Programmed cell death in plants in response to pathogen attack
In recent years, evidence has emerged to suggest that many developmental and physiological processes in plants, including the hypersensitive response (HR) to pathogen attack, involve programmed cell death (PCD) similar to apoptosis in animals. Morphological features of apoptosis, such as membrane bleb-bing, chromatin condensation and DNA cleavage, can be observed in the HR. Changes in gene transcription, protein phosphorylation, the oxidative burst, ion fluxes and lipid-based signalling are detected at early stages of PCD in both animals and plants. Furthermore, sequencing projects are revealing plant genes with intriguing similarities to apoptosis-related animal genes. In animals, specific cysteine proteases, termed caspases, are involved in the execution phase of apoptosis. Although cysteine proteases appear to have a role in plant PCD, and at least one of the targets of caspases, poly (ADP-ribose) polymerase, is cleaved during the HR, caspase-encoding genes have not yet been identified in plants. Moreover, a role for the mitochondrion, central in either regulating or effecting apoptosis in animals, has yet to emerge in plant PCD. Evidence exists for more ancient PCD mechanisms than those executed by caspases, and these may ultimately be what is evolutionarily conserved between plants and animals.