Bcl-2 family members localize to tobacco chloroplasts and inhibit programmed cell death induced by chloroplast-targeted herbicides

Metacaspase-8 modulates programmed cell death induced by ultraviolet light and H2O2 in Arabidopsis

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.

Heat stress: an inducer of programmed cell death in Chlorella saccharophila

Programmed cell death (PCD) has been recognized as a fundamental cellular process conserved in metazoans, plants and yeast. However, the cellular mechanisms leading to PCD have not been fully elucidated in unicellular organisms. Evidence is presented that heat stress induces PCD in Chlorella saccharophila cells. Our results demonstrate that heat shock triggers a PCD pathway occurring with characteristics features such as chromatin condensation, DNA fragmentation, cell shrinkage and detachment of the plasma membrane from the cell wall, and suggest the presence of caspase 3-like activity. The caspase 3 inhibitor Ac-DEVD-CHO gave significant protection against heat shock-induced cell death. Moreover, a reduction in photosynthetic pigment contents associated with alteration of chloroplast morphology and a fairly rapid disappearance of the ribulose-1,5-bisphosphate carboxylase/oxygenase large subunit and the light-harvesting complex of PSII have been observed. The timing of events in the signaling cascade associated with the C. saccharophila heat shock PCD response is discussed. Insights into this field may have general implications for understanding the pathway of cell death in unicellular green algae.

Mitochondrial redox biology and homeostasis in plants

Mitochondria are key players in plant cell redox homeostasis and signalling. Earlier concepts that regarded mitochondria as secondary to chloroplasts as the powerhouses of photosynthetic cells, with roles in cell proliferation, death and ageing described largely by analogy to animal paradigms, have been replaced by the new philosophy of integrated cellular energy and redox metabolism involving mitochondria and chloroplasts. Thanks to oxygenic photosynthesis, plant mitochondria often operate in an oxygen- and carbohydrate-rich environment. This rather unique environment necessitates extensive flexibility in electron transport pathways and associated NAD(P)-linked enzymes. In this review, mitochondrial redox metabolism is discussed in relation to the integrated cellular energy and redox function that controls plant cell biology and fate.

Ptr ToxA interacts with a chloroplast-localized protein

Pyrenophora tritici-repentis, causal agent of tan spot of wheat, produces host-selective toxins that are determinants of pathogenicity or virulence. Ptr ToxA (ToxA), a proteinaceous toxin produced by P. tritici-repentis, is a necrotizing toxin produced by the most common races isolated from infected wheat. Recent studies have shown that ToxA is internalized into the mesophyll cells and localizes to chloroplasts of sensitive wheat cultivars only. We employed a yeast two-hybrid screen in an effort to determine plant proteins that interact with ToxA and found that ToxA interacts with a chloroplast protein, designated ToxA binding protein 1 (ToxABP1). ToxABP1 contains a lysine-rich region within a coiled-coil domain that is similar to phosphotidyl-inositol binding sites present in animal proteins involved in endocytosis. In both ToxA-sensitive and -insensitive cultivars, ToxABP1 is expressed at similar levels and encodes an identical protein. ToxABP1 protein is present in both chloroplast membranes and chloroplast stroma. ToxA appears to interact primarily with a multimeric complex of ToxABP1 protein associated with the chloroplast membrane.

The diphenylether herbicide lactofen induces cell death and expression of defense-related genes in soybean

Lactofen belongs to the diphenylether class of herbicides, which targets protoporphyrinogen oxidase, which in turn causes singlet oxygen generation. In tolerant plants like soybean (Glycine max), the chemical nonetheless causes necrotic patches called "bronzing" in contact areas. Here it is shown that such bronzing is accompanied by cell death, which was quantified from digital microscopic images using Assess Software. Cellular autofluorescence accompanied cell death, and a homolog of the cell death marker gene, Hsr203j, was induced by lactofen in treated soybean tissues. Thus, this form of chemically induced cell death shares some hallmarks of certain types of programmed cell death. In addition to the cell death phenotype, lactofen caused enhanced expressions of chalcone synthase and chalcone reductase genes, mainly in the exposed and immediately adjacent (proximal) cells. Furthermore, isoflavone synthase genes, which are wound inducible in soybean, were up-regulated by lactofen in both proximal and distal cell zones in minimally wounded cotyledons and further enhanced in wounded tissues. Moreover, if the wall glucan elicitor from Phytophthora sojae was present during lactofen treatment, the induction of isoflavone synthase was even more rapid. These results are consistent with the fact that lactofen triggers massive isoflavone accumulations and activates the capacity for glyceollin elicitation competency. In addition, lactofen induces late expression of a selective set of pathogenesis-related (PR) protein genes, including PR-1a, PR-5, and PR-10, mainly in treated proximal tissues. These various results are discussed in the context of singlet oxygen-induced responses and lactofen's potential as a disease resistance-inducing agent.

Ascorbic acid deficiency activates cell death and disease resistance responses in Arabidopsis

Programmed cell death, developmental senescence, and responses to pathogens are linked through complex genetic controls that are influenced by redox regulation. Here we show that the Arabidopsis (Arabidopsis thaliana) low vitamin C mutants, vtc1 and vtc2, which have between 10% and 25% of wild-type ascorbic acid, exhibit microlesions, express pathogenesis-related (PR) proteins, and have enhanced basal resistance against infections caused by Pseudomonas syringae. The mutants have a delayed senescence phenotype with smaller leaf cells than the wild type at maturity. The vtc leaves have more glutathione than the wild type, with higher ratios of reduced glutathione to glutathione disulfide. Expression of green fluorescence protein (GFP) fused to the nonexpressor of PR protein 1 (GFP-NPR1) was used to detect the presence of NPR1 in the nuclei of transformed plants. Fluorescence was observed in the nuclei of 6- to 8-week-old GFP-NPR1 vtc1 plants, but not in the nuclei of transformed GFP-NPR1 wild-type plants at any developmental stage. The absence of senescence-associated gene 12 (SAG12) mRNA at the time when constitutive cell death and basal resistance were detected confirms that elaboration of innate immune responses in vtc plants does not result from activation of early senescence. Moreover, H2O2-sensitive genes are not induced at the time of systemic acquired resistance execution. These results demonstrate that ascorbic acid abundance modifies the threshold for activation of plant innate defense responses via redox mechanisms that are independent of the natural senescence program.