Apoptosis, programmed cell death and the hypersensitive response

Apoptosis is typically a morphologically identifiable form of programmed cell death in mammals that is regulated by genes with homologues in other animal Phyla. Although both plants and fungal plant pathogens exhibit forms of developmental programmed cell death, demonstrated morphological or genetic homologies with mammalian apoptosis are still generally lacking. Because of its ubiquity and the involvement of signal transduction pathways in its induction, a strong case is developing that the hypersensitive response is a specific form of plant programmed cell death evolved as a defense against microbial parasites. Data suggest that separate signalling pathways may lead to the cell death and the defense gene activation that characterize this response and that parasite-specific resistance genes represent only one of many types of genes involved in response regulation. However, despite some biochemical similarities between the hypersensitive response, forms of developmental programmed cell death in plants, and animal apoptosis, no unique and consistent markers for the hypersensitive response (or plant programmed cell death in general) have yet been found. Whether any of these forms of plant cell death should be called apoptosis depends on how the term is defined. Assuming the hypersensitive response is a form of programmed cell death and is the 'default state' upon pathogen entry into a cell, it seems likely that intracellular biotrophic plant pathogens resemble some animal viruses in being able to suppress this response in susceptible hosts.