaging technologies combined with genetics, micro-mechanical approaches and mathematical modelling. Hereby we are focusing on the shoot apical meristem in higher plants. This is a population of stem cells which continuously initiates aerial organs and, therefore, is a basic determinant of plant architecture. The link between mechanical constraints and a major structural cellular component, the microtubules, receives particular attention. Microtubules are filamentous elements of the cytoskeleton, which play a major role in structuring the extracellular matrix, or plant cell wall. We have found the presence of highly dynamic and stereotypic microtubule orientations in the meristem. The microtubule arrays are aligned to predicted stress patterns at the meristem surface and react to externally applied constraints in a cell autonomous way. We show, using mathematical modeling, that a cell autonomous reaction to stress patterns would be sufficient to generate the observed behaviour of microtubules. When microtubules are depolymerized using drug treatments, the cells loose their capacity to grow anisotropically. As a result, the meristematic cells adopt a number of growth properties usually observed in foams and the tissue is no longer able to carry out certain morphogenetic processes such as tissue folding. We are currently testing the hypothesis that microtubules react to physical stress vectors that arise during anisotropic growth. Via directing cellulose synthase complexes in the membrane they would subsequently structure the cell wall in such a way that the cells would resist to stress. This, in turn, would allow specific events such as directional organ outgrowth and tissue folding to happen.

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