Concurrent activation of cell death-regulating signaling pathways by singlet oxygen in Arabidopsis thaliana

Plant oxylipins: role of jasmonic acid during programmed cell death, defence and leaf senescence

Plants are continuously challenged by a variety of abiotic and biotic cues. To deter feeding insects, nematodes and fungal and bacterial pathogens, plants have evolved a plethora of defence strategies. A central player in many of these defence responses is jasmonic acid. It is the aim of this minireview to summarize recent findings that highlight the role of jasmonic acid during programmed cell death, plant defence and leaf senescence.

Singlet oxygen-dependent translational control in the tigrina-d.12 mutant of barley

The tigrina (tig)-d.12 mutant of barley is impaired in the negative control limiting excess protochlorophyllide (Pchlide) accumulation in the dark. Upon illumination, Pchlide operates as photosensitizer and triggers singlet oxygen production and cell death. Here, we show that both Pchlide and singlet oxygen operate as signals that control gene expression and metabolite accumulation in tig-d.12 plants. In vivo labeling, Northern blotting, polysome profiling, and protein gel blot analyses revealed a selective suppression of synthesis of the small and large subunits of ribulose-1,5-bisphosphate carboxylase/oxygenase (RBCSs and RBCLs), the major light-harvesting chlorophyll a/b-binding protein of photosystem II (LHCB2), as well as other chlorophyll-binding proteins, in response to singlet oxygen. In part, these effects were caused by an arrest in translation initiation of photosynthetic transcripts at 80S cytoplasmic ribosomes. The observed changes in translation correlated with a decline in the phosphorylation level of ribosomal protein S6. At later stages, ribosome dissociation occurred. Together, our results identify translation as a major target of singlet oxygen-dependent growth control and cell death in higher plants.

Arabidopsis mutants reveal multiple singlet oxygen signaling pathways involved in stress response and development

Shortly after the release of singlet oxygen ((1)O(2)) in chloroplasts drastic changes in nuclear gene expression occur in the conditional flu mutant of Arabidopsis that reveal a rapid transfer of signals from the plastid to the nucleus. Factors involved in this retrograde signaling were identified by mutagenizing a transgenic flu line expressing a (1)O(2)-responsive reporter gene. The reporter gene consisted of the luciferase open reading frame and the promoter of an AAA-ATPase gene (At3g28580) that was selectively activated by (1)O(2) but not by superoxide or hydrogen peroxide. A total of eight second-site mutants were identified that either constitutively activate the reporter gene and the endogenous AAA-ATPase irrespectively of whether (1)O(2) was generated or not (constitutive activators of AAA-ATPase, caa) or abrogated the (1)O(2)-dependent up-regulation of these genes as seen in the transgenic parental flu line (non-activators of AAA-ATPase, naa). The characterization of the mutants strongly suggests that (1)O(2)-signaling does not operate as an isolated linear pathway but rather forms an integral part of a signaling network that is modified by other signaling routes and impacts not only stress responses of plants but also their development.

Light-independent cell death induced by accumulation of pheophorbide a in Arabidopsis thaliana

Tetrapyrroles are well-known photosensitizers. In plants, various intermediate molecules of tetrapyrrole metabolism have been reported to induce cell death in a light-dependent manner. In contrast to these reports, we found that pheophorbide a, a key intermediate of chlorophyll catabolism, causes cell death in complete darkness in a transgenic Arabidopsis plant, As-ACD1. In this plant, expression of mRNA for pheophorbide a oxygenase was suppressed by expression of Acd1 antisense RNA; thus, As-ACD1 accumulated an excessive amount of pheophorbide a when chlorophyll breakdown occurred. We observed that when senescence was induced by a continuous dark period, leaves of As-ACD1 plants became dehydrated. By measuring electrolyte leakage, we estimated that >50% of the leaf cells underwent cell death within a 5 d period of darkness. Light and electron microscopic observations indicated that the cellular structure had collapsed in a large population of cells. Partially covering a leaf with aluminum foil resulted in light-independent cell death in the covered region and induced bleaching in the uncovered regions. These results indicate that accumulation of pheophorbide a induces cell death under both darkness and illumination, but the mechanisms of cell death under these conditions may differ. We discuss the possible mechanism of light-independent cell death and the involvement of pheophorbide a in the signaling pathway for programmed cell death.

Characterization of soldat8, a suppressor of singlet oxygen-induced cell death in Arabidopsis seedlings

The flu mutant of Arabidopsis thaliana overaccumulates in the dark the immediate precursor of chlorophyllide, protochlorophyllide (Pchlide), a potent photosensitizer, that upon illumination generates singlet oxygen ((1)O2). Once (1)O2 has been released in plastids of the flu mutant, mature plants stop growing, while seedlings die. Several suppressor mutations, dubbed singlet oxygen-linked death activator (soldat), were identified that specifically abrogate (1)O2-mediated stress responses in young flu seedlings without grossly affecting (1)O2-mediated stress responses of mature flu plants. One of the soldat mutations, soldat8, was shown to impair a gene encoding the SIGMA6 factor of the plastid RNA polymerase. Reintroduction of a wild-type copy of the SOLDAT8 gene into the soldat8/flu mutant restored the phenotype of the flu parental line. In contrast to flu, seedlings of soldat8/flu did not bleach when grown under non-permissive dark/light conditions, despite their continuous overaccumulation of the photosensitizer Pchlide in the dark. The activity of SIGMA6 is confined primarily to the very early stage of seedling development. Inactivation of SIGMA6 in soldat8 mutants disturbed plastid homeostasis, drastically reduced the non-photochemical quenching capacity and enhanced the light sensitivity of young soldat8 seedlings. Surprisingly, after being grown under very low light, soldat8 seedlings showed an enhanced resistance against a subsequent severe light stress that was significantly higher than in wild-type seedlings. In order to reach a similar enhanced stress resistance, wild-type seedlings had to be exposed to a brief higher light treatment that triggered an acclimatory response. Such a mild pre-stress treatment did not further enhance the stress resistance of soldat8 seedlings. Suppression of (1)O2-mediated cell death in young flu/soldat8 seedlings seems to be due to a transiently enhanced acclimation at the beginning of seedling development caused by the initial disturbance of plastid homeostasis.

Singlet oxygen in plants: production, detoxification and signaling

Singlet oxygen ((1)O(2)) is a singular reactive oxygen species (ROS) that is produced constitutively in plant leaves in light via chlorophylls that act as photosensitizers. This (1)O(2) production is spatially resolved within thylakoid membranes and is enhanced under light stress conditions. (1)O(2) can also be produced by phytotoxins during plant-pathogen interactions. (1)O(2) is highly reactive, can be toxic to cells and can be involved in the signaling of programmed cell death or acclimation processes. Here, we summarize current knowledge on (1)O(2) management in plants and on the biological effects of this peculiar ROS. Compared with other ROS, (1)O(2) has received relatively little attention, but recent developments indicate that it has a crucial role in the responses of plants to light.

Potential regulation of gene expression in photosynthetic cells by redox and energy state: approaches towards better understanding

BACKGROUND

Photosynthetic electron transport is performed by a chain of redox components that are electrochemically connected in series. Its efficiency depends on the balanced action of the photosystems and on the interaction with the dark reaction. Plants are sessile and cannot escape from environmental conditions such as fluctuating illumination, limitation of CO(2) fixation by low temperatures, salinity, or low nutrient or water availability, which disturb the homeostasis of the photosynthetic process. Photosynthetic organisms, therefore, have developed various molecular acclimation mechanisms that maintain or restore photosynthetic efficiency under adverse conditions and counteract abiotic stresses. Recent studies indicate that redox signals from photosynthetic electron transport and reactive oxygen species (ROS) or ROS-scavenging molecules play a central role in the regulation of acclimation and stress responses.

SCOPE

The underlying signalling network of photosynthetic redox control is largely unknown, but it is already apparent that gene regulation by redox signals is of major importance for plants. Signalling cascades controlling the expression of chloroplast and nuclear genes have been identified and dissection of the different pathways is advancing. Because of the direction of information flow, photosynthetic redox signals can be defined as a distinct class of retrograde signals in addition to signals from organellar gene expression or pigment biosynthesis. They represent a vital signal of mature chloroplasts that report their present functional state to the nucleus. Here we describe possible problems in the elucidation of redox signalling networks and discuss some aspects of plant cell biology that are important for developing suitable experimental approaches.

CONCLUSIONS

The photosynthetic function of chloroplasts represents an important sensor that integrates various abiotic changes in the environment into corresponding molecular signals, which, in turn, regulate cellular activities to counterbalance the environmental changes or stresses.

Retrograde signaling and plant stress: plastid signals initiate cellular stress responses

Retrograde signaling coordinates the expression of nuclear genes encoding organellar proteins with the metabolic and developmental state of the organelle. These plastid signals are essential not only for coordinating photosynthetic gene expression in both the nucleus and in the chloroplasts but also for mediating plant stress responses. The chloroplasts therefore act as sensors of environmental changes and complex networks of plastid signals coordinate cellular activities and assist the cell during plant stress responses. Recent work suggests that information from both cytosolic-signaling and plastid-signaling networks must be integrated for the plant cell to respond optimally to environmental stress.

The Arabidopsis sweetie mutant is affected in carbohydrate metabolism and defective in the control of growth, development and senescence

Sugars modulate many vital metabolic and developmental processes in plants, from seed germination to flowering, senescence and protection against diverse abiotic and biotic stresses. However, the exact mechanisms involved in morphogenesis, developmental signalling and stress tolerance remain largely unknown. Here we report the characterization of a novel Arabidopsis thaliana mutant, sweetie, with drastically altered morphogenesis, and a strongly modified carbohydrate metabolism leading to elevated levels of trehalose, trehalose-6-phosphate and starch. We additionally show that the disruption of SWEETIE causes significant growth and developmental alterations, such as severe dwarfism, lancet-shaped leaves, early senescence and flower sterility. Genes implicated in sugar metabolism, senescence, ethylene biosynthesis and abiotic stress were found to be upregulated in sweetie. Our physiological, biochemical, genetic and molecular data indicate that the mutation in sweetie was nuclear, single and recessive. The effects of metabolizable sugars and osmolytes on sweetie morphogenesis were distinct; in light, sweetie was hypersensitive to sucrose and glucose during vegetative growth and a partial phenotypic reversion took place in the presence of high sorbitol concentrations. However, SWEETIE encodes a protein that is unrelated to any known enzyme involved in sugar metabolism. We suggest that SWEETIE plays an important regulatory function that influences multiple metabolic, hormonal and stress-related pathways, leading to altered gene expression and pronounced changes in the accumulation of sugar, starch and ethylene.

No single way to understand singlet oxygen signalling in plants

When plant cells are under environmental stress, several chemically distinct reactive oxygen species (ROS) are generated simultaneously in various intracellular compartments and these can cause oxidative damage or act as signals. The conditional flu mutant of Arabidopsis, which generates singlet oxygen in plastids during a dark-to-light transition, has allowed the biological activity of singlet oxygen to be determined, and the criteria to distinguish between cytotoxicity and signalling of this particular ROS to be defined. The genetic basis of singlet-oxygen-mediated signalling has been revealed by the mutation of two nuclear genes encoding the plastid proteins EXECUTER (EX)1 and EX2, which are sufficient to abrogate singlet-oxygen-dependent stress responses. Conversely, responses due to higher cytotoxic levels of singlet oxygen are not suppressed in the ex1/ex2 background. Whether singlet oxygen levels lower than those that trigger genetically controlled cell death activate acclimation is now under investigation.

Enzymatic, but not non-enzymatic, 1O2-mediated peroxidation of polyunsaturated fatty acids forms part of the EXECUTER1-dependent stress response program in the flu mutant of Arabidopsis thaliana

The conditional flu mutant of Arabidopsis accumulates excess amounts of protochlorophyllide within plastid membranes in the dark and generates singlet oxygen upon light exposure. By varying the length of the dark period, the level of the photosensitizer protochlorophyllide may be modulated, and conditions have been established that either endorse the cytotoxicity of (1)O(2) or reveal its signaling role. Two criteria have been used to distinguish between these two modes of activity of (1)O(2): the impact of the EXECUTER1 mutation and the prevalence of either non-enzymatic or enzymatic lipid peroxidation. During illumination of etiolated flu seedlings, toxic effects of (1)O(2) prevail and non-enzymatic lipid peroxidation proceeds rapidly. In contrast, in light-grown flu plants that were subjected to an 8 h dark/light shift, lipid peroxidation occurred almost exclusively enzymatically. The resulting oxidation product, 13-hydroperoxy octadecatrienoic acid (13-HPOT), serves as a substrate for synthesis of 12-oxo phytodienoic acid (OPDA) and jasmonic acid (JA), both of which are known to control various metabolic and developmental processes in plants. Inactivation of the EXECUTER1 protein abrogates not only (1)O(2)-mediated cell death and growth inhibition of flu plants, but also enzymatic lipid peroxidation. However, inactivation of jasmonate biosynthesis in the aos/flu double mutant does not affect (1)O(2)-mediated growth inhibition and cell death. Hence, JA and OPDA do not act as second messengers during (1)O(2) signaling, but form an integral part of a stress-related signaling cascade activated by (1)O(2) that encompasses several signaling pathways known to be activated by abiotic and biotic stressors.

[Reactive oxygen species, stress and cell death in plants]

Plants are constantly exposed to changes in environmental conditions. During periods of stress, the cellular redox homeostasis is altered as a result of reactive oxygen species accumulation. The change in redox is responsible for the symptoms commonly observed during periods of stress and reflects the phytotoxic nature of oxygen radical accumulation. However, oxygen radicals have recently been identified as key actors in the response to stress and their role as secondary messengers is now clearly established. The identification of their role in gene regulation has allowed one to identify them as key regulators in the induction and execution of programmed cell death typically observed during developmental processes as well as during stress responses. This review presents recent advances in the characterisation of the role of reactive oxygen species in plants.

Veinal necrosis induced by turnip mosaic virus infection in Arabidopsis is a form of defense response accompanying HR-like cell death

In the pathosystems of Turnip mosaic virus (TuMV) with Brassicaceae crops, various symptoms, including mosaic and necrosis, are observed. We previously reported a necrosis-inducing factor TuNI in Arabidopsis thaliana, a model species. In this study, we show that the necrotic symptom induced by TuNI, observed along the veins, was actually a form of defense response accompanying a hypersensitive reaction (HR)-like cell death in the veinal area. The virus is often localized in the necrotic region. The necrotic response is associated with the production of H2O2, accumulation of salicylic acid (SA), emission of ethylene, and subsequent expression of defense-related genes. Additionally, this HR-like cell death is eased or erased by a shading treatment. These features are similar to the HR-associated resistance reaction to pathogens. However, unlike HR, two phytohormones--SA and ethylene--are involved in the necrosis induction, and both SA- and ethylene-dependent pathogenesis-related genes are activated. We concluded that the veinal necrosis induced by TuMV is regulated by a complex and unique network of at least two signaling pathways, which differs from the signal transduction for the known HR-associated resistance.

AtMCP1b, a chloroplast-localised metacaspase, is induced in vascular tissue after wounding or pathogen infection

cDNA corresponding to the Arabidopsis type I metacaspase AtMCP1b was isolated from plants infected with Pseudomonas syringae. A positive correlation between AtMCP1b expression and cell death was observed in the presence of staurosporine, a protein kinase inhibitor that induces programmed cell death. The tissue localisation of an AtMCP1b promoter-GUS fusion was observed in the vascular tissue of transgenic plants. GUS activity increased in response to an incompatible DC3000 (avrRpm1) or a compatible DC3000 P. syringae infection, or to wounding. Confocal and immunohistochemical analysis of Arabidopsis thaliana (L.) leaves showed that an AtMCP1b-GFP fusion protein was localised in the chloroplasts. Our data support a positive correlation between AtMCP1b gene expression and cell death in response to wounding or pathogenic interactions. Moreover, the localisation of AtMCP1b gene expression within vascular tissue and cells of abscission regions strongly supports a role for AtMCP1b in programmed cell dismantling events in response to environmental and developmental triggers. The AtMCP1b-GFP subcellular localisation infers a role for the plastid organelles in PCD and, thus, in responses to pathogen attack and development.

Jasmonate signalling network in Arabidopsis thaliana: crucial regulatory nodes and new physiological scenarios

Plant development and stress responses are regulated by complex signalling networks that mediate specific and dynamic plant responses upon activation by various types of exogenous and endogenous signal. In this review, we focus on the latest published work on jasmonate (JA) signalling components and new regulatory nodes in the transcriptional network that regulates a number of diverse plant responses to developmental and environmental cues. Not surprisingly, the majority of the key revelations in the field have been made in Arabidopsis thaliana. However, for comparative reasons, we integrate information on Arabidopsis with recent reports for other plant species (when available). Recent findings on the regulation of plant responses to pathogens by JAs, as well as new evidence implicating JAs in the regulation of senescence, suggest a common mechanism of JA action in these responses via distinct groups of transcription factors. Moreover, a significant increase in the amount of evidence has allowed placing of specific mitogen-activated protein kinases (MAPKs) as crucial regulatory nodes in the defence signalling network. In addition, we report on new physiological scenarios for JA signalling, such as organogenesis of nitrogen-fixing nodules and anticancer therapy.

EXECUTER1- and EXECUTER2-dependent transfer of stress-related signals from the plastid to the nucleus of Arabidopsis thaliana

Shortly after the release of singlet oxygen ((1)O2), drastic changes in nuclear gene expression occur in the conditional flu mutant of Arabidopsis that reveal a rapid transfer of signals from the plastid to the nucleus. In contrast to retrograde control of nuclear gene expression by plastid signals described earlier, the primary effect of (1)O2 generation in the flu mutant is not the control of chloroplast biogenesis but the activation of a broad range of signaling pathways known to be involved in biotic and abiotic stress responses. This activity of a plastid-derived signal suggests a new function of the chloroplast, namely that of a sensor of environmental changes that activates a broad range of stress responses. Inactivation of the plastid protein EXECUTER1 attenuates the extent of (1)O2-induced up-regulation of nuclear gene expression, but it does not fully eliminate these changes. A second related nuclear-encoded protein, dubbed EXECUTER2, has been identified that is also implicated with the signaling of (1)O2-dependent nuclear gene expression changes. Like EXECUTER1, EXECUTER2 is confined to the plastid. Inactivation of both EXECUTER proteins in the ex1/ex2/flu triple mutant is sufficient to suppress the up-regulation of almost all (1)O2-responsive genes. Retrograde control of (1)O2-responsive genes requires the concerted action of both EXECUTER proteins within the plastid compartment.

Regulation of the Arabidopsis thaliana vitamin B6 biosynthesis genes by abiotic stress

Vitamin B(6) (pyridoxine and its vitamers) plays an essential role as a co-factor for enzymatic reactions and has also recently been implicated in defense against cellular oxidative stress. The biosynthetic pathway was thoroughly characterized in Escherichia coli, however most organisms, including plants, utilize an alternate pathway involving two genes, PDX1 and PDX2. Arabidopsis thaliana contains one copy of PDX2, but three full-length copies of PDX1, one each on chromosomes 2, 3, and 5 (referred to as PDX1.1, PDX1.2, and PDX1.3, respectively). Phylogenetic analysis of the PDX1 homologues in A. thaliana showed that PDX1.1 and PDX1.3 clustered with the homologues from the other dicots, whereas PDX1.2 was more divergent, and did not cluster with either the dicots or monocots. Expression analysis using quantitative PCR showed that PDX1.1 and PDX1.3 were highly expressed in A. thaliana rosettes, while PDX1.2 showed only low level expression. All three PDX1 genes and PDX2 were responsive to abiotic stressors including high light, chilling, drought, and ozone, however, the response of PDX1.2 was disparate from that of the other PDX genes, showing a lessened response to high light, chilling, and drought, but an increased response to ozone. Green fluorescent protein fusion studies demonstrated that PDX2 localizes in the nucleus and membranes of cells, consistent with recent published data for PDX1. Insight into regulation of the biosynthetic genes during abiotic stress could have important applications in the development of stress-tolerant crops.

Inducible cell death in plant immunity

Programmed cell death (PCD) occurs during vegetative and reproductive plant growth, as typified by autumnal leaf senescence and the terminal differentiation of the endosperm of cereals which provide our major source of food. PCD also occurs in response to environmental stress and pathogen attack, and these inducible PCD forms are intensively studied due their experimental tractability. In general, evidence exists for plant cell death pathways which have similarities to the apoptotic, autophagic and necrotic forms described in yeast and metazoans. Recent research aiming to understand these pathways and their molecular components in plants are reviewed here.

PDX1 is essential for vitamin B6 biosynthesis, development and stress tolerance in Arabidopsis

Vitamin B6 is an essential coenzyme for numerous metabolic enzymes and is a potent antioxidant. In plants, very little is known about its contribution to viability, growth and development. The de novo pathway of vitamin B6 biosynthesis has only been described recently and involves the protein PDX1 (pyridoxal phosphate synthase protein). Arabidopsis thaliana has three homologs of PDX1, two of which, PDX1.1 and PDX1.3, have been demonstrated as functional in vitamin B6 biosynthesis in vitro and by yeast complementation. In this study, we show that the spatial and temporal expression patterns of PDX1.1 and PDX1.3, investigated at the transcript and protein level, largely overlap, but PDX1.3 is more abundant than PDX1.1. Development of single pdx1.1 and pdx1.3 mutants is partially affected, whereas disruption of both genes causes embryo lethality at the globular stage. Detailed examination of the single mutants, in addition to those that only have a single functional copy of either gene, indicates that although these genes are partially redundant in vitamin B6 synthesis, PDX1.3 is more requisite than PDX1.1. Developmental distinctions correlate with the vitamin B6 content. Furthermore, we provide evidence that in addition to being essential for plant growth and development, vitamin B6 also plays a role in stress tolerance and photoprotection of plants.

Cryptochrome-1-dependent execution of programmed cell death induced by singlet oxygen in Arabidopsis thaliana

Programmed cell death (PCD) plays an important role during the life cycle of higher organisms. Although several regulatory mechanisms governing PCD are thought to be conserved in animals and plants, light-dependent cell death represents a form of PCD that is unique to plants. The light requirement of PCD has often been associated with the production of reactive oxygen species during photosynthesis. In support of this hypothesis, hydrogen peroxide and superoxide have been shown to be involved in triggering a PCD response. In the present work, we have used the conditional flu mutant of Arabidopsis to analyze the impact of another reactive oxygen species, singlet oxygen, on cell death. Unexpectedly, the light-dependent release of singlet oxygen alone is not sufficient to induce PCD of flu seedlings but has to act together with a second concurrent blue light reaction. This blue-light-specific trigger of PCD could not be attributed to a photosynthetic reaction or redox change within the chloroplast but to the activation of the blue light/UVA-specific photoreceptor cryptochrome. The singlet oxygen-mediated and cryptochrome-dependent cell death response differs in several ways from PCD triggered by hydrogen peroxide/superoxide.

The role of EDS1 (enhanced disease susceptibility) during singlet oxygen-mediated stress responses of Arabidopsis

Upon a dark/light shift the conditional flu mutant of Arabidopsis starts to generate singlet oxygen (1O2) that is restricted to the plastid compartment. Distinct sets of genes are activated that are different from those induced by hydrogen peroxide/superoxide. One of the genes that is rapidly upregulated is EDS1 (enhanced disease susceptibility). The EDS1 protein has been shown to be required for the resistance to biotrophic pathogens and the accumulation of salicylic acid (SA) that enhances the defenses of a plant by inducing the synthesis of pathogen-related (PR) proteins. Because of the similarity of its N-terminal portion to the catalytic site of lipases, EDS1 has also been implicated with the release of polyunsaturated fatty acids and the subsequent formation of various oxylipins. The release of singlet oxygen in the flu mutant triggers a drastic increase in the concentration of free SA and activates the expression of PR1 and PR5 genes. These changes depend on the activity of EDS1 and are suppressed in flu/eds1 double mutants. Soon after the beginning of singlet oxygen production, the synthesis of oxylipins such as jasmonic acid (JA) and 12-oxophytodienoic acid (OPDA) also start and plants stop growing and induce a cell-death response. The inactivation of EDS1 does not affect oxylipin synthesis, growth inhibition and the initiation of cell death, but it does allow plants to recover much faster from singlet oxygen-mediated growth inhibition and it also suppresses the spread of necrotic lesions in leaves. Hence, singlet oxygen activates a complex stress-response program with EDS1 playing a key role in initiating and modulating several steps of it. This program includes not only responses to oxidative stress, but also responses known to be activated during plant-pathogen interactions and wounding.

Growth condition-dependent sensitivity, photodamage and stress response of Chlamydomonas reinhardtii exposed to high light conditions

Different substrate conditions, such as varying CO(2) concentrations or the presence of acetate, strongly influence the efficiency of photosynthesis in Chlamydomonas reinhardtii. Altered photosynthetic efficiencies affect the susceptibility of algae to the deleterious effects of high light stress, such as the production of reactive oxygen species (ROS) and PSII photodamage. In this study, we investigated the effect of high light on C. reinhardtii grown under photomixotrophy, i.e. in the presence of acetate, as well as under photoautotrophic growth conditions with either low or high CO(2) concentrations. Different parameters such as growth rate, chlorophyll bleaching, singlet oxygen generation, PSII photodamage and the total genomic stress response were analyzed. Although showing a similar degree of PSII photodamage, a much stronger singlet oxygen-specific response and a broader general stress response was observed in acetate and high CO(2)-supplemented cells compared with CO(2)-limited cells. These different photooxidative stress responses were correlated with the individual cellular PSII content and probably directly influenced the ROS production during exposure to high light. In addition, growth of high CO(2)-supplemented cells was more susceptible to high light stress compared with cells grown under CO(2) limitation. The growth of acetate-supplemented cultures, on the other hand, was less affected by high light treatment than cultures grown under high CO(2) concentrations, despite the similar cellular stress. This suggests that the production of ATP by mitochondrial acetate respiration protects the cells from the deleterious effects of high light stress, presumably by providing energy for an effective defense.

Transcriptomic footprints disclose specificity of reactive oxygen species signaling in Arabidopsis

Reactive oxygen species (ROS) are key players in the regulation of plant development, stress responses, and programmed cell death. Previous studies indicated that depending on the type of ROS (hydrogen peroxide, superoxide, or singlet oxygen) or its subcellular production site (plastidic, cytosolic, peroxisomal, or apoplastic), a different physiological, biochemical, and molecular response is provoked. We used transcriptome data generated from ROS-related microarray experiments to assess the specificity of ROS-driven transcript expression. Data sets obtained by exogenous application of oxidative stress-causing agents (methyl viologen, Alternaria alternata toxin, 3-aminotriazole, and ozone) and from a mutant (fluorescent) and transgenic plants, in which the activity of an individual antioxidant enzyme was perturbed (catalase, cytosolic ascorbate peroxidase, and copper/zinc superoxide dismutase), were compared. In total, the abundance of nearly 26,000 transcripts of Arabidopsis (Arabidopsis thaliana) was monitored in response to different ROS. Overall, 8,056, 5,312, and 3,925 transcripts showed at least a 3-, 4-, or 5-fold change in expression, respectively. In addition to marker transcripts that were specifically regulated by hydrogen peroxide, superoxide, or singlet oxygen, several transcripts were identified as general oxidative stress response markers because their steady-state levels were at least 5-fold elevated in most experiments. We also assessed the expression characteristics of all annotated transcription factors and inferred new candidate regulatory transcripts that could be responsible for orchestrating the specific transcriptomic signatures triggered by different ROS. Our analysis provides a framework that will assist future efforts to address the impact of ROS signals within environmental stress conditions and elucidate the molecular mechanisms of the oxidative stress response in plants.

Chlorophyll metabolism

Since the 1970s, researchers have proposed several regulatory pathways governing chlorophyll metabolism, but only recently have the underlying molecular mechanisms been elucidated. The recent data indicate that such regulatory systems are more complex than originally anticipated. For instance, the pathways involve a series of protein-protein interactions, including complex formation, the dual localization of enzymes within chloroplasts, and a novel protein degradation mechanism that is triggered by pigments. Furthermore, several lines of evidence suggest that chlorophyll metabolism might not only significantly impact the assembly of photosynthetic machineries but also influence processes such as programmed cell death, the 'stay-green' phenomenon, and chloroplast-nucleus communication.

Jasmonates in cancer therapy

Several groups have reported in recent years that members of the plant stress hormones family of jasmonates, and some of their synthetic derivatives, exhibit anti-cancer activity in vitro and in vivo. Jasmonates increased the life span of EL-4 lymphoma-bearing mice, and exhibited selective cytotoxicity towards cancer cells while sparing normal blood lymphocytes, even when the latter were part of a mixed population of leukemic and normal cells drawn from the blood of chronic lymphocytic leukemia (CLL) patients. Jasmonates join a growing number of old and new cancer chemotherapeutic compounds of plant origin. Three mechanisms of action have been proposed to explain the anti-cancer activity of jasmonates. These include: (1) The bio-energetic mechanism-jasmonates induce severe ATP depletion in cancer cells via mitochondrial perturbation; (2) The re-differentiation mechanism-jasmonates induce re-differentiation in human myeloid leukemia cells via mitogen-activated protein kinase (MAPK) activity; (3) The reactive oxygen species (ROS)-mediated mechanism-jasmonates induce apoptosis in lung carcinoma cells via the generation of hydrogen peroxide, and pro-apoptotic proteins of the Bcl-2 family. Several similarities between the effects of jasmonates on plant and cancer cells have been recorded, suggesting that additional analysis of jasmonate effects in plant cells may contribute to a deeper understanding of the anti-cancer actions of these compounds. Those similarities include: induction of cell death, suppression of proliferation and cell cycle arrest, MAPK induction, ROS generation, and enhancement of heat-shock proteins (HSP) expression. Finally, jasmonates can induce death in drug-resistant cells. The drug resistance was conferred by either p53 mutation or P-glycoprotein (P-gp) over-expression. In summary, the jasmonate family of novel anti-cancer agents presents new hope for the development of cancer therapeutics, which should attract further scientific and pharmaceutical interest.

The outcomes of concentration-specific interactions between salicylate and jasmonate signaling include synergy, antagonism, and oxidative stress leading to cell death

Salicylic acid (SA) has been proposed to antagonize jasmonic acid (JA) biosynthesis and signaling. We report, however, that in salicylate hydroxylase-expressing tobacco (Nicotiana tabacum) plants, where SA levels were reduced, JA levels were not elevated during a hypersensitive response elicited by Pseudomonas syringae pv phaseolicola. The effects of cotreatment with various concentrations of SA and JA were assessed in tobacco and Arabidopsis (Arabidopsis thaliana). These suggested that there was a transient synergistic enhancement in the expression of genes associated with either JA (PDF1.2 [defensin] and Thi1.2 [thionin]) or SA (PR1 [PR1a-beta-glucuronidase in tobacco]) signaling when both signals were applied at low (typically 10-100 microm) concentrations. Antagonism was observed at more prolonged treatment times or at higher concentrations. Similar results were also observed when adding the JA precursor, alpha-linolenic acid with SA. Synergic effects on gene expression and plant stress were NPR1- and COI1-dependent, SA- and JA-signaling components, respectively. Electrolyte leakage and Evans blue staining indicated that application of higher concentrations of SA + JA induced plant stress or death and elicited the generation of apoplastic reactive oxygen species. This was indicated by enhancement of hydrogen peroxide-responsive AoPR10-beta-glucuronidase expression, suppression of plant stress/death using catalase, and direct hydrogen peroxide measurements. Our data suggests that the outcomes of JA-SA interactions could be tailored to pathogen/pest attack by the relative concentration of each hormone.

Regulation of tetrapyrrole biosynthesis in higher plants

Plant tetrapyrroles are the most abundant biomolecules on the earth and are cofactors of many apoproteins essential for plant function. The four end-products sirohaem, chlorophyll, haem and phytochromobilin are synthesized by a common branched pathway, which is tightly regulated to ensure a continuous supply to the cognate apoproteins. This may induce strong competition between different branches of the pathway for common substrates. In addition, the intermediates, which are phototoxic, must not be allowed to accumulate in the cell. The major control points are during the synthesis of the initial precursor, ALA (5-aminolaevulinic acid), and at the branch points for the insertion of metal ions into the porphyrin macrocycle. A recent study has also suggested that tetrapyrroles are involved in the communication between the chloroplast and the nucleus, strengthening the necessity for tight regulation. However, intermediates remain difficult to quantify mainly due to their low content and the different properties of the intermediates. In this paper, we summarize the regulation of this pathway and we detail why it is important to have an accurate method for the determination of tetrapyrroles in plants.

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.