Programmed cell death (PCD) is a complex process involving transcriptional, biochemical and cellular changes and is in this distinguished from sudden necrotic death. PCD is an integral part of plant development: the death of specific groups of cells is essential for the correct development of, for example, the anther and the xylem. PCD is also the final step in organ senescence, although the transition between senescence and PCD has been difficult to define precisely. Pathogen attack, through the hypersensitive response, clearly activates a programme ending in the PCD of the cells surrounding the point of pathogen entry. Another form of biotic interaction, the self incompatibility response, also clearly activates a regulated cell death programme. Abiotic stress can also activate a form of PCD as long as the stress is not too severe. Morphologically some types of plant PCD resemble animal apoptosis including DNA degradation and chromatin condensation, and the formation of apoptotic bodies. However in many cases the cellular changes are more similar to autophagic cell death. These include the formation of vesicles which fuse with the vacuole, enlarging it, and ultimately lead to rupture of the tonoplast and release of hydrolytic enzymes into the cytoplasm. One of the problems in constructing a model for plant PCD has been the absence of genes homologous to plant caspases, key enzymes in animal PCD. However caspase- like activities have been reported in numerous PCD systems, and other proteases such as vacuolar processing enzymes have been proposed as candidates for at least some of this caspase activity. However it seems likely that additional proteases with caspase activity remain to be discovered. Likewise although there is evidence for the participation of mitochondria in some plant PCD systems, through the release of cytochrome c, this phenomenon is as yet not confirmed in many examples of plant PCD and whether the cytochrome c has a regulatory role is also uncertain. Changes in intracellular calcium levels do seem to be emerging as a widespread feature and may be an important regulator of plant PCD, while the role of reactive oxygen species remains as yet unclear. A number of plant growth regulators participate in the regulation of plant PCD such as ethylene, jasmonic acid and salicylic acid, but it has not been possible to identify a single molecule that is a universal regulator for all forms of PCD. Thus, although a complete model for plant PCD is elusive, we are beginning to have enough data to start building models that can be tested experimentally.
Keywords: autophagy, apoptosis, caspases, hypersensitive response, plant growth regulators