Nitration of gamma-tocopherol in plant tissues

The light and hypoxia induced gene ZmPORB1 determines tocopherol content in the maize kernel

Tocopherol is an important lipid-soluble antioxidant beneficial for both human health and plant growth. Here, we fine mapped a major QTL-qVE1 affecting γ-tocopherol content in maize kernel, positionally cloned and confirmed the underlying gene ZmPORB1 (por1), as a protochlorophyllide oxidoreductase. A 13.7 kb insertion reduced the tocopherol and chlorophyll content, and the photosynthetic activity by repressing ZmPORB1 expression in embryos of NIL-K22, but did not affect the levels of the tocopherol precursors HGA (homogentisic acid) and PMP (phytyl monophosphate). Furthermore, ZmPORB1 is inducible by low oxygen and light, thereby involved in the hypoxia response in developing embryos. Concurrent with natural hypoxia in embryos, the redox state has been changed with NO increasing and H2O2 decreasing, which lowered γ-tocopherol content via scavenging reactive nitrogen species. In conclusion, we proposed that the lower light-harvesting chlorophyll content weakened embryo photosynthesis, leading to fewer oxygen supplies and consequently diverse hypoxic responses including an elevated γ-tocopherol consumption. Our findings shed light on the mechanism for fine-tuning endogenous oxygen concentration in the maize embryo through a novel feedback pathway involving the light and low oxygen regulation of ZmPORB1 expression and chlorophyll content.

Nitric Oxide Acts as a Key Signaling Molecule in Plant Development under Stressful Conditions

Nitric oxide (NO), a colorless gaseous molecule, is a lipophilic free radical that easily diffuses through the plasma membrane. These characteristics make NO an ideal autocrine (i.e., within a single cell) and paracrine (i.e., between adjacent cells) signalling molecule. As a chemical messenger, NO plays a crucial role in plant growth, development, and responses to biotic and abiotic stresses. Furthermore, NO interacts with reactive oxygen species, antioxidants, melatonin, and hydrogen sulfide. It regulates gene expression, modulates phytohormones, and contributes to plant growth and defense mechanisms. In plants, NO is mainly produced via redox pathways. However, nitric oxide synthase, a key enzyme in NO production, has been poorly understood recently in both model and crop plants. In this review, we discuss the pivotal role of NO in signalling and chemical interactions as well as its involvement in the mitigation of biotic and abiotic stress conditions. In the current review, we have discussed various aspects of NO including its biosynthesis, interaction with reactive oxygen species (ROS), melatonin (MEL), hydrogen sulfide, enzymes, phytohormones, and its role in normal and stressful conditions.

Bioactive Nitrosylated and Nitrated N-(2-hydroxyphenyl)acetamides and Derived Oligomers: An Alternative Pathway to 2-Amidophenol-Derived Phytotoxic Metabolites

Incubation of Aminobacter aminovorans, Paenibacillus polymyxa, and Arthrobacter MPI764 with the microbial 2-benzoxazolinone (BOA)-degradation-product 2-acetamido-phenol, produced from 2-aminophenol, led to the recently identified N-(2-hydroxy-5-nitrophenyl) acetamide, to the hitherto unknown N-(2-hydroxy-5-nitrosophenyl)acetamide, and to N-(2-hydroxy-3-nitrophenyl)acetamide. As an alternative to the formation of phenoxazinone derived from aminophenol, dimers- and trimers-transformation products have been found. Identification of the compounds was carried out by LC/HRMS and MS/MS and, for the new structure N-(2-hydroxy-5-nitrosophenyl)acetamide, additionally by 1D- and 2D-NMR. Incubation of microorganisms, such as the soil bacteria Pseudomonas laurentiana, Arthrobacter MPI763, the yeast Papiliotrema baii and Pantoea ananatis, and the plants Brassica oleracea var. gongylodes L. (kohlrabi) and Arabidopsis thaliana Col-0, with N-(2-hydroxy-5-nitrophenyl) acetamide, led to its glucoside derivative as a prominent detoxification product; in the case of Pantoea ananatis, this was together with the corresponding glucoside succinic acid ester. In contrast, Actinomucor elegans consortium synthesized 2-acetamido-4-nitrophenyl sulfate. 1 mM bioactive N-(2-hydroxy-5-nitrophenyl) acetamide elicits alterations in the Arabidopsis thaliana expression profile of several genes. The most responsive upregulated gene was pathogen-inducible terpene synthase TPS04. The bioactivity of the compound is rapidly annihilated by glucosylation.

ROS-Scavengers, Osmoprotectants and Violaxanthin De-Epoxidation in Salt-Stressed Arabidopsis thaliana with Different Tocopherol Composition

To determine the role of α- and γ-tocopherol (TC), this study compared the response to salt stress (200 mM NaCl) in wild type (WT) Arabidopsis thaliana (L.) Heynh. And its two mutants: (1) totally TC-deficient vte1; (2) vte4 accumulating γ-TC instead of α-TC; and (3) tmt transgenic line overaccumulating α-TC. Raman spectra revealed that salt-exposed α-TC accumulating plants were more flexible in regulating chlorophyll, carotenoid and polysaccharide levels than TC deficient mutants, while the plants overaccumulating γ-TC had the lowest levels of these biocompounds. Tocopherol composition and NaCl concentration affected xanthophyll cycle by changing the rate of violaxanthin de-epoxidation and zeaxanthin formation. NaCl treated plants with altered TC composition accumulated less oligosaccharides than WT plants. α-TC deficient plants increased their oligosaccharide levels and reduced maltose amount, while excessive accumulation of α-TC corresponded with enhanced amounts of maltose. Salt-stressed TC-deficient mutants and tmt transgenic line exhibited greater proline levels than WT plants, lower chlorogenic acid levels, and lower activity of catalase and peroxidases. α-TC accumulating plants produced more methylated proline- and glycine- betaines, and showed greater activity of superoxide dismutase than γ-TC deficient plants. Under salt stress, α-TC demonstrated a stronger regulatory effect on carbon- and nitrogen-related metabolites reorganization and modulation of antioxidant patterns than γ-TC. This suggested different links of α- and γ-TCs with various metabolic pathways via various functions and metabolic loops.

A physiological perspective on targets of nitration in NO-based signaling networks in plants

Although peroxynitrite (ONOO-) has been well documented as a nitrating cognate of nitric oxide (NO) in plant cells, modifications of proteins, fatty acids, and nucleotides by nitration are relatively under-explored topics in plant NO research. As a result, they are seen mainly as hallmarks of redox processes or as markers of nitro-oxidative stress under unfavorable conditions, similar to those observed in human and other animal systems. Protein tyrosine nitration is the best-known nitrative modification in the plant system and can be promoted by the action of both ONOO- and related NO-derived oxidants within the cell environment. Recent progress in 'omics' and modeling tools have provided novel biochemical insights into the physiological and pathophysiological fate of nitrated proteins. The nitration process can be specifically involved in various cell regulatory mechanisms that control redox signaling via nitrated cGMP or nitrated fatty acids. In addition, there is evidence to suggest that nitrative modifications of nucleotides embedded in DNA and RNA can be considered as smart switches of gene expression that fine-tune adaptive cellular responses to stress. This review highlights recent advances in our understanding of the potential implications of biotargets in the regulation of intracellular traffic and plant biological processes.

Rapid enhancement of α-tocopherol content in Nicotiana benthamiana by transient expression of Arabidopsis thaliana Tocopherol cyclase and Homogentisate phytyl transferase genes

Agrobacterium-mediated transient gene expression have become a method of choice over stable plant genetic transformation. Tocopherols are a family of vitamin E compounds, which are categorized along with tocotrienols occurring naturally in vegetable oils, nuts and leafy green vegetables. This is the first report involving AtTC and AtHPT transient expression in Nicotiana benthamiana and this system can be used efficiently for large scale production of vitamin E. Agroinfiltration studies were carried out in N.benthamiana for the expression of Arabidopsis thaliana (At) genes encoding homogentisate phytyltransferase (HPT) and tocopherol cyclase (TC) individually and in combination (HPT + TC). The transgene presence was analyzed by reverse transcription PCR, which showed the presence of both the vitamin E biosynthetic pathway genes. The gene expression analysis was carried out by (reverse transcription quantitative real-time polymerase chain reaction) RT-qPCR and α-tocopherol content was quantified using high performance liquid chromatography (HPLC). The relative gene expression analysis by RT-qPCR confirmed an increased expression pattern where TC + HPT combination recorded the highest of 231 fold, followed by TC gene with 186 fold, whereas the HPT gene recorded 178 fold. The α-tocopherol content in leaves expressing HPT, TC, and HPT + TC was increased by 4.2, 5.9 and 11.3 fold, respectively, as compared to the control. These results indicate that the transient expression of HPT and TC genes has enhanced the vitamin E levels and stable expression of both A. thaliana genes could be an efficient strategy to enhance vitamin E biosynthesis in agricultural crop breeding.

Novel and rare prenyllipids - Occurrence and biological activity

The data presented indicate that there is a variety of unique prenyllipids, often of very limited taxonomic distribution, whose origin, biosynthesis, metabolism and biological function deserves to be elucidated. These compounds include tocoenols, tocochromanol esters, tocochromanol acids, plastoquinones and ubiquinones. Additionally, based on the available data, it can be assumed that there are still unrecognized prenyllipids, like prenylquinols fatty acid esters of the hydroquinone ring, including prenylquinol phosphates, and others, whose biological function might be of great importance. Our knowledge of these compounds is not only important from the scientific point of view, but may also be of practical significance to medicine, pharmacy or cosmetics.

Molecular characterization of tocopherol biosynthetic genes in sweetpotato that respond to stress and activate the tocopherol production in tobacco

Tocopherol (vitamin E) is a chloroplast lipid that is presumed to be involved in the plant response to oxidative stress. In this study, we isolated and characterized five tocopherol biosynthetic genes from sweetpotato (Ipomoea batatas [L.] Lam) plants, including genes encoding 4-hydroxyphenylpyruvate dioxygenase (IbHPPD), homogentisate phytyltransferase (IbHPT), 2-methyl-6-phytylbenzoquinol methyltransferase (IbMPBQ MT), tocopherol cyclase (IbTC) and γ-tocopherol methyltransferase (IbTMT). Fluorescence microscope analysis indicated that four proteins localized into the chloroplast, whereas IbHPPD observed in the nuclear. Quantitative RT-PCR analysis revealed that the expression patterns of the five tocopherol biosynthetic genes varied in different plant tissues and under different stress conditions. All five genes were highly expressed in leaf tissues, whereas IbHPPD and IbHPT were highly expressed in the thick roots. The expression patterns of these five genes significantly differed in response to PEG, NaCl and H2O2-mediated oxidative stress. IbHPPD was strongly induced following PEG and H2O2 treatment and IbHPT was strongly induced following PEG treatment, whereas IbMPBQ MT and IbTC were highly expressed following NaCl treatment. Upon infection of the bacterial pathogen Pectobacterium chrysanthemi, the expression of IbHPPD increased sharply in sweetpotato leaves, whereas the expression of the other genes was reduced or unchanged. Additionally, transient expression of the five tocopherol biosynthetic genes in tobacco (Nicotiana bentamiana) leaves resulted in increased transcript levels of the transgenes expressions and tocopherol production. Therefore, our results suggested that the five tocopherol biosynthetic genes of sweetpotato play roles in the stress defense response as transcriptional regulators of the tocopherol production.

Natural variation in tocochromanols content in Arabidopsis thaliana accessions - the effect of temperature and light intensity

In this study, 25 accessions of Arabidopsis thaliana originating from a variety of climate conditions were grown under controlled circumstances of different light intensity and temperature. The accessions were analyzed for prenyllipids content and composition, as well as expression of the genes involved in tocochromanol biosynthesis (vte1-5). It was found that the applied conditions did not strongly affect total tocochromanols content and there was no apparent correlation of the tocochromanol content with the origin of the accessions. However, the presented results indicate that the temperature, more than the light intensity, affects the expression of the vte1-5 genes and the content of some prenyllipids. An interesting observation was that under low growth temperature, the hydroxy-plastochromanol (PC-OH) to plastochromanol (PC) ratio was considerably increased regardless of the light intensity in most of the accessions. PC-OH is known to be formed as a result of singlet oxygen stress, therefore this observation indicates that the singlet oxygen production is enhanced under low temperature. Unexpectedly, the highest increase in the PC-OH/PC ratio was found for accessions originating from cold climate (Shigu, Krazo-1 and Lov-5), even though such plants could be expected to be more resistant to low temperature stress.

Homogentisate phytyltransferase from the unicellular green alga Chlamydomonas reinhardtii

Homogentisate phytyltransferase (HPT) (EC 2.5.1.-) catalyzes the first committed step of tocopherol biosynthesis in all photosynthetic organisms. This paper presents the molecular characterization and expression analysis of HPT1 gene, and a study on the accumulation of tocopherols under different environmental conditions in the unicellular green alga Chlamydomonas reinhardtii. The Chlamydomonas HPT1 protein conserves all the prenylphosphate- and divalent cation-binding sites that are found in polyprenyltransferases and all the amino acids that are essential for its catalytic activity. Its hydrophobicity profile confirms that HPT is a membrane-bound protein. Chlamydomonas genomic DNA analysis suggests that HPT is encoded by a single gene, HPT1, whose promoter region contains multiple motifs related to regulation by jasmonate, abscisic acid, low temperature and light, and an ATCTA motif presents in genes involved in tocopherol biosynthesis and some photosynthesis-related genes. Expression analysis revealed that HPT1 is strongly regulated by dark and low-temperature. Under the same treatments, α-tocopherol increased in cultures exposed to darkness or heat, whereas γ-tocopherol did it in low temperature. The regulatory expression pattern of HPT1 and the changes of tocopherol abundance support the idea that different tocopherols play specific functions, and suggest a role for γ-tocopherol in the adaptation to growth under low-temperature.

Physiological and antioxidant responses of two accessions of Arabidopsis thaliana in different light and temperature conditions

During their lifetime, plants need to adapt to a changing environment, including light and temperature. To understand how these factors influence plant growth, we investigated the physiological and antioxidant responses of two Arabidopsis accessions, Shahdara (Sha) from the Shahdara valley (Tajikistan, Central Asia) in a mountainous area and Lovvik-5 (Lov-5) from northern Sweden to different light and temperature conditions. These accessions originate from different latitudes and have different life strategies, both of which are known to be influenced by light and temperature. We showed that both accessions grew better in high-light and at a lower temperature (16°C) than in low light and at 23°C. Interestingly, Sha had a lower chlorophyll content but more efficient non-photochemical quenching than Lov-5. Sha, also showed a higher expression of vitamin E biosynthetic genes. We did not observe any difference in the antioxidant prenyllipid level under these conditions. Our results suggest that the mechanisms that keep the plastoquinone (PQ)-pool in more oxidized state could play a role in the adaptation of these accessions to their local climatic conditions.

Redox signaling and stress tolerance in plants: a focus on vitamin E

Plants are subject to specific redox processes, in which photosynthesis plays a prominent role. Chloroplasts function in light at high oxygen tensions and are enormous generators of reactive oxygen species, mainly singlet oxygen. This side product of photosynthesis inflicts damage to thylakoid membranes at high concentrations, but at the same time it is an essential component of cellular signaling. Detoxification of singlet oxygen is achieved by different means, including quenching and scavenging by tocopherols, responsible for controlling singlet oxygen levels, and the extent of lipid peroxidation in chloroplasts. Here, environmental conditions leading to excess light in chloroplasts will be used to show the importance of singlet oxygen, tocopherols, and lipid peroxidation in cell signaling. Defects in antioxidant protection (e.g., tocopherol deficiency) can lead to increased photo-oxidative damage, but also to the activation of defense pathways, illustrating the phenotypic plasticity evolved by plants to withstand stress. Most importantly, these studies show how redox signaling processes are integrated within the cell and illustrate the great capacity of plants to adapt to their environment.

Nitric oxide, antioxidants and prooxidants in plant defence responses

In plant cells the free radical nitric oxide (NO) interacts both with anti- as well as prooxidants. This review provides a short survey of the central roles of ascorbate and glutathione-the latter alone or in conjunction with S-nitrosoglutathione reductase-in controlling NO bioavailability. Other major topics include the regulation of antioxidant enzymes by NO and the interplay between NO and reactive oxygen species (ROS). Under stress conditions NO regulates antioxidant enzymes at the level of activity and gene expression, which can cause either enhancement or reduction of the cellular redox status. For instance chronic NO production during salt stress induced the antioxidant system thereby increasing salt tolerance in various plants. In contrast, rapid NO accumulation in response to strong stress stimuli was occasionally linked to inhibition of antioxidant enzymes and a subsequent rise in hydrogen peroxide levels. Moreover, during incompatible Arabidopsis thaliana-Pseudomonas syringae interactions ROS burst and cell death progression were shown to be terminated by S-nitrosylation-triggered inhibition of NADPH oxidases, further highlighting the multiple roles of NO during redox-signaling. In chemical reactions between NO and ROS reactive nitrogen species (RNS) arise with characteristics different from their precursors. Recently, peroxynitrite formed by the reaction of NO with superoxide has attracted much attention. We will describe putative functions of this molecule and other NO derivatives in plant cells. Non-symbiotic hemoglobins (nsHb) were proposed to act in NO degradation. Additionally, like other oxidases nsHb is also capable of catalyzing protein nitration through a nitrite- and hydrogen peroxide-dependent process. The physiological significance of the described findings under abiotic and biotic stress conditions will be discussed with a special emphasis on pathogen-induced programmed cell death (PCD).

Hypothesis: Nitro-fatty acids play a role in plant metabolism

The free radical molecule nitric oxide (NO) is involved in a wide range of plant functions such as growth, senescence, fruit ripening, and responses to adverse environmental conditions. NO and NO-derived molecules peroxynitrite and S-nitrosoglutathione are reactive nitrogen species (RNS) that can directly or indirectly interact with a broad spectrum of biomolecules that affect their biological functions. Plant NO research has focused on post-translational modifications in proteins, mainly S-nitrosylation and nitration. There are other potential target biomolecules in plants that have not been studied, which have been studied in animal systems, such as lipids. Nitro-fatty acids (NO(2)-FAs) are involved in pleiotropic activities in animal systems, including modulation of macrophage activation, prevention of leukocyte and platelet activation, and promotion of blood vessel relaxation. NO(2)-FAs are therefore novel mediators in NO signaling pathways and metabolism. This review will focus on these molecules and will highlight their potential in relation to the physiology of higher plants.

The role of nitric oxide in the germination of plant seeds and pollen

Two complex physiological processes, with opposite positions in the plant's life-cycle, seed and pollen germination, are vital to the accomplishment of successful plant growth and reproduction. This review summarizes the current state of knowledge of the intersection of NO signalling with the signalling pathways of ABA, GA, and ethylene; plant hormones that control the release of plant seeds from dormancy and germination. The cross-talk of NO and ROS is involved in the light- and hormone-specific regulation of seeds' developmental processes during the initiation of plant ontogenesis. Similarly to seed germination, the mechanisms of plant pollen hydration, germination, tube growth, as well as pollen-stigma recognition are tightly linked to the proper adjustment of NO and ROS levels. The interaction of NO with ROS and secondary messengers such as Ca(2+), cAMP and cGMP discovered in pollen represent a common mechanism of NO signalling. The involvement of NO in both breakpoints of plant physiology, as well as in the germination of spores within fungi and oomycetes, points toward NO as a component of an evolutionary conserved signalling pathway.

Understanding the fate of peroxynitrite in plant cells--from physiology to pathophysiology

Peroxynitrite (ONOO(-)) is a potent oxidant and nitrating species, generated by the reaction of nitric oxide and superoxide in one of the most rapid reactions known in biology. It is widely accepted that an enhanced ONOO(-) formation contributes to oxidative and nitrosative stress in various biological systems. However, an increasing number of studies have reported that ONOO(-) cannot only be considered as a mediator of cellular dysfunction, but also behaves as a potent modulator of the redox regulation in various cell signal transduction pathways. Although the formation of ONOO(-) has been demonstrated in vivo in plant cells, the relevance of this molecule during plant physiological responses is still far from being clarified. Admittedly, the detection of protein tyrosine nitration phenomena provides some justification to the speculations that ONOO() is generated during various plant stress responses associated with pathophysiological mechanisms. On the other hand, it was found that ONOO(-) itself is not as toxic for plant cells as it is for animal ones. Based on the concepts of the role played by ONOO(-) in biological systems, this review is focused mainly on the search for potential functions of ONOO(-) in plants. Moreover, it is also an attempt to stimulate a discussion on the significance of protein nitration as a paradigm in signal modulation, since the newest reports identified proteins associated with signal transduction cascades within the plant nitroproteome.

Reactive oxygen species and antioxidant machinery in abiotic stress tolerance in crop plants

Various abiotic stresses lead to the overproduction of reactive oxygen species (ROS) in plants which are highly reactive and toxic and cause damage to proteins, lipids, carbohydrates and DNA which ultimately results in oxidative stress. The ROS comprises both free radical (O(2)(-), superoxide radicals; OH, hydroxyl radical; HO(2), perhydroxy radical and RO, alkoxy radicals) and non-radical (molecular) forms (H(2)O(2), hydrogen peroxide and (1)O(2), singlet oxygen). In chloroplasts, photosystem I and II (PSI and PSII) are the major sites for the production of (1)O(2) and O(2)(-). In mitochondria, complex I, ubiquinone and complex III of electron transport chain (ETC) are the major sites for the generation of O(2)(-). The antioxidant defense machinery protects plants against oxidative stress damages. Plants possess very efficient enzymatic (superoxide dismutase, SOD; catalase, CAT; ascorbate peroxidase, APX; glutathione reductase, GR; monodehydroascorbate reductase, MDHAR; dehydroascorbate reductase, DHAR; glutathione peroxidase, GPX; guaicol peroxidase, GOPX and glutathione-S- transferase, GST) and non-enzymatic (ascorbic acid, ASH; glutathione, GSH; phenolic compounds, alkaloids, non-protein amino acids and α-tocopherols) antioxidant defense systems which work in concert to control the cascades of uncontrolled oxidation and protect plant cells from oxidative damage by scavenging of ROS. ROS also influence the expression of a number of genes and therefore control the many processes like growth, cell cycle, programmed cell death (PCD), abiotic stress responses, pathogen defense, systemic signaling and development. In this review, we describe the biochemistry of ROS and their production sites, and ROS scavenging antioxidant defense machinery.

Homolog of tocopherol C methyltransferases catalyzes N methylation in anticancer alkaloid biosynthesis

Madagascar periwinkle (Catharanthus roseus) is the sole source of the anticancer drugs vinblastine and vincristine, bisindole alkaloids derived from the dimerization of the terpenoid indole alkaloids vindoline and catharanthine. Full elucidation of the biosynthetic pathways of these compounds is a prerequisite for metabolic engineering efforts that will improve production of these costly molecules. However, despite the medical and commercial importance of these natural products, the biosynthetic pathways remain poorly understood. Here we report the identification and characterization of a C. roseus cDNA encoding an S-adenosyl-L-methionine-dependent N methyltransferase that catalyzes a nitrogen methylation involved in vindoline biosynthesis. Recombinant enzyme produced in Escherichia coli is highly substrate specific, displaying a strict requirement for a 2,3-dihydro bond in the aspidosperma skeleton. The corresponding gene transcript is induced in methyl jasmonate-elicited seedlings, along with the other known vindoline biosynthetic transcripts. Intriguingly, this unique N methyltransferase is most similar at the amino acid level to the plastidic γ-tocopherol C methyltransferases of vitamin E biosynthesis, suggesting an evolutionary link between these two functionally disparate methyltransferases.

Tocochromanol functions in plants: antioxidation and beyond

Tocopherols and tocotrienols, collectively known as tocochromanols, are lipid-soluble molecules that belong to the group of vitamin E compounds and are essential in the human diet. Not surprisingly, most of what is known about the biological functions of tocochromanols comes from studies of mammalian systems, yet they have been shown to be synthesized only by photosynthetic organisms. The last decade has seen a radical change in the appreciation of the biological role of tocochromanols in plants thanks to a detailed characterization of mutant and transgenic plants, including several Arabidopsis thaliana mutants, the sucrose export defective1 (sxd1) maize mutant, and some transgenic potato and tobacco lines altered in tocochromanol biosynthesis. Recent findings indicate that tocopherols may play important roles in plants beyond their antioxidant function in photosynthetic membranes. Plants deficient in tocopherols show alterations in germination and export of photoassimilates, and growth, leaf senescence, and plant responses to abiotic stresses, thus suggesting that tocopherols may influence a number of physiological processes in plants. Thus, in this review not only the antioxidant function of tocochromanols in plants, but also these new emerging possible roles will be considered. Particular attention will be paid to specific roles attributed to different tocopherol homologues (particularly alpha- and gamma-tocopherol) and the possible functions of tocotrienols, which in contrast to tocopherols are only present in a range of unrelated plant groups and are almost exclusively found in seeds and fruits.

Biosynthesis, regulation and functions of tocochromanols in plants

Tocopherols and tocotrienols have been originally identified as essential nutrients in mammals based on their vitamin E activity. These lipid-soluble compounds are potent antioxidants that protect polyunsaturated fatty acids from lipid peroxidation. The biosynthesis of tocopherols and tocotrienols occurs exclusively in photosynthetic organisms. The biosynthetic precursors and the different pathway intermediates have been identified by biochemical studies and the different vitamin E biosynthetic genes (VTE genes) have been isolated in several plants and cyanobacteria. The characterization of transgenic plants overexpressing one or multiple VTE genes combined with the study of vitamin E deficient mutants allows from now on understanding the regulation and the function of tocopherols and tocotrienols in plants.

Protein tyrosine nitration: a new challenge in plants

Nitric oxide metabolism in plant cells has a relative short history. Nitration is a chemical process which consists of introducing a nitro group (-NO(2)) into a chemical compound. In biological systems, this process has been found in different molecules such as proteins, lipids and nucleic acids that can affect its function. This mini-review offers an overview of this process with special emphasis on protein tyrosine nitration in plants and its involvement in the process of nitrosative stress.

Inactivation of genes, encoding tocopherol biosynthetic pathway enzymes, results in oxidative stress in outdoor grown Arabidopsis thaliana

Tocopherols (alpha-, beta-, gamma- and delta-tocopherols) represent a group of lipophilic antioxidants which are synthesized only by photosynthetic organisms. It is widely believed that protection of pigments and proteins of photosynthetic system and polyunsaturated fatty acids from oxidative damage caused by reactive oxygen species (ROS) is the main function of tocopherols. The wild type Columbia and two mutants of Arabidopsis thaliana with T-DNA insertions in tocopherol biosynthesis genes - tocopherol cyclase (vte1) and gamma-tocopherol methyltransferase (vte4) - were analyzed after long-term outdoor growth. The concentration of total tocopherol was up to 12-fold higher in outdoor growing wild type and vte4 plant lines than in plants grown under laboratory conditions. The vte4 mutant plants had a lower concentration of chlorophylls and carotenoids, whereas the mutant plants had a higher level of total glutathione than of wild type. The activities of antioxidant enzymes superoxide dismutase (SOD, EC 1.15.1.1) and ascorbate oxidase (AO, EC 1.10.3.3) were lower in both mutants, whereas activities of catalase (EC 1.11.1.6) and ascorbate peroxidase (APx, EC 1.11.1.11) were lower only in vte1 mutant plants in comparison to wild type plants. However, the activity of guaiacol peroxidase (GuPx, EC 1.11.1.7) was higher in vte1 and vte4 mutants than that in wild type. Additionally, both mutant plant lines had higher concentration of protein carbonyl groups and oxidized glutathione compared to the wild type, indicating the development of oxidative stress. These results demonstrate in plants that tocopherols play a crucial role for growth of plants under outdoor conditions by preventing oxidation of cellular components.

Gamma-tocopherol dominates in young leaves of runner bean (Phaseolus coccineus) under a variety of growing conditions: the possible functions of gamma-tocopherol

It has been shown that young leaves of runner bean (Phaseolus coccineus) plants grown under natural conditions have an unusually high content of gamma-tocopherol, accounting for up to 90% of all tocopherols and 50% of the chlorophyll content. The level of gamma-tocopherol gradually decreased during the first two weeks of leaf development. The high content of gamma-tocopherol in young leaves was not significantly influenced by growth conditions. In contrast to seeds, gamma-tocopherol was also the main tocopherol found in light-grown and etiolated primary leaves of runner bean. The obtained results suggest that gamma-tocopherol decline during leaf development is not only due to conversion of gamma- to alpha-tocopherol but probably also due to degradation of gamma-tocopherol to non-tocochromanol compounds. We have also shown that gamma-tocopherol found in young leaves is mainly localized in thylakoid membranes within chloroplast. In the primary leaves subjected to different abiotic stresses, only during simultaneous drought and light stress, gamma-tocopherolquinone, an oxidation product of gamma-tocopherol, was preferentially accumulated. Since one of the other possible functions of gamma-tocopherol could be its action as a nitric oxide scavenger, young leaves were analyzed for the presence of nitro-gamma-tocopherol. However, despite the use of a sensitive detection method, it was not found. The possible physiological function of the increased level of gamma-tocopherol in the young leaves was discussed.

Tocopherol metabolism, oxidation and recycling under high light stress in Arabidopsis

Tocopherols are synthesized and accumulated by all plants and many cyanobacteria. The quenching and scavenging of reactive oxygen species and lipid peroxy radicals by tocopherols can result in the formation of various tocopherol oxidation compounds. A targeted GC/MS profiling method was developed to quantify all tocopherols and pathway intermediates, and 23 potential alpha- and gamma-tocopherol oxidation products. This method was used to study the response of wild-type Arabidopsis (Col) and the tocopherol biosynthetic mutants vte1, vte2 and vte4 during 12 h low- and high-light treatments (LL and HL, 90 and 1500 mumol photon m(-2) sec(-1), respectively) and a subsequent 12 h dark recovery period. All tocopherols and pathway intermediates exhibited HL-dependent increases except 2,3-dimethyl-6-phytyl-1,4-benzoquinone (DMPBQ) in vte1 and beta-tocopherol in Col. Profiling of potential tocopherol oxidation products during HL treatment indicated the presence of only alpha-tocopherolquinol (alpha-TQH(2)) in Col and only gamma-tocopherolquinol (gamma-TQH(2)) in vte4, both of which accumulated to similar levels and with similar kinetics the two genotypes. However, during dark recovery, the level of alpha-TQH(2) in Col decreased several times faster than that of gamma-TQH(2) in vte4, suggesting the presence of biochemical processes with higher specificity for alpha-TQH(2). (14)C-labeled alpha-tocopherolquinone (alpha-TQ) applied to isolated Col chloroplasts was converted to (14)C-alpha-tocopherol, demonstrating the existence of a plastid-based system for recycling oxidized alpha-tocopherol. The accumulation of (14)C-trimethylphytylbenzoquinone (TMPBQ) by isolated vte1 plastids treated with (14)C-labeled alpha-TQ is consistent with the tocopherolquinone-recycling pathway utilizing a yet to be identified plastid-localized dehydratase that converts tocopherolquinone to TMPBQ.