XYLOGENESIS: INITIATION, PROGRESSION, AND CELL DEATH.

Xylem cells develop from procambial or cambial initials in situ, and they can also be induced from parenchyma cells by wound stress and/or a combination of phytohormones in vitro. Recent molecular and biochemical studies have identified some of the genes and proteins involved in xylem differentiation, which have led to an understanding of xylem differentiation based on comparisons of events in situ and in vitro. As a result, differentiation into tracheary elements (TEs) has been divided into two processes. The "early" process involves the origination and development of procambial initials in situ. In vitro, the early process of transdifferentiation involves the dedifferentiation of cells and subsequent differentiation of dedifferentiated cells into TE precursor cells. The "late" process, observed both in situ and in vitro, involves a variety of events specific to TE formation, most of which have been observed in association with secondary wall thickenings and programmed cell death. In this review, I summarize these events, including coordinated expression of genes that are involved in secondary wall formation.