Programmed cell death (PCD) is an integral part of plant development and of responses to abiotic stress or pathogens. Although the morphology of plant PCD is, in some cases, well characterised and molecular mechanisms controlling plant PCD are beginning to emerge, there is still confusion about the classification of PCD in plants. Here we suggest a classification based on morphological criteria. According to this classification, the use of the term 'apoptosis' is not justified in plants, but at least two classes of PCD can be distinguished: vacuolar cell death and necrosis. During vacuolar cell death, the cell contents are removed by a combination of autophagy-like process and release of hydrolases from collapsed lytic vacuoles. Necrosis is characterised by early rupture of the plasma membrane, shrinkage of the protoplast and absence of vacuolar cell death features. Vacuolar cell death is common during tissue and organ formation and elimination, whereas necrosis is typically found under abiotic stress. Some examples of plant PCD cannot be ascribed to either major class and are therefore classified as separate modalities. These are PCD associated with the hypersensitive response to biotrophic pathogens, which can express features of both necrosis and vacuolar cell death, PCD in starchy cereal endosperm and during self-incompatibility. The present classification is not static, but will be subject to further revision, especially when specific biochemical pathways are better defined. Research on plant cell death has grown considerably in the past few years, owing to the importance of cell death for plant development and defense. Just as animal cells engage several mechanisms leading to death, the road to cell demise in plants can also vary. The long evolutionary distance and distinct cellular architecture between the two kingdoms may account for the differences between the mechanisms of plant and animal cell death. It is therefore appropriate to assess the relevance of animal cell death nomenclature 1 to plants. At present, there is confusion in cell death terminology in plant biology, which drives our attempt to formulate a more logical classification. Although our molecular understanding of plant cell death regulation and execution is insufficient to create definitive classifications based on precise biochemical pathways, it is possible to begin classifying plant cell death scenarios based on morphological criteria, as was initially the case in animal cell death research 2,3 and is still used for the classification of cell death in animal science. 1 This document attempts to provide a classification of plant cell death. We urge authors, reviewers and editors to follow this classification to facilitate communication between scientists and accelerate research in this field.

Metacaspases are cysteine-dependent proteases found in protozoa, fungi and plants and are distantly related to metazoan caspases. Although metacaspases share structural properties with those of caspases, they lack Asp specificity and cleave their targets after Arg or Lys residues. Studies performed over the past 10 years have demonstrated that metacaspases are multifunctional proteases essential for normal physiology of non-metazoan organisms. This article provides a comprehensive overview of the metacaspase function and molecular regulation during programmed cell death, stress and cell proliferation, as well as an analysis of the first metacaspase-mediated proteolytic pathway. To prevent further misapplication of caspase-specific molecular probes for measuring and inhibiting metacaspase activity, we provide a list of probes suitable for metacaspases.

Tracheary elements (TEs) are cells in the xylem that are highly specialized for transporting water and solutes up the plant. TEs undergo a very well-defined process of differentiation that involves specification, enlargement, patterned cell wall deposition, programmed cell death and cell wall removal. This process is coordinated such that adjacent TEs are joined together to form a continuous network. Expression studies on model systems as diverse as trees and cell cultures have contributed to providing a flood of candidate genes with potential roles in TE differentiation. Analysis of some of these genes has yielded important information on processes such as patterned secondary cell wall deposition. The current challenge is to continue this functional analysis and to use these data and build an integrated model of TE development.

Programmed cell death (PCD) is a genetically controlled cell death that is regulated during development and activated in response to environmental stresses or pathogen infection. The degree of conservation of PCD across kingdoms and phylum is not yet clear; however, whereas caspases are proteases that act as key components of animal apoptosis, plants have no orthologous caspase sequences in their genomes. The discovery of plant and fungi metacaspases as proteases most closely related to animal caspases led to the hypothesis that metacaspases are the functional homologues of animal caspases in these organisms. Arabidopsis thaliana has nine metacaspase genes, and so far it is unknown which members of the family if any are involved in the regulation of PCD. We show here that metacaspase-8 (AtMC8) is a member of the gene family strongly up-regulated by oxidative stresses caused by UVC, H 2 O 2 , or methyl viologen. This up-regulation was dependent of RCD1, a mediator of the oxidative stress response. Recombinant metacaspase-8 cleaved after arginine, had a pH optimum of 8, and complemented the H 2 O 2 no-death phenotype of a yeast metacaspase knock-out. Overexpressing AtMC8 up-regulated PCD induced by UVC or H 2 O 2 , and knocking out AtMC8 reduced cell death triggered by UVC and H 2 O 2 in protoplasts. Knock-out seeds and seedlings had an increased tolerance to the herbicide methyl viologen. We suggest that metacaspase-8 is part of an evolutionary conserved PCD pathway activated by oxidative stress.In some instances, programmed cell death (PCD) 4 in plants is comparable with animal apoptosis at the cellular level. However, sequencing the Arabidopsis genome revealed that very few of the animal PCD regulators are conserved in plants. This suggests a greater divergence of the PCD pathways across kingdoms than thought. Initial reports seemed to provide indirect evidence supporting the existence of caspase orthologues in plants, with several caspase-like activities detected in plant extracts and inhibitor studies that show them to be required for PCD (for review, see Ref. 1). Although several research groups reported the absence of orthologous caspase sequences in plant genomes, a more in depth analysis revealed a greater diversity of caspase-related proteases than previously suspected (2). In particular, two families of predicted proteases were identified that are more closely related to animal caspases than to other proteases: the paracaspases and metacaspases. Paracaspases and caspases appear animal specific, whereas metacaspases are present in other eukaryotes, including plants. Plant metacaspases are subdivided in type I and type II on the basis of their structure; type I have an N-terminal prodomain that is not present in type II. A role for metacaspases in plant PCD was proposed (3) for four reasons; 1) a common origin with caspases, 2) the absence of closer caspase homologues in plants, 3) the proliferation of the genes coding for metacaspases in plant genomes mirrors the pattern of the proliferation and speciali...

Plants, animals, and several branches of unicellular eukaryotes use programmed cell death (PCD) for defense or developmental mechanisms. This argues for a common ancestral apoptotic system in eukaryotes. However, at the molecular level, very few regulatory proteins or protein domains have been identified as conserved across all eukaryotic PCD forms. A very important goal is to determine which molecular components may be used in the execution of PCD in plants, which have been conserved during evolution, and which are plant-specific. Using Arabidopsis thaliana, we have shown that UV radiation can induce apoptosis-like changes at the cellular level and that a UV experimental system is relevant to the study of PCD in plants. We report here that UV induction of PCD required light and that a protease cleaving the caspase substrate Asp-GluVal-Asp (DEVDase activity) was induced within 30 min and peaked at 1 h. This DEVDase appears to be related to animal caspases at the biochemical level, being insensitive to broad-range cysteine protease inhibitors. In addition, caspase-1 and caspase-3 inhibitors and the pan-caspase inhibitor p35 were able to suppress DNA fragmentation and cell death. These results suggest that a YVADase activity and an inducible DEVDase activity possibly mediate DNA fragmentation during plant PCD induced by UV overexposure. We also report that At-DAD1 and At-DAD2, the two A. thaliana homologs of Defender against Apoptotic Death-1, could suppress the onset of DNA fragmentation in A. thaliana, supporting an involvement of the endoplasmic reticulum in this form of the plant PCD pathway.