Into another dimension: how streptophyte algae gained morphological complexity

Land plants with elaborated three-dimensional (3D) body plans have evolved from streptophyte algae. The streptophyte algae are known to exhibit varying degrees of morphological complexity, ranging from single-celled flagellates to branched macrophytic forms exhibiting tissue-like organization. In this review, I discuss mechanisms by which, during evolution, filamentous algae may have gained 2D and eventually 3D body plans. There are, in principle, two mechanisms by which an additional dimension may be added to an existing algal filament or cell layer: first, by tip growth-mediated branching. An example of this mechanism is the emergence and polar expansion of root hairs from land plants. The second possibility is the rotation of the cell division plane. In this case, the plane of the forthcoming cell division is rotated within the parental cell wall. This type of mechanism corresponds to the formative cell division seen in meristems of land plants. This literature review shows that of the extant streptophyte algae, the Charophyceae and Coleochaetophyceae are capable of performing both mechanisms, while the Zygnematophyceae (the actual sister to land plants) show tip growth-based branching only. I finally discuss how apical cells with two or three cutting faces, as found in mosses, may have evolved from algal ancestors.