Regulation of tracheary element differentiation

Tracheary elements (TEs) are highly specialized cells that play central roles in the development of vascular plants. TEs function as components of xylem vessels or tracheids to deliver water throughout plant parts and to confer mechanical strength on a plant body. As a result of distinctive cytological features, patterned secondary walls or the loss of the protoplast, a differentiated TE cell is easily recognized, and has long provided a simple analytical model system of plant cell differentiation. Evidence obtained from this simple system indicates, however, that the regulatory mechanisms underlying TE differentiation are quite complicated. Various factors such as plant hormones and unidentified offspring of cell-cell interactions are involved in the induction of TE differentiation. With the help of some signal transduction machinery, such developmental or environmental cues activate the gene expression cascades required for TE wall material synthesis and protoplast digestion. This review covers recent advances in physiological and genetic studies on this paradigm. The detailed mechanisms of the regulation of TE differentiation are discussed, especially analyses using in vitro culture systems for TE differentiation, transgenic plants, or mutants defective in leaf vascular pattern formation that are accelerating the expansion of our knowledge on the process of vascular cell fate specification.