Tracheary element formation: building up to a dead end

Many differentiated plant cells are able to re differentiate into new types of cell. In contrast, tracheary element formation is a terminal fate, culminating in an end to further development. In vivo, a variety of cell types can be induced by treatment with plant growth regulators to undergo a programmed series of events including cytoskeletal rearrangements, secondary wall formation, lignification and autolysis to form a hollow, water-conducting tube. Mesophyll cells freshly isolated from the leaves of Zinnia elegans can also be induced by auxin and cytokinin to form such tracheary elements. This has proven particularly useful as a model system for studying differentiation, because the process is both inducible and highly synchronous. Recent progress in identifying the cytological, biochemical and molecular markers of xylem formation has allowed questions of cell commitment to a particular fate to be addressed.