Programmed cell death (PCD) in plants is a crucial component of development and defence mechanisms. In animals, different types of cell death (apoptosis, autophagy, and necrosis) have been distinguished morphologically and discussed in these morphological terms. PCD is largely used to describe the processes of apoptosis and autophagy (although some use PCD and apoptosis interchangeably) while necrosis is generally described as a chaotic and uncontrolled mode of death. In plants, the term PCD is widely used to describe most instances of death observed. At present, there is a vast array of plant cell culture models and developmental systems being studied by different research groups and it is clear from what is described in this mass of literature that, as with animals, there does not appear to be just one type of PCD in plants. It is fundamentally important to be able to distinguish between different types of cell death for several reasons. For example, it is clear that, in cell culture systems, the window of time in which ‘PCD’ is studied by different groups varies hugely and this can have profound effects on the interpretation of data and complicates attempts to compare different researcher's data. In addition, different types of PCD will probably have different regulators and modes of death. For this reason, in plant cell cultures an apoptotic-like PCD (AL-PCD) has been identified that is fairly rapid and results in a distinct corpse morphology which is visible 4–6 h after release of cytochrome c and other apoptogenic proteins. This type of morphology, distinct from autophagy and from necrosis, has also been observed in examples of plant development. In this review, our model system and how it is used to distinguish specifically between AL-PCD and necrosis will be discussed. The different types of PCD observed in plants will also be discussed and the importance of distinguishing between different forms of cell death will be highlighted.

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.

Cells in a plant differentiate according to their positions and use cell-cell communication to assess these positions. Similarly, single cells in suspension cultures can develop into somatic embryos, and cell-cell communication is thought to control this process. The monoclonal antibody JIM8 labels an epitope on cells in specific positions in plants. JIM8 also labels certain cells in carrot embryogenic suspension cultures. We have used JIM8 and secondary antibodies coupled to paramagnetic beads to label and immunomagnetically sort single cells in a carrot embryogenic suspension culture into pure populations. Cells in the JIM8(+) population develop into somatic embryos, whereas cells in the JIM8(-) population do not form somatic embryos. However, certain cells in JIM8(+) cultures (state B cells) undergo asymmetric divisions, resulting in daughter cells (state C cells) that do not label with JIM8 and that sort to JIM8(-) cultures. State C cells are competent to form somatic embryos, and we show here that a conditioned growth medium from a culture of JIM8(+) cells allows state C cells in a JIM8(-) culture to go on and develop into somatic embryos. JIM8 labels cells in suspension cultures at the cell wall. Therefore, a cell with a role in cell-cell communication and early cell fate selection can be identified by an epitope in its cell wall.