The Genetic Basis of Singlet Oxygen-Induced Stress Responses of Arabidopsis thaliana

Cytoplasmic genome substitution in wheat affects the nuclear-cytoplasmic cross-talk leading to transcript and metabolite alterations

Background

Alloplasmic lines provide a unique tool to study nuclear-cytoplasmic interactions. Three alloplasmic lines, with nuclear genomes from Triticum aestivum and harboring cytoplasm from Aegilops uniaristata, Aegilops tauschii and Hordeum chilense, were investigated by transcript and metabolite profiling to identify the effects of cytoplasmic substitution on nuclear-cytoplasmic signaling mechanisms.

Results

In combining the wheat nuclear genome with a cytoplasm of H. chilense, 540 genes were significantly altered, whereas 11 and 28 genes were significantly changed in the alloplasmic lines carrying the cytoplasm of Ae. uniaristata or Ae. tauschii, respectively. We identified the RNA maturation-related process as one of the most sensitive to a perturbation of the nuclear-cytoplasmic interaction. Several key components of the ROS chloroplast retrograde signaling, together with the up-regulation of the ROS scavenging system, showed that changes in the chloroplast genome have a direct impact on nuclear-cytoplasmic cross-talk. Remarkably, the H. chilense alloplasmic line down-regulated some genes involved in the determination of cytoplasmic male sterility without expressing the male sterility phenotype. Metabolic profiling showed a comparable response of the central metabolism of the alloplasmic and euplasmic lines to light, while exposing larger metabolite alterations in the H. chilense alloplasmic line as compared with the Aegilops lines, in agreement with the transcriptomic data. Several stress-related metabolites, remarkably raffinose, were altered in content in the H. chilense alloplasmic line when exposed to high light, while amino acids, as well as organic acids were significantly decreased. Alterations in the levels of transcript, related to raffinose, and the photorespiration-related metabolisms were associated with changes in the level of related metabolites.

Conclusion

The replacement of a wheat cytoplasm with the cytoplasm of a related species affects the nuclear-cytoplasmic cross-talk leading to transcript and metabolite alterations. The extent of these modifications was limited in the alloplasmic lines with Aegilops cytoplasm, and more evident in the alloplasmic line with H. chilense cytoplasm. We consider that, this finding might be linked to the phylogenetic distance of the genomes.

The roles of two transcription factors, ABI4 and CBFA, in ABA and plastid signalling and stress responses

Genetic and physiological studies have revealed evidences for multiple signaling pathways by which the plastid exerts retrograde control over photosynthesis-associated-nuclear-genes. In this study we have examined the mechanisms of control of transcription by plastid signals, focusing on transcription factors. We have also further addressed the physical nature of plastid signals and the physiological role, in stress acclimation of this regulatory pathway. ABI4, a master Apetala 2 (AP2)-type transcription factor (TF), is targeted by multiple signalling pathways in plant cells, such as abscisic acid (ABA) signals, sugar signals and plastid signals derived from reactive oxygen species (ROS) and chlorophyll intermediates. ABI4 binds the promoter of target genes to prevent their transcription by competing with other competitive TFs. However, we found that once ABI4 bound the element (CCACGT), it may not be bound by other TFs, therefore making the signalling long-lasting. Downstream of ABI4, CBFA (CCAAT binding factor A) is a subunit of the HAP2/HAP3/HAP5 (Heme activator protein) trimeric transcription complex. CBFA however is a redundant HAP3 subunit. When emergency occurs (such as herbicide treatments or environmental stresses followed by ABA and ROS accumulation), the master transcription factor ABI4 down-regulates some TFs, like CBFA, and then some other TF subunits enter the transcription complex and transcriptional efficiency of stress-responsive genes (including the transcription co-factor CBP) is improved instantaneously. abi4, cbfA and cbp mutants showed weaker drought-tolerance after a herbicide norflurazon treatment, which indicated the physiological role of these key transcription factors.

Functional roles of flavonoids in photoprotection: new evidence, lessons from the past

We discuss on the relative significance of different functional roles potentially served by flavonoids in photoprotection, with special emphasis to their ability to scavenge reactive oxygen species (ROS) and control the development of individual organs and whole plant. We propose a model in which chloroplast-located flavonoids scavenge H2O2 and singlet oxygen generated under excess light-stress, thus avoiding programmed cell death. We also draw a picture in which vacuolar flavonoids in conjunction with peroxidases and ascorbic acid constitute a secondary antioxidant system aimed at detoxifying H2O2, which may diffuse out of the chloroplast at considerable rates and enter the vacuole following excess light stress-induced depletion of ascorbate peroxidase. We hypothesize for flavonols key roles as developmental regulators in early and current-day land-plants, based on their ability to modulate auxin movement and auxin catabolism. We show that antioxidant flavonoids display the greatest capacity to regulate key steps of cell growth and differentiation in eukaryotes. These regulatory functions of flavonoids, which are shared by plants and animals, are fully accomplished in the nM concentration range, as likely occurred in early land plants. We therefore conclude that functions of flavonoids as antioxidants and/or developmental regulators flavonoids are of great value in photoprotection. We also suggest that UV-B screening was just one of the multiple functions served by flavonoids when early land-plants faced an abrupt increase in sunlight irradiance.

Redox control of plant growth and development

Redox changes determined by genetic and environmental factors display well-organized interactions in the control of plant growth and development. Diurnal and seasonal changes in the environmental conditions are important for the normal course of these physiological processes and, similarly to their mild irregular alterations, for stress adaptation. However, fast or large-scale environmental changes may lead to damage or death of sensitive plants. The spatial and temporal redox changes influence growth and development due to the reprogramming of metabolism. In this process reactive oxygen and nitrogen species and antioxidants are involved as components of signalling networks. The control of growth, development and flowering by reactive oxygen and nitrogen species and antioxidants in interaction with hormones at organ, tissue, cellular and subcellular level will be discussed in the present review. Unsolved problems of the field, among others the need for identification of new components and interactions in the redox regulatory network at various organization levels using systems biology approaches will be also indicated.

RNAi based simultaneous silencing of all forms of light-dependent NADPH:protochlorophyllide oxidoreductase genes result in the accumulation of protochlorophyllide in tobacco (Nicotiana tabacum)

Conversion of protochlorophyllide (Pchlide) into chlorophyllide (Chlide), a key step in chlorophyll biosynthesis, is mediated by a light-dependent NADPH:protochlorophyllide oxidoreductase (POR). POR exists in multiple isoforms that share high degree of homology. RNAi-mediated gene silencing approach was used to suppress the expression of POR genes in order to study its role in the Chls biosynthesis in tobacco (Nicotiana tabacum L.). The transgenic plants were devoid of chlorophyll pigments and resembled albino plants. Northern blot analysis confirmed the degradation of POR transcripts into 21-23 bp fragments. Pigment analysis showed the accumulation of various intermediate compounds of Chl biosynthesis pathway including Pchlide. However, no trace of chlorophyll was observed. As compared to wild type, POR-silenced plants accumulated larger (60%) amounts of Pchlide from its endogenous substrate. When leaf discs of WT and POR-silenced plants were treated with exogenous ALA both WT and POR-silenced plants accumulated large amounts of tetrapyrrolic intermediates demonstrating that Pchlide biosynthesis potential was not suppressed in POR-silenced plants. Upon illumination, WT plants photo-transformed large amounts of Pchlide to Chlide. However, POR-silenced plants almost completely failed to do so. Results demonstrate that the antisense approaches to drop expression of individual POR isoforms have provided valuable insights into the role of distinct PORs during greening. Moreover, data illustrate that the POR is the only enzyme that can convert the Pchlide to Chlide and there is no alternate enzyme that can substitute the POR in higher plants. Thus, this investigation describes ideal mechanism for the silencing of POR isozymes in tobacco.

Singlet oxygen-mediated signaling in plants: moving from flu to wild type reveals an increasing complexity

Singlet oxygen ((1)O2)-mediated signaling has been established in the conditional fluorescent (flu) mutant of Arabidopsis. In the dark, the flu mutant accumulates free protochlorophyllide (Pchlide), a photosensitizer that in the light generates (1)O2. The release of (1)O2 leads to growth inhibition of mature plants and bleaching of seedlings. These (1)O2-mediated responses depend on two plastid proteins, EXECUTER (EX) 1 and 2. An ex1/ex2/flu mutant accumulates in the dark Pchlide and upon illumination generates similar amounts of (1)O2 as flu, but (1)O2-mediated responses are abrogated in the triple mutant. The (1)O2- and EX-dependent signaling pathway operates also in wild type placed under light stress. However, it does not act alone as in flu, but interacts with other signaling pathways that modulate (1)O2-mediated responses. Depending on how severe the light stress is, (1)O2- and EX-dependent signaling may be superimposed by (1)O2-mediated signaling that does not depend on EX and is associated with photo-oxidative damage. Because of its high reactivity and short half-life, (1)O2 is unlikely to be a signal that is translocated across the chloroplast envelope, but is likely to interact with other plastid components close to its site of production and to generate more stable signaling molecules during this interaction. Depending on the site of (1)O2 production and the severity of stress, different signaling molecules may be expected that give rise to different (1)O2-mediated responses.

1O2-mediated and EXECUTER-dependent retrograde plastid-to-nucleus signaling in norflurazon-treated seedlings of Arabidopsis thaliana

Chloroplast development depends on the synthesis and import of a large number of nuclear-encoded proteins. The synthesis of some of these proteins is affected by the functional state of the plastid via a process known as retrograde signaling. Retrograde plastid-to-nucleus signaling has been often characterized in seedlings of Arabidopsis thaliana exposed to norflurazon (NF), an inhibitor of carotenoid biosynthesis. Results of this work suggested that, throughout seedling development, a factor is released from the plastid to the cytoplasm that indicates a perturbation of plastid homeostasis and represses nuclear genes required for normal chloroplast development. The identity of this factor is still under debate. Reactive oxygen species (ROS) were among the candidates discussed as possible retrograde signals in NF-treated plants. In the present work, this proposed role of ROS has been analyzed. In seedlings grown from the very beginning in the presence of NF, ROS-dependent signaling was not detectable, whereas, in seedlings first exposed to NF after light-dependent chloroplast formation had been completed, enhanced ROS production occurred and, among others, (1)O2-mediated and EXECUTER-dependent retrograde signaling was induced. Hence, depending on the developmental stage at which plants are exposed to NF, different retrograde signaling pathways may be activated, some of which are also active in non-treated plants under light stress.

A TIR-NBS protein encoded by Arabidopsis Chilling Sensitive 1 (CHS1) limits chloroplast damage and cell death at low temperature

Survival of plants at low temperature depends on mechanisms for limiting physiological damage and maintaining growth. We mapped the chs1-1 (chilling sensitive1-1) mutation in Arabidopsis accession Columbia to the TIR-NBS gene At1g17610. In chs1-1, a single amino acid exchange at the CHS1 N-terminus close to the conserved TIR domain creates a stable mutant protein that fails to protect leaves against chilling stress. The sequence of another TIR-NBS gene (At5g40090) named CHL1 (CHS1-like 1) is related to that of CHS1. Over-expression of CHS1 or CHL1 alleviates chilling damage and enhances plant growth at moderate (24°C) and chilling (13°C) temperatures, suggesting a role for both proteins in growth homeostasis. chs1-1 mutants show induced salicylic acid production and defense gene expression at 13°C, indicative of autoimmunity. Genetic analysis of chs1-1 in combination with defense pathway mutants shows that chs1-1 chilling sensitivity requires the TIR-NBS-LRR and basal resistance regulators encoded by EDS1 and PAD4 but not salicylic acid. By following the timing of metabolic, physiological and chloroplast ultrastructural changes in chs1-1 leaves during chilling, we have established that alterations in photosynthetic complexes and thylakoid membrane integrity precede leaf cell death measured by ion leakage. At 24°C, the chs1-1 mutant appears normal but produces a massive necrotic response to virulent Pseudomonas syringae pv. tomato infection, although this does not affect bacterial proliferation. Our results suggest that CHS1 acts at an intersection between temperature sensing and biotic stress pathway activation to maintain plant performance over a range of conditions.

Production, detection, and signaling of singlet oxygen in photosynthetic organisms

SIGNIFICANCE

In photosynthetic organisms, excited chlorophylls (Chl) can stimulate the formation of singlet oxygen ((1)O(2)), a highly toxic molecule that acts in addition to its damaging nature as an important signaling molecule. Thus, due to this dual role of (1)O(2), its production and detoxification have to be strictly controlled.

RECENT ADVANCES

Regulation of pigment synthesis is essential to control (1)O(2) production, and several components of the Chl synthesis and pigment insertion machineries to assemble and disassemble protein/pigment complexes have recently been identified. Once produced, (1)O(2) activates a signaling cascade from the chloroplast to the nucleus that can involve multiple mechanisms and stimulate a specific gene expression response. Further, (1)O(2) signaling was shown to interact with signal cascades of other reactive oxygen species, oxidized carotenoids, and lipid hydroperoxide-derived reactive electrophile species.

CRITICAL ISSUES

Despite recent progresses, hardly anything is known about how and where the (1)O(2) signal is sensed and transmitted to the cytoplasm. One reason for that is the limitation of available detection methods challenging the reliable quantification and localization of (1)O(2) in plant cells. In addition, the process of Chl insertion into the reaction centers and antenna complexes is still unclear.

FUTURE DIRECTIONS

Unraveling the mechanisms controlling (1)O(2) production and signaling would help clarifying the specific role of (1)O(2) in cellular stress responses. It would further enable to investigate the interaction and sensitivity to other abiotic and biotic stress signals and thus allow to better understand why some stressors activate an acclimation, while others provoke a programmed cell death response.

A mutation in a coproporphyrinogen III oxidase gene confers growth inhibition, enhanced powdery mildew resistance and powdery mildew-induced cell death in Arabidopsis

A gene encoding a coproporphyrinogen III oxidase mediates disease resistance in plants by the salicylic acid pathway. A number of genes that regulate powdery mildew resistance have been identified in Arabidopsis, such as ENHANCED DISEASE RESISTANCE 1 to 3 (EDR1 to 3). To further study the molecular interactions between the powdery mildew pathogen and Arabidopsis, we isolated and characterized a mutant that exhibited enhanced resistance to powdery mildew. The mutant also showed dramatic powdery mildew-induced cell death as well as growth defects and early senescence in the absence of pathogens. We identified the affected gene by map-based cloning and found that the gene encodes a coproporphyrinogen III oxidase, a key enzyme in the tetrapyrrole biosynthesis pathway, previously known as LESION INITIATION 2 (LIN2). Therefore, we designated the mutant lin2-2. Further studies revealed that the lin2-2 mutant also displayed enhanced resistance to Hyaloperonospora arabidopsidis (H.a.) Noco2. Genetic analysis showed that the lin2-2-mediated disease resistance and spontaneous cell death were dependent on PHYTOALEXIN DEFICIENT 4 (PAD4), SALICYLIC ACID INDUCTION-DEFICIENT 2 (SID2), and NONEXPRESSOR OF PATHOGENESIS-RELATED GENES 1 (NPR1), which are all involved in salicylic acid signaling. Furthermore, the relative expression levels of defense-related genes were induced after powdery mildew infection in the lin2-2 mutant. These data indicated that LIN2 plays an important role in cell death control and defense responses in plants.

Antagonistic basic helix-loop-helix/bZIP transcription factors form transcriptional modules that integrate light and reactive oxygen species signaling in Arabidopsis

The critical developmental switch from heterotrophic to autotrophic growth of plants involves light signaling transduction and the production of reactive oxygen species (ROS). ROS function as signaling molecules that regulate multiple developmental processes, including cell death. However, the relationship between light and ROS signaling remains unclear. Here, we identify transcriptional modules composed of the basic helix-loop-helix and bZIP transcription factors PHYTOCHROME-INTERACTING FACTOR1 (PIF1), PIF3, ELONGATED HYPOCOTYL5 (HY5), and HY5 HOMOLOGY (HYH) that bridge light and ROS signaling to regulate cell death and photooxidative response. We show that pif mutants release more singlet oxygen and exhibit more extensive cell death than the wild type during Arabidopsis thaliana deetiolation. Genome-wide expression profiling indicates that PIF1 represses numerous ROS and stress-related genes. Molecular and biochemical analyses reveal that PIF1/PIF3 and HY5/HYH physically interact and coordinately regulate the expression of five ROS-responsive genes by directly binding to their promoters. Furthermore, PIF1/PIF3 and HY5/HYH function antagonistically during the seedling greening process. In addition, phytochromes, cryptochromes, and CONSTITUTIVE PHOTOMORPHOGENIC1 act upstream to regulate ROS signaling. Together, this study reveals that the PIF1/PIF3-HY5/HYH transcriptional modules mediate crosstalk between light and ROS signaling and sheds light on a new mechanism by which plants adapt to the light environments.

Redox regulation of photosynthetic gene expression

Redox chemistry and redox regulation are central to the operation of photosynthesis and respiration. However, the roles of different oxidants and antioxidants in the regulation of photosynthetic or respiratory gene expression remain poorly understood. Leaf transcriptome profiles of a range of Arabidopsis thaliana genotypes that are deficient in either hydrogen peroxide processing enzymes or in low molecular weight antioxidant were therefore compared to determine how different antioxidant systems that process hydrogen peroxide influence transcripts encoding proteins targeted to the chloroplasts or mitochondria. Less than 10 per cent overlap was observed in the transcriptome patterns of leaves that are deficient in either photorespiratory (catalase (cat)2) or chloroplastic (thylakoid ascorbate peroxidase (tapx)) hydrogen peroxide processing. Transcripts encoding photosystem II (PSII) repair cycle components were lower in glutathione-deficient leaves, as were the thylakoid NAD(P)H (nicotinamide adenine dinucleotide (phosphate)) dehydrogenases (NDH) mRNAs. Some thylakoid NDH mRNAs were also less abundant in tAPX-deficient and ascorbate-deficient leaves. Transcripts encoding the external and internal respiratory NDHs were increased by low glutathione and low ascorbate. Regulation of transcripts encoding specific components of the photosynthetic and respiratory electron transport chains by hydrogen peroxide, ascorbate and glutathione may serve to balance non-cyclic and cyclic electron flow pathways in relation to oxidant production and reductant availability.

Light-induced acclimation of the Arabidopsis chlorina1 mutant to singlet oxygen

Singlet oxygen (¹O₂) is a reactive oxygen species that can function as a stress signal in plant leaves leading to programmed cell death. In microalgae, ¹O₂-induced transcriptomic changes result in acclimation to ¹O₂. Here, using a chlorophyll b-less Arabidopsis thaliana mutant (chlorina1 [ch1]), we show that this phenomenon can also occur in vascular plants. The ch1 mutant is highly photosensitive due to a selective increase in the release of ¹O₂ by photosystem II. Under photooxidative stress conditions, the gene expression profile of ch1 mutant leaves very much resembled the gene responses to ¹O₂ reported in the Arabidopsis mutant flu. Preexposure of ch1 plants to moderately elevated light intensities eliminated photooxidative damage without suppressing ¹O₂ formation, indicating acclimation to ¹O₂. Substantial differences in gene expression were observed between acclimation and high-light stress: A number of transcription factors were selectively induced by acclimation, and contrasting effects were observed for the jasmonate pathway. Jasmonate biosynthesis was strongly induced in ch1 mutant plants under high-light stress and was noticeably repressed under acclimation conditions, suggesting the involvement of this hormone in ¹O₂-induced cell death. This was confirmed by the decreased tolerance to photooxidative damage of jasmonate-treated ch1 plants and by the increased tolerance of the jasmonate-deficient mutant delayed-dehiscence2.

Organelle signaling: how stressed chloroplasts communicate with the nucleus

Plastids are able to relay information to the nucleus to regulate stress responses. A new genetic screen has identified an isoprenoid intermediate that accumulates in stressed plastids and acts as a novel retrograde signal.

Plastid-to-nucleus communication, signals controlling the running of the plant cell

The presence of genes encoding organellar proteins in both the nucleus and the organelle necessitates tight coordination of expression by the different genomes, and this has led to the evolution of sophisticated intracellular signaling networks. Organelle-to-nucleus signaling, or retrograde control, coordinates the expression of nuclear genes encoding organellar proteins with the metabolic and developmental state of the organelle. Complex networks of retrograde signals orchestrate major changes in nuclear gene expression and coordinate cellular activities and assist the cell during plant development and stress responses. It has become clear that, even though the chloroplast depends on the nucleus for its function, plastid signals play important roles in an array of different cellular processes vital to the plant. Hence, the chloroplast exerts significant control over the running of the cell. This article is part of a Special Issue entitled: Protein Import and Quality Control in Mitochondria and Plastids.

Molecular components of stress-responsive plastid retrograde signaling networks and their involvement in ammonium stress

Plastid retrograde signaling (chloroplast to nucleus) has been proposed to play an important role in the acclimation of plant function to environmental stress. Although several pathways and molecular components, as well as some signals, have been identified in recent years, our understanding of the communication between plastid and nucleus under stress remains fragmentary. This mini-review summarizes the properties of currently proposed candidate signals, chief molecular components, and their roles in the plastid retrograde signaling network in a variety of stress responses. We provide special emphasis on the recently characterized AMOS1/EGY1-dependent plastid retrograde signaling pathways engaged during ammonium stress.

Hypothesis: increase of the ratio singlet oxygen plus superoxide radical to hydrogen peroxide changes stress defense response to programmed leaf death

The level of reactive oxygen species (ROS) increases under different stresses and, by destroying cellular components, may cause cell death. In addition, ROS are part of the complex network of transduction signals that induce defense reactions against stress or, alternatively, trigger programmed cell death, and key questions are the levels of each ROS that, respectively determine defense and death responses of the cell. The answer to those questions is difficult because there are several patterns of cell death that frequently appear mixed and are hardly distinguishable. Moreover, although considerable progresses have been achieved in the determination of the levels of specific ROS, critical questions remain on the ROS level in specific cell compartments. By considering chloroplasts as the main source of ROS in photosynthetic tissues at light, a comparison of the levels in stress and senescence of the chloroplastic activities involved in the generation and scavenging of ROS suggests plausible differences in the levels of specific ROS between stress defense and death. In effect, the three activities of the chlororespiratory chain increase similarly in stress defense response. However, in senescence, superoxide dismutase (SOD), that converts superoxide anion radical ([Formula: see text]) to hydrogen peroxide (H2O2,) decreases, while the thylakoid Ndh complex, that favors the generation of singlet oxygen ((1)O2) and [Formula: see text], and peroxidase (PX), that consumes H2O2, increase. The obvious inference is that, in respect to defense response, the ratio ((1)O2 plus [Formula: see text])/H2O2 is increased in the senescence previous to cell death. We hypothesize that the different ROS ratios, probably through changes in the jasmonic acid/H2O2 ratio, could determine the activation of the defense network or the death network response of the cell.

Sigma factor-mediated plastid retrograde signals control nuclear gene expression

Retrograde signalling from plastids to the nucleus is necessary to regulate the organelle's proteome during the establishment of photoautotrophy and fluctuating environmental conditions. Studies that used inhibitors of chloroplast biogenesis have revealed that hundreds of nuclear genes are regulated by retrograde signals emitted from plastids. Plastid gene expression is the source of at least one of these signals, but the number of signals and their mechanisms used to regulate nuclear gene expression are unknown. To further examine the effects of plastid gene expression on nuclear gene expression, we analyzed Arabidopsis mutants that were defective in each of the six sigma factor (SIG) genes that encode proteins utilized by plastid-encoded RNA polymerase to transcribe specific sets of plastid genes. We showed that SIG2 and SIG6 have partially redundant roles in plastid transcription and retrograde signalling to control nuclear gene expression. The loss of GUN1 (a plastid-localized pentatricopeptide repeat protein) is able to restore nuclear (but not plastid) gene expression in both sig2 and sig6, whereas an increase in heme synthesis is able to restore nuclear gene expression in sig2 mutants only. These results demonstrate that sigma factor function is the source of at least two retrograde signals to the nucleus; one likely to involve the transcription of tRNA(Glu) . A microarray analysis showed that these two signals accounted for at least one subset of the nuclear genes that are regulated by the plastid biogenesis inhibitors norflurazon and lincomycin. Together these data suggest that such inhibitors can induce retrograde signalling by affecting transcription in the plastid.

Centrality of host cell death in plant-microbe interactions

Programmed cell death (PCD) is essential for proper growth, development, and cellular homeostasis in all eukaryotes. The regulation of PCD is of central importance in plant-microbe interactions; notably, PCD and features associated with PCD are observed in many host resistance responses. Conversely, pathogen induction of inappropriate cell death in the host results in a susceptible phenotype and disease. Thus, the party in control of PCD has a distinct advantage in these battles. PCD processes appear to be of ancient origin, as indicated by the fact that many features of cell death strategy are conserved between animals and plants; however, some of the details of death execution differ. Mammalian core PCD genes, such as caspases, are not present in plant genomes. Similarly, pro- and antiapoptotic mammalian regulatory elements are absent in plants, but, remarkably, when expressed in plants, successfully impact plant PCD. Thus, subtle structural similarities independent of sequence homology appear to sustain operational equivalence. The vacuole is emerging as a key organelle in the modulation of plant PCD. Under different signals for cell death, the vacuole either fuses with the plasmalemma membrane or disintegrates. Moreover, the vacuole appears to play a key role in autophagy; evidence suggests a prosurvival function for autophagy, but other studies propose a prodeath phenotype. Here, we describe and discuss what we know and what we do not know about various PCD pathways and how the host integrates signals to activate salicylic acid and reactive oxygen pathways that orchestrate cell death. We suggest that it is not cell death as such but rather the processes leading to cell death that contribute to the outcome of a given plant-pathogen interaction.

Intracellular signaling from plastid to nucleus

Intracellular signaling from plastids to the nucleus, called retrograde signaling, coordinates the expression of nuclear and plastid genes and is essential for plastid biogenesis and for maintaining plastid function at optimal levels. Recent identification of several components involved in plastid retrograde generation, transmission, and control of nuclear gene expression has provided significant insight into the regulatory network of plastid retrograde signaling. Here, we review the current knowledge of multiple plastid retrograde signaling pathways, which are derived from distinct sources, and of possible plastid signaling molecules. We describe the retrograde signaling-dependent regulation of nuclear gene expression, which involves multilayered transcriptional control, as well as the transcription factors involved. We also summarize recent advances in the identification of key components mediating signal transduction from plastids to the nucleus.

Sigma factor-mediated plastid retrograde signals control nuclear gene expression

Retrograde signalling from plastids to the nucleus is necessary to regulate the organelle's proteome during the establishment of photoautotrophy and fluctuating environmental conditions. Studies that used inhibitors of chloroplast biogenesis have revealed that hundreds of nuclear genes are regulated by retrograde signals emitted from plastids. Plastid gene expression is the source of at least one of these signals, but the number of signals and their mechanisms used to regulate nuclear gene expression are unknown. To further examine the effects of plastid gene expression on nuclear gene expression, we analyzed Arabidopsis mutants that were defective in each of the six sigma factor (SIG) genes that encode proteins utilized by plastid-encoded RNA polymerase to transcribe specific sets of plastid genes. We showed that SIG2 and SIG6 have partially redundant roles in plastid transcription and retrograde signalling to control nuclear gene expression. The loss of GUN1 (a plastid-localized pentatricopeptide repeat protein) is able to restore nuclear (but not plastid) gene expression in both sig2 and sig6, whereas an increase in heme synthesis is able to restore nuclear gene expression in sig2 mutants only. These results demonstrate that sigma factor function is the source of at least two retrograde signals to the nucleus; one likely to involve the transcription of tRNA(Glu) . A microarray analysis showed that these two signals accounted for at least one subset of the nuclear genes that are regulated by the plastid biogenesis inhibitors norflurazon and lincomycin. Together these data suggest that such inhibitors can induce retrograde signalling by affecting transcription in the plastid.

Nonenzymic carotenoid oxidation and photooxidative stress signalling in plants

Carotenoids play a crucial protective role in photosynthetic organisms as quenchers of singlet oxygen ((1)O(2)). This function occurs either via a physical mechanism involving thermal energy dissipation or via a chemical mechanism involving direct oxidation of the carotenoid molecule. The latter mechanism can produce a variety of aldehydic or ketonic cleavage products containing a reactive carbonyl group. One such molecule, the volatile β-carotene derivative β-cyclocitral, triggers changes in the expression of (1)O(2)-responsive genes and leads to an enhancement of photooxidative stress tolerance. Thus, besides their well-known functions in light harvesting and photoprotection, carotenoids can also play a role through their nonenzymic oxidation in the sensing and signalling of reactive oxygen species and photooxidative stress in photosynthetic organisms. Enzymic carotenoid oxidation does not seem to play a significant role in this phenomenon. Elucidation of the carotenoid-mediated (1)O(2) signalling pathway could provide new targets for improving photooxidative stress tolerance of plants.

Reactive oxygen species generation and signaling in plants

The introduction of molecular oxygen into the atmosphere was accompanied by the generation of reactive oxygen species (ROS) as side products of many biochemical reactions. ROS are permanently generated in plastids, peroxisomes, mitochiondria, the cytosol and the apoplast. Imbalance between ROS generation and safe detoxification generates oxidative stress and the accumulating ROS are harmful for the plants. On the other hand, specific ROS function as signaling molecules and activate signal transduction processes in response to various stresses. Here, we summarize the generation of ROS in the different cellular compartments and the signaling processes which are induced by ROS.

Reactive oxygen and oxidative stress: N-formyl kynurenine in photosystem II and non-photosynthetic proteins

While light is the essential driving force for photosynthetic carbon fixation, high light intensities are toxic to photosynthetic organisms. Prolonged exposure to high light results in damage to the photosynthetic membrane proteins and suboptimal activity, a phenomenon called photoinhibition. The primary target for inactivation is the photosystem II (PSII) reaction center. PSII catalyzes the light-induced oxidation of water at the oxygen-evolving complex. Reactive oxygen species (ROS) are generated under photoinhibitory conditions and induce oxidative post translational modifications of amino acid side chains. Specific modification of tryptophan residues to N-formylkynurenine (NFK) occurs in the CP43 and D1 core polypeptides of PSII. The NFK modification has also been detected in other proteins, such as mitochondrial respiratory enzymes, and is formed by a non-random, ROS-targeted mechanism. NFK has been shown to accumulate in PSII during conditions of high light stress in vitro. This review provides a summary of what is known about the generation and function of NFK in PSII and other proteins. Currently, the role of ROS in photoinhibition is under debate. Furthermore, the triggers for the degradation and accelerated turnover of PSII subunits, which occur under high light, are not yet identified. Owing to its unique optical and Raman signal, NFK provides a new marker to use in the identification of ROS generation sites in PSII and other proteins. Also, the speculative hypothesis that NFK, and other oxidative modifications of tryptophan, play a role in the PSII damage and repair cycle is discussed. NFK may have a similar function during oxidative stress in other biologic systems.

Flavonoids as antioxidants in plants: location and functional significance

Stress-responsive dihydroxy B-ring-substituted flavonoids have great potential to inhibit the generation of reactive oxygen species (ROS) and reduce the levels of ROS once they are formed, i.e., to perform antioxidant functions. These flavonoids are located within or in the proximity of centers of ROS generation in severely stressed plants. Efficient mechanisms have been recently identified for the transport of flavonoids from the endoplasmic reticulum, the site of their biosynthesis, to different cellular compartments. The mechanism underlying flavonoid-mediated ROS reduction in plants is still unclear. 'Antioxidant' flavonoids are found in the chloroplast, which suggests a role as scavengers of singlet oxygen and stabilizers of the chloroplast outer envelope membrane. Dihydroxy B-ring substituted flavonoids are present in the nucleus of mesophyll cells and may inhibit ROS-generation making complexes with Fe and Cu ions. The genes that govern the biosynthesis of antioxidant flavonoids are present in liverworts and mosses and are mostly up-regulated as a consequence of severe stress. This suggests that the antioxidant flavonoid metabolism is a robust trait of terrestrial plants. Vacuolar dihydroxy B-ring flavonoids have been reported to serve as co-substrates for vacuolar peroxidases to reduce H(2)O(2) escape from the chloroplast, following the depletion of ascorbate peroxidase activity. Antioxidant flavonoids may effectively control key steps of cell growth and differentiation, thus acting regulating the development of the whole plant and individual organs.

Integration of stress-related and reactive oxygen species-mediated signals by Topoisomerase VI in Arabidopsis thaliana

Environmental stress often leads to an increased production of reactive oxygen species that are involved in plastid-to-nucleus retrograde signaling. Soon after the release of singlet oxygen ((1)O(2)) in chloroplasts of the flu mutant of Arabidopsis, reprogramming of nuclear gene expression reveals a rapid transfer of signals from the plastid to the nucleus. We have identified extraplastidic signaling constituents involved in (1)O(2)-initiated plastid-to-nucleus signaling and nuclear gene activation after mutagenizing a flu line expressing the luciferase reporter gene under the control of the promoter of a (1)O(2)-responsive AAA-ATPase gene (At3g28580) and isolating second-site mutations that lead to a constitutive up-regulation of the reporter gene or abrogate its (1)O(2)-dependent up-regulation. One of these mutants, caa39, turned out to be a weak mutant allele of the Topoisomerase VI (Topo VI) A-subunit gene with a single amino acid substitution. Transcript profile analysis of flu and flu caa39 mutants revealed that Topo VI is necessary for the full activation of AAA-ATPase and a set of (1)O(2)-responsive transcripts in response to (1)O(2). Topo VI binds to the promoter of the AAA-ATPase and other (1)O(2)-responsive genes, and hence could directly regulate their expression. Under photoinhibitory stress conditions, which enhance the production of (1)O(2) and H(2)O(2), Topo VI regulates (1)O(2)-responsive and H(2)O(2)-responsive genes in a distinct manner. These results suggest that Topo VI acts as an integrator of multiple signals generated by reactive oxygen species formed in plants under adverse environmental conditions.

Ubiquity and quantitative significance of detoxification catabolism of chlorophyll associated with protistan herbivory

Chlorophylls are essential components of the photosynthetic apparati that sustain all of the life forms that ultimately depend on solar energy. However, a drawback of the extraordinary photosensitizing efficiency of certain chlorophyll species is their ability to generate harmful singlet oxygen. Recent studies have clarified the catabolic processes involved in the detoxification of chlorophylls in land plants, but little is understood about these strategies in aquatic ecosystem. Here, we report that a variety of heterotrophic protists accumulate the chlorophyll a catabolite 13(2),17(3)-cyclopheophorbide a enol (cPPB-aE) after their ingestion of algae. This chlorophyll derivative is nonfluorescent in solution, and its inability to generate singlet oxygen in vitro qualifies it as a detoxified catabolite of chlorophyll a. Using a modified analytical method, we show that cPPB-aE is ubiquitous in aquatic environments, and it is often the major chlorophyll a derivative. Our findings suggest that cPPB-aE metabolism is one of the most important, widely distributed processes in aquatic ecosystems. Therefore, the herbivorous protists that convert chlorophyll a to cPPB-aE are suggested to play more significant roles in the modern oceanic carbon flux than was previously recognized, critically linking microscopic primary producers to the macroscopic food web and carbon sequestration in the ocean.

Retrograde signaling by the plastidial metabolite MEcPP regulates expression of nuclear stress-response genes

Plastid-derived signals are known to coordinate expression of nuclear genes encoding plastid-localized proteins in a process termed retrograde signaling. To date, the identity of retrograde-signaling molecules has remained elusive. Here, we show that methylerythritol cyclodiphosphate (MEcPP), a precursor of isoprenoids produced by the plastidial methylerythritol phosphate (MEP) pathway, elicits the expression of selected stress-responsive nuclear-encoded plastidial proteins. Genetic and pharmacological manipulations of the individual MEP pathway metabolite levels demonstrate the high specificity of MEcPP as an inducer of these targeted stress-responsive genes. We further demonstrate that abiotic stresses elevate MEcPP levels, eliciting the expression of the aforementioned genes. We propose that the MEP pathway, in addition to producing isoprenoids, functions as a stress sensor and a coordinator of expression of targeted stress-responsive nuclear genes via modulation of the levels of MEcPP, a specific and critical retrograde-signaling metabolite.

Assembly of NADPH: protochlorophyllide oxidoreductase complex is needed for effective greening of barley seedlings

NADPH:protochlorophyllide (Pchlide) oxidoreductase (POR) is the key enzyme in the light-induced greening of higher plants. A unique light-harvesting POR:Pchlide complexes (LHPP) has been found in barley etioplasts, but not in other plant species. Why PORs from barley, but not from other plants, can form LHPP? And its function is not well understood. We modeled the barley and Arabidopsis POR proteins and compared molecular surface. The results confirm the idea that barley PORA can form a five-unit oligomer that interacts with a single PORB. Chemical treatment experiments indicated that POR complex may be formed by dithiol oxidation of cysteines of two adjacent proteins. We further showed that LHPP assembly was needed for barley POR functions and seedling greening. On the contrary, Arabidopsis POR proteins only formed dimers, which were not related to the functions or the greening. Finally, POR complex assembly (including LHPP and POR dimers) did not affect the formation of prolamellar bodies (PLBs) that function for efficient capture of light energy for photo conversion in etioplasts.

Modulation of photosynthetic energy conversion efficiency in nature: from seconds to seasons

Modulation of the efficiency with which leaves convert absorbed light to photochemical energy [intrinsic efficiency of open photosystem II (PSII) centers, as the ratio of variable to maximal chlorophyll fluorescence] as well as leaf xanthophyll composition (interconversions of the xanthophyll cycle pigments violaxanthin and zeaxanthin) were characterized throughout single days and nights to entire seasons in plants growing naturally in contrasting light and temperature environments. All pronounced decreases of intrinsic PSII efficiency took place in the presence of zeaxanthin. The reversibility of these PSII efficiency changes varied widely, ranging from reversible-within-seconds (in a vine experiencing multiple sunflecks under a eucalypt canopy) to apparently permanently locked-in for entire seasons (throughout the whole winter in a subalpine conifer forest at 3,000 m). While close association between low intrinsic PSII efficiency and zeaxanthin accumulation was ubiquitous, accompanying features (such as trans-thylakoid pH gradient, thylakoid protein composition, and phosphorylation) differed among contrasting conditions. The strongest and longest-lasting depressions in intrinsic PSII efficiency were seen in the most stress-tolerant species. Evergreens, in particular, showed the most pronounced modulation of PSII efficiency and thermal dissipation, and are therefore suggested as model species for the study of photoprotection. Implications of the responses of field-grown plants in nature for mechanistic models are discussed.

Brassinolide alleviates salt stress and increases antioxidant activity of cowpea plants (Vigna sinensis)

Soil salinity is one of the most severe factors limiting growth and physiological response in Vigna sinensis plants. Plant salt stress tolerance requires the activation of complex metabolic activities including antioxidative pathways, especially reactive oxygen species and scavenging systems within the cells which can contribute to continued growth under water stress. The present investigation was carried out to study the role of brassinolide in enhancing tolerance of cowpea plants to salt stress (NaCl). Treatment with 0.05 ppm brassinolide as foliar spray mitigated salt stress by inducing enzyme activities responsible for antioxidation, e.g., superoxide dismutase, peroxidase, polyphenol oxidase, and detoxification as well as by elevating contents of ascorbic acid, tocopherol, and glutathione. On the other hand, total soluble proteins decreased with increasing NaCl concentrations in comparison with control plants. However, lipid peroxidation increased with increasing concentrations of NaCl. In addition to, the high concentrations of NaCl (100 and 150 mM) decreased total phenol of cowpea plants as being compared with control plants. SDS-PAGE of protein revealed that NaCl treatments alone or in combination with 0.05 ppm brassinolide were associated with the disappearance of some bands or appearance of unique ones in cowpea plants. Electrophoretic studies of α-esterase, β-esterase, polyphenol oxidase, peroxidase, acid phosphatase, and superoxide dismutase isoenzymes showed wide variations in their intensities and densities among all treatments.

Coordination of plastid and light signaling pathways upon development of Arabidopsis leaves under various photoperiods

Plants synchronize their cellular and physiological functions according to the photoperiod (the length of the light period) in the cycle of 24 h. Photoperiod adjusts several traits in the plant life cycle, including flowering and senescence in annuals and seasonal growth cessation in perennials. Photoperiodic development is controlled by the coordinated action of photoreceptors and the circadian clock. During the past 10 years, remarkable progress has been made in understanding the molecular mechanism of the circadian clock, especially with regard to the transition of Arabidopsis from the vegetative growth to the reproductive phase. Besides flowering photoperiod also modifies plant photosynthetic structures and traits. Light signals controlling biogenesis of chloroplasts and development of leaf photosynthetic structures are perceived both by photoreceptors and in chloroplasts. In this review, we provide evidence suggesting that the photoperiodic development of Arabidopsis leaves mimics the acclimation of plant to various light intensities. Furthermore, the chloroplast-to-nucleus retrograde signals that adjust acclimation to light intensity are proposed to contribute also to the signaling pathways that control photoperiodic acclimation of leaves.

Plastid-to-nucleus communication, signals controlling the running of the plant cell

The presence of genes encoding organellar proteins in both the nucleus and the organelle necessitates tight coordination of expression by the different genomes, and this has led to the evolution of sophisticated intracellular signaling networks. Organelle-to-nucleus signaling, or retrograde control, coordinates the expression of nuclear genes encoding organellar proteins with the metabolic and developmental state of the organelle. Complex networks of retrograde signals orchestrate major changes in nuclear gene expression and coordinate cellular activities and assist the cell during plant development and stress responses. It has become clear that, even though the chloroplast depends on the nucleus for its function, plastid signals play important roles in an array of different cellular processes vital to the plant. Hence, the chloroplast exerts significant control over the running of the cell. This article is part of a Special Issue entitled: Protein Import and Quality Control in Mitochondria and Plastids.

Xenobiotic sensing and signalling in higher plants

Anthropogenic changes and chemical pollution confront plant communities with various xenobiotic compounds or combinations of xenobiotics, involving chemical structures that are at least partially novel for plant species. Plant responses to chemical challenges and stimuli are usually characterized by the approaches of toxicology, ecotoxicology, and stress physiology. Development of transcriptomics and proteomics analysis has demonstrated the importance of modifications to gene expression in plant responses to xenobiotics. It has emerged that xenobiotic effects could involve not only biochemical and physiological disruption, but also the disruption of signalling pathways. Moreover, mutations affecting sensing and signalling pathways result in modifications of responses to xenobiotics, thus confirming interference or crosstalk between xenobiotic effects and signalling pathways. Some of these changes at gene expression, regulation and signalling levels suggest various mechanisms of xenobiotic sensing in higher plants, in accordance with xenobiotic-sensing mechanisms that have been characterized in other phyla (yeast, invertebrates, vertebrates). In higher plants, such sensing systems are difficult to identify, even though different lines of evidence, involving mutant studies, transcription factor analysis, or comparative studies, point to their existence. It remains difficult to distinguish between the hypothesis of direct xenobiotic sensing and indirect sensing of xenobiotic-related modifications. However, future characterization of xenobiotic sensing and signalling in higher plants is likely to be a key element for determining the tolerance and remediation capacities of plant species. This characterization will also be of interest for understanding evolutionary dynamics of stress adaptation and mechanisms of adaptation to novel stressors.

Thioredoxin redox regulates ATPase activity of magnesium chelatase CHLI subunit and modulates redox-mediated signaling in tetrapyrrole biosynthesis and homeostasis of reactive oxygen species in pea plants

The chloroplast thioredoxins (TRXs) function as messengers of redox signals from ferredoxin to target enzymes. In this work, we studied the regulatory impact of pea (Pisum sativum) TRX-F on the magnesium (Mg) chelatase CHLI subunit and the enzymatic activation of Mg chelatase in vitro and in vivo. In vitro, reduced TRX-F activated the ATPase activity of pea CHLI and enhanced the activity of Mg chelatase reconstituted from the three recombinant subunits CHLI, CHLD, and CHLH in combination with the regulator protein GENOMES UNCOUPLED4 (GUN4). Yeast two-hybrid and bimolecular fluorescence complementation assays demonstrated that TRX-F physically interacts with CHLI but not with either of the other two subunits or GUN4. In vivo, virus-induced TRX-F gene silencing (VIGS-TRX-F) in pea plants did not result in an altered redox state of CHLI. However, simultaneous silencing of the pea TRX-F and TRX-M genes (VIGS-TRX-F/TRX-M) resulted in partially and fully oxidized CHLI in vivo. VIGS-TRX-F/TRX-M plants demonstrated a significant reduction in Mg chelatase activity and 5-aminolevulinic acid synthesizing capacity as well as reduced pigment content and lower photosynthetic capacity. These results suggest that, in vivo, TRX-M can compensate for a lack of TRX-F and that both TRXs act as important redox regulators of Mg chelatase. Furthermore, the silencing of TRX-F and TRX-M expression also affects gene expression in the tetrapyrrole biosynthesis pathway and leads to the accumulation of reactive oxygen species, which may also serve as an additional signal for the transcriptional regulation of photosynthesis-associated nuclear genes.

Reactive Oxygen Species, Oxidative Damage, and Antioxidative Defense Mechanism in Plants under Stressful Conditions

Reactive oxygen species (ROS) are produced as a normal product of plant cellular metabolism. Various environmental stresses lead to excessive production of ROS causing progressive oxidative damage and ultimately cell death. Despite their destructive activity, they are well-described second messengers in a variety of cellular processes, including conferment of tolerance to various environmental stresses. Whether ROS would serve as signaling molecules or could cause oxidative damage to the tissues depends on the delicate equilibrium between ROS production, and their scavenging. Efficient scavenging of ROS produced during various environmental stresses requires the action of several nonenzymatic as well as enzymatic antioxidants present in the tissues. In this paper, we describe the generation, sites of production and role of ROS as messenger molecules as well as inducers of oxidative damage. Further, the antioxidative defense mechanisms operating in the cells for scavenging of ROS overproduced under various stressful conditions of the environment have been discussed in detail.

SINGLET OXYGEN RESISTANT 1 links reactive electrophile signaling to singlet oxygen acclimation in Chlamydomonas reinhardtii

Acclimation of Chlamydomonas reinhardtii cells to low levels of singlet oxygen, produced either by photoreactive chemicals or high light treatment, induces a specific genetic response that strongly increases the tolerance of the algae to subsequent exposure to normally lethal singlet oxygen-producing conditions. The genetic response includes the increased expression of various oxidative stress response and detoxification genes, like the glutathione peroxidase homologous gene GPXH/GPX5 and the σ-class glutathione-S-transferase gene GSTS1. To identify components involved in the signal transduction and activation of the singlet oxygen-mediated response, a mutant selection was performed. This selection led to the isolation of the singlet oxygen resistant 1 (sor1) mutant, which is more tolerant to singlet oxygen-producing chemicals and shows a constitutively higher expression of GPXH and GSTS1. Map-based cloning revealed that the SOR1 gene encodes a basic leucine zipper transcription factor, which controls its own expression and the expression of a large number of oxidative stress response and detoxification genes. In the promoter region of many of these genes, a highly conserved 8-bp palindromic sequence element was found to be enriched. This element was essential for GSTS1 induction by increased levels of lipophilic reactive electrophile species (RES), suggesting that it functions as an electrophile response element (ERE). Furthermore, GSTS1 overexpression in sor1 requires the ERE, although it is unknown whether it occurs through direct binding of SOR1 to the ERE. RES can be formed after singlet oxygen-induced lipid peroxidation, indicating that RES-stimulated and SOR1-mediated responses of detoxification genes are part of the singlet oxygen-induced acclimation process in C. reinhardtii.

Carotenoid oxidation products are stress signals that mediate gene responses to singlet oxygen in plants

(1)O(2) (singlet oxygen) is a reactive O(2) species produced from triplet excited chlorophylls in the chloroplasts, especially when plants are exposed to excess light energy. Similarly to other active O(2) species, (1)O(2) has a dual effect: It is toxic, causing oxidation of biomolecules, and it can act as a signal molecule that leads to cell death or to acclimation. Carotenoids are considered to be the main (1)O(2) quenchers in chloroplasts, and we show here that light stress induces the oxidation of the carotenoid β-carotene in Arabidopsis plants, leading to the accumulation of different volatile derivatives. One such compound, β-cyclocitral, was found to induce changes in the expression of a large set of genes that have been identified as (1)O(2) responsive genes. In contrast, β-cyclocitral had little effect on the expression of H(2)O(2) gene markers. β-Cyclocitral-induced reprogramming of gene expression was associated with an increased tolerance to photooxidative stress. The results indicate that β-cyclocitral is a stress signal produced in high light that is able to induce defense mechanisms and represents a likely messenger involved in the (1)O(2) signaling pathway in plants.

H2O2-triggered retrograde signaling from chloroplasts to nucleus plays specific role in response to stress

Recent findings have suggested that reactive oxygen species (ROS) are important signaling molecules for regulating plant responses to abiotic and biotic stress and that there exist source- and kind-specific pathways for ROS signaling. In plant cells, a major source of ROS is chloroplasts, in which thylakoid membrane-bound ascorbate peroxidase (tAPX) plays a role in the regulation of H(2)O(2) levels. Here, to clarify the signaling function of H(2)O(2) derived from the chloroplast, we created a conditional system for producing H(2)O(2) in the organelle by chemical-dependent tAPX silencing using estrogen-inducible RNAi. When the expression of tAPX was silenced in leaves, levels of oxidized protein in chloroplasts increased in the absence of stress. Microarray analysis revealed that tAPX silencing affects the expression of a large set of genes, some of which are involved in the response to chilling and pathogens. In response to tAPX silencing, the transcript levels of C-repeat/DRE binding factor (CBF1), a central regulator for cold acclimation, was suppressed, resulting in a high sensitivity of tAPX-silenced plants to cold. Furthermore, tAPX silencing enhanced the levels of salicylic acid (SA) and the response to SA. Interestingly, we found that tAPX silencing-responsive genes were up- or down-regulated by high light (HL) and that tAPX silencing had a negative effect on expression of ROS-responsive genes under HL, suggesting synergistic and antagonistic roles of chloroplastic H(2)O(2) in HL response. These findings provide a new insight into the role of H(2)O(2)-triggered retrograde signaling from chloroplasts in the response to stress in planta.

ROS and redox signalling in the response of plants to abiotic stress

The redox state of the chloroplast and mitochondria, the two main powerhouses of photosynthesizing eukaryotes, is maintained by a delicate balance between energy production and consumption, and affected by the need to avoid increased production of reactive oxygen species (ROS). These demands are especially critical during exposure to extreme environmental conditions, such as high light (HL) intensity, heat, drought or a combination of different environmental stresses. Under these conditions, ROS and redox cues, generated in the chloroplast and mitochondria, are essential for maintaining normal energy and metabolic fluxes, optimizing different cell functions, activating acclimation responses through retrograde signalling, and controlling whole-plant systemic signalling pathways. Regulation of the multiple redox and ROS signals in plants requires a high degree of coordination and balance between signalling and metabolic pathways in different cellular compartments. In this review, we provide an update on ROS and redox signalling in the context of abiotic stress responses, while addressing their role in retrograde regulation, systemic acquired acclimation and cellular coordination in plants.

Plant responses to insect herbivory: interactions between photosynthesis, reactive oxygen species and hormonal signalling pathways

Under herbivore attack plants mount a defence response characterized by the accumulation of secondary metabolites and inhibitory proteins. Significant changes are observed in the transcriptional profiles of genes encoding enzymes of primary metabolism. Such changes have often been interpreted in terms of a requirement for an increased investment of resources to 'fuel' the synthesis of secondary metabolites. While enhanced secondary metabolism undoubtedly exerts an influence on primary metabolism, accumulating evidence suggests that rather than stimulating photosynthesis insect herbivory reduces photosynthetic carbon fixation and this response occurs by a re-programming of gene expression. Within this context, reactive oxygen species (ROS) and reductant/oxidant (redox) signalling play a central role. Accumulating evidence suggests that ROS signalling pathways are closely interwoven with hormone-signalling pathways in plant-insect interactions. Here we consider how insect infestation impacts on the stress signalling network through effects on ROS and cellular redox metabolism with particular emphasis on the roles of ROS in the plant responses to phloem-feeding insects.

Photosynthesis, photorespiration, and light signalling in defence responses

Visible light is the basic energetic driver of plant biomass production through photosynthesis. The constantly fluctuating availability of light and other environmental factors means that the photosynthetic apparatus must be able to operate in a dynamic fashion appropriate to the prevailing conditions. Dynamic regulation is achieved through an array of homeostatic control mechanisms that both respond to and influence cellular energy and reductant status. In addition, light availability and quality are continuously monitored by plants through photoreceptors. Outside the laboratory growth room, it is within the context of complex changes in energy and signalling status that plants must regulate pathways to deal with biotic challenges, and this can be influenced by changes in the highly energetic photosynthetic pathways and in the turnover of the photosynthetic machinery. Because of this, defence responses are neither simple nor easily predictable, but rather conditioned by the nutritional and signalling status of the plant cell. This review discusses recent data and emerging concepts of how recognized defence pathways interact with and are influenced by light-dependent processes. Particular emphasis is placed on the potential roles of the chloroplast, photorespiration, and photoreceptor-associated pathways in regulating the outcome of interactions between plants and pathogenic organisms.

The chloroplast division mutant caa33 of Arabidopsis thaliana reveals the crucial impact of chloroplast homeostasis on stress acclimation and retrograde plastid-to-nucleus signaling

Retrograde plastid-to-nucleus signaling tightly controls and coordinates the nuclear and plastid gene expression that is required for plastid biogenesis and chloroplast activity. As chloroplasts act as sensors of environmental changes, plastid-derived signaling also modulates stress responses of plants by transferring stress-related signals and altering nuclear gene expression. Various mutant screens have been undertaken to identify constituents of plastid signaling pathways. Almost all mutations identified in these screens target plastid-specific but not extraplastidic functions. They have been suggested to define either genuine constituents of retrograde signaling pathways or components required for the synthesis of plastid signals. Here we report the characterization of the constitutive activator of AAA-ATPase (caa33) mutant, which reveals another way of how mutations that affect plastid functions may modulate retrograde plastid signaling. caa33 disturbs a plastid-specific function by impeding plastid division, and thereby perturbing plastid homeostasis. This results in preconditioning plants by activating the expression of stress genes, enhancing pathogen resistance and attenuating the capacity of the plant to respond to plastid signals. Our study reveals an intimate link between chloroplast activity and the susceptibility of the plant to stress, and emphasizes the need to consider the possible impact of preconditioning on retrograde plastid-to-nucleus signaling.