Summary

• Controlled cellular suicide in plants is important for the sculpting of organ shapes and the differentiation of specific cell types with diverse functions, as well as the removal of damaged or infected cells. These various types of programmed cell death (PCD) can have diverse morphotypes that are defined by their sequence of cytological changes, as well as the expression of various biochemical and molecular markers.
• With the presence of a cell wall, the content of the dying cell inevitably leaks into the extracellular milieu and is then presumably recycled during the late stages of cell death in plants. This is a key differentiating feature compared with animal apoptosis where engulfment of the contents of the dead cell by neighbouring cells or macrophages prevents the activation of the inflammatory response.
• Various phytohormones are known to function as signal molecules for cell-death pathways that are activated during pathogen defence or at a particular stage of plant development. However, their action could be dependent on the cellular context and environmental conditions.
• Reactive oxygen species (ROS) and the mitochondrion are common mediators of PCD in plants and animals, whereas plastids provide an additional source of cell-death signalling that is specific for plant cells. Precise mechanisms and pathways through which ROS and organelle-derived signals regulate PCD in plants remain to be elucidated.
• In spite of the lack of apparent orthologues for core regulators of cell death in animals, plants seem to use some similar molecules to serve analogous functions. These include caspase-like activities detected in different plant-cell-death model systems, the demonstration that expression of animal BAX can induce PCD in plants and fungi, and the identification of a conserved protein, BAX inhibitor-1 (BI-1), that can suppress cell death in both plants and animals.
• The prominent vacuole and its associated process of autophagy could have important roles in plant PCD, as has been found for the animal lysosome in developmental cell death. In the future, genetic and molecular approaches with model plants such as Arabidopsis thaliana should help to delineate the possible overlaps in the autophagic and cell-death pathways in plant cells.