Activities displaying
caspase cleavage specificity have been well documented in various plant programmed cell death (PCD) models. However, plant genome analyses have not revealed clear orthologues of
caspase genes, indicating that
enzyme(s) structurally unrelated yet possessing
caspase specificity have functions in plant PCD. Here, we review recent data showing that some
caspase-like activities are attributable to the plant
subtilisin-like
proteases, saspases and phytaspases. These
proteases hydrolyze a range of tetrapeptide
caspase substrates following the
aspartate residue. Data obtained with saspases implicate them in the proteolytic degradation of
ribulose-1,5-bisphosphate carboxylase/
oxygenase (
Rubisco) during biotic and abiotic PCD, whereas
phytaspase overproducing and silenced transgenics provide evidence that
phytaspase regulates PCD during both abiotic (oxidative and osmotic stresses) and biotic (
virus infection) insults. Like
caspases, phytaspases and saspases are synthesized as
proenzymes, which are autocatalytically processed to generate a mature
enzyme. However, unlike
caspases, phytaspases and saspases appear to be constitutively processed and secreted from healthy plant cells into the intercellular space. Apoplastic localization presumably prevents
enzyme-mediated
protein fragmentation in the absence of PCD. In response to death-inducing stimuli,
phytaspase has been shown to re-localize to the cell interior. Thus, plant PCD-related
proteases display both common (D-specific
protein fragmentation during PCD) and distinct (
enzyme structure and activity regulation) features with animal PCD-related
proteases.