Tracheary Element Differentiation.

Vascular plants, which are adapted for life on land, first appeared in the late Silurian period, some 400 million years ago. Since then they have evolved to fill a diverse range of habitats all over the earth. The vascular systems of land plants are composed of specialized conducting tissues, the xylem and the phloem, which provide both a pathway for water and nutrient transport and mechanical support for slender plants. The vascular system is also an important conduit for signal-transducing molecules. Tracheary elements (TEs), which are the distinctive cells of the xylem, are characterized by the formation of a secondary cell wall with annular, spiral, reticulate, or pitted wall thickenings. In the primary xylem, TEs differentiate from procambial cells, whereas in the secondary xylem, they arise from cells produced by the vascular cambium. As they mature, TEs lose their nuclei and cell contents, leaving hollow dead cells that form vessels or tracheids. The final stage of TE differentiation represents a typical example of programmed cell death in higher plants (see Pennell and Lamb, 1997, in this issue). TEs can also be induced to form in vitro from various types of cells, including cells of the phloem parenchyma and the cortex in roots, the pith parenchyma in shoots, the tuber parenchyma, and the mesophyll and epidermis in leaves (Roberts et al., 1988; Fukuda, 1992). In Zinnia elegans cell cultures, single mesophyll cells transdifferentiate directly into TEs without cell division in response to phytohormones (Fukuda and Komamine, 1980). The Zinnia system has proven to be particularly useful for studies of the sequence of events during TE differentiation. This is largely because differentiation occurs at a high frequency in Zinnia cultures and because the process can be followed in single cells (Chasan, 1994; Fukuda, 1994, 1996). Recently, I presented a general overview of xylogenesis (Fukuda, 1996). In this article, I focus on efforts to elucidate the molecular mechanisms underlying the in vitro differentiation of parenchyma cells into TEs.