Masterminds or minions? Cysteine proteinases in plant programmed cell death

Cysteine proteinases are ubiquitously involved in programmed cell death (PCD) in multicellular organisms. Ill animals, one group of cysteine proteinases, the cysteine-dependent aspartate-specific proteinases (caspases), are involved ill a proteolytic signalling cascade that controls apoptosis, the Most Studied form of PCD. The enzymes act as both masterminds and executioners ill apoptotic cell death. In plants, members of the metacaspase family, as well as those of the papain-like and legumain families, of cysteine proteinases have all been implicated in PCD. These enzymes often belong to sizeable gene families, with Arabidopsis having 9 metacaspase, 32 papain-like, and 4 legumain genes, This redundancy has made it difficult to ascertain the functional importance of any particular enzyme in plant PCD. as many are often expressed in a given tissue undergoing PCD. As yet, mechanisms similar to the apoptotic caspase cascade in annuals have not been Uncovered in plants and, indeed, may not exist. Are the various cysteine proteinases, so often implicated ill plant PCD, merely acting as minions in the process'? This review Will Outline reports of cysteine proteinases associated With plant PCD, discuss problems in determining the function of specific proteases, and suggest avenues for determining, how these enzymes might be regulated and how PCD pathways upstream of protease expression and activation might operate.