Box 1 | PCD mechanisms in eukaryotes: plants versus animals

Despite the widespread interest, the study of programmed cell death (PCD) mechanisms in plants remains hampered by the lack of a clear set of 'core regulators' that function as the key executioners of various death signals. In metazoans, from humans to Caenorhabditis elegans, the three known classes of conserved core regulators for apoptosis are the BCL-2/CED-9, APAF1/CED-4 and caspase/CED-3 families of proteins^{1, 2, 7}. The absence of structural orthologues for these proteins in the complete genome sequences of Arabidopsis thaliana and yeast species indicates that alternative, structurally distinct, regulators must have evolved to control cell-death activation in plants and fungi. However, recent experimental evidence has reinforced the idea that caspase-like proteases (CLPs) exist in plants to mediate some forms of developmental and induced cell death. Also, the effects of pro-apoptotic and pro-survival BCL-2-related proteins on plant PCD have been reported using transgenic approaches. These results indicate that, despite differences in the inherent morphology (see Fig. 2 for comparison), physiological requirements and regulators at the sequence level, similar strategies with functionally related molecules might have been conserved between animals and plants to orchestrate controlled cell death. One candidate for such a conserved cell-death regulator is the BAX inhibitor-1 (BI-1) class of proteins, which has been found to function as a cell-death suppressor in different eukaryotes such as animals, plants and fungi. Interestingly, although BI-1 can function in Saccharomyces cerevisiae, it is apparently absent from its genome. Whether it could be found in the genome of other fungi might help determine the evolutionary flux of this highly conserved gene within the eukaryote lineages.