A PROGRAMMED CELL-DEATH PATHWAY ACTIVATED IN CARROT CELLS CULTURED ATLOW CELL-DENSITY

Programmed cell death (PCD) occurs in plants during development and de fense, but the processes and mechanisms are not yet defined. Culture o f carrot single cells at a cell density of <10(4) cells ml(-1) activat es a cell death process involving condensation and shrinkage of the cy toplasm and nucleus and fragmentation of the DNA. Modest abiotic stres s treatments also cause cell condensation and shrinkage and the format ion of DNA fragments, but the same abiotic stresses at high levels cau se rapid necrosis with cell swelling and lysis. The common morphologic al features of cells dying at tow cell density and following modest ab iotic stress treatments suggest that these features reveal a PCD pathw ay in carrot. The addition of a cell-conditioned growth medium allows cells at low cell density to remain alive, demonstrating that cell-der ived signal molecules suppress a pathway that is otherwise induced by default. Differences in the morphology of the dead cells suggest that proteolysis during PCD differs in detail in plants and animals; howeve r, these findings show that plants, like animals, can control PCD by s ocial signaling, and imply that the mechanism of PCD in plants and ani mals may be similar. Consistent with this, manipulation of signal path way intermediates that regulate PCD in animals shows that Ca2+ and pro tein phosphorylation events are PCD pathway intermediates in carrot.