Plant proteolytic enzymes: possible roles during programmed cell death

Proteolytic enzymes are known to be associated with developmentally program med cell death during organ senescence and tracheary element differentiatio n. Recent evidence also links proteinases with some types of pathogen- and stress-induced cell suicide. The precise roles of proteinases in these and other plant programmed cell death processes are not understood, however. To provide a framework for consideration of the importance of proteinases dur ing plant cell suicide, characteristics of the best-known proteinases from plants including subtilisin-type and papain-type enzymes, phytepsins, metal loproteinases and the 26S proteasome are summarized. Examples of serine, cy steine, aspartic, metallo- and threonine proteinases linked to animal progr ammed cell death are cited and the potential for plant proteinases to act a s mediators of signal transduction and as effectors of programmed cell deat h is discussed.