Antisense suppression of proline degradation improves tolerance to freezing and salinity in Arabidopsis thaliana

Sterol glycosyltransferases--the enzymes that modify sterols

Sterols are important components of cell membranes, hormones, signalling molecules and defense-related biotic and abiotic chemicals. Sterol glycosyltransferases (SGTs) are enzymes involved in sterol modifications and play an important role in metabolic plasticity during adaptive responses. The enzymes are classified as a subset of family 1 glycosyltransferases due to the presence of a signature motif in their primary sequence. These enzymes follow a compulsory order sequential mechanism forming a ternary complex. The diverse applications of sterol glycosides, like cytotoxic and apoptotic activity, anticancer activity, medicinal values, anti-stress roles and anti-insect and antibacterial properties, draws attention towards their synthesis mechanisms. Many secondary metabolites are derived from sterol pathways, which are important in defense mechanisms against pathogens. SGTs in plants are involved in changed sensitivity to stress hormones and their agrochemical analogs and changed tolerance to biotic and abiotic stresses. SGTs that glycosylate steroidal hormones, such as brassinosteroids, function as growth and development regulators in plants. In terms of metabolic roles, it can be said that SGTs occupy important position in plant metabolism and may offer future tools for crop improvement.

Alleviation of cold stress in inoculated wheat (Triticum aestivum L.) seedlings with psychrotolerant Pseudomonads from NW Himalayas

Twelve psychrotolerant Pseudomonad strains were selected on the basis of various plant growth-promoting (PGP) activities at cold temperature (4°C). The effect of inoculation with Pseudomonad strains on cold alleviation and growth of wheat seedling at cold temperature (8°C) was investigated under greenhouse condition. Inoculation with Pseudomonad strains significantly enhanced root/shoot biomass and nutrients uptake as compared to non-bacterized control at 60 days of plant growth. Bacterization significantly improved the level of cellular metabolites like chlorophyll, anthocyanin, free proline, total phenolics, starch content, physiologically available iron, proteins, and amino acids that are sign of alleviation of cold stress in wheat plants. Increased relative water content, reduced membrane injury (electrolyte leakage), and Na(+)/K(+) ratio were also recorded in bacterized wheat plants. Electrolyte leakage and Na(+)/K(+) were found inversely proportional to plant growth at cold temperature. Statistical analysis of twenty-three measured parameters revealed that uninoculated control was under cold stress while eight bacterial strains were positively alleviating cold stress in wheat plants. Thus, the psychrotrophic Pseudomonad strains could effectively provide a promising solution to overcome cold stress, which is major factor hindering wheat productivity under cold climatic condition.

Proteomics analysis reveals post-translational mechanisms for cold-induced metabolic changes in Arabidopsis

Cold-induced changes of gene expression and metabolism are critical for plants to survive freezing. Largely by changing gene expression, exposure to a period of non-freezing low temperatures increases plant tolerance to freezing-a phenomenon known as cold acclimation. Cold also induces rapid metabolic changes, which provide instant protection before temperature drops below freezing point. The molecular mechanisms for such rapid metabolic responses to cold remain largely unknown. Here, we use two-dimensional difference gel electrophoresis (2-D DIGE) analysis of sub-cellular fractions of Arabidopsis thaliana proteome coupled with spot identification by tandem mass spectrometry to identify early cold-responsive proteins in Arabidopsis. These proteins include four enzymes involved in starch degradation, three HSP100 proteins, several proteins in the tricarboxylic acid cycle, and sucrose metabolism. Upon cold treatment, the Disproportionating Enzyme 2 (DPE2), a cytosolic transglucosidase metabolizing maltose to glucose, increased rapidly in the centrifugation pellet fraction and decreased in the soluble fraction. Consistent with cold-induced inactivation of DPE2 enzymatic activity, the dpe2 mutant showed increased freezing tolerance without affecting the C-repeat binding transcription factor (CBF) transcriptional pathway. These results support a model that cold-induced inactivation of DPE2 leads to rapid accumulation of maltose, which is a cold-induced compatible solute that protects cells from freezing damage. This study provides evidence for a key role of rapid post-translational regulation of carbohydrate metabolic enzymes in plant protection against sudden temperature drop.

Root-specific transcript profiling of contrasting rice genotypes in response to salinity stress

Elevated salinity imposes osmotic and ion toxicity stresses on living cells and requires a multitude of responses in order to enable plant survival. Building on earlier work profiling transcript levels in rice (Oryza sativa) shoots of FL478, a salt-tolerant indica recombinant inbred line, and IR29, a salt-sensitive cultivar, transcript levels were compared in roots of these two accessions as well as in the roots of two additional salt-tolerant indica genotypes, the landrace Pokkali and the recombinant inbred line IR63731. The aim of this study was to compare transcripts in the sensitive and the tolerant lines in order to identify genes likely to be involved in plant salinity tolerance, rather than in responses to salinity per se. Transcript profiles of several gene families with known links to salinity tolerance are described (e.g. HKTs, NHXs). The putative function of a set of genes identified through their salt responsiveness, transcript levels, and/or chromosomal location (i.e. underneath QTLs for salinity tolerance) is also discussed. Finally, the parental origin of the Saltol region in FL478 is further investigated. Overall, the dataset presented appears to be robust and it seems likely that this system could provide a reliable strategy for the discovery of novel genes involved in salinity tolerance.

Mechanisms independent of abscisic acid (ABA) or proline feedback have a predominant role in transcriptional regulation of proline metabolism during low water potential and stress recovery

Proline accumulation in response to abiotic stress is controlled partially by transcriptional regulation of key enzymes including Δ¹-pyrroline-carboxylate synthetase1 (P5CS1), proline dehydrogenase (ProDH), ornithine amino transferase (OAT) and Δ¹-pyrroline-carboxylate dehydrogenase (P5CDH). For these genes, the role of abscisic acid (ABA), role of feedback regulation by high proline and the mechanisms of gene regulation upon stress release remain unclear. An ABA-deficient (aba2-1) mutant, mutants deficient in proline accumulation (p5cs1), as well as double mutants deficient in both, were used to determine the importance of these factors in transcriptional regulation of proline metabolism. Upregulation of P5CS1 by low water potential was less dependent on ABA than that of stress-marker genes used for comparison. ProDH downregulation by low water potential and upregulation by stress release was not impaired in aba2-1, p5cs1 or p5cs1/aba2-1 compared with wild type despite differing ABA and proline levels in these mutants. Thus, ProDH is a model for characterization of novel regulatory mechanisms associated with low water potential and stress recovery. Both OAT and P5CDH were upregulated during low water potential. This contrasts with previous salt stress experiments and raises questions about the flux of metabolites through proline metabolism under low water potential when high levels of proline accumulate.

Isolation and expression analysis of salt induced genes from contrasting grapevine (Vitis vinifera L.) cultivars

Salt stress adversely affects the growth of grapevine plants. In order to understand the molecular basis of salt stress response in grapevine plants, suppression subtractive hybridization (SSH) and microarray based screening approaches were combined. Two leaf-specific subtractive cDNA libraries were constructed from grapevine plants subjected to a moderate, incremental salt stress treatment. SSH were performed 6h and 24h after NaCl peaked at 100mM using cDNAs prepared from leaves of a salt tolerant cultivar (Razegui) as testers and cDNAs from unstressed leaves as drivers. Then, a pre-screened subset of cDNA clones from these SSH libraries were used to construct a Vitis vinifera cDNA array, in order to verify the expression changes of the genes upon salt treatment. Expression profiles were compared between the salt tolerant and a susceptible cultivar (Syrah) under both control conditions and after salt stress treatment. Seven cDNA clones were identified which were up-regulated by salt stress in two independent growth experiments and confirmed by RNA blot analysis. The transcript expression patterns of the selected genes differed between the contrasting grapevine cultivars tested with respect to stress-regulation. The possible relationship of individual cDNAs with salinity tolerance mechanisms is discussed.

Pleiotropy, plasticity, and the evolution of plant abiotic stress tolerance

Progress in understanding the mechanisms of adaptive plant abiotic stress response has historically come from two separate fields. Molecular biologists employ mutagenic screens, experimental manipulations, and controlled stress treatment to identify genes that, when perturbed, have fairly large effects on phenotype. By contrast, quantitative and evolutionary geneticists generally study naturally occurring variants to inform multigenic models of trait architecture in an effort to predict, for example, the evolutionary response to selection. We discuss five emerging themes from the molecular study of osmotic stress response: the multigenic nature of adaptive response, the modular organization of response to specific cues, the pleiotropic effects of key signaling proteins, the integration of many environmental signals, and the abundant cross-talk between signaling pathways. We argue that these concepts can be incorporated into existing models of trait evolution and provide examples of what may constitute the molecular basis of plasticity and evolvability of abiotic stress response. We conclude by considering future directions in the study of the functional molecular evolution of abiotic stress response that may facilitate new discoveries in molecular biology, evolutionary studies, and plant breeding.

Root growth inhibition by NH(4)(+) in Arabidopsis is mediated by the root tip and is linked to NH(4)(+) efflux and GMPase activity

Root growth in higher plants is sensitive to excess ammonium (NH(4)(+)). Our study shows that contact of NH(4)(+) with the primary root tip is both necessary and sufficient to the development of arrested root growth under NH(4)(+) nutrition in Arabidopsis. We show that cell elongation and not cell division is the principal target in the NH(4)(+) inhibition of primary root growth. Mutant and expression analyses using DR5:GUS revealed that the growth inhibition is furthermore independent of auxin and ethylene signalling. NH(4)(+) fluxes along the primary root, measured using the Scanning Ion-selective Electrode Technique, revealed a significant stimulation of NH(4)(+) efflux at the elongation zone following treatment with elevated NH(4)(+), coincident with the inhibition of root elongation. Stimulation of NH(4)(+) efflux and inhibition of cell expansion were significantly more pronounced in the NH(4)(+)-hypersensitive mutant vtc1-1, deficient in the enzyme GDP-mannose pyrophosphorylase (GMPase). We conclude that both restricted transmembrane NH(4)(+) fluxes and proper functioning of GMPase in roots are critical to minimizing the severity of the NH(4)(+) toxicity response in Arabidopsis.

Metabolic response to cold and freezing of Osteospermum ecklonis overexpressing Osmyb4

The constitutive expression of the rice Osmyb4 gene in Arabidopsis plants gives rise to enhanced abiotic and biotic stress tolerance, probably by activating several stress-inducible pathways. However, the effect of Osmyb4 on stress tolerance likely depends on the genetic background of the transformed species. In this study, we explored the potential of Osmyb4 to enhance the cold and freezing tolerance of Osteospermum ecklonis, an ornamental and perennial plant native to South Africa, because of an increasing interest in growing this species in Europe where winter temperatures are low. Transgenic O. ecklonis plants were obtained through transformation with the Osmyb4 rice gene under the control of the CaMV35S promoter. We examined the phenotypic adaptation of transgenic plants to cold and freezing stress. We also analysed the ability of wild-type and transgenic Osteospermum to accumulate several solutes, such as proline, amino acids and sugars. Using nuclear magnetic resonance, we outlined the metabolic profile of this species under normal growth conditions and under stress for the first time. Indeed, we found that overexpression of Osmyb4 improved the cold and freezing tolerance and produced changes in metabolite accumulation, especially of sugars and proline. Based on our data, it could be of agronomic and economic interest to use this gene to produce Osteospermum plants capable of growing in open field, even during the winter season in climatic zone Z9.

Proline: a multifunctional amino acid

Proline accumulates in many plant species in response to environmental stress. Although much is now known about proline metabolism, some aspects of its biological functions are still unclear. Here, we discuss the compartmentalization of proline biosynthesis, accumulation and degradation in the cytosol, chloroplast and mitochondria. We also describe the role of proline in cellular homeostasis, including redox balance and energy status. Proline can act as a signaling molecule to modulate mitochondrial functions, influence cell proliferation or cell death and trigger specific gene expression, which can be essential for plant recovery from stress. Although the regulation and function of proline accumulation are not yet completely understood, the engineering of proline metabolism could lead to new opportunities to improve plant tolerance of environmental stresses.

Proline metabolism and its implications for plant-environment interaction

Proline has long been known to accumulate in plants experiencing water limitation and this has driven studies of proline as a beneficial solute allowing plants to increase cellular osmolarity during water limitation. Proline metabolism also has roles in redox buffering and energy transfer and is involved in plant pathogen interaction and programmed cell death. Some of these unique roles of proline depend on the properties of proline itself, whereas others depend on the "proline cycle" of coordinated proline synthesis in the chloroplast and cytoplasm with proline catabolism in the mitochondria. The regulatory mechanisms controlling proline metabolism, intercellular and intracellular transport and connections of proline to other metabolic pathways are all important to the in vivo functions of proline metabolism. Connections of proline metabolism to the oxidative pentose phosphate pathway and glutamate-glutamine metabolism are of particular interest. The N-acetyl glutamate pathway can also produce ornithine and, potentially, proline but its role and activity are unclear. Use of model systems such as Arabidopsis thaliana to better understand both these long studied and newly emerging functions of proline can help in the design of next-generation experiments testing whether proline metabolism is a promising metabolic engineering target for improving stress resistance of economically important plants.

Methods for determination of proline in plants

Accumulation of proline in higher plants is an indication of disturbed physiological condition, triggered by biotic or abiotic stress condition. Free proline content can increase upon exposure of plants to drought, salinity, cold, heavy metals, or certain pathogens. Determination of free proline levels is a useful assay to monitor physiological status and to assess stress tolerance of higher plants. Here we describe three methods suitable for determination of free proline content. The isatin paper assay is simple and is suitable to assay proline content in large number of samples. The colorimetric measurement is quantitative and provides reliable data about proline content. The HPLC-based amino acid analysis can be employed when concentration of all amino acids has to be compared.

Implementation of two high through-put techniques in a novel application: detecting point mutations in large EMS mutated plant populations

BACKGROUND

The establishment of mutant populations together with the strategies for targeted mutation detection has been applied successfully to a large number of organisms including many species in the plant kingdom. Considerable efforts have been invested into research on tomato as a model for berry-fruit plants. With the progress of the tomato sequencing project, reverse genetics becomes an obvious and achievable goal.

RESULTS

Here we describe the treatment of Solanum lycopersicum seeds with 1% EMS and the development of a new mutated tomato population. To increase targeted mutant detection throughput an automated seed DNA extraction has been combined with novel mutation detection platforms for TILLING in plants. We have adapted two techniques used in human genetic diagnostics: Conformation Sensitive Capillary Electrophoresis (CSCE) and High Resolution DNA Melting Analysis (HRM) to mutation screening in DNA pools. Classical TILLING involves critical and time consuming steps such as endonuclease digestion reactions and gel electrophoresis runs. Using CSCE or HRM, the only step required is a simple PCR before either capillary electrophoresis or DNA melting curve analysis. Here we describe the development of a mutant tomato population, the setting up of two polymorphism detection platforms for plants and the results of the first screens as mutation density in the populations and estimation of the false-positives rate when using HRM to screen DNA pools.

CONCLUSION

These results demonstrate that CSCE and HRM are fast, affordable and sensitive techniques for mutation detection in DNA pools and therefore allow the rapid identification of new allelic variants in a mutant population. Results from the first screens indicate that the mutagen treatment has been effective with an average mutation detection rate per diploid genome of 1.36 mutation/kb/1000 lines.

Nitric reductase-dependent nitric oxide production is involved in cold acclimation and freezing tolerance in Arabidopsis

Nitric oxide (NO) is an important signaling molecule involved in many physiological processes in plants. We evaluated the role of NO in cold acclimation and freezing tolerance using Arabidopsis (Arabidopsis thaliana) wild type and mutants nia1nia2 (for nitrate reductase [NR]-defective double mutant) and Atnoa1/rif1 (for nitric oxide associated1/resistant to inhibition by fosmidomycin1) that exhibit defects in NR and reduced NO production, respectively. Cold acclimation induced an increase in endogenous NO production in wild-type and Atnoa1/rif1 leaves, while endogenous NO level in nia1nia2 leaves was lower than in wild-type ones and was little changed during cold acclimation. Cold acclimation stimulated NR activity and induced up-regulation of NIA1 gene expression. In contrast, cold acclimation reduced the quantity of NOA1/RIF1 protein and inhibited NO synthase (NOS) activity. These results indicate that up-regulation of NR-dependent NO synthesis underpins cold acclimation-induced NO production. Seedlings of nia1nia2 were less tolerant to freezing than wild-type plants. Pharmacological studies using NR inhibitor, NO scavenger, and NO donor showed that NR-dependent NO level was positively correlated with freezing tolerance. Furthermore, cold acclimation up- and down-regulated expression of P5CS1 and ProDH genes, respectively, resulting in enhanced accumulation of proline (Pro) in wild-type plants. The stimulation of Pro accumulation by cold acclimation was reduced by NR inhibitor and NO scavenger, while Pro accumulation by cold acclimation was not affected by the NOS inhibitor. In contrast to wild-type plants, cold acclimation up-regulated ProDH gene expression in nia1nia2 plants, leading to less accumulation in nia1nia2 plants than in wild-type plants. These findings demonstrate that NR-dependent NO production plays an important role in cold acclimation-induced increase in freezing tolerance by modulating Pro accumulation in Arabidopsis.

Molecular characterization of ThIPK2, an inositol polyphosphate kinase gene homolog from Thellungiella halophila, and its heterologous expression to improve abiotic stress tolerance in Brassica napus

Inositol polyphosphate kinases play important roles in diverse cellular processes. In this study, the function of an inositol polyphosphate kinase gene homolog named ThIPK2 from a dicotyledonous halophyte Thellungiella halophila was investigated. The deduced translation product (ThIPK2) shares 85% identity with the Arabidopsis inositol polyphosphate kinase AtIPK2beta. Transient expression of ThIPK2-YFP fusion protein in tobacco (Nicotiana tabacum) protoplasts indicates that the protein is localized to the nucleus and plasma membrane, with a minor localization to the cytosol. Heterologous expression of ThIPK2 in ipk2Delta (also known as arg82Delta), a yeast mutant strain that lacks inositol polyphosphate multikinase (Ipk2) activity, rescued the mutant's salt-, osmotic- and temperature-sensitive growth defects. Semi-quantitative reverse transcription polymerase chain reaction (RT-PCR) revealed ubiquitous expression of ThIPK2 in various tissues, including roots, rosette leaves, cauline leaves, stem, flowers and siliques, and shoot ThIPK2 transcript was strongly induced by NaCl or mannitol in T. halophila as exhibited by real-time PCR analysis. Transgenic expression of ThIPK2 in Brassica napus led to significantly improved salt-, dehydration- and oxidative stress resistance. Furthermore, the transcripts of various stress responsive marker genes increased in ThIPK2 transgenic plants under salt stress condition. These results suggest that ThIPK2 is involved in plant stress responses, and for the first time demonstrate that ThIPK2 could be a useful candidate gene for improving drought and salt tolerance in important crop plants by genetic transformation.

Screening of genes induced by salt stress from Alfalfa

An alfalfa cDNA library induced by salt stress was constructed by suppression subtraction hybridization (SSH) technology. Total RNA from 10-day-old seedlings was used as a "driver," and total RNA from seedlings induced by salt was used as a "tester". One hundred and nineteen clones identified as positive clones by reverse Northern dot-blotting resulted in 82 uni-ESTs comprised of 16 contigs and 66 singletons. Blast analysis of deduced protein sequences revealed that 51 ESTs had identity similar to proteins with known function, while 24 could not be annotated at all. Most of the annotated sequences were homologous to genes involved in abiotic or biotic stress in plants. Among these proteins, beta-amylase, fructose-1,6-bisphosphate, aldolase, and sucrose synthase are related to osmolyte synthesis; a CCCH-type zinc finger protein, DNA binding protein, His-Asp phosphotransfer protein, and the RelA/SpoT protein partake in transcription regulation and signal transduction; and ribulose-l,5-bisphosphate carboxylase/oxygenase, chlorophyll a/b binding proteins, and an early light-inducible proteins are related to photosynthesis. In addition, several ESTs, similar to genes from other plant species, closely involved in salt stress were isolated from alfalfa, such as an aquaporin protein, a late embryogenesis-abundant protein, and glutathione peroxidase.

Comparative expression and transcript initiation of three peach dehydrin genes

Dehydrin genes encode proteins with demonstrated cryoprotective and antifreeze activity, and they respond to a variety of abiotic stress conditions that have dehydration as a common component. Two dehydrins from peach (Prunus persica L. [Batsch.]) have been previously characterized; here, we describe the characterization of a third dehydrin from peach bark, PpDhn3, isolated by its response to low temperature. The expression of all three dehydrin genes was profiled by semi-quantitative reverse transcription PCR, and transcript initiation was mapped for all three genes using the RNA ligase-mediated 5' rapid amplification of cDNA ends technique. PpDhn3 transcripts from bark collected in December or July, as well as transcripts from developing fruit, initiated at a single site. Although most of the PpDhn1 transcripts initiated at a similar position, those from young fruit initiated much further upstream of the consensus TATA box. Bark and fruit transcripts encoding PpDhn2 initiated ca. 30 bases downstream of a consensus TATA box; however, transcripts from ripe fruit initiated further upstream. Ripe fruit transcripts of PpDhn2 contain a 5' leader intron which is predicted to add some 34 amino acids to the N-terminal methionine of the cognate protein when properly processed. Secondary structure prediction of sequences surrounding the TATA box suggests that conformational transitions associated with decreasing temperature contribute to the regulation of expression of the cold-responsive dehydrin genes. Taken together these results reveal new, unexpected levels of gene regulation contributing to the overall expression pattern of peach dehydrins.

Proline accumulation in plants: a review

Proline (Pro) accumulation is a common physiological response in many plants in response to a wide range of biotic and abiotic stresses. Controversy has surrounded the possible role(s) of proline accumulation. In this review, knowledge on the regulation of Pro metabolism during development and stress, results of genetic manipulation of Pro metabolism and current debate on Pro toxicity in plants are presented.

ITN1, a novel gene encoding an ankyrin-repeat protein that affects the ABA-mediated production of reactive oxygen species and is involved in salt-stress tolerance in Arabidopsis thaliana

Salt stress and abscisic acid (ABA) induce accumulation of reactive oxygen species (ROS) in plant cells. ROS not only act as second messengers for the activation of salt-stress responses, but also have deleterious effects on plant growth due to their cytotoxicity. Therefore, the timing and degree of activation of ROS-producing or ROS-scavenging enzymes must be tightly regulated under salt-stress conditions. We identified a novel locus of Arabidopsis, designated itn1 (increased tolerance to NaCl1), whose disruption leads to increased salt-stress tolerance in vegetative tissues. ITN1 encodes a transmembrane protein with an ankyrin-repeat motif that has been implicated in diverse cellular processes such as signal transduction. Comparative microarray analysis between wild-type and the itn1 mutant revealed that induction of genes encoding the ROS-producing NADPH oxidases (RBOHC and RBOHD) under salt-stress conditions was suppressed in the mutant. This suppression was accompanied by a corresponding reduction in ROS accumulation. The ABA-induced expression of RBOHC and RBOHD was also suppressed in the mutant, as was the case for RD29A, an ABA-inducible marker gene. However, the ABA-induced expression of another marker gene, RD22, was not impaired in the mutant. These results suggest that the itn1 mutation partially impairs ABA signaling pathways, possibly leading to the reduction in ROS accumulation under salt-stress conditions. We discuss the possible mechanisms underlying the salt-tolerant phenotype of the itn1 mutant.

A simplified model of local structure in aqueous proline amino acid revealed by first-principles molecular dynamics simulations

Aqueous proline solutions are deceptively simple as they can take on complex roles such as protein chaperones, cryoprotectants, and hydrotropic agents in biological processes. Here, a molecular level picture of proline/water mixtures is developed. Car-Parrinello ab initio molecular dynamics (CPAIMD) simulations of aqueous proline amino acid at the B-LYP level of theory, performed using IBM's Blue Gene/L supercomputer and massively parallel software, reveal hydrogen-bonding propensities that are at odds with the predictions of the CHARMM22 empirical force field but are in better agreement with results of recent neutron diffraction experiments. In general, the CPAIMD (B-LYP) simulations predict a simplified structural model of proline/water mixtures consisting of fewer distinct local motifs. Comparisons of simulation results to experiment are made by direct evaluation of the neutron static structure factor S(Q) from CPAIMD (B-LYP) trajectories as well as to the results of the empirical potential structure refinement reverse Monte Carlo procedure applied to the neutron data.

Free amino acids in botanicals and botanical preparations

Numerous studies were carried out about aminoacidic composition of vegetable proteins, but information about the free amino acid pool and the role of these substances is very incomplete. The aim of this paper was to contribute to the scarce knowledge concerning the composition of free amino acids in botanicals and botanical preparations widely used as food, in dietary supplements, and in pharmaceutical products. This work studied the composition of free amino acids, identified the major components of 19 species of plants, and evaluated the influence of different types of extraction on the amino acid profile. Amino acids were determined using an automatic precolumn derivatization with fluorenylmethyl-chloroformate and reversed-phase liquid chromatography with fluorescence and ultraviolet detection. The amounts of total free amino acids varied widely between plants, from approximately 12 g in 100 g of Echinacea pallida extract to less than 60 mg in the same amount of Coleus forskohlii, Garcinia cambogia, and Glycine max. In 13 plants arginine, asparagine, glutamine, proline, and gamma-aminobutyric acid were the free amino acids found in preponderant quantities. The levels of free amino acids above the quantification limit in 36 assayed samples of botanicals, extracts, and supplements are shown.

Ectopic AtCBF1 over-expression enhances freezing tolerance and induces cold acclimation-associated physiological modifications in potato

We studied the effect of ectopic AtCBF over-expression on physiological alterations that occur during cold exposure in frost-sensitive Solanum tuberosum and frost-tolerant Solanum commersonii. Relative to wild-type plants, ectopic AtCBF1 over-expression induced expression of COR genes without a cold stimulus in both species, and imparted a significant freezing tolerance gain in both species: 2 degrees C in S. tuberosum and up to 4 degrees C in S. commersonii. Transgenic S. commersonii displayed improved cold acclimation potential, whereas transgenic S. tuberosum was still incapable of cold acclimation. During cold treatment, leaves of wild-type S. commersonii showed significant thickening resulting from palisade cell lengthening and intercellular space enlargement, whereas those of S. tuberosum did not. Ectopic AtCBF1 activity induced these same leaf alterations in the absence of cold in both species. In transgenic S. commersonii, AtCBF1 activity also mimicked cold treatment by increasing proline and total sugar contents in the absence of cold. Relative to wild type, transgenic S. commersonii leaves were darker green, had higher chlorophyll and lower anthocyanin levels, greater stomatal numbers, and displayed greater photosynthetic capacity, suggesting higher productivity potential. These results suggest an endogenous CBFpathway is involved in many of the structural, biochemical and physiological alterations associated with cold acclimation in these Solanum species.

Transgenic approaches for abiotic stress tolerance in plants: retrospect and prospects

Abiotic stresses including drought are serious threats to the sustainability of crop yields accounting for more crop productivity losses than any other factor in rainfed agriculture. Success in breeding for better adapted varieties to abiotic stresses depend upon the concerted efforts by various research domains including plant and cell physiology, molecular biology, genetics, and breeding. Use of modern molecular biology tools for elucidating the control mechanisms of abiotic stress tolerance, and for engineering stress tolerant crops is based on the expression of specific stress-related genes. Hence, genetic engineering for developing stress tolerant plants, based on the introgression of genes that are known to be involved in stress response and putative tolerance, might prove to be a faster track towards improving crop varieties. Far beyond the initial attempts to insert "single-action" genes, engineering of the regulatory machinery involving transcription factors has emerged as a new tool now for controlling the expression of many stress-responsive genes. Nevertheless, the task of generating transgenic cultivars is not only limited to the success in the transformation process, but also proper incorporation of the stress tolerance. Evaluation of the transgenic plants under stress conditions, and understanding the physiological effect of the inserted genes at the whole plant level remain as major challenges to overcome. This review focuses on the recent progress in using transgenic technology for the improvement of abiotic stress tolerance in plants. This includes discussion on the evaluation of abiotic stress response and the protocols for testing the transgenic plants for their tolerance under close-to-field conditions.

Enhancement of stress tolerance in transgenic tobacco plants constitutively expressing AtIpk2beta, an inositol polyphosphate 6-/3-kinase from Arabidopsis thaliana

Inositol phosphates (IPs) and their turnover products have been implicated to play important roles in stress signaling in eukaryotic cells. In higher plants genes encoding inositol polyphosphate kinases have been identified previously, but their physiological functions have not been fully resolved. Here we expressed Arabidopsis inositol polyphosphate 6-/3-kinase (AtIpk2beta) in two heterologous systems, i.e. the yeast Saccharomyces cerevisiae and in tobacco (Nicotiana tabacum), and tested the effect on abiotic stress tolerance. Expression of AtIpk2beta rescued the salt-, osmotic- and temperature-sensitive growth defects of a yeast mutant strain (arg82Delta) that lacks inositol polyphosphate multikinase activity encoded by the ARG82/IPK2 gene. Transgenic tobacco plants constitutively expressing AtIpk2beta under the control of the Cauliflower Mosaic Virus 35S promoter were generated and found to exhibit improved tolerance to diverse abiotic stresses when compared to wild type plants. Expression patterns of various stress responsive genes were enhanced, and the activities of anti-oxidative enzymes were elevated in transgenic plants, suggesting a possible involvement of AtIpk2beta in plant stress responses.

Modulation of intracellular proline levels affects flowering time and inflorescence architecture in Arabidopsis

We reported previously that the plant oncogene rolD anticipates and stimulates flowering in Nicotiana tabacum, and encodes ornithine cyclodeaminase, an enzyme catalysing the conversion of ornithine to proline. To investigate on the possible role of proline in flowering, we altered the expression of AtP5CS1, encoding the rate-limiting enzyme of proline biosynthesis in plants. Accordingly we characterized a mutant line containing a T-DNA insertion into AtP5CS1 and introduced in Arabidopsis thaliana AtP5CS1 under the control of the CaMV35S promoter. As expected homozygous p5cs1 mutants behaved as late flowering. In addition p5cs1 mutants exhibited a shorter size and contained lower levels of proline, compared to wild type. 35S-P5CS1 plants, manifested, early in development, overexpression of P5CS1 and accumulation of proline, leading to early flowering, both under long- and short-day conditions. Later in development, down-regulation of P5CS1 occurred in 35S-P5CS1 leaves, leading to proline reduction, and, in turn, impaired bolting and stunted growth. Salt-stress restored expression of P5CS1 and proline accumulation in P5CS1-transformed plants, as well as rescuing growth. Our data suggest that proline plays a key role in flower transition, bolting and coflorescence formation.

Altered expression of barley proline transporter causes different growth responses in Arabidopsis

A compatible solute, proline is accumulated in various kinds of plants and microorganisms under environmental stresses. The function of proline is thought to be an osmotic regulator under water stress, and its transport into cells is mediated by a proline transporter. Here, we report the effects of expressing the barley proline transporter (HvProT) under the control of either the CaMV35S promoter (35Sp) or a root cap promoter (RCp), on Arabidopsis growth. In Arabidopsis, transformed HvProT functions in the plasma membrane, like other amino acid transporters. Reduction in biomass production was observed in aerial parts of 35Sp-HvProT plants, and it was accompanied with decreased proline accumulation in leaves. Impaired growth of 35Sp-HvProT plants was restored by exogenously adding L: -proline. These results suggested that growth reduction was caused by a deficiency of endogenous proline. In 35Sp-HvProT plants, the amount of proline dehydrogenase (PDH) transcript was increased compared to wild type (WT) plants, with a consequent enhancement of the activity of PDH. On the other hand, the transgenic RCp-HvProT plants accumulated 2- to 3-fold more proline in the root tip region compared to WT, and root elongation was enhanced at the same time. Thus, different physiological responses were caused by the altered location in accumulation of proline using two different promoters for heterologous expression of HvProT. These results indicate the importance of proline distribution at the tissue level during vegetative development.

Adaptive response to salt involving carbohydrate metabolism in leaves of a salt-sensitive tomato cultivar

A salt-sensitive genotype of Solanum lycopersicum cv. Volgogradskij was submitted to a 6-day treatment with high salt (100, 200 mM NaCl), allowed to recover for 6 days and then submitted to a second period of salt stress in order to study changes in carbohydrate metabolism related to salt adaptation. The ion, soluble sugar and starch contents, as well as sucrose biosynthetic and sugar mobilizing enzyme activities and transcript levels were determined during the salt stress/recovery/stress cycle. Sodium ions were found to accumulate preferentially in old leaves. Young leaves accumulated lower levels of sodium ions but maintained control levels of potassium ions. Hexoses accumulated to higher levels and starch was better maintained in young compared to old leaves during the two salt treatments. Sucrose accumulated dramatically only in old leaves during the initial salt treatment. Sugar accumulation was not related to decreases in the activities of sugar mobilizing enzymes, acid (EC 3.2.1.25) and neutral (EC 3.2.1.26) invertases, sucrose synthase (EC 2.4.1.13) and hexokinase (EC 2.7.1.1). The activity of the biosynthetic enzyme sucrose phosphate synthase (EC 2.3.1.14) was linked to changes in sucrose levels but not with transcript levels. These results point to the importance of post-transcriptional regulation. Transcriptional regulation could nevertheless be seen in the down-regulation of ribulose bisphosphate carboxylase small subunit (EC 4.1.1.39) in old compared to young leaves, but this was not related to sugar levels.

Arabidopsis rd29A::DREB1A enhances freezing tolerance in transgenic potato

The freezing tolerance of 38 independent transgenic potato lines derived from the cultivar Desiree was tested in vitro using plantlets. The lines were transgenic for the DREB1A gene under control of the rd29A promoter, both of which were derived from Arabidopsis thaliana. The level of damage caused by freezing varied significantly among the transgenic clones and a non-transgenic control (cv. Desiree). Phenotypic evaluation indicated that the variable responses to freezing were attributable to genotypic variation, but freezing tolerance was not dependent on the number of insertions. Northern blot analysis using a DREB1A cDNA probe revealed high levels of DREB1A expression among the transgenic clones during the initial cold exposure at 4 degrees C (after 2 h) and in the early stages of freezing (-20 degrees C, 1-10 min). Furthermore, a linear correlation was detected between the level of expression and the phenotypic response for all lines except D138. Thus, in the case of potato, a significant increase in freezing tolerance was observed in vitro on a small scale following the introduction of rd29A::DREB1A. Additional testing will show whether this strategy can be used for tolerance breeding in potato and to increase the freezing tolerance of other agriculturally important crops.

Exogenous proline mitigates the detrimental effects of salt stress more than exogenous betaine by increasing antioxidant enzyme activities

Proline and betaine accumulate in plant cells under environmental stresses including salt stress. Here, we investigated effects of proline and betaine on the growth and activities of antioxidant enzymes in tobacco Bright Yellow-2 (BY-2) culture cells in suspension under salt stress. Both proline and betaine mitigated the inhibition of growth of BY-2 cells under salt stress and the mitigating effect of proline was more than that of betaine. Salt stress significantly decreased the activities of superoxide dismutase (SOD), catalase and peroxidase in BY-2 cells. Exogenous application of proline or betaine alleviated the reduction in catalase and peroxidase activities but not SOD activity under salt stress. In addition, proline was found to be effective in alleviating the inhibition of salt stress-induced catalase and peroxidase activities in BY-2 cells. Neither proline nor betaine directly scavenged superoxide (O(2)(-)) or hydrogen peroxide (H(2)O(2)). It is concluded that exogenous proline mitigates the detrimental effects of salt stress more than exogenous betaine because of its superior ability to increase the activities of antioxidant enzymes.

Elevated atmospheric CO2 and strain of rhizobium alter freezing tolerance and cold-induced molecular changes in alfalfa (Medicago sativa)

BACKGROUND AND AIMS

The objective of the study was to assess the impact of elevated CO2 in interaction with rhizobial strains on freezing tolerance and cold-induced molecular changes in alfalfa.

METHODS

Alfalfa inoculated with two different strains of rhizobium (A2 and NRG34) was grown and cold acclimated (2 weeks at 2 degrees C) under either 400 (ambient) or 800 micromol mol(-1) (elevated) CO2.

KEY RESULTS

Plants acclimated under 400 micromol mol(-1) CO2 were more freezing tolerant than those maintained under 800 micromol mol(-1). Cryoprotective sugars typically linked with the acquisition of freezing tolerance such as sucrose, stachyose and raffinose increased in roots in response to low temperature but did not differ between CO2 treatments. Similarly high CO2 did not alter the expression of many cold-regulated (COR) genes although it significantly increased the level of transcripts encoding a COR gene homologous to glyceraldehyde-3-phosphate-dehydrogenase (GAPDH). A significant effect of rhizobial strain was observed on both freezing tolerance and gene expression. Plants of alfalfa inoculated with strain A2 were more freezing tolerant than those inoculated with strain NRG34. Transcripts of COR genes homologous to a pathogenesis-related protein (PR-10) and to a nuclear-targeted protein were markedly enhanced in roots of alfalfa inoculated with strain A2 as compared with strain NRG34. Transcripts encoding the vegetative storage proteins (VSPs) beta-amylase and chitinase were more abundant in roots of non-acclimated plants inoculated with strain NRG34 than with strain A2.

CONCLUSIONS

Taken together, the results suggest that elevated CO2 stimulates plant growth and reduces freezing tolerance. The acquisition of cold tolerance is also influenced by the rhizobial strain, as indicated by lower levels of expression of COR genes and sustained accumulation of VSP-encoding transcripts in alfalfa inoculated with strain NRG34 as compared with strain A2.

A Tolerant Behavior in Salt-Sensitive Tomato Plants can be Mimicked by Chemical Stimuli

Lycopersicon esculentum plants exhibit increased salt stress tolerance following treatment with adipic acid monoethylester and 1,3-diaminepropane (DAAME), known as an inducer of resistance against biotic stress in tomato and pepper. For an efficient water and nutrient uptake, plants should adapt their water potential to compensate a decrease in water soil potential produced by salt stress. DAAME-treated plants showed a faster and stronger water potential reduction and an enhanced proline accumulation. Salinity-induced oxidative stress was also ameliorated by DAAME treatments. Oxidative membrane damage and ethylene emission were both reduced in DAAME-treated plants. This effect is probably a consequence of an increase of both non-enzymatic antioxidant activity as well as peroxidase activity. DAAME-mediated tolerance resulted in an unaltered photosynthetic rate and a stimulation of the decrease in transpiration under stress conditions without a cost in growth due to salt stress. The reduction in transpiration rate was concomitant with a reduction in phytotoxic Na(+) and Cl(-) accumulation under saline stress. Interestingly, the ABA deficient tomato mutant sitiens was insensitive to DAAME-induced tolerance following NaCl stress exposure. Additionally, DAAME treatments increased the ABA content of leaves, therefore, an intact ABA signalling pathway seems to be important to express DAAME-induced salt tolerance. Here, we show a possibility of enhance tomato stress tolerance by chemical induction of the major plant defences against salt stress. DAAME-induced tolerance against salt stress could be complementary to or share elements with induced resistance against biotic stress. This might be the reason for the observed wide spectrum of effectiveness of this compound.

Nitrogen uptake and metabolism in Populus x canescens as affected by salinity

External salinization can affect different steps of nitrogen (N) metabolism (ion uptake, N assimilation, and amino acid and protein synthesis) depending on the inorganic N source. Here, we assessed the net uptake of N supplied as nitrate or ammonium and N assimilation (combining metabolite analyses with molecular biological approaches) in grey poplar (Populus x canescens) plants grown under saline (75 mM NaCl) and control conditions. The specific (micromol N g(-1) dry weight fine roots h(-1)) and total plant (micromol N per plant h(-1)) N net uptake rates, total plant N content, total plant biomass and total leaf protein concentration were reduced under saline conditions when plants were supplied with ammonium. In both nutritional groups, salt treatment caused pronounced accumulation of soluble N compounds in the leaves. The mRNAs of genes coding for enzymes catalyzing rate-limiting steps of both proline synthesis and degradation (delta-1-pyrroline-5-carboxylate synthase and proline dehydrogenase) as well as for NADH-dependent glutamate synthase were accumulated under saline conditions. Whereas under control conditions the plant N status seemed to be superior when ammonium was supplied, the N balance of ammonium-fed plants was more severely affected by salt stress than that of plants supplied with nitrate. Possible metabolic implications of stress-related accumulation of particular amino acids are discussed.

Salt tolerance, salt accumulation, and ionic homeostasis in an epidermal bladder-cell-less mutant of the common ice plant Mesembryanthemum crystallinum

The aerial surfaces of the common or crystalline ice plant Mesembryanthemum crystallinum L., a halophytic, facultative crassulacean acid metabolism species, are covered with specialized trichome cells called epidermal bladder cells (EBCs). EBCs are thought to serve as a peripheral salinity and/or water storage organ to improve survival under high salinity or water deficit stress conditions. However, the exact contribution of EBCs to salt tolerance in the ice plant remains poorly understood. An M. crystallinum mutant lacking EBCs was isolated from plant collections mutagenized by fast neutron irradiation. Light and electron microscopy revealed that mutant plants lacked EBCs on all surfaces of leaves and stems. Dry weight gain of aerial parts of the mutant was almost half that of wild-type plants after 3 weeks of growth at 400 mM NaCl. The EBC mutant also showed reduced leaf succulence and leaf and stem water contents compared with wild-type plants. Aerial tissues of wild-type plants had approximately 1.5-fold higher Na(+) and Cl(-) content than the mutant grown under 400 mM NaCl for 2 weeks. Na(+) and Cl(-) partitioning into EBCs of wild-type plants resulted in lower concentrations of these ions in photosynthetically active leaf tissues than in leaves of the EBC-less mutant, particularly under conditions of high salt stress. Potassium, nitrate, and phosphate ion content decreased with incorporation of NaCl into tissues in both the wild type and the mutant, but the ratios of Na(+)/K(+) and Cl(-)/NO(3)(-)content were maintained only in the leaf and stem tissues of wild-type plants. The EBC mutant showed significant impairment in plant productivity under salt stress as evaluated by seed pod and seed number and average seed weight. These results clearly show that EBCs contribute to succulence by serving as a water storage reservoir and to salt tolerance by maintaining ion sequestration and homeostasis within photosynthetically active tissues of M. crystallinum.

Abiotic stress generates ROS that signal expression of anionic glutamate dehydrogenases to form glutamate for proline synthesis in tobacco and grapevine

Glutamate dehydrogenase (GDH) may be a stress-responsive enzyme, as GDH exhibits considerable thermal stability, and de novo synthesis of the alpha-GDH subunit is induced by exogenous ammonium and senescence. NaCl treatment induces reactive oxygen species (ROS), intracellular ammonia, expression of tobacco (Nicotiana tabacum cv Xanthi) gdh-NAD;A1 encoding the alpha-subunit of GDH, increase in immunoreactive alpha-polypeptide, assembly of the anionic isoenzymes, and in vitro GDH aminating activity in tissues from hypergeous plant organs. In vivo aminating GDH activity was confirmed by gas chromatorgraphy-mass spectrometry monitoring of (15)N-Glu, (15)N-Gln, and (15)N-Pro in the presence of methionine sulfoximine and amino oxyacetic acid, inhibitors of Gln synthetase and transaminases, respectively. Along with upregulation of alpha-GDH by NaCl, isocitrate dehydrogenase genes, which provide 2-oxoglutarate, are also induced. Treatment with menadione also elicits a severalfold increase in ROS and immunoreactive alpha-polypeptide and GDH activity. This suggests that ROS participate in the signaling pathway for GDH expression and protease activation, which contribute to intracellular hyperammonia. Ammonium ions also mimic the effects of salinity in induction of gdh-NAD;A1 expression. These results, confirmed in tobacco and grape (Vitis vinifera cv Sultanina) tissues, support the hypothesis that the salinity-generated ROS signal induces alpha-GDH subunit expression, and the anionic iso-GDHs assimilate ammonia, acting as antistress enzymes in ammonia detoxification and production of Glu for Pro synthesis.

Transgenic Medicago truncatula plants that accumulate proline display nitrogen-fixing activity with enhanced tolerance to osmotic stress

Legume root nodule nitrogen-fixing activity is severely affected by osmotic stress. Proline accumulation has been shown to induce tolerance to salt stress, and transgenic plants over-expressing Delta(1)-pyrroline-5-carboxylate synthetase (P5CS), which accumulates high levels of proline, display enhanced osmotolerance. Here, we transformed the model legume Medicago truncatula with the P5CS gene from Vigna aconitifolia, and nodule activity was evaluated under osmotic stress in transgenic plants that showed high proline accumulation levels. Nitrogen fixation was significantly less affected by salt treatment compared to wild-type (WT) plants. To our knowledge, this is the first time that transgenic legumes have been produced that display nitrogen-fixing activity with enhanced tolerance to osmotic stress. We studied the expression of M. truncatula proline-related endogenous genes M. truncatulaDelta(1)-pyrroline-5-carboxylate synthetase 1 (MtP5CS1), M. truncatulaDelta(1)-pyrroline-5-carboxylate synthetase 2 (MtP5CS2), M. truncatula ornithine delta-aminotransferase (MtOAT), M. truncatula proline dehydrogenase (MtProDH) and a proline transporter gene in both WT and transgenic plants. Our results indicate that proline metabolism is finely regulated in response to osmotic stress in an organ-specific manner. The transgenic model allowed us to analyse some of the biochemical and molecular mechanisms that are activated in the nodule in response to high salt conditions, and to ascertain the essential role of proline in the maintenance of nitrogen-fixing activity under osmotic stress.

Enhancement of chilling resistance of inoculated grapevine plantlets with a plant growth-promoting rhizobacterium, Burkholderia phytofirmans strain PsJN

In vitro inoculation of Vitis vinifera L. cv. Chardonnay explants with a plant growth-promoting rhizobacterium, Burkholderia phytofirmans strain PsJN, increased grapevine growth and physiological activity at a low temperature. There was a relationship between endophytic bacterial colonization of the grapevine plantlets and their growth at both ambient (26 degrees C) and low (4 degrees C) temperatures and their sensitivities to chilling. The major benefits of bacterization were observed on root growth (11.8- and 10.7-fold increases at 26 degrees C and 4 degrees C, respectively) and plantlet biomass (6- and 2.2-fold increases at 26 degrees C and 4 degrees C, respectively). The inoculation with PsJN also significantly improved plantlet cold tolerance compared to that of the nonbacterized control. In nonchilled plantlets, bacterization enhanced CO(2) fixation and O(2) evolution 1.3 and 2.2 times, respectively. The nonbacterized controls were more sensitive to exposure to low temperatures than were the bacterized plantlets, as indicated by several measured parameters. Moreover, relative to the noninoculated controls, bacterized plantlets had significantly increased levels of starch, proline, and phenolics. These increases correlated with the enhancement of cold tolerance of the grapevine plantlets. In summary, B. phytofirmans strain PsJN inoculation stimulates grapevine growth and improves its ability to withstand cold stress.

The Arabidopsis Mutant stg1 Identifies a Function for TBP-Associated Factor 10 in Plant Osmotic Stress Adaptation

Plant salt tolerance is a complex trait involving many genes. To identify new salt tolerance determinants during seed germination, we have screened a population of chemically inducible activation-tagged Arabidopsis mutants. A mutant, designated stg1 (salt tolerance during germination 1), was obtained. The stg1 mutant is less sensitive than the wild type to NaCl and osmotic stress inhibition of germination in the presence of the inducer. Germination assays on media containing various salts upon inducer application indicate that the stg1 mutation enhances tolerance to Na(+) and K(+). Under salt stress, stg1 maintains a higher K(+)/Na(+) ratio and accumulates less proline than the wild-type control, suggesting that its salt tolerance mechanisms are mainly involved in the regulation of ion balance. STG1 encodes a putative Arabidopsis TATA box-binding protein (TBP)-associated factor 10 (atTAF10), which constitutes the transcriptional factor IID (TFIID) complex. Overexpression of atTAF10 under the control of the 35S promoter in Arabidopsis improves seed tolerance to salt stress during germination and the knocked-down mutant is more sensitive to salt stress, indicating the transcription initiation factor as a physiological target of salt toxicity in plants.