Ascorbate regulation of 2-Cys peroxiredoxin-A promoter activity is light-dependent

The ascorbate-glutathione cycle coming of age

Concepts regarding the operation of the ascorbate-glutathione cycle and the associated water/water cycle in the processing of metabolically generated hydrogen peroxide and other forms of reactive oxygen species (ROS) are well established in the literature. However, our knowledge of the functions of these cycles and their component enzymes continues to grow and evolve. Recent insights include participation in the intrinsic environmental and developmental signalling pathways that regulate plant growth, development, and defence. In addition to ROS processing, the enzymes of the two cycles not only support the functions of ascorbate and glutathione, they also have 'moonlighting' functions. They are subject to post-translational modifications and have an extensive interactome, particularly with other signalling proteins. In this assessment of current knowledge, we highlight the central position of the ascorbate-glutathione cycle in the network of cellular redox systems that underpin the energy-sensitive communication within the different cellular compartments and integrate plant signalling pathways.

Determining the ROS and the Antioxidant Status of Leaves During Cold Acclimation

Cold slows down Calvin cycle activity stronger than photosynthetic electron transport, which supports production of reactive oxygen species (ROS). Even under extreme temperature conditions, most ROS are detoxified by the combined action of low-molecular weight antioxidants and antioxidant enzymes. Subsequent regeneration of the low-molecular weight antioxidants by NAD(P)H and thioredoxin/thiol-dependent pathways relaxes the electron pressure in the photosynthetic electron transport chain. In general, the chloroplast antioxidant system protects plants from severe damage of enzymes, metabolites, and cellular structures by both ROS detoxification and antioxidant recycling. Various methods have been developed to quantify ROS and antioxidant levels in photosynthetic tissues. Here, we summarize a series of exceptionally fast and easily applicable methods that show local ROS accumulation and provide information on the overall availability of reducing sugars, mainly ascorbate, and of thiols.

Selective silencing of 2Cys and type-IIB Peroxiredoxins discloses their roles in cell redox state and stress signaling

Peroxiredoxins (Prx) catalyse the reduction of hydrogen peroxide (H2O2) and, in association with catalases and other peroxidases, may participate in signal transduction by regulating intercellular H2O2 concentration that in turn can control gene transcription and cell signaling. Using virus-induced-gene-silencing (VIGS), 2-Cys Peroxiredoxin (2CysPrx) family and type-II Peroxiredoxin B (PrxIIB) gene were silenced in Nicotiana benthamiana, to study the impact that the loss of function of each Prx would have in the antioxidant system under control (22 °C) and severe heat stress conditions (48 °C). The results showed that both Prxs, although in different organelles, influence the regeneration of ascorbate to a significant extent, but with different purposes. 2CysPrx affects abscisic acid (ABA) biosynthesis through ascorbate, while PrxIIB does it probably through the xanthophyll cycle. Moreover, 2CysPrx is key in H2O2 scavenging and in consequence in the regulation of ABA signaling downstream of reactive oxygen species and PrxIIB provides an important assistance for H2O2 peroxisome scavenges.

The thioredoxin/peroxiredoxin/sulfiredoxin system: current overview on its redox function in plants and regulation by reactive oxygen and nitrogen species

In plants, the presence of thioredoxin (Trx), peroxiredoxin (Prx), and sulfiredoxin (Srx) has been reported as a component of a redox system involved in the control of dithiol-disulfide exchanges of target proteins, which modulate redox signalling during development and stress adaptation. Plant thiols, and specifically redox state and regulation of thiol groups of cysteinyl residues in proteins and transcription factors, are emerging as key components in the plant response to almost all stress conditions. They function in both redox sensing and signal transduction pathways. Scarce information exists on the transcriptional regulation of genes encoding Trx/Prx and on the transcriptional and post-transcriptional control exercised by these proteins on their putative targets. As another point of control, post-translational regulation of the proteins, such as S-nitrosylation and S-oxidation, is of increasing interest for its effect on protein structure and function. Special attention is given to the involvement of the Trx/Prx/Srx system and its redox state in plant signalling under stress, more specifically under abiotic stress conditions, as an important cue that influences plant yield and growth. This review focuses on the regulation of Trx and Prx through cysteine S-oxidation and/or S-nitrosylation, which affects their functionality. Some examples of redox regulation of transcription factors and Trx- and Prx-related genes are also presented.

Coordination of plant mitochondrial biogenesis: keeping pace with cellular requirements

Plant mitochondria are complex organelles that carry out numerous metabolic processes related with the generation of energy for cellular functions and the synthesis and degradation of several compounds. Mitochondria are semiautonomous and dynamic organelles changing in shape, number, and composition depending on tissue or developmental stage. The biogenesis of functional mitochondria requires the coordination of genes present both in the nucleus and the organelle. In addition, due to their central role, all processes held inside mitochondria must be finely coordinated with those in other organelles according to cellular demands. Coordination is achieved by transcriptional control of nuclear genes encoding mitochondrial proteins by specific transcription factors that recognize conserved elements in their promoter regions. In turn, the expression of most of these transcription factors is linked to developmental and environmental cues, according to the availability of nutrients, light-dark cycles, and warning signals generated in response to stress conditions. Among the signals impacting in the expression of nuclear genes, retrograde signals that originate inside mitochondria help to adjust mitochondrial biogenesis to organelle demands. Adding more complexity, several nuclear encoded proteins are dual localized to mitochondria and either chloroplasts or the nucleus. Dual targeting might establish a crosstalk between the nucleus and cell organelles to ensure a fine coordination of cellular activities. In this article, we discuss how the different levels of coordination of mitochondrial biogenesis interconnect to optimize the function of the organelle according to both internal and external demands.

Linking chloroplast antioxidant defense to carbohydrate availability: the transcript abundance of stromal ascorbate peroxidase is sugar-controlled via ascorbate biosynthesis

All genes encoding chloroplast antioxidant enzymes are nuclear-encoded and posttranscriptionally targeted to chloroplasts. The transcript levels of most of them decreased upon sucrose feeding like the transcript levels of many genes encoding components of the photosynthetic electron transport chain. However, the transcript abundance of stromal ascorbate peroxidase (s-APX; At4g08390) increased. Due to mild sugar application conditions, the plants kept the phosphorylation status of the ADP+ATP pool and the redox states of the NADPH+NADP+ and the ascorbate pools under control, which excludes them as signals in s-APX regulation. Correlation with ascorbate pool size regulation and comparison of transcript abundance regulation in the starch-biosynthetic mutant adg1, the ascorbate biosynthesis mutant vtc1, and the abscisic acid (ABA) biosynthetic mutant aba2 showed a link between sugar induction of s-APX and ascorbate biosynthesis.

The radical induced cell death protein 1 (RCD1) supports transcriptional activation of genes for chloroplast antioxidant enzymes

The rimb1 (redox imbalanced 1) mutation was mapped to the RCD1 locus (radical-induced cell death 1; At1g32230) demonstrating that a major factor involved in redox-regulation genes for chloroplast antioxidant enzymes and protection against photooxidative stress, RIMB1, is identical to the regulator of disease response reactions and cell death, RCD1. Discovering this link let to our investigation of its regulatory mechanism. We show in yeast that RCD1 can physically interact with the transcription factor Rap2.4a which provides redox-sensitivity to nuclear expression of genes for chloroplast antioxidant enzymes. In the rimb1 (rcd1-6) mutant, a single nucleotide exchange results in a truncated RCD1 protein lacking the transcription factor binding site. Protein-protein interaction between full-length RCD1 and Rap2.4a is supported by H2O2, but not sensitive to the antioxidants dithiotreitol and ascorbate. In combination with transcript abundance analysis in Arabidopsis, it is concluded that RCD1 stabilizes the Rap2.4-dependent redox-regulation of the genes encoding chloroplast antioxidant enzymes in a widely redox-independent manner. Over the years, rcd1-mutant alleles have been described to develop symptoms like chlorosis, lesions along the leaf rims and in the mesophyll and (secondary) induction of extra- and intra-plastidic antioxidant defense mechanisms. All these rcd1 mutant characteristics were observed in rcd1-6 to succeed low activation of the chloroplast antioxidant system and glutathione biosynthesis. We conclude that RCD1 protects plant cells from running into reactive oxygen species (ROS)-triggered programs, such as cell death and activation of pathogen-responsive genes (PR genes) and extra-plastidic antioxidant enzymes, by supporting the induction of the chloroplast antioxidant system.

Proteomic perspective of Quercus suber somatic embryogenesis

Quercus suber L. is a forest tree with remarkable ecological, social and economic value in the southern Europe ecosystems. To circumvent the difficulties of breeding such long-lived species like Q. suber in a conventional fashion, clonal propagation of Q. suber elite trees can be carried out, although this process is sometimes unsuccessful. To help decipher the complex program underlying the development of Q. suber somatic embryos from the first early stage until maturity, a proteomic approach based on DIGE and MALDI-MS has been envisaged. Results highlighted several key processes involved in the three developmental stages (proliferative, cotyledonary and mature) of Q. suber somatic embryogenesis studied. Results show that the proliferation stage is characterized by fermentation as an alternative energy source at the first steps of somatic embryo development, as well as by up-regulation of proteins involved in cell division. In this stage reactive oxygen species play a role in proliferation, while other proteins like CAD and PR5 seem to be implied in embryonic competence. In the transition to the cotyledonary stage diverse ROS detoxification enzymes are activated and reserve products (mainly carbohydrates and proteins) are accumulated, whereas energy production is increased probably to participate in the synthesis of primary metabolites such as amino acids and fatty acids. Finally, in the mature stage ethylene accumulation regulates embryo development.

BIOLOGICAL SIGNIFICANCE

Quercus suber L. is a forest tree with remarkable ecological, social and economic value in the southern Europe ecosystems. To circumvent the difficulties of breeding such long-lived species like Q. suber in a conventional fashion, clonal propagation of Q. suber elite trees can be carried out, although this process is sometimes unsuccessful. To help decipher the complex program underlying the development of Q. suber somatic embryos from the first early stage until maturity, in deep studies become necessary. This article is part of a Special Issue entitled: Translational Plant Proteomics.

Ascorbate as seen through plant evolution: the rise of a successful molecule?

Ascorbate is a widespread and efficient antioxidant that has multiple functions in plants, traditionally associated with the reactions of photosynthesis. This review aims to look at ascorbate from an evolutionary perspective. Cyanobacteria, algae, and bryophytes contain lower concentrations of ascorbate than higher plants, where the molecule accumulates in high concentrations in both photosynthetic and non-photosynthetic organs and tissues. This increase in ascorbate concentration is paralleled by an increase in the number of isoforms of ascorbate peroxidase and the ascorbate regenerating enzymes mono- and dehydroascorbate reductase. One way of understanding the rise in ascorbate concentrations is to consider ascorbate as a molecule among others that has been subject to selection pressures during evolution, due to its cost or benefit for the cell and the organism. Ascorbate has a low cost in terms of synthesis and toxicity, and its benefits include protection of the glutathione pool and proper functioning of a range of enzymes. The hypothesis presented here is that these features would have favoured increasing roles for the molecule in the development and growth of multicellular organisms. This review then focuses on this diversity of roles for ascorbate in both photosynthetic and non-photosynthetic tissues of higher plants, including fruits and seeds, as well as further functions the molecule may possess by looking at other species. The review also highlights one of the trade-offs of domestication, which has often reduced or diluted ascorbate content in the quest for increased fruit growth and yield, with unknown consequences for the corresponding functional diversity, particularly in terms of stress resistance and adaptive responses to the environment.

Natural genetic variation in the expression regulation of the chloroplast antioxidant system among Arabidopsis thaliana accessions

Photosynthesis is the predominant source of reactive oxygen species in light. In order to prevent the negative influence of reactive oxygen species (ROS) on cell functionality, chloroplasts have evolved a highly efficient antioxidant protection system. Here, we present the first study on natural variation in this system. Comparison of temperature and developmental responses in seven accessions of Arabidopsis thaliana from northern habitats showed that the regulation is widely genetically manifested, but hardly correlates with geographic parameters. Transcript, polysomal RNA (pRNA) and protein data showed that the ecotypes use different strategies to adjust the chloroplast antioxidative defense system, either by regulating transcript abundance or initiation of translation. Comparison of mRNA and pRNA levels showed that Col-0 invests more into transcript accumulation, while Van-0, WS and C24 regulates the chloroplast antioxidant protection system more on the level of pRNA. Nevertheless, both strategies of regulation led to the expression of chloroplast antioxidant enzymes at sufficient level to efficiently protect plants from ROS accumulation in Col-0, WS, C24 and Van-0. On the contrary, Cvi-0, Ms-0 and Kas-1 accumulated high amounts of ROS. The expression of copper/zinc superoxide dismutase (Csd2), ascorbate peroxidases and 2-Cys peroxiredoxins was higher in Cvi-0 on the transcriptional level, while Csd2, peroxiredoxin Q, type II peroxiredoxin E and glutathione peroxidase 1 were induced in Ms-0 on the mRNA level. Similar to Kas-1, in which mRNA levels were less than or similar to Col-0 gene, specific support for translation was observed in Ms-0, showing that the ecotypes use different strategies to adjust the antioxidant system.