The Induction of Vascular Tissues by Auxin and Cytokinin

Abstract

The vascular system of the plant connects the leaves and other parts of the shoot, with the roots, and enables efficient long-distance transport between the organs. In higher plants it is composed of two kinds of conducting tissues: thephloemthrough which organic materials are transported and thexylemwhich is the pathway for water and soil nutrients. In angiosperms, the functional conduits of thephloemare thesieve tubes;and those of thexylemare thevessels(4, 37). Vascular development in the plant is an open type of differentiation, it continues as long as the plant grows from apical and lateral meristems. The continuous development of new vascular tissues enables regeneration of the plant and its adaptation to changes in the environment. This differentiation of vascular tissues along the plant is induced and controlled by longitudinal streams of inductive signals (4, 42). In spite of the complexity of structure and development of the vascular tissues (37), there is evidence that the differentiation of both the sieve tubes and the vessels is induced by two hormonal signals, namely: (i) auxin, indole-3-acetic acid (IAA), produced mainly by young leaves (4, 6, 26, 27, 42), and (ii) cytokinin produced by root apices (8, 9, 18). This fact raises the question how these two hormonal signals control the differentiation of complex patterns of phloem and xylem? Nevertheless, it should be emphasized that additional growth regulators, like gibberellin (1) and ethylene (7, 45), may also be involved in vascular differentiation. They are beyond the scope of this article and the reader is directed to reviews on the topic (4, 5, 6, 24, 39, 42, 46).