Plants, like animals, employ programmed cell death (PCD) for a variety of purposes including development (for example, xylem formation, seed germination and senescence), stress responses (for example, salt, cold, heat) and defence (for example, hypersensitive response).

While it is becoming increasingly evident that multiple forms of PCD exist in both plants and animals, certain forms of PCD in both kingdoms share a number of hallmarks, such as DNA fragmentation, cytochrome c release from mitochondria and cell shrinkage. However, despite extensive similarities, the details of the molecular mechanisms regulating plant PCD are less clearly understood.

Among the more surprising findings is an absence of caspase orthologues in plants. Because caspases, a family of cysteine-dependent aspartate-specific proteases, are known to play key roles in the initiation and execution of PCD in animals through cleavage of a number of protein targets, their apparent absence in plants poses a dilemma.

Recently, in collaboration with Moscow State University (Professor A. Vartapetian and co-workers), we have identified a plant protein with caspase-like protease activity which is a functional analogue of animal caspase. This protein has been purified from plants, identified as putative subtilisin-like protease and named phytaspase (plant aspartate specific protease).

We have provided evidence that phytaspase is essential for PCD-related responses to biotic (virus) and abiotic (environmental) stresses. We suggest a model whereby, after translation, phytaspase is activated and secreted into the apoplast where it may be sequestered before PCD and/or fulfils a guarding function. In response to a variety of biotic or abiotic stresses, phytaspase is re-localised from the apoplast to the inside of the cell, where it functions as an executioner of PCD.