Recent advance in the study of caspase-like proteases and Bax inhibitor-1 in plants: their possible roles as regulator of programmed cell death

Programmed cell death (PCD) is a common process in multicellular organisms during development and in response to pathogens and stress signals. In animal systems, it is well established that a class of cysteine protease called caspase plays a central role in signalling and executing PCD. No caspase orthologue has been uncovered from any plant genomic database so far, but recent structural element-based iterative searches have uncovered the presence of a putative caspase-related protease family designated as metacaspases in plants. Furthermore, a structural homologue of Bcl-2-like proteins such as Bax and Bcl-2, which act as important regulators to modulate caspase action in animal cells, has not been found in any plant genomic databases. Clearly, the molecular components of plant PCD machinery and many key interactions in the regulatory network of plant PCD remain obscure. However, recent discoveries from numerous laboratories have provided good evidence for the involvement of caspase-like proteases in the control of cell death activation in plants, and there is accumulating evidence for a plant homologue of Bax inhibitor-1, which was first identified as an antagonist of Bax action in yeast, as an evolutionarily conserved protein and that may act as a key regulator for a common cell death pathway in plants and animals.