Of Cells, Strands, and Networks: Auxin and the Patterned Formation of the Vascular System

Throughout plant development, vascular cells continually form from within a population of seemingly equivalent cells.Vascular cells connect end to end to form continuous strands, and vascular strands connect at both or either end to form networks of exquisite complexity and mesmerizing beauty.Here we argue that experimental evidence gained over the past few decades implicates the plant hormone auxin-its production, transport, perception, and response-in all the steps that lead to the patterned formation of the plant vascular system, from the formation of vascular cells to their connection into vascular networks.We emphasize the organizing principles of the cell-and tissue-patterning process, rather than its molecular subtleties.In the picture that emerges, cells compete for an auxin-dependent, cellpolarizing signal; positive feedback between cell polarization and cell-to-cell movement of the polarizing signal leads to gradual selection of cell files; and selected cell files differentiate into vascular strands that drain the polarizing signal from the neighboring cells.Although the logic of the patterning process has become increasingly clear, the molecular details remain blurry; the future challenge will be to bring them into razor-sharp focus. AUXIN AND THE PLANT VASCULAR SYSTEMI n most multicellular organisms, water, nutri- ents, and signals are transported by tissue networks.In animals, this essential transport function is distributed over separate tissue networks-for example, the nervous, circulatory, and respiratory systems.By contrast, in plants, all transport functions are performed by the only tissue network: the vascular system.The plant vascular system is a network of vascular strands that connect the different parts of an organ and the different organs of a plant (Fig. 1; Esau 1965).Vascular strands are bundles of files of vascular cells that are elongated along the file and connected at their ends (Fig. 1).Vascular strands are named differently in different organs: veins in flat organs like leaves, petals, sepals, and cotyledons; vascular bundles in the stem; and vascular cylinder or stele in the root.The relation between the plant hormone auxin and vascular development is old (e.g., Kraus et al. 1936) and varied (e.g., Snow 1935;Camus 1949;Jacobs 1952;Wangermann 1967).Here we discuss evidence that suggests that auxin controls the patterned formation of the vascular system at all the system's organization levels: the cells', the strands', and the network's.

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