Phyllotaxis: a remarkable example of developmental canalization in plants
Résumé
Why living forms develop in a relatively robust manner, despite various sources of internal or external variability, is a fundamental question in developmental biology. Part of the answer relies on the notion of developmental constraints: at any stage of ontogenenesis, morphogenetic processes are constrained to operate within the context of the current organism being built, which is thought to bias or to limit phenotype variability. One universal aspect of this context is the shape of the organism itself that progressively channels the development of the organism toward its final shape. Here, we illustrate this notion with plants, where conspicuous patterns are formed by the lateral organs produced by apical meristems. These patterns, called phyllotaxis, traditionally fall into two broad categories, spiral or whorled that present striking symmetries and regularities. These properties suggest that plant development is strongly canalized and cannot escape specific attraction patterns. Since the early 19th century, researchers have looked for biological or physical explanations for such amazing and specific form "attractors". Thanks to this collective and sustained effort, we now have gained much insight on this self-organizing process, and uncovered important parts of the mystery. This paper aims to provide an easy-to-read overview of the main concepts that have been developed to explain phyllotaxis and to make clear their connections in a step-by-step progression, while keeping the mathematics light. We suggest that altogether a view emerges where phyllotaxis appears as a remarkable example of how shapes may be canalized during development.
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