Activation of cysteine proteases in cowpea plants during the hypersensitive response--a form of programmed cell death.

There is increasing evidence that the hypersensitive response during plant-pathogen interactions is a form of programmed cell death. In an attempt to understand the biochemical nature of this form of programmed cell death in the cowpea-cowpea rust fungus system, proteolytic activity in extracts of fungus-infected and uninfected cowpea plants was investigated, using exogenously added poly(ADP-ribose) polymerase as a marker. Unlike the proteolytic cleavage pattern of endogenous poly(ADP-ribose) polymerase in apoptotic animal cells, exogenously added poly(ADP-ribose) polymerase in extracts of fungus-infected plants was proteolytically cleaved into fragments of molecular masses 77, 52, 47, and 45 kDa. In vitro and in vivo protease inhibitor experiments revealed the activation of cysteine proteases, and possibly a regulatory role, during the hypersensitive response.