Do Plant Caspases Exist?

Programmed cell death (PCD) is a functional concept that refers to cell death that is part of the normal life of a multicellular organism; it involves controlled disassembly of the cell. In animal systems PCD is synonymous with apoptosis, a cell death process characterized by a distinct set of morphological and biochemical features, mediated by a class of specific Cys proteases called cysteinyl aspartate-specific proteinases (caspases). Although to date no functional homologs of animal caspases have been identified in plants, a vast amount of indirect evidence suggesting the existence in plants of true caspase-like activity and its functional involvement in plant cell death has accumulated. Apoptosis in animal cells is characterized by specific features such as cell shrinkage, blebbing of the plasma membrane, condensation and fragmentation of the nucleus, and internucleosomal cleavage of DNA. The final stage of apoptosis is the fragmentation of the cell into cellular debris-containing vesicles called “apototic bodies” that are being phagocytosed by other cells (for review, seeHengartner, 2000). Inappropriate apoptosis has been implicated in many human diseases, including a number of birth defects, ischemic vascular diseases (e.g. heart attack and stroke), neurodegenerative diseases (e.g. Alzheimer's and Parkinson's diseases), autoimmune diseases (e.g. rheumatoid arthritis), AIDS, and diabetes mellitus type I.