[A Picasso among brown algae: the arduous conquest of symmetry by Ectocarpus].

In response to environmental constraints, living organisms organise their body according to axes, rotation and translation plans, or asymmetries. Cellular and molecular processes are involved in the establishment of this architecture. Hence, this review aims at presenting the molecular mechanisms controlling the main symmetries and axes in plants. Several genes, coding for transcription factors, have been identified in land plants (mainly Arabidopsis thaliana), as controlling the establishment of apico-basal and adaxial-abaxial axes mainly. The establishment of these axes allows the development in other spatial directions of radial or bilateral symmetries. These processes seem in most cases to be under the control of the phytohormone auxin. In brown algae, which are all multicellular marine plants, polarity plans are less obvious than in land plants. The development of the model brown alga Ectocarpus siliculosus is currently being studied. E. siliculosus develops a filamentous architecture, and primary observations show that branching along the main axis occurs in a non-stereotyped and regular way, even though it is mainly centred. However, more detailed morphometrical studies, accompanied by probabilistic analyses, have shown that, among the overall population of individuals, organisms obey yet unidentified biological constraints, that aim at refining the radial symmetry as the organism grows. The role of this symmetry in the adaptation of E. siliculosus to its environment, as well as the molecular actors involved in this process, are currently under study in our laboratory.