Final and fatal step of tracheary element differentiation

Abstract

ABSTRACT The process of terminal differentiation which produces the hollow tracheary element cell corpse requires strict coordination of two developmental events, construction of a rigid, persistent secondary cell wall and programmed cell death. We show that tracheary element programmed cell death involves an influx of Ca 2+ into the cell which may play a causative role in executing cell death. Ca 2+ influx is triggered by an extracellular signal, and leads to the rapid cessation of cytoplasmic streaming and collapse of the large hydrolytic vacuole. This specific means of effecting cell death is a necessary prerequisite for postmortem developmental events including autolysis and chromatin degradation. A protease (“trigger protease) is secreted during secondary cell wall synthesis which may be the primary trigger of cell death, because specific proteolysis of the extracellular matrix is necessary and sufficient to trigger Ca 2+ influx, vacuole collapse, cell death, and chromatin degradation. We propose a model in which secondary cell wall synthesis and cell death are coordinated by the concomitant secretion of the trigger protease with secondary cell wall precursors. Subsequent cell death is triggered upon realization of a critical extracellular activity of protease corresponding with completion of a functional secondary cell wall. Because increased Ca 2+ levels are associated with cell death involving what has been termed the mitochondrial pathway in animals, we investigated the possible role of released mitochondrial factors in the death mechanism in tracheary elements. We find that some cytochrome c is released to the cytosol at a time when death occurs and induced by calcium influx. Concomitant with this release are changes in the inner membrane voltage potential and the morphology of the mitochondria. However, cytochrome c release is insufficient to induce death in these cells. This suggests that the events triggered by the extracellular “trigger” protease may set in motion events shared by the mitochondrial pathway for apoptosis in animal cells.