The Role of Hormones in Controlling Vascular Differentiation

The vascular system in plants is induced and controlled by streams of inductive hormonal signals. Auxin produced in young leaves is the primary controlling hormone in vascular differentiation. Its downward transport pathways, major controlling mechanisms, and sensitivity of cells to auxin are clarified. Cytokinin, from the root cap moves upward, increases the sensitivity to auxin and stimulates cambial cell divisions. Gibberellin produced in mature leaves moves non-polarly and promotes elongation, regulates cambium activity, and induces long fibers and long tracheids. Transgenic plants with elevated bioactive gibberellin concentrations grow rapidly and yield numerous longer fibers and longer tracheids. Centrifugal movement of ethylene from maturing vessels induces the radial vascular rays. In conifer trees, jasmonate, which promotes defense response, is mediated by ethylene and induces traumatic resin ducts. In addition the role of the hormonal signals in regulating gradients of cell size and density, in controlling the type of differentiating vascular element and how they have shaped wood evolution are elucidated.