Tracheary element differentiation uses a novel mechanism coordinating programmed cell death and secondary cell wall synthesis

Tracheary element differentiation requires strict coordination of secondary cell wall synthesis and programmed cell death (PCD) to produce a functiona l cell corpse. The execution of cell death involves an influx of Ca2+ into the cell and is manifested by rapid collapse of the large hydrolytic vacuol e and cessation of cytoplasmic streaming. This precise means of effecting c ell death is a prerequisite for postmortem developmental events, including autolysis and chromatin degradation. A 40-kD serine protease is secreted du ring secondary cell wall synthesis, which may be the coordinating factor be tween secondary cell wall synthesis and PCD. Specific proteolysis of the ex tracellular matrix is necessary and sufficient to trigger Ca2+ influx, vacu ole collapse, cell death, and chromatin degradation, suggesting that extrac ellular proteolysis plays a key regulatory role during PCD. We propose a mo del in which secondary cell wall synthesis and cell death are coordinated b y the concomitant secretion of the 40-kD protease and secondary cell wall p recursors. Subsequent cell death is triggered by a critical activity of pro tease or the arrival of substrate signal precursor corresponding with the c ompletion of a functional secondary cell wall.