Plants, animals and several unicellular eukaryotes use programmed cell death (PCD) for defense and developmental mechanisms. While cell death pathways in animals have been well characterized, relatively little is known about the molecular mechanism of such a strategy in plants. Although, very few regulatory proteins or protein domains have been identified as conserved across all eukaryotic PCD forms, still plants and animals share many hallmarks of PCD, both at cellular and molecular levels. Morphological and biochemical features like chromatin condensation, nuclear DNA fragmentation, and participation of caspase like proteases in plant PCD appear to be similar across the eukaryotic kingdom and in conformity with the process in metazoans as well. Transgenic expression of mammalian anti- and pro-apoptotic proteins in plants has been shown to influence the regulatory pathways of cell death activation and suppression, indicating the existence of functional counterparts of such genes in plants, several of which have now been cloned and characterized to various extents. This suggests that despite differences, there may be a fair level of functional similarity between the mechanistic components of plant and animal apoptosis. Although genome scan of Arabidopsis thaliana seems to rule out the existence of major mammalian apoptotic counterparts in plants, the identification of caspase like proteins and other structural homolgs (metacaspases) together with mildly conserved apoptotic players like Bax-1 inhibitor may seemed to suggest some degree of common grounds both in execution and in the regulation of the cell death phenomenon. The overall review of the available data pertaining to mechanism of PCD in plants primarily supports an ancestral relationship with animal apoptosis rather than any common executional or regulational strategies. The establishment of mechanistic details of the phenomenon in plants is certain to throw up many surprises to necessitate a fresh review of this intriguing phenomenon. Metacaspases and Paracaspases having been ruled out to possess caspase activity is the beginning for this surprise to unfold.
Key words: Programmed cell death, apoptosis, caspases.