Ultraviolet-B-radiation-induced changes in nicotinamide and glutathione metabolism and gene expression in plants

The Role of Reactive Oxygen Species in Plant Response to Radiation

Radiation is widespread in nature, including ultraviolet radiation from the sun, cosmic radiation and radiation emitted by natural radionuclides. Over the years, the increasing industrialization of human beings has brought about more radiation, such as enhanced UV-B radiation due to ground ozone decay, and the emission and contamination of nuclear waste due to the increasing nuclear power plants and radioactive material industry. With additional radiation reaching plants, both negative effects including damage to cell membranes, reduction of photosynthetic rate and premature aging and benefits such as growth promotion and stress resistance enhancement have been observed. ROS (Reactive oxygen species) are reactive oxidants in plant cells, including hydrogen peroxide (H₂O₂), superoxide anions (O₂^•-) and hydroxide anion radicals (·OH), which may stimulate the antioxidant system of plants and act as signaling molecules to regulate downstream reactions. A number of studies have observed the change of ROS in plant cells under radiation, and new technology such as RNA-seq has molecularly revealed the regulation of radiative biological effects by ROS. This review summarized recent progress on the role of ROS in plant response to radiations including UV, ion beam and plasma, and may help to reveal the mechanisms of plant responses to radiation.

Foliar and Root Comparative Metabolomics and Phenolic Profiling of Micro-Tom Tomato (Solanum lycopersicum L.) Plants Associated with a Gene Expression Analysis in Response to Short Daily UV Treatments

Tomato (Solanum lycopersicum L.) is globally recognised as a high-value crop both for commercial profit and nutritional benefits. In contrast to the extensive data regarding the changes in the metabolism of tomato fruit exposed to UV radiation, less is known about the foliar and root metabolome. Using an untargeted metabolomic approach through UHPLC-ESI-QTOF-MS analysis, we detected thousands of metabolites in the leaves (3000) and roots (2800) of Micro-Tom tomato plants exposed to 11 days of short daily UV radiation, applied only on the aboveground organs. Multivariate statistical analysis, such as OPLS-DA and volcano, were performed to allow a better understanding of the modifications caused by the treatment. Based on the unexpected modulation to the secondary metabolism, especially the phenylpropanoid pathway, of which compounds were down and up accumulated respectively in leaves and roots of treated plants, a phenolic profiling was carried out for both organs. The phenolic profile was associated with a gene expression analysis to check the transcription trend of genes involved in the UVR8 signalling pathway and the early steps of the phenolic biosynthesis. The retention of the modifications at metabolic and phenolic levels was also investigated 3 days after the UV treatment, showing a prolonged effect on the modulation once the UV treatment had ceased.

Genome-Wide Association Study for Ultraviolet-B Resistance in Soybean (Glycine max L.)

The depletion of the stratospheric ozone layer is a major environmental issue and has increased the dosage of ultraviolet-B (UV-B) radiation reaching the Earth’s surface. Organisms are negatively affected by enhanced UV-B radiation, and especially in crop plants this may lead to severe yield losses. Soybean (Glycine max L.), a major legume crop, is sensitive to UV-B radiation, and therefore, it is required to breed the UV-B-resistant soybean cultivar. In this study, 688 soybean germplasms were phenotyped for two categories, Damage of Leaf Chlorosis (DLC) and Damage of Leaf Shape (DLS), after supplementary UV-B irradiation for 14 days. About 5% of the germplasms showed strong UV-B resistance, and GCS731 was the most resistant genotype. Their phenotypic distributions showed similar patterns to the normal, suggesting UV-B resistance as a quantitative trait governed by polygenes. A total of 688 soybean germplasms were genotyped using the Axiom^® Soya 180K SNP array, and a genome-wide association study (GWAS) was conducted to identify SNPs significantly associated with the two traits, DLC and DLS. Five peaks on chromosomes 2, 6, 10, and 11 were significantly associated with either DLC or DLS, and the five adjacent genes were selected as candidate genes responsible for UV-B resistance. Among those candidate genes, Glyma.02g017500 and Glyma.06g103200 encode cryptochrome (CRY) and cryptochrome 1 (CRY1), respectively, and are known to play a role in DNA repair during photoreactivation. Real-time quantitative RT-PCR (qRT-PCR) results revealed that CRY1 was expressed significantly higher in the UV-B-resistant soybean compared to the susceptible soybean after 6 h of UV-B irradiation. This study is the first GWAS report on UV-B resistance in soybean, and the results will provide valuable information for breeding UV-B-resistant soybeans in preparation for climate change.

Different levels of UV-B resistance in Vaccinium corymbosum cultivars reveal distinct backgrounds of phenylpropanoid metabolites

UV-B radiation induces several physiological and biochemical effects that can influence regulatory plant processes. Vaccinium corymbosum responds differently to UV-B radiation depending on the UV-B resistance of cultivars, according to their physiological and biochemical features. In this work, the effect of two levels of UV-B radiation during long-term exposure on the phenylpropanoid biosynthesis, and the expression of genes associated with flavonoid biosynthesis as well as the absolute quantification of secondary metabolites were studied in two contrasting UV-B-resistant cultivars (Legacy, resistant and Bluegold, sensitive). Multivariate analyses were performed to understand the role of phenylpropanoids in UV-B defense mechanisms. The amount of phenylpropanoid compounds was generally higher in Legacy than in Bluegold. Different expression levels of flavonoid biosynthetic genes for both cultivars were transiently induced, showing that even in longer period of UV-B exposure; plants are still adjusting their phenylpropanoids at the transcription levels. Multivariate analysis in Legacy indicated no significant correlation between gene expression and the levels of the flavonoids and phenolic acids. By contrast, in the Bluegold cultivar higher number of correlations between secondary metabolite and transcript levels was found. Taken together, the results indicated different adjustments between the cultivars for a successful UV-B acclimation. While the sensitive cultivar depends on metabolite adjustments to respond to UV-B exposure, the resistant cultivar also possesses an intrinsically higher antioxidant and UV-B screening capacity. Thus, we conclude that UV-B resistance involves not only metabolite level adjustments during the acclimation period, but also depends on the intrinsic metabolic status of the plant and metabolic features of the phenylpropanoid compounds.

Nicotinamide; antioxidative and DNA hypomethylation effects in plant cells

The effects of nicotinamide (NIC) and its natural plant metabolites nicotinic acid (NIA) and trigonelline (TRIG) were studied with respect to defense in plant cell cultures. NIC and NIA could protect against oxidative stress damage caused by 2,2'-azobis(2-amidinopropane) dihydrochloride (AAPH), which generates free radicals. Damage was analyzed as DNA strand breaks in cell cultures of Pisum sativum (garden pea), Daucus carota (carrot), Populus tremula L. × P. tremuloides (hybrid aspen) and Catharanthus roseus (Madagascar periwinkle), monitored by single cell gel electrophoresis (comet assay), and assays of cell leakage in C. roseus. The activities of aconitase and fumarase enzymes, which have key roles in energy metabolism, were analyzed in P. sativum cultures after treatment with NIC or NIA. Aconitase activity was increased by NIA, and fumarase activity was increased by both compounds. These compounds were shown to promote glutathione metabolism in P. sativum cultures, and NIC was shown to have a global DNA hypomethylating effect. Neither TRIG nor poly(ADP-ribose) polymerase (PARP) inhibitor 3-aminobenzamide offered any protection against DNA damage or cell leakage, nor did they promote aconitase or fumarase activities, or glutathione metabolism. By this broad approach addressing multiple biochemical factors and different plant species, we demonstrate that NIC and NIA protect plant cells from oxidative stress, and that NIC clearly exerts an epigenetic effect; decreased DNA methylation. This indicates that these compounds have important roles in the regulation of metabolism in plant cells, especially in connection to stress.

Hormone-controlled UV-B responses in plants

Ultraviolet B (UV-B) light is a portion of solar radiation that has significant effects on the development and metabolism of plants. Effects of UV-B on plants can be classified into photomorphogenic effects and stress effects. These effects largely rely on the control of, and interactions with, hormonal pathways. The fairly recent discovery of the UV-B-specific photoreceptor UV RESISTANCE LOCUS 8 (UVR8) allowed evaluation of the role of downstream hormones, leading to the identification of connections with auxin and gibberellin. Moreover, a substantial overlap between UVR8 and phytochrome responses has been shown, suggesting that part of the responses caused by UVR8 are under PHYTOCHROME INTERACTING FACTOR control. UV-B effects can also be independent of UVR8, and affect different hormonal pathways. UV-B affects hormonal pathways in various ways: photochemically, affecting biosynthesis, transport, and/or signaling. This review concludes that the effects of UV-B on hormonal regulation can be roughly divided in two: inhibition of growth-promoting hormones; and the enhancement of environmental stress-induced defense hormones.

UV-B exposure, ROS, and stress: inseparable companions or loosely linked associates?

Ultraviolet-B (UV-B) radiation has long been perceived as a stressor. However, a conceptual U-turn has taken place, and UV-B damage is now considered rare. We question whether UV-stress and UV-B-induced reactive oxygen species (ROS) are still relevant concepts, and if ROS-mediated signaling contributes to UV-B acclimation. Measurements of antioxidants and of antioxidant genes show that both low and high UV-B doses alter ROS metabolism. Yet, there is no evidence that ROS control gene expression under low UV-B. Instead, expression of antioxidant genes is linked to the UV RESISTANCE LOCUS 8 pathway. We hypothesize that low UV-B doses cause 'eustress' (good stress) and that stimuli-specific signaling pathways pre-dispose plants to a state of low alert that includes activation of antioxidant defenses.

Assessment of impact of solar UV components on growth and antioxidant enzyme activity in cotton plant

A field experiment was conducted to study the impact of solar UV-B (280-315 nm) and UV-A (315-400 nm) components on the growth and antioxidant enzyme activity in cotton plant (Gossypium hirsutum var. Vikram). Solar UV components were excluded by filtering the sunlight through selective UV-B (<315 nm) and UV-B/A (<400 nm) absorbing polyester films. Plants grown under filters that transmitted solar UV served as controls. Exclusion of UV-B and UV-B/A enhanced plant height, leaf area and total biomass of plants. The activity of antioxidant enzymes superoxide dismutase (SOD), ascorbic acid peroxidase (APX), glutathione reductase (GR) and guaiacol peroxidase (GPx) assayed in the leaves were lesser in the UV excluded plants. The level of ascorbic acid and UV absorbing substances were also decreased by the exclusion of UV. Solar UV components exerted a limitation on the potential growth of cotton plants. Reduction in the antioxidant enzyme activity and ascorbic acid after UV exclusion indicated that ambient UV components exert a significant stress on cotton plants. Reduction in the production of UAS indicated a changed pattern of metabolism leading to improved primary metabolism. Exclusion of UV components is advantageous from the agricultural point to enhance the growth of cotton plants.

The role of the pyridoxine (vitamin B6) biosynthesis enzyme PDX1 in ultraviolet-B radiation responses in plants

Ultraviolet-B radiation regulates plant growth and morphology at low and ambient fluence rates but can severely impact on plants at higher doses. Some plant UV-B responses are related to the formation of reactive oxygen species (ROS) and pyridoxine (vitamin B(6)) has been reported to be a quencher of ROS. UV-B irradiation of Arabidopsis Col-0 plants resulted in increased levels of PDX1 protein, compared with UV-A-exposed plants. This was shown by immunoblot analysis using specific polyclonal antibodies raised against the recombinant PDX1.3 protein and confirmed by mass spectrometry analysis of immunoprecipitated PDX1. The protein was located mainly in the cytosol but also to a small extent in the membrane fraction of plant leaves. Immunohistochemical analysis performed in pea revealed that PDX1 is present in UV-B-exposed leaf mesophyll and palisade parenchyma but not in epidermal cells. Pyridoxine production increased in Col-0 plants exposed to 3 days of UV-B, whereas in an Arabidopsis pdx1.3 mutant UV-B did not induce pyridoxine biosynthesis. In gene expression studies performed after UV-B exposure, the pdx1.3 mutant showed elevated transcript levels for the LHCB1*3 gene (encoding a chlorophyll a/b-binding protein of the photosystem II light-harvesting antenna complex) and the pathogenesis-related protein 5 (PR-5) gene, compared with wild type.

The cellular redox state in plant stress biology--a charging concept

Different redox-active compounds, such as ascorbate, glutathione, NAD(P)H and proteins from the thioredoxin superfamily, contribute to the general redox homeostasis in the plant cell. The myriad of interactions between redox-active compounds, and the effect of environmental parameters on them, has been encapsulated in the concept of a cellular redox state. This concept has facilitated progress in understanding stress signalling and defence in plants. However, despite the proven usefulness of the concept of a redox state, there is no single, operational definition that allows for quantitative analysis and hypothesis testing.

Expression of Pisum sativum SAD polypeptides in production hosts and in planta: tetrameric organization of the protein

In Pisum sativum, the short-chain alcohol dehydrogenase-like protein (SAD) gene family consists of at least three members (SAD-A, -B, and -C). Expression of two of these genes (SAD-A and -C) in Escherichia coli or Pichia pastoris resulted in full-length soluble proteins. Purified SAD-A was used as antigen for antibody production in rabbits. With these antibodies the recombinant SAD-C protein (which was most highly expressed of the two isoforms) was shown to be a tetramer consisting of a dimer of dimers. The SAD genes are transiently expressed in plants by short exposures to ultraviolet-B radiation (UV-B), as judged by northern blotting. In turn, mRNA accumulation leads to formation of SAD protein in leaf and stem tissue upon prolonged UV-B irradiation.

Trigonelline (N-methylnicotinic acid) Biosynthesis and its Biological Role in Plants

Trigonelline is synthesized from nicotinic acid in many plant species. This compound accumulates in seeds of leguminous plants and Coffea sp. The present article reviews the distribution of nicotinic acid conjugates and biosynthetic pathways of trigonelline, including the pyridine nucleotide cycle and de novo synthesis. The function of trigonelline in plants is also discussed.

Two separate UV-B radiation wavelength regions control expression of different molecular markers in Arabidopsis thaliana

Fluence-response curves were obtained at nine wavelengths in the interval 280-360 nm for mRNA transcripts of four molecular markers induced by ultraviolet-B (UV-B) radiation in Arabidopsis thaliana (L.) Heynh.: CHS (encoding chalcone synthase), PDX1.3 (encoding an enzyme involved in formation of pyridoxine), MEB5.2 (encoding a protein with unknown function but which is strongly upregulated by UV-B), and LHCB1*3 (encoding a chlorophyll a/b binding protein). Intact Arabidopsis plants were irradiated for 3 h using a high intensity deuterium radiation source and narrow bandwith filters without supplementary PAR. The results obtained suggest the existence of two distinct UV-B signal responses: one sensitive between 300 and 310 nm and the other sensitive around 280-290 nm. Among the investigated molecular markers, CHS and PDX1.3 were regulated through the chromophore absorbing around 300 nm, whereas MEB5.2 and LHCB1*3 were regulated through the chromophore absorbing at 280-290 nm. The results obtained show that at least two signal transduction pathways exist that regulate gene expression as a result of absorption of UV-B radiation in plants.

Transcriptional profiling of summer wheat, grown under different realistic UV-B irradiation regimes

There is limited information on the impact of present-day ultraviolet-B (UV-B) radiation on a reprogramming of gene expression in crops. Summer wheat was cultivated in controlled environmental facilities under simulated realistic climatic conditions. We investigated the effect of different regimes of UV-B radiation on summer wheat (Triticum aestivum L.) cultivars Nandu, Star and Turbo. Until recently, these were most important in Bavaria. Different cultivars of crops often show great differences in their sensitivity towards UV-B radiation. To identify genes that might be involved in UV-B defence mechanisms, we first analyzed selected genes known to be involved in plant defence mechanisms. RNA gel blot analysis of RNA isolated from the flag leaf of 84-day-old plants showed differences in transcript levels among the cultivars. Flag leaves are known to be important for grain development, which was completed at 84 days post-anthesis. Catalase 2 (Cat2) transcripts were elevated by increased UV irradiation in all cultivars with highest levels in cv. Nandu. Pathogenesis-related protein 1 (PR1) transcripts were elevated only in cv. Star. A minor influence on transcripts for phenylalanine ammonia-lyase (PAL) was observed in all three cultivars. This indicates different levels of acclimation to UV-B radiation in the wheat cultivars studied. To analyze these responses in more detail, UV-B-exposed flag leaves of 84-day-old wheat (cv. Nandu) were pooled to isolate cDNAs of induced genes by suppression-subtractive hybridization (SSH). Among the initially isolated cDNA clones, 13 were verified by RNA gel blot analysis showing an up-regulation at elevated levels of UV-B radiation. Functional classification revealed genes encoding proteins associated with protein assembly, chaperonins, programmed cell death and signal transduction. We also studied growth, flowering time, ear development and yield as more typical agricultural parameters. Plant growth of young plants was reduced at increased UV-B radiation. Flowering and ear development were delayed concomitantly, whereas total grain weight was not influenced at any of the UV-B irradiation regimes.

Genes up-regulated during red coloration in UV-B irradiated lettuce leaves

Molecular analysis of gene expression differences between green and red lettuce leaves was performed using the SSH method. BlastX comparisons of subtractive expressed sequence tags (ESTs) indicated that 7.6% of clones encoded enzymes involved in secondary metabolism. Such clones had a particularly high abundance of flavonoid-metabolism proteins (6.5%). Following SSH, 566 clones were rescreened for differential gene expression using dot-blot hybridization. Of these, 53 were found to overexpressed during red coloration. The up-regulated expression of six genes was confirmed by Northern blot analyses. The expression of chalcone synthase (CHS), flavanone 3-hydroxylase (F3H), and dihydroflavonol 4-reductase (DFR) genes showed a positive correlation with anthocyanin accumulation in UV-B-irradiated lettuce leaves; flavonoid 3',5'-hydroxylase (F3',5'H) and anthocyanidin synthase (ANS) were expressed continuously in both samples. These results indicated that the genes CHS, F3H, and DFR coincided with increases in anthocyanin accumulation during the red coloration of lettuce leaves. This study show a relationship between red coloration and the expression of up-regulated genes in lettuce. The subtractive cDNA library and EST database described in this study represent a valuable resource for further research for secondary metabolism in the vegetable crops.

Heme oxygenase up-regulation in ultraviolet-B irradiated soybean plants involves reactive oxygen species

Ultraviolet-B (UV-B) radiation has a negative impact on plant cells, and leads to the generation of reactive oxygen species (ROS). Heme oxygenase (HO, EC 1.14.99.3) plays a protective role against oxidative stress in mammals, but little is known about this issue in plants. Here, we report for the first time the response of HO in leaves of soybean (Glycine max L.) plants subjected to UV-B radiation. Under 7.5 and 15 kJ m(-2 )UV-B doses, HO, catalase (CAT, EC 1.11.1.6) and ascorbate peroxidase (APX, EC 1.11.1.11) activities were increased and the production of thiobarbituric acid reactive substances (TBARS) regain control values after 4 h of plant recuperation. Treatment with 30 kJ m(-2) UV-B provoked a decrease in these antioxidant enzyme activities. Immunoblot analysis showed a 4.3 and 3.7-fold increase in HO-1 protein expression after irradiation with 7.5 and 15 kJ m(-2), respectively. HO-1 transcript levels were enhanced (up to 77%) at these doses, as assessed by semi-quantitative RT-PCR. These data demonstrated that increased HO activity was associated with augmented protein expression and transcript levels. Plants pre-treated with the antioxidant ascorbic acid did not show the UV-B-induced up-regulation of HO-1 mRNA, but hydrogen peroxide treatment could mimic this reaction. Our results indicate that HO is up-regulated in a dose-depending manner as a mechanism of cell protection against oxidative damage and that such response occurred as a consequence of HO-1 mRNA enhancement involving ROS.

The role of NADPH oxidase and MAP kinase phosphatase in UV-B-dependent gene expression in Arabidopsis

Plant responses to supplementary UV-B irradiation have been reported to include formation of reactive oxygen species (ROS), hydrogen peroxide, in particular, and regulation by mitogen-activated protein kinase (MAPK) cascades which in turn are fine-tuned by MAPK phosphatases (MKPs). Here we present direct genetic evidence for the involvement of plasma membrane NADPH oxidase, a source of superoxide and hydrogen peroxide in the apoplasts, in UV-B signalling in Arabidopsis thaliana, by analysis of gene expression of the UV-B molecular markers in NADPH oxidase (atrbohD, F and DF) and MAP kinase phosphatase 1 (MKP1) knockout mutants (mkp1). Whereas the NADPH oxidase mutants were affected in UV-B-dependent CHS, PYROA and MEB5.2 gene expression, the mkp1 mutant was affected in the general expression pattern of the pathogenesis-related (PR) and PDF1.2 genes. The results indicate involvement of MKP1 in repressive action on gene expression of more general stress response pathways, similar to those activated by pathogen attack, while NADPH oxidase is involved in quantitative (rather than absolute) regulation of more UV-B-specific genes. The expressions of the molecular markers in the knockout mutant mkp1 and in its complemented lines (lines 6 and 10) were similar, as opposed to the responses of the corresponding wild-type Wassilewskija-4 (Ws-4). Lines 6 and 10 showed much higher MKP1 mRNA than Ws-4 but did not complement the mutant. This suggests a complex dependency of the MAPK phosporylation level of the PR and PDF1.2 genes. Both NADPH oxidase mutants and the mkp1 mutant phenotypically responded to UV-B by growth retardation.

Supplementary ultraviolet-B irradiation reveals differences in stress responses between Arabidopsis thaliana ecotypes

Irradiation of Arabidopsis thaliana ecotypes C24, Wassilewskija (Ws) and Columbia-0 (Col-0) with supplementary ultraviolet-A+B (UV-A+B) radiation revealed ecotype-specific differences in expression of the gene for the pathogenesis-related protein PR-5. C24 showed an increased expression level of PR-5 (5- and 20-fold higher compared with Col-0 and Ws, respectively). Expression of other molecular markers such as CHS (encoding chalcone synthase), MEB5.2 [encoding a gene strongly up-regulated by ultraviolet-B (UV-B)] and PYROA [encoding a pyridoxine (Vitamin B6) biosynthesis enzyme] only showed slight differences between ecotypes. Oxidative stress during UVA+B exposure was monitored by staining for H2O2. This analysis also revealed important ecotype-specific differences. 'H2O2 hot spots' were found in C24, whereas an even distribution of H2O2 was found in Ws and Col-0. Necrotic lesions also appeared on C24 leaves after prolonged UV-B exposure. There was a reverse correlation between the H2O2 steady-state concentration and the PR-5 gene expression; Ws showed the highest level of H2O2 accumulation but the lowest expression level of the PR-5 gene. Furthermore, application of paraquat on the rosettes led to similar PR-5 expression and H2O2 accumulation patterns as were found after UV-A+B irradiation. The observed ecotypic differences were also reflected in a statistically significant UV-B-dependent decrease in biomass, rosette size and leaf area for Ws, but not for C24 and Col-0. Our results show that a significant ecotype-specific genetic variability in general UV-B responses in Arabidopsis exists. Moreover, the signal transduction or gene regulation pathway for PR-5 differs from the other molecular markers used in this study.

UV-B radiation and acclimation in timberline plants

Research has shown that some plants respond to enhanced UV-B radiation by producing smaller and thicker leaves, by increasing the thickness of epidermis and concentration of UV-B absorbing compounds of their surface layers and activation of the antioxidant defence system. The response of high-altitude plants to UV-B radiation in controlled conditions is often less pronounced compared to low-altitude plants, which shows that the alpine timberline plants are adapted to UV-B. These plants may have a simultaneous co-tolerance for several stress factors: acclimation or adaptation to the harsh climate can also increase tolerance to UV-B radiation, and vice versa. On the other hand, alpine timberline plants of northern latitudes may be less protected against increasing UV-B radiation than plants from more southern latitudes and higher elevations due to harsh conditions and weaker preadaptation resulting from lower UV-B radiation exposure. It is evident that more long-term experimental field research is needed in order to study the interaction of climate, soil and UV-B irradiance on the timberline plants.

Effects of UV-B in biological and chemical systems: Equipment for wavelength dependence determination

The thinning of the stratospheric ozone layer has prompted a large number of studies of UV-B-induced effects in biological and chemical systems. The wavelength dependency of such effects is of interest from mechanistic, physiological or economic points of view. Here, we describe an apparatus for determining the wavelength dependency of UV-B effects in biological and chemical systems. The apparatus consists of a high intensity UV radiation source and narrow bandpass filters to produce UV radiation in even intervals (between 280 and 360 nm). The usefulness of the equipment is demonstrated in two different systems: 1) Chalcone synthase (CHS) gene is up-regulated by UV-B radiation. Therefore quantitative analysis of the CHS gene expression was chosen in the present investigation for studies of the wavelength dependency of gene expression regulation in plants. Maximum induction of CHS expression was found at 300 nm with a 12-fold induction compared with the control; 2) The wavelength dependency of formation of dioxin-like photoproducts from the brominated flame retardant decabrominated diphenyl ether (DeBDE) is described. This is an example of UV-B-induced conversion of non-toxic species into a number of products of which some may be toxic in the environment. In the UV interval studied, the highest dioxin-like activity was found in the sample irradiated at 330 nm and therefore this wavelength is most important for the mechanism involved in photoconversion of DeBDE.

Anthocyanin accumulation and changes in CHS and PR-5 gene expression in Arabidopsis thaliana after removal of the inflorescence stem (decapitation)

Anthocyanin accumulation occurs in rosette leaves of Arabidopsis thaliana within 8 days after removal of the inflorescence stem. Expression of stress-induced genes CHS and PR-5 was strongly up-regulated by stem removal, and was highest on day 4 after decapitation. These levels were 10 and 5 times higher than in the control, respectively. No obvious difference was found in expression of LHCB, MEB5.2, or PYROA genes following stem removal. We demonstrate that removing the inflorescence stem triggers events in Arabidopsis, including pigment accumulation and changes in gene expression of a subset of stress-induced genes, in a tissue distant from the wound site.

Ultraviolet-B signalling: Arabidopsis brassinosteroid mutants are defective in UV-B regulated defence gene expression

The involvement of brassinosteroids in signalling events in plants during UV-B stress (280-315 nm) was investigated in Arabidopsis thaliana. Brassinosteroids are involved in growth and development in plants and have also been shown to enhance stress tolerance. Three mutants deficient in the biosynthetic pathway of brassinolide (BL; det2, dim1 and cpd) and the BL insensitive mutant (bri1) were together with visible light irradiated with 3 or 9 h of UV-B radiation (biologically effective radiation normalised to 300 nm being 0.24 W m(-2)). Also, a small size control, irx1, and Columbia 0 (Col-0) wild-type plants were examined under identical conditions. Gene expression patterns were established for these mutants with a set of four molecular markers (the defence genes chalcone synthase (CHS), PYROA, pathogenesis-related protein PR-5, and a gene regulated by very low levels of UV-B, MEB5.2). Although the genes in the brassinodefective mutants were still induced by UV-B radiation, they all also showed reduced levels of mRNA transcripts compared with Col-0 and irx1. The bri1 and cpd were the mutants with lowest levels of molecular marker mRNA transcripts. The effects of impairment of brassinosteroid signalling also differed between the genes studied, indicating a need for a complete brassinosteroid pathway in UV-B signalling.

A Pisum sativum glyoxysomal malate dehydrogenase induced by cadmium exposure

The glyoxysomal malate dehydrogenase (gMDH) catalyses the formation of oxaloacetate from malate during beta-oxidation of fatty acids in the glyoxysome. A partial Pisum sativum L. (cv. Greenfeast) cDNA was first isolated from a suppression subtractive hybridisation cDNA library obtained from heavy metal stressed plants. The full length cDNA was then isolated by rapid amplification of cDNA ends. The translated sequence showed strong similarity to Cucumis sativus and Citrullus lanatus gMDH including a typical glyoxysome-targeting presequence comprising the PTS2 motif and a cleavage site for a cystein-directed protease. Exposure of pea plants to Cd2+ induced expression of the gMDH gene in mature pea leaves indicating that the enzyme is under environmental control in addition to the normal developmental regulation pattern.

Six genes strongly regulated by mercury in Pisum sativum roots

Suppression subtractive hybridisation was used to isolate heavy metal-induced genes from Pisum sativum roots hydroponically exposed to 5 microM HgCl2 and 10 microM EDTA. Six genes were induced out of which one, PsHMIP6B, was novel. The other genes (PsSAMT, PsI2'H, PsNDA, PsAPSR, PsPOD) had not previously been isolated from pea and sequenced. All six genes were also induced after exposure to 5 microM HgCl2 in the absence of EDTA. The induction pattern was in some cases different for the two Hg species, demonstrating a quicker response to-free Hg2+ than Hg-EDTA. The stress-specificity of the gene regulation was investigated by hydroponically adding 5 microM Cd2+. Most Hg-induced cDNAs were also induced by Cd2+ but to a smaller extent than after Hg exposure. In addition, the gene expression was also probed for tissue specificity, which showed that all six genes were expressed in roots and not in leaves.

Gene regulation by low level UV-B radiation: identification by DNA array analysis

UV-B radiation alters transcript levels of various defence genes and photosynthetic genes in plants. Utilising a DNA array with 5000 ESTs and cDNAs from Arabidopsis thaliana, 70 genes were found to show a greater than two-fold induction or repression of transcript levels. Six genes (MEB5.2, PyroA, Ubq3, Lhcb6, F5D21.10 and the gene for an RNA polymerase II subunit) were tested for stress specific gene regulation on northern blots with RNA from plants exposed to low dose UV-B radiation, ozone or wounding. Transcript levels for PyroA, Uhq3 and the gene for a RNA polymerase II subunit were all specifically increased by UV-B. MEB5.2 mRNA levels also rose, whereas Lhcb6 and FSD21.10 transcript levels decreased under all stresses. The PyroA gene product in fungi is needed for biosynthesis of pyridoxine, and might have a role in protection against singlet oxygen. The Ubq3 gene encodes the ubiquitin protein that is attached to proteins destined for degradation. MEB5.2 and F5D21.10 represent novel gene products whose function have not yet been identified. Pairwise comparisons between the UV-B inducible promoters have identified a series of elements present in the MEB5.2 and PyroA promoters, absent from promoters of genes for early phenylpropanoid metabolism and that may be responsible for modulating their UV-B responses.

Regulation of gene expression by low levels of ultraviolet-B radiation in Pisum sativum: isolation of novel genes by suppression subtractive hybridisation

Suppression subtractive hybridisation was used to isolate genes differentially regulated by low levels (UV-B(BE,300) 0.13 W m(-2)) of ultraviolet-B radiation (UV-B; 290-320 nm) in Pisum sativum. Six genes were regulated, two of which were novel. The mRNA levels for these two (PsTSDC and PsUOS1) were increased and depressed by UV-B treatment, respectively. Domains in the PsTSDC translation product was similar to TIR (Toll-Interleukin-1 receptor-similar) domains and a NB-ARC domain (nucleotide-binding domain in APAF-1, R gene products and CED-4). The PsUOS1 translation product was similar to an open reading frame in Arabidopsis. Genes encoding embryo-abundant protein (PsEMB) and S-adenosyl-L-methionine synthase (PsSAMS) were induced by UV-B, whereas the transcript levels for genes encoding sucrose transport protein (PsSUT) or ribulose-5-phosphate 3-epimerase (PsR5P3E) were decreased. These regulation patterns are novel, and the PsEMB and PsR5P3E sequences are reported for the first time. The stress-specificity of regulation of these genes were tested by ozone fumigation (100 ppb O(3)). Qualitatively, the similarity of expression after both UV-B and ozone exposure suggests that, for these genes, similar stress-response pathways are in action.

The glutathione status of mature Scots pines during the third season of UV-B radiation exposure

Impacts of UV-B radiation on the glutathione level were studied in mature Scots pine needles (Pinus sylvestris L.) during the third season of a UV-B field experiment. Studies were made on 4-week-old (July) to 14-week-old (September) current-year needles and 3-year-old needles which had their third UV-B-exposure season in progress. Depending on the season and the year (1996-98), the supplemental UV-B dose varied from 0.92 to 5.09 kJ m-2 day-1 UV-BBE compared to 0.47-2.44 kJ m-2 day-1 UV-BBE under the ambient treatment. Fully grown UV-B-treated current-year needles showed lower total glutathione concentrations after the vegetation period in September, whereas in UV-B-treated 3-year-old needles the total glutathione content was significantly lower and the proportion of oxidized glutathione (GSSG%) 56% higher in July. The significant differences in total glutathione in current-year needles in September and in active 3-year-old needles in July seem to indicate that the effect of enhanced UV-B radiation on glutathione status could be cumulative.

Free amino acids present in commercially available seedlings sold for human consumption. A potential hazard for consumers

The importance of fresh seedlings for human consumption on European markets continues to increase. Although the contents of free amino acids and potentially toxic free nonprotein amino acids in these fresh and supposedly healthy seedlings is very different from those of the seeds, the crude composition is never mentioned on commercial packages. A commercial product containing seven different kinds of fresh seedlings including kamut, adzuki bean, chickpea, mungbean, pinto bean, garden pea, and lentil has been analyzed by HPLC. Per 100 g of fresh product, 548.2 mg of total free amino acids was found, of which 56.7 mg is free nonprotein amino acids including beta-(isoxazolin-5-on-2-yl)alanine, homoserine, and isowillardiine and the plant hormone trigonelline (N-methylnicotinic acid). The highest amounts of free nonprotein amino acids and trigonelline are found in garden pea (28.3 mg/100 g), mungbean (9.59 mg/100 g), and lentil (7.50 mg/100 g) seedlings. Trigonelline is present in all legume seedlings examined.

Transcriptional activation of the parsley chalcone synthase promoter in heterologous pea and yeast systems

Introduction by electroporation of different parsley (Petroselinum crispum) CHS-promoter/beta-glucuronidase(GUS)-reporter constructs into pea (Pisum sativum L.) protoplasts leads to a high constitutive GUS-expression and to the loss of the light-inducibility seen in the homologous parsley protoplast system. These results indicate that Unit 1 of the parsley CHS-promoter is only partly responsible for the GUS-expression detected. Instead, additional cis-elements, which are located downstream within 100 bp from the transcriptional start site, mediate the de-repression in pea protoplasts. In contrast, in yeast (Saccharomyces cerevisiae) cells, the GUS expression from the heterologous CHS/GUS construct is controlled by elements between Unit 1 and -100 bp. In both pea and yeast cells, transcription factors different from those regulating UV-responsiveness in parsley, are probably mediating the constitutive expression from the heterologous construct. The results with pea protoplasts imply that protoplastation of pea leaf cells itself induces de-repression as a result of stress to the protoplasts. This notion was strengthened by the finding that mRNA levels of the endogenous chalcone synthase were drastically increased as the result of the protoplastation procedure.

Molecular markers for UV-B stress in plants: alteration of the expression of four classes of genes in Pisum sativum and the formation of high molecular mass RNA adducts

Sixteen ultraviolet-B radiation-regulated pea genes were identified. Functionally, the corresponding proteins were divided into four groups. (i) Chloroplast-localized proteins. Genes for these proteins were down-regulated, underlining the deleterious effects of UV-B on this organelle. A novel down-regulated photosystem I light-harvesting chlorophyll a/b-binding protein gene (PsLhcA4), was cloned and sequenced. (ii) Protein turnover enzymes. Levels of mature mRNAs for the PU1 and PsUBC4 genes, encoding proteins of the ubiquitin protein degradation pathway, were up- and down-regulated, respectively, implying alteration of plant cell protein content by changes in both gene expression and protein degradation. (iii) Proteins involved in intracellular signalling. Expression of genes for small GTPases, rab and rho homologues, were altered. (iv) Phenylpropanoid or flavonoid biosynthesis. Expression of three genes encoding enzymes in these pathways were up-regulated and one of them, the novel PsC450R1, was cloned and sequenced. Moreover, unexpected high molecular mass psbA RNA adducts were found to appear after UV-B exposure. In addition, a large increase in corresponding high molecular mass adducts were also found for PsLhcA4, and PsUBC4 mRNA and 23S rRNA. These RNA species do not contain protein and probably appear due to cross-linking of two or more RNA molecules, or are the result of UV-B-induced failure of transcription termination.

Cloning, expression, and molecular characterization of a small pea gene family regulated by low levels of ultraviolet B radiation and other stresses

A pea (Pisum sativum) DNA fragment (termed MB3) was isolated by differential display of cDNAs obtained from total leaf RNA of ultraviolet B (UV-B) radiation-treated plants. Longer cDNAs were cloned by rapid amplification of cDNA ends in the 3' to 5' direction. Three different, but very similar, cDNAs were cloned, sadA, sadB, and sadC, the major difference between them being a 36-bp deletion in the coding region of sadB. Southern blotting confirmed the occurrence of at least three genes in the pea genome. Database comparisons of the SAD protein sequences revealed high identity (46%) and similarity (77%) with a putative tomato (Lycopersicon esculentum) short-chain alcohol dehydrogenase. Very low levels of UV-B radiation (the biologically effective radiation normalized to 300 nm = 0.08 W m(-2)) was shown to up-regulate expression, a dose considerably lower than that needed to induce expression of the well-known UV-B defensive chalcone synthase and phenylalanine ammonia lyase genes. RNase protection assay revealed that primarily sadA and sadC mRNA accumulation was enhanced by UV-B. In addition to UV-B irradiation, ozone fumigation, wounding, aluminum stress, and salt stress induced increased transcript levels of the sad genes in pea.

The mRNA-binding ribosomal protein S26 as a molecular marker in plants: molecular cloning, sequencing and differential gene expression during environmental stress

One condition for using a gene as a transcriptional marker for environmental stress is its specific and differential expression. In order to be used as such a marker, the ribosomal protein S26 cDNA from pea (Pisum sativum L.) was cloned and fully sequenced. The gene (PsRPS26) was shown to be differentially regulated by ozone and UV-B radiation in opposite ways. Ozone gave rise to increased mRNA levels, whereas UV-B led to a decrease in S26 transcript abundance. Thus, the expression of PsRPS26 can be used as a molecular marker to differentiate between these two environmental stresses.