Xylogenesis: the birth of a corpse.

Xylogenesis is a complex developmental process culminating in programmed cell death as a truly terminal differentiation event. In Arabidopsis, the availability of vascular-patterning mutants, and the identification of genes and their products that are involved in cell specification, secondary-wall deposition and lignification, are providing clues to the functions of some of the sequences in the large expressed sequence tag databases derived from the xylem-rich tissues of trees. An in vitro system, the Zinnia mesophyll cell system, provides an alternative system for those cell-biological experiments that are difficult to tackle in intact plants. In particular, a combination of molecular-genetic and cell-biological approaches has made possible the elucidation of some of the features of plant programmed cell death.