A programmed cell death pathway activated in carrot cells cultured at low cell density

Programmed cell death (PCD) occurs in plants during development and defense, but the processes and mechanisms are not yet defined. Culture of carrot single cells at a cell density of <10⁴ cells ml^-1 activates a cell death process involving condensation and shrinkage of the cytoplasm and nucleus and fragmentation of the DNA. Modest abiotic stress treatments also cause cell condensation and shrinkage and the formation of DNA fragments, but the same abiotic stresses at high levels cause rapid necrosis with cell swelling and lysis. The common morphological features of cells dying at low cell density and following modest abiotic stress treatments suggest that these features reveal a PCD pathway in carrot. The addition of a cell-conditioned growth medium allows cells at low cell density to remain alive, demonstrating that cell-derived 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; however, these findings show that plants, like animals, can control PCD by social signaling, and imply that the mechanism of PCD in plants and animals may be similar. Consistent with this, manipulation of signal pathway intermediates that regulate PCD in animals shows that Ca²⁺ and protein phosphorylation events are PCD pathway intermediates in carrot.