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.

Host-selective toxins, a group of structurally complex and chemically diverse metabolites produced by plant pathogenic strains of certain fungal species, function as essential determinants of pathogenicity or virulence. Investigations into the molecular and biochemical responses to these disease determinants reveal responses typically associated with host defense and incompatibility induced by avirulence determinants. The characteristic responses that unify these disparate disease phenotypes are numerous, yet the evidence implicating a causal relationship of these responses, whether induced by host-selective toxins or avirulence factors, in determining the consequences of the host-pathogen interaction is equivocal. This review summarizes some examples of the action of host-selective toxins to illustrate the similarity in responses with those to avirulence determinants.

Victoria blight of Avena sativa (oat) is caused by the fungus Cochliobolus victoriae, which is pathogenic because of the production of the toxin victorin. The victorin-induced response in sensitive A. sativa has been characterized as a form of programmed cell death (PCD) and displays morphological and biochemical features similar to apoptosis, including chromatin condensation, DNA laddering, cell shrinkage, altered mitochondrial function, and ordered, substrate-specific proteolytic events. Victorin-induced proteolysis of ribulose-1,5-bisphosphate carboxylase/oxygenase (Rubisco) is shown to be prevented by caspase-specific and general protease inhibitors. Evidence is presented for a signaling cascade leading to Rubisco proteolysis that involves multiple proteases. Furthermore, two proteases that are apparently involved in the Rubisco proteolytic cascade were purified and characterized. These proteases exhibit caspase specificity and display amino acid sequences homologous to plant subtilisin-like Ser proteases. The proteases are constitutively present in an active form and are relocalized to the extracellular fluid after induction of PCD by either victorin or heat shock. The role of the enzymes as processive proteases involved in a signal cascade during the PCD response is discussed.

The molecular nature of many plant disease resistance (R) genes is known; the largest class encodes nucleotide-binding site-leucinerich repeat (NBS-LRR) proteins that are structurally related to proteins involved in innate immunity in animals. Few genes conferring disease susceptibility, on the other hand, have been identified. Recent identification of susceptibility to the fungus Cochliobolus victoriae in Arabidopsis thaliana has enabled our cloning of LOV1, a disease susceptibility gene that, paradoxically, is a member of the NBS-LRR resistance gene family. We found LOV1 mediates responses associated with defense, but mutations in known defense response pathways do not prevent susceptibility to C. victoriae. These findings demonstrate that NBS-LRR genes can condition disease susceptibility and resistance and may have implications for R gene deployment.Cochliobolus victoriae ͉ disease susceptibility ͉ nucleotide binding site-leucine-rich repeat ͉ victorin

Typically pathogens deploy virulence effectors to disable defense. Plants defeat effectors with resistance proteins that guard effector targets. Here we show that a pathogen exploits a resistance protein by activating it to confer susceptibility. Interactions of victorin, an effector produced by the necrotrophic fungus Cochliobolus victoriae, TRX-h5, a defense-associated thioredoxin, and LOV1, an Arabidopsis susceptibility protein, recapitulate the guard mechanism of plant defense. In LOV1’s absence, victorin inhibits TRX-h5 resulting in compromised defense but not disease by C. victoriae. In LOV1’s presence, victorin binding to TRX-h5 activates LOV1 and elicits a resistance-like response that confers disease susceptibility. We propose victorin is or mimics a conventional pathogen virulence effector that was defeated by LOV1 and confers virulence to C. victoriae solely because it incites defense.

Victorin is a host-selective toxin produced by Cochliobolus victoriae, the causal agent of victoria blight of oats. Previously, victorin was shown to be bound specifically by two proteins of the mitochondrial glycine decarboxylase complex, at least one of which binds victorin only in toxin-sensitive genotypes in vivo. This enzyme complex is involved in the photorespiratory cycle and is inhibited by victorin, with an effective concentration for 50% inhibition of 81 pM. The photorespiratory cycle begins with ribulose-1,5-bisphosphate carboxylase/oxygenase (Rubisco), and victorin was found to induce a specific proteolytic cleavage of the Rubisco large subunit (LSU). Leaf slices incubated with victorin for 4 hr in the dark accumulated a form of the LSU that is cleaved after the 14th amino acid. This proteolytic cleavage was prevented by the protease inhibitors E-64 and calpeptin. Another primary symptom of victorin treatment is chlorophyll loss, which along with the specific LSU cleavage is suggestive of a victorin-induced, senescence-like response. DNA from victorin-treated leaf slices showed a pronounced laddering effect, which is typical of apoptosis. Calcium appeared to play a role in mediating the plant response to victorin because LaCl3 gave near-complete protection against victorin, preventing both leaf symptoms and LSU cleavage. The ethylene inhibitors aminooxyacetic acid and silver thiosulfate also gave significant protection against victorin-induced leaf symptoms and prevented LSU cleavage. The symptoms resulting from victorin treatment suggest that victorin causes premature senescence of leaves.

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The fungus Cochliobolus victoriae causes Victoria blight of oats (Avena sativa) and is pathogenic due to its production of victorin, which induces programmed cell death in sensitive plants. Victorin sensitivity has been identified in Arabidopsis thaliana and is conferred by the dominant gene LOCUS ORCHESTRATING VICTORIN EFFECTS1 (LOV1), which encodes a coiled-coil-nucleotide binding site-leucine-rich repeat protein. We isolated 63 victorin-insensitive mutants, including 59 lov1 mutants and four locus of insensitivity to victorin1 (liv1) mutants. The LIV1 gene encodes thioredoxin h5 (ATTRX5), a member of a large family of disulfide oxidoreductases. To date, very few plant thioredoxins have been assigned specific, nonredundant functions. We found that the victorin response was highly specific to ATTRX5, as the closely related ATTRX3 could only partially compensate for loss of ATTRX5, even when overexpressed. We also created chimeric ATTRX5/ATTRX3 proteins, which identified the central portion of the protein as important for conferring specificity to ATTRX5. Furthermore, we found that ATTRX5, but not ATTRX3, is highly induced in sensitive Arabidopsis following victorin treatment. Finally, we determined that only the first of the two active-site Cys residues in ATTRX5 is required for the response to victorin, suggesting that ATTRX5 function in the victorin pathway involves an atypical mechanism of action.