TRACHEARY ELEMENT FORMATION AS A MODEL SYSTEM OF CELL-DIFFERENTIATION

This chapter reviews progress on tracheary-element (TE) differentiation, with particular emphasis on the control mechanism of differentiation at cellular and molecular levels. Differentiated cells in higher plants can redifferentiate in vitro to other types of cells and can eventually form tissues, organs, and even whole plants. TE differentiation is an excellent example of redifferentiation occurring at the cellular level in higher plants. TEs are the distinctive constituents of xylem and are derived in situ from the cells of the procambium of roots and shoots in the primary xylem and from the cells produced by the vascular cambium in the secondary xylem. Xylem is formed by the redifferentiation of parenchyma cells as well as by a programmed differentiation of procambium or cambium. This occurs where lateral roots interconnect with the vascular system of the main axis in situ. Parenchyma cells in plants can redifferentiate to TEs when excised from intact plants and cultured under adequate conditions. Various factors lead to the induction of differentiation, such as wounds, phytohormones, calcium, and calmodulin. During the ontogenetic stages of TE differentiation, cell enlargement occurs after the induction of differentiation in primary meristems, and it is followed by secondary wall formation and autolysis. © 1992, Academic Press Inc.