Metabolic engineering to simultaneously activate anthocyanin and proanthocyanidin biosynthetic pathways in Nicotiana spp

Proanthocyanidins (PAs), or condensed tannins, are powerful antioxidants that remove harmful free oxygen radicals from cells. To engineer the anthocyanin and proanthocyanidin biosynthetic pathways to de novo produce PAs in two Nicotiana species, we incorporated four transgenes to the plant chassis. We opted to perform a simultaneous transformation of the genes linked in a multigenic construct rather than classical breeding or retransformation approaches. We generated a GoldenBraid 2.0 multigenic construct containing two Antirrhinum majus transcription factors (AmRosea1 and AmDelila) to upregulate the anthocyanin pathway in combination with two Medicago truncatula genes (MtLAR and MtANR) to produce the enzymes that will derivate the biosynthetic pathway to PAs production. Transient and stable transformation of Nicotiana benthamiana and Nicotiana tabacum with the multigenic construct were respectively performed. Transient expression experiments in N. benthamiana showed the activation of the anthocyanin pathway producing a purple color in the agroinfiltrated leaves and also the effective production of 208.5 nmol (-) catechin/g FW and 228.5 nmol (-) epicatechin/g FW measured by the p-dimethylaminocinnamaldehyde (DMACA) method. The integration capacity of the four transgenes, their respective expression levels and their heritability in the second generation were analyzed in stably transformed N. tabacum plants. DMACA and phoroglucinolysis/HPLC-MS analyses corroborated the activation of both pathways and the effective production of PAs in T0 and T1 transgenic tobacco plants up to a maximum of 3.48 mg/g DW. The possible biotechnological applications of the GB2.0 multigenic approach in forage legumes to produce "bloat-safe" plants and to improve the efficiency of conversion of plant protein into animal protein (ruminal protein bypass) are discussed.

Unraveling Salt Tolerance Mechanisms in Halophytes: A Comparative Study on Four Mediterranean Limonium Species with Different Geographic Distribution Patterns

We have performed an extensive study on the responses to salt stress in four related Limonium halophytes with different geographic distribution patterns, during seed germination and early vegetative growth. The aims of the work were twofold: to establish the basis for the different chorology of these species, and to identify relevant mechanisms of salt tolerance dependent on the control of ion transport and osmolyte accumulation. Seeds were germinated in vitro, in the presence of increasing NaCl concentrations, and subjected to "recovery of germination" tests; germination percentages and velocity were determined to establish the relative tolerance and competitiveness of the four Limonium taxa. Salt treatments were also applied to young plants, by 1-month irrigation with NaCl up to 800 mM; then, growth parameters, levels of monovalent and divalent ions (in roots and leaves), and leaf contents of photosynthetic pigments and common osmolytes were determined in control and stressed plants of the four species. Seed germination is the most salt-sensitive developmental phase in Limonium. The different germination behavior of the investigated species appears to be responsible for their geographical range size: L. narbonense and L. virgatum, widespread throughout the Mediterranean, are the most tolerant and the most competitive at higher soil salinities; the endemic L. santapolense and L. girardianum are the most sensitive and more competitive only at lower salinities. During early vegetative growth, all taxa showed a strong tolerance to salt stress, although slightly higher in L. virgatum and L. santapolense. Salt tolerance is based on the efficient transport of Na+ and Cl- to the leaves and on the accumulation of fructose and proline for osmotic adjustment. Despite some species-specific quantitative differences, the accumulation patterns of the different ions were similar in all species, not explaining differences in tolerance, except for the apparent activation of K+ transport to the leaves at high external salinity, observed only in the most tolerant L. narbonense and L. virgatum. This specific response may be therefore relevant for salt tolerance in Limonium. The ecological implications of these results, which can contribute to a more efficient management of salt marshes conservation/regeneration programs, are also discussed.

Drought Tolerance in Pinus halepensis Seed Sources As Identified by Distinctive Physiological and Molecular Markers

Drought is one of the main constraints determining forest species growth, survival and productivity, and therefore one of the main limitations for reforestation or afforestation. The aim of this study is to characterize the drought response at the physiological and molecular level of different Pinus halepensis (common name Aleppo pine) seed sources, previously characterized in field trials as drought-sensitive or drought-tolerant. This approach aims to identify different traits capable of predicting the ability of formerly uncharacterized seedlings to cope with drought stress. Gas-exchange, water potential, photosynthetic pigments, soluble sugars, free amino acids, glutathione and proteomic analyses were carried out on control and drought-stressed seedlings in greenhouse conditions. Gas-exchange determinations were also assessed in field-planted seedlings in order to validate the greenhouse experimental conditions. Drought-tolerant seed sources presented higher values of photosynthetic rates, water use efficiency, photosynthetic pigments and soluble carbohydrates concentrations. We observed the same pattern of variation of photosynthesis rate and maximal efficiency of PSII in field. Interestingly drought-tolerant seed sources exhibited increased levels of glutathione, methionine and cysteine. The proteomic profile of drought tolerant seedlings identified two heat shock proteins and an enzyme related to methionine biosynthesis that were not present in drought sensitive seedlings, pointing to the synthesis of sulfur amino acids as a limiting factor for drought tolerance in Pinus halepensis. Our results established physiological and molecular traits useful as distinctive markers to predict drought tolerance in Pinus halepensis provenances that could be reliably used in reforestation programs in drought prone areas.

Metabolomic profiling of plant tissues

Metabolomics is a powerful discipline aimed at a comprehensive and global analysis of the metabolites present in a cell, tissue, or organism, and to which increasing attention has been paid in the last few years. Given the high diversity in physical and chemical properties of plant metabolites, not a single method is able to analyze them all.Here we describe two techniques for the profiling of two quite different groups of metabolites: polar and semi-polar secondary metabolites, including many of those involved in plant response to biotic and abiotic stress, and volatile compounds, which include those responsible of most of our perception of food flavor. According to these techniques, polar and semi-polar metabolites are extracted in methanol, separated by liquid chromatography (UPLC), and detected by a UV-VIS detector (PDA) and a time-of-flight (ToF) mass spectrometer. Volatile compounds, on the other hand, are extracted by headspace solid phase microextraction (HS-SPME), and separated and detected by gas chromatography coupled to mass spectrometry (GC-MS).

Diversity and relationships in key traits for functional and apparent quality in a collection of eggplant: fruit phenolics content, antioxidant activity, polyphenol oxidase activity, and browning

Eggplant (Solanum melongena) varieties with increased levels of phenolics in the fruit present enhanced functional quality, but may display greater fruit flesh browning. We evaluated 18 eggplant accessions for fruit total phenolics content, chlorogenic acid content, DPPH scavenging activity, polyphenol oxidase (PPO) activity, liquid extract browning, and fruit flesh browning. For all the traits we found a high diversity, with differences among accessions of up to 3.36-fold for fruit flesh browning. Variation in total content in phenolics and in chlorogenic acid content accounted only for 18.9% and 6.0% in the variation in fruit flesh browning, and PPO activity was not significantly correlated with fruit flesh browning. Liquid extract browning was highly correlated with chlorogenic acid content (r = 0.852). Principal components analysis (PCA) identified four groups of accessions with different profiles for the traits studied. Results suggest that it is possible to develop new eggplant varieties with improved functional and apparent quality.

A noncoding plant pathogen provokes both transcriptional and posttranscriptional alterations in tomato

Viroids are single-stranded, circular, noncoding RNAs that infect plants, causing devastating diseases. In this work, we employed 2D DIGE, followed by MS identification, to analyze the response of tomato plants infected by Citrus exocortis viroid (CEVd). Among the differentially expressed proteins detected, 45 were successfully identified and classified into different functional categories. Validation results by RT-PCR allowed us to classify the proteins into two expression groups. First group included genes with changes at the transcriptional level upon CEVd infection, such as an endochitinase, a β-glucanase, and pathogenesis-related proteins, PR10 and P69G. All these defense proteins were also induced by gentisic acid, a pathogen-induced signal in compatible interactions. The second group of proteins showed no changes at the transcriptional level and included several ribosomal proteins and translation factors, such as the elongation factors 1 and 2 and the translation initiation factor 5-alpha. These results were validated by 2D Western blot, and possible PTMs caused by CEVd infection were detected. Moreover, an interaction between eukaryotic elongation factor 1 and CEVd was observed by 2D Northwestern. The present study provides new protein-related information on the mechanisms of plant resistance to pathogens.

Shared and novel molecular responses of mandarin to drought

Drought is the most important stress experienced by citrus crops. A citrus cDNA microarray of about 6.000 genes has been utilized to identify transcriptomic responses of mandarin to water stress. As observed in other plant species challenged with drought stress, key genes for lysine catabolism, proline and raffinose synthesis, hydrogen peroxide reduction, vacuolar malate transport, RCI2 proteolipids and defence proteins such as osmotin, dehydrins and heat-shock proteins are induced in mandarin. Also, some aquaporin genes are repressed. The osmolyte raffinose could be detected in stressed roots while the dehydrin COR15 protein only accumulated in stressed leaves but not in roots. Novel drought responses in mandarin include the induction of genes encoding a new miraculin isoform, chloroplast beta-carotene hydroxylase, oleoyl desaturase, ribosomal protein RPS13A and protein kinase CTR1. These results suggest that drought tolerance in citrus may benefit from inhibition of proteolysis, activation of zeaxanthin and linolenoyl synthesis, reinforcement of ribosomal structure and down-regulation of the ethylene response.

Accumulation of gentisic acid as associated with systemic infections but not with the hypersensitive response in plant-pathogen interactions

In the present work we have studied the accumulation of gentisic acid (2,5-dihydroxybenzoic acid, a metabolic derivative of salicylic acid, SA) in the plant-pathogen systems, Cucumis sativus and Gynura aurantiaca, infected with either prunus necrotic ringspot virus (PNRSV) or the exocortis viroid (CEVd), respectively. Both pathogens produced systemic infections and accumulated large amounts of the intermediary signal molecule gentisic acid as ascertained by electrospray ionization mass spectrometry (ESI-MS) coupled on line with high performance liquid chromatography (HPLC). The compound was found mostly in a conjugated (beta-glucoside) form. Gentisic acid has also been found to accumulate (although at lower levels) in cucumber inoculated with low doses of Pseudomonas syringae pv. tomato, producing a nonnecrotic reaction. In contrast, when cucumber was inoculated with high doses of this pathogen, a hypersensitive reaction occurred, but no gentisic-acid signal was induced. This is consistent with our results supporting the idea that gentisic-acid signaling may be restricted to nonnecrotizing reactions of the host plant (Bellés et al. in Mol Plant-Microbe Interact 12:227-235, 1999). In cucumber and Gynura plants, the activity of gentisic acid as inducing signal was different to that of SA, thus confirming the data found for tomato. Exogenously supplied gentisic acid was able to induce peroxidase activity in both Gynura and cucumber plants in a similar way as SA or pathogens. However, gentisic-acid treatments strongly induced polyphenol oxidase activity in cucumber, whereas pathogen infection or SA treatment resulted in a lower induction of this enzyme. Nevertheless, gentisic acid did not induce other defensive proteins which are induced by SA in these plants. This indicates that gentisic acid could act as an additional signal to SA for the activation of plant defenses in cucumber and Gynura plants.

Lithium treatment induces a hypersensitive-like response in tobacco

Treatment of tobacco ( Nicotiana tabacum L.) plants with lithium induces the formation of necrotic lesions and leaf curling as in the case of incompatible pathogen interactions. Further similarities at the molecular level include accumulation of ethylene and of salicylic and gentisic acids, and induced expression of pathogenesis-related PR-P, PR5 and PR1 genes. With the exception of PR1 induction, lithium produced the same effects in transgenic tobacco plants that do not accumulate salicylate because of overexpression of the bacterial hydroxylase gene nahG. On the other hand, inhibition of ethylene biosynthesis with aminoethoxyvinylglycine prevented lithium-induced cell death and PR5 expression. These results suggest that lithium triggers a hypersensitive-like response where ethylene signalling is essential.

Yeast inositol mono- and trisphosphate levels are modulated by inositol monophosphatase activity and nutrients

Yeast lithium-sensitive inositol monophosphatase (IMPase) is encoded by a non-essential gene pair (IMP1 and IMP2). Inhibition of IMPase with either Li(+) or Na(+) or a double null mutation imp1 imp2 causes increased levels of inositol monophosphates and reduced level of inositol 1,4,5-trisphosphate. Overexpression of the IMP2 gene has the opposite effects and these results suggest that IMPase activity is limiting for the inositol cycle. Addition of ammonium to cells starved for this nutrient results in a decrease of inositol monophosphates and an increase of inositol 1,4,5-triphosphate, pointing to simultaneous regulation of both inositol 1,4,5-triphosphate production and IMPase activity.

The short-chain alcohol dehydrogenase ABA2 catalyzes the conversion of xanthoxin to abscisic aldehyde

Mutants able to germinate and perform early growth in medium containing a high NaCl concentration were identified during the course of two independent screenings and named salt resistant (sre) and salobreño (sañ). The sre and sañ mutants also were able to germinate in high-osmoticum medium, indicating that they are osmotolerant in a germination assay. Complementation analyses revealed that sre1-1, sre1-2, sañ3-1, and sañ3-2 were alleles of the abscisic acid (ABA) biosynthesis ABA2 gene. A map-based cloning strategy allowed the identification of the ABA2 gene and molecular characterization of four new aba2 alleles. The ABA2 gene product belongs to the family of short-chain dehydrogenases/reductases, which are known to be NAD- or NADP-dependent oxidoreductases. Recombinant ABA2 protein produced in Escherichia coli exhibits a K(m) value for xanthoxin of 19 micro M and catalyzes in a NAD-dependent manner the conversion of xanthoxin to abscisic aldehyde, as determined by HPLC-mass spectrometry. The ABA2 mRNA is expressed constitutively in all plant organs examined and is not upregulated in response to osmotic stress. The results of this work are discussed in the context of previous genetic and biochemical evidence regarding ABA biosynthesis, confirming the xanthoxin-->abscisic aldehyde-->ABA transition as the last steps of the major ABA biosynthetic pathway.

A novel target of lithium therapy

Phosphatases converting 3'-phosphoadenosine 5'-phosphate (PAP) into adenosine 5'-phosphate are of fundamental importance in living cells as the accumulation of PAP is toxic to several cellular systems. These enzymes are lithium-sensitive and we have characterized a human PAP phosphatase as a potential target of lithium therapy. A cDNA encoding a human enzyme was identified by data base screening, expressed in Escherichia coli and the 33 kDa protein purified to homogeneity. The enzyme exhibits high affinity for PAP (K(m)<1 microM) and is sensitive to subtherapeutic concentrations of lithium (IC(50)=0.3 mM). The human enzyme also hydrolyzes inositol-1, 4-bisphosphate with high affinity (K(m)=0.4 microM), therefore it can be considered as a dual specificity enzyme with high affinity (microM range) for both PAP and inositol-1,4-bisphosphate. Hydrolysis of inositol-1,4-bisphosphate was also inhibited by lithium (IC(50)=0.6 mM). Thus, we present experimental evidence for a novel target of lithium therapy, which could explain some of the side effects of this therapy.

Arabidopsis thaliana AtHAL3: a flavoprotein related to salt and osmotic tolerance and plant growth

We have isolated two Arabidopsis thaliana genes, AtHAL3a and AtHAL3b, showing homology with HAL3, a yeast protein which regulates the cell cycle and tolerance to salt stress through inhibition of the PPZ1 type-1 protein phosphatase. Expression of AtHAL3a in yeast hal3 mutants partially complements their LiCl sensitivity, suggesting possible conserved functions between both proteins. AtHAL3a and AtHAL3b are induced by salt stress and AtHAL3a is the most expressed in non-stressed plants, particularly in seeds. In situ hybridization demonstrates enrichment of AtHAL3a mRNA in seed embryos and in the vascular phloem of different plant tissues. AtHAL3 proteins show striking homology with a group of proteins found in fungi, plants and animals and some homology with a large family of prokaryotic flavoproteins. Recombinant AtHAL3a protein purified from Escherichia coli was yellow because it contained a non-covalently bound chromophore revealed as flavin mononucleotide. Trans- genic Arabidopsis plants, with gain of AtHAL3a function, show altered growth rates and improved tolerance to salt and osmotic stress.

A novel mammalian lithium-sensitive enzyme with a dual enzymatic activity, 3'-phosphoadenosine 5'-phosphate phosphatase and inositol-polyphosphate 1-phosphatase

We report the molecular cloning in Rattus norvegicus of a novel mammalian enzyme (RnPIP), which shows both 3'-phosphoadenosine 5'-phosphate (PAP) phosphatase and inositol-polyphosphate 1-phosphatase activities. This enzyme is the first PAP phosphatase characterized at the molecular level in mammals, and it represents the first member of a novel family of dual specificity enzymes. The phosphatase activity is strictly dependent on Mg2+, and it is inhibited by Ca2+ and Li+ ions. Lithium chloride inhibits the hydrolysis of both PAP and inositol-1,4-bisphosphate at submillimolar concentration; therefore, it is possible that the inhibition of the human homologue of RnPIP by lithium ions is related to the pharmacological action of lithium. We propose that the PAP phosphatase activity of RnPIP is crucial for the function of enzymes sensitive to inhibition by PAP, such as sulfotransferase and RNA processing enzymes. Finally, an unexpected connection between PAP and inositol-1,4-bisphosphate metabolism emerges from this work.

The Arabidopsis HAL2-like gene family includes a novel sodium-sensitive phosphatase

The yeast HAL2 gene encodes a lithium- and sodium-sensitive phosphatase that hydrolyses 3'-phosphoadenosine-5'-phosphate (PAP). Salt toxicity in yeast results from Hal2 inhibition and accumulation of PAP, which inhibits sulphate assimilation and RNA processing. We have investigated whether the model plant Arabidopsis thaliana contains sodium-sensitive PAP phosphatases. The Arabidopsis HAL2-like gene family is composed of three members: AtAHL and AtSAL2, characterized in the present work, and the previously identified AtSAL1. The AtAHL and AtSAL2 cDNAs complement the auxotrophy for methionine of the yeast hal2 mutant and the recombinant proteins catalyse the conversion of PAP to AMP in a Mg(2+)-dependent reaction sensitive to inhibition by Ca2+ and Li+. The PAP phosphatase activity of AtAHL is sensitive to physiological concentrations of Na+, whereas the activities of AtSAL1 and AtSAL2 are not. Another important difference is that AtAHL is very specific for PAP while AtSAL1 and AtSAL2 also act as inositol polyphosphate 1-phosphatases. AtAHL constitutes a novel type of sodium-sensitive PAP phosphatase which could act co-ordinately with plant sulphotransferases and serve as target of salt toxicity in plants.

Expression of the yeast trehalose-6-phosphate synthase gene in transgenic tobacco plants: pleiotropic phenotypes include drought tolerance

The yeast trehalose-6-phosphate synthase gene (TPS1) was engineered under the control of the cauliflower mosaic virus regulatory sequences (CaMV35S) for expression in plants. Using Agrobacterium-mediated transfer, the gene was incorporated into the genomic DNA and constitutively expressed in Nicotiana tabacum L. plants. Trehalose was determined in the transformants, by anion-exchange chromatography coupled to pulsed amperometric detection. The non-reducing disaccharide accumulated up to 0.17 mg per g fresh weight in leaf extracts of transgenic plants. Trehaloseaccumulating plants exhibited multiple phenotypic alterations, including stunted growth, lancet-shaped leaves, reduced sucrose content and improved drought tolerance. These pleiotropic effects, and the fact that water loss from detached leaves was not significantly affected by trehalose accumulation, suggest that synthesis of this sugar, rather than leading to an osmoprotectant effect, had altered sugar metabolism and regulatory pathways affecting plant development and stress tolerance.

The yeast HAL2 nucleotidase is an in vivo target of salt toxicity

The yeast halotolerance gene HAL2 encodes a nucleotidase that dephosphorylates 3'-phosphoadenosine 5'-phosphate (PAP) and 3'-phosphoadenosine 5'-phosphosulfate (PAPS), intermediates of the sulfate assimilation pathway. This nucleotidase is inhibited by Na+ and Li+ but not by K+. Incubation of wild-type yeast cells with NaCl and LiCl, but not with KCl, increased intracellular PAP to millimolar concentrations. No depletion of the pool of adenine nucleotides (AMP, ADP, ATP) was observed. Other stresses such as heat shock or oxidative stress did not result in PAP accumulation. PAPS concentrations also increased during salt stress but remained lower than 0.5 microM. S-Adenosylmethionine concentrations decreased by 50%, reflecting inhibition of sulfate assimilation during salt stress. Salt-induced PAP accumulation was attenuated in a yeast strain overexpressing HAL2. This strain grew better than the wild type under salt stress. These results suggest that the cation sensitivity of the HAL2 nucleotidase is an important determinant of the inhibition of yeast growth by sodium and lithium salts. In addition to blocking sulfate assimilation by product inhibition of PAPS reductase, PAP accumulation may have other unidentified toxic effects.

A salt-sensitive 3'(2'),5'-bisphosphate nucleotidase involved in sulfate activation

Overexpression of a yeast gene, HAL2, allows the cells to tolerate higher than normal extracellular salt concentrations. HAL2 encodes a 3'(2')5'-bisphosphate nucleotidase that serves to remove the end products of sulfate transfer during cellular metabolism. The enzyme is inhibited by lithium and sodium and is activated by potassium. Metabolic systems that are sensitive to salt, as well as those governing osmolyte synthesis and ion transport, offer routes by which genetic engineering can be used to improve the tolerance of various organisms to salt.

Pathogenesis-related proteins and polyamines in a developmental mutant of tomato, epinastic

The polyamine level and the accumulation of pathogenesis-related (PR) proteins were studied in the ethylene overproducing Epinastic (Epi) tomato (Lycopersicon esculentum Mill.) mutant, as compared with its parent, cv VFN8. Neither a decreased putrescine level nor an enhanced production of PR proteins were detected in Epi, contrary to what could be expected from our previous studies (JM Bellés, J Carbonell, V Conejero [1991] Plant Physiol 96: 1053-1059). However, treatment with the ethylene-releasing compound 2-chloroethylphosphonic acid (ethephon) or silver nitrate at high doses induced a decrease in putrescine content and an enhancing of the synthesis of PR proteins in Epi as ascertained by immunoblot analysis using antisera raised against Rutgers tomato PR proteins.

Polyamines in plants infected by citrus exocortis viroid or treated with silver ions and ethephon

The levels of polyamines in leaves of Gynura aurantiaca DC and tomato, Lycopersicon esculentum Mill. cv Rutgers, infected with citrus exocortis viroid (CEVd) or treated with silver nitrate or ethephon (2-chloroethylphosphonic acid) were measured by HPLC in relation to development of symptoms. Previously it had been demonstrated that treatment of G. aurantiaca plants with silver nitrate or ethephon closely mimicked the effects of viroid infection in the plants. In the studies reported here, a marked decrease in putrescine level was observed in plants infected by CEVd or treated with silver ions or ethephon. There was no significant change in either spermidine or spermine levels. Treatment of G. aurantiaca plants with specific inhibitors of ethylene biosynthesis (aminoethoxyvinylglycine, Co(2+)) or ethylene action (norbornadiene) prevented the decrease of putrescine associated with silver nitrate treatment and had no effect on spermidine or spermine levels. The development of viroid-like symptoms, the production of associated pathogenesis-related proteins, and the rise in protease activity induced by silver nitrate, were all suppressed by exogenous application of putrescine. The decreased level of putrescine as an ethylene-mediated step in the transduction of the viroid and silver or ethephon signaling is discussed.