Proline does not quench singlet oxygen: evidence to reconsider its protective role in plants

Determination of Free Proline in Plants

Proline metabolism has been associated with the induction of reactive oxygen species (ROS), antioxidant enzymes, and the control of cellular redox status. Moreover, proline accumulation is a highly evolutionarily conserved response to diverse abiotic stresses in plants. Thus, proline quantification has been helpful in abiotic stress research as a stress marker. The need for a reliable, fast, and simple method to detect proline in plant tissues is a powerful resource to imply the physiological status of plants under abiotic stress. This chapter summarizes the main strategies for proline extraction and quantification, highlighting their limitations and advantages, and recommends and details a specific protocol for proline extraction and quantification. The chapter provides a friendly version of this protocol with notes useful for researchers to perform the protocol.

Alleviating the toxic effects of Cd and Co on the seed germination and seedling biochemistry of wheat (Triticum aestivum L.) using Azolla pinnata

One of the most significant environmental challenges in the twenty-first century is heavy metal pollution. The potential use of fresh Azolla pinnata to alleviate the toxic effects of Cd and Co on the germination measurements of wheat seeds (Triticum aestivum L.) and the biochemistry of seedlings was studied. Two concentrations (80 and 100 mg L^-1 solutions) of CdNO₃ and CoCl₂ were used before and after treatment with A. pinnata. The highest removal efficiency (RE) by A. pinnata was obtained on the fifth day, with a Cd RE = 55.9 and 49.9% at 80 and 100 mg L^-1, respectively. Cadmium and cobalt solutions reduced the germination percentage, and the measured variables of wheat seeds meanwhile increased the radicle phytotoxicity. In contrast, the presence of A. pinnata in the germination medium increased all the measured variables and decreased radicle phytotoxicity. At 80 and 100 mg L^-1, Cd significantly reduced the fresh and dry biomass, and height of wheat seedlings after 21 days of cultivation compared to Co. Cadmium and high concentrations of cobalt increased the contents of H₂O₂, proline, MDA, phenolic, and flavonoid compounds. The application of treated Cd and Co solutions by A. pinnata showed a decrease in H₂O₂, proline, phenolic, and flavonoid compounds levels accompanied by a reduction in catalase and peroxidase activities compared to the control. This study showed the positive role of A. pinnata in alleviating the metal impacts, particularly Cd, on the seedling growth of wheat and its germination.

Assessment of proline function in higher plants under extreme temperatures

Climate change and abiotic stress factors are key players in crop losses worldwide. Among which, extreme temperatures (heat and cold) disturb plant growth and development, reduce productivity and, in severe cases, lead to plant death. Plants have developed numerous strategies to mitigate the detrimental impact of temperature stress. Exposure to stress leads to the accumulation of various metabolites, e.g. sugars, sugar alcohols, organic acids and amino acids. Plants accumulate the amino acid 'proline' in response to several abiotic stresses, including temperature stress. Proline abundance may result from de novo synthesis, hydrolysis of proteins, reduced utilization or degradation. Proline also leads to stress tolerance by maintaining the osmotic balance (still controversial), cell turgidity and indirectly modulating metabolism of reactive oxygen species. Furthermore, the crosstalk of proline with other osmoprotectants and signalling molecules, e.g. glycine betaine, abscisic acid, nitric oxide, hydrogen sulfide, soluble sugars, helps to strengthen protective mechanisms in stressful environments. Development of less temperature-responsive cultivars can be achieved by manipulating the biosynthesis of proline through genetic engineering. This review presents an overview of plant responses to extreme temperatures and an outline of proline metabolism under such temperatures. The exogenous application of proline as a protective molecule under extreme temperatures is also presented. Proline crosstalk and interaction with other molecules is also discussed. Finally, the potential of genetic engineering of proline-related genes is explained to develop 'temperature-smart' plants. In short, exogenous application of proline and genetic engineering of proline genes promise ways forward for developing 'temperature-smart' future crop plants.

Exogenous Proline Optimizes Osmotic Adjustment Substances and Active Oxygen Metabolism of Maize Embryo under Low-Temperature Stress and Metabolomic Analysis

Maize (Zea mays L.) is more sensitive to low-temperature stress in the early growth period. The study was to explore the response mechanism of proline to low-temperature stress during maize seed germination. Maize varieties Xinxin 2 (low-temperature insensitive) and Damin 3307 (low-temperature sensitive) were chosen as the test materials, setting the normal temperature for germination (22 °C/10 °C, 9d), low-temperature germination (4 °C/4 °C, 5d) and normal temperature recovery (22 °C/10 °C, 4d), combined with proline (15 mmol·L−1) soaking treatment, to study its effects on the osmotic regulation system and antioxidant protection system of maize embryos. Metabolomics analysis was carried out to initially reveal the basis of the metabolic regulation mechanism. The results showed that the activities of superoxide dismutase (SOD), peroxidase (POD), ascorbic acid peroxidase (APX) and glutathione reductase (GR) were induced to some extent under low-temperature stress. The activities of SOD, POD, APX and GR were further enhanced in the soaking seeds with proline. Proline treatment improved the activities of catalase (CAT), monodehydrated ascorbic acid reductase (MDHAR) and dehydroascorbic acid (DHAR), increased the contents of ascorbic acid (AsA) and glutathione (GSH) and decreased the contents of oxidized ascorbic acid (DHA) and reduced glutathione (GSSG) under low-temperature stress. The ratio of AsA/DHA and GSH/GSSG increased. The increase in antioxidant enzyme activity and the content of antioxidants can help to maintain the stability of the AsA-GSH cycle, and effectively reduce the production rate of superoxide anion (O2•−), hydrogen peroxide (H2O2) and malondialdehyde (MDA). Based on the UPLC-MS/MS detection platform and self-built database, 589 metabolites were detected in each treated maize embryo; 262 differential metabolites were obtained, including 32 organic acids, 28 amino acids, 20 nucleotides and their derivatives, 26 sugars and alcohols, 46 lipids, 51 alkaloids, 44 phenols and 15 other metabolites. Sixty-eight metabolic pathways involving different metabolites were obtained by KEGG enrichment analysis. The results showed that proline increased the accumulation of sorbitol, planteose, erythritose 4-phosphate, arabinose and other saccharides and alcohols in response to low-temperature stress, increased the content of osmoregulation substances under low-temperature stress. Proline also restored the TCA cycle by increasing the content of α-ketoglutarate and fumaric acid. Proline increased the contents of some amino acids (ornithine, proline, glycine, etc.), alkaloids (cocamidopropyl betaine, vanillylamine, 6-hydroxynicotinic acid, etc.), phenols (phenolic ayapin, chlorogenic acid, etc.) and vitamins (ascorbic acid, etc.) in the embryo under low-temperature stress. Combined with pathway enrichment analysis, proline could enhance the low-temperature stress resistance of germinated maize embryos by enhancing starch and sucrose metabolism, arginine and proline metabolism, biosynthesis of secondary metabolites, flavonoid biosynthesis and pentose phosphate pathway.

Intriguing Role of Proline in Redox Potential Conferring High Temperature Stress Tolerance

Proline is a proteinogenic amino acid synthesized from glutamate and ornithine. Pyrroline-5-carboxylate synthetase and pyrroline-5-carboxylate reductase are the two key enzymes involved in proline synthesis from glutamate. On the other hand, ornithine-δ-aminotransferase converts ornithine to pyrroline 5-carboxylate (P5C), an intermediate in the synthesis of proline as well as glutamate. Both proline dehydrogenase and P5C dehydrogenase convert proline back to glutamate. Proline accumulation is widespread in response to environmental challenges such as high temperatures, and it is known to defend plants against unpropitious situations promoting plant growth and flowering. While proline accumulation is positively correlated with heat stress tolerance in some crops, it has detrimental consequences in others. Although it has been established that proline is a key osmolyte, its exact physiological function during heat stress and plant ontogeny remains unknown. Emerging evidence pointed out its role as an overriding molecule in alleviating high temperature stress (HTS) by quenching singlet oxygen and superoxide radicals. Proline cycle acts as a shuttle and the redox couple (NAD+/NADH, NADP+/NADPH) appears to be highly crucial for energy transfer among different cellular compartments during plant development, exposure to HTS conditions and also during the recovery of stress. In this review, the progress made in recent years regarding its involvement in heat stress tolerance is highlighted.

The Influence of the Partitioning of Sugars, Starch, and Free Proline in Various Organs of Cyclamen graecum on the Biology of the Species and Its Resistance to Abiotic Stressors

The geophyte Cyclamen graecum is native to the eastern Mediterranean. Its beautiful flowers with upswept pink petals appear during early autumn, after the summer drought period and before leaf expansion in late autumn. The floral and leaf development alternates with their cessation in early winter and late spring, respectively. Ecophysiological parameters and processes underlining the life-cycle of C. graecum have not previously been published. Seasonal fluctuations of sugars, starch, and free proline have been investigated in tubers, leaves, pedicels, and petals, as well as petal and leaf water status. At the whole plant level, the seasonal co-existence of leaves and flowers is marked by an elevated soluble sugar content, which was gradually reduced as the above-ground plant parts shed. The sugar content of petals and pedicels was lower than that of leaves and tubers. Leaf starch content increased from late autumn to spring and was comparable to that of tubers. The starch content in petals and pedicels was substantially lower than that of tubers and leaves. In tubers, monthly proline accumulation was sustained at relatively constant values. Although the partitioning of proline in various organs did not show a considerable seasonal variation, resulting in an unchanged profile of the trends between tubers, leaves, and flowers, the seasonal differences in proline accumulation were remarkable at the whole plant level. The pronounced petal proline content during the flowering period seems to be associated with the maintenance of floral turgor. Leaf proline content increased with the advance of the growth season. The values of leaf relative water content were sustained fairly constant before the senescence stage, but lower than the typical values of turgid and transpiring leaves. Relationships of the studied parameters with rainfall indicate the responsiveness of C. graecum to water availability in its habitat in the Mediterranean ecosystem.

Proline metabolism as regulatory hub

Proline is a multifunctional amino acid that is accumulated in high concentrations in plants under various stress conditions. Proline accumulation is intimately connected to many cellular processes, such as osmotic pressure, energy status, nutrient availability, changes in redox balance, and defenses against pathogens. Proline biosynthesis and catabolism is linked to photosynthesis and mitochondrial respiration, respectively. Proline can function as a signal, modulating gene expression and certain metabolic processes. We review important findings on proline metabolism and function of the last decade, giving a more informative picture about the function of this unusual amino acid in maintaining cellular homeostasis, modulating plant development, and promoting stress acclimation.

Citric Acid-Mediated Abiotic Stress Tolerance in Plants

Several recent studies have shown that citric acid/citrate (CA) can confer abiotic stress tolerance to plants. Exogenous CA application leads to improved growth and yield in crop plants under various abiotic stress conditions. Improved physiological outcomes are associated with higher photosynthetic rates, reduced reactive oxygen species, and better osmoregulation. Application of CA also induces antioxidant defense systems, promotes increased chlorophyll content, and affects secondary metabolism to limit plant growth restrictions under stress. In particular, CA has a major impact on relieving heavy metal stress by promoting precipitation, chelation, and sequestration of metal ions. This review summarizes the mechanisms that mediate CA-regulated changes in plants, primarily CA’s involvement in the control of physiological and molecular processes in plants under abiotic stress conditions. We also review genetic engineering strategies for CA-mediated abiotic stress tolerance. Finally, we propose a model to explain how CA’s position in complex metabolic networks involving the biosynthesis of phytohormones, amino acids, signaling molecules, and other secondary metabolites could explain some of its abiotic stress-ameliorating properties. This review summarizes our current understanding of CA-mediated abiotic stress tolerance and highlights areas where additional research is needed.

Day and Night Fluctuations in GABA Biosynthesis Contribute to Drought Responses in Nicotiana tabacum L

When synchronized with the light/dark cycle the circadian rhythm is termed a diurnal rhythm and this organizes an organism's daily life cycle in relation to the metabolic shifts during the day/night cycles. This is a complex task, particularly under stress conditions. Accurate maintenance of the diurnal rhythm becomes an issue under environmental extremes, such as drought due to the impairment of metabolism, redox balance, and structural integrity. In plants, the non-proteinogenic amino acid GABA accumulates to high levels in response to several stress factors but this is not always dependent on the activation of its biosynthesis. Here we propose a regulatory role to GABA during the diurnal rhythm in plants which is similar to its function in animals where it adjusts the circadian rhythm. Here we investigated whether GABA-biosynthesis was affected by drought stress during the diurnal cycle. For this, we took samples from leaves of N. tabacum plants subjected to PEG-mediated drought stress (-0.73 MPa) during the day and night cycle during a 24 hour period. Glutamate, GABA, and proline contents, along with GDH, GAD enzyme activities and transcript profiles were analyzed. Overall, we conclude that the oscillations in GABA biosynthesis during day and night cycle have an impact on drought stress responses which needs to be elucidated by further analysis.

Proline is a quencher of singlet oxygen and superoxide both in in vitro systems and isolated thylakoids

Proline is a versatile plant metabolite, which is produced in large amounts in plants exposed to osmotic and oxidative stress. Proline has been shown to provide protection against various reactive oxygen species (ROS), such as hydrogen peroxide and hydroxyl radicals. On the other hand, its protective effect against singlet oxygen has been debated, and it is considered ineffective against superoxide. Here we used various methods for the detection of singlet oxygen (electron paramagnetic resonance, EPR, spin trapping by 2,2,6,6-tetramethyl-4-piperidone, fluorescence probing by singlet oxygen sensor green, SOSG, and oxygen uptake due to chemical trapping) and superoxide (oxygen uptake due to oxygen reduction) in vitro and in isolated thylakoids. We demonstrated that proline does quench both singlet oxygen and superoxide in vitro. By comparing the effects of chemical scavengers and physical quenchers, we concluded that proline eliminates singlet oxygen via a physical mechanism, with a bimolecular quenching rate of ca. 1.5-4 10⁶  M^-1 s^-1 . Our data also show that proline can eliminate superoxide in vitro in a process that is likely to proceed via an electron transfer reaction. We could also show that proline does quench both singlet oxygen and superoxide produced in isolated thylakoids. The scavenging efficiency of proline is relatively small on a molar basis, but considering its presence in high amounts in plant cells under stress conditions it may provide a physiologically relevant contribution to ROS scavenging, supplementing other nonenzymatic ROS scavengers of plant cells.

The Relationship between the Antioxidant System and Proline Metabolism in the Leaves of Cucumber Plants Acclimated to Salt Stress

The study examines the effect of acclimation on the antioxidant system and proline metabolism in cucumber leaves subjected to 100 and 150 NaCl stress. The levels of protein carbonyl group, thiobarbituric acid reactive substances, α-tocopherol, and activity of ascorbate and glutathione peroxidases, catalase, glutathione S-transferase, pyrroline-5-carboxylate: synthetase and reductase as well as proline dehydrogenase were determined after 24 and 72 h periods of salt stress in the acclimated and non-acclimated plants. Although both groups of plants showed high α-tocopherol levels, in acclimated plants was observed higher constitutive concentration of these compounds as well as after salt treatment. Furthermore, the activity of enzymatic antioxidants grew in response to salt stress, mainly in the acclimated plants. In the acclimated plants, protein carbonyl group levels collapsed on a constitutive level and in response to salt stress. Although both groups of plants showed a decrease in proline dehydrogenase activity, they differed with regard to the range and time. Differences in response to salt stress between the acclimated and non-acclimated plants may suggest a relationship between increased tolerance in acclimated plants and raised activity of antioxidant enzymes, high-level of α-tocopherol as well, as decrease enzyme activity incorporates in proline catabolism.

Towards Developing Drought-smart Soybeans

Drought is one of the significant abiotic stresses threatening crop production worldwide. Soybean is a major legume crop with immense economic significance, but its production is highly dependent on optimum rainfall or abundant irrigation. Also, in dry periods, it may require supplemental irrigation for drought-susceptible soybean varieties. The effects of drought stress on soybean including osmotic adjustments, growth morphology and yield loss have been well studied. In addition, drought-resistant soybean cultivars have been investigated for revealing the mechanisms of tolerance and survival. Advanced high-throughput technologies have yielded remarkable phenotypic and genetic information for producing drought-tolerant soybean cultivars, either through molecular breeding or transgenic approaches. Further, transcriptomics and functional genomics have led to the characterisation of new genes or gene families controlling drought response. Interestingly, genetically modified drought-smart soybeans are just beginning to be released for field applications cultivation. In this review, we focus on breeding and genetic engineering approaches that have successfully led to the development of drought-tolerant soybeans for commercial use.

Oxidative damage of proline residues by nitrate radicals (NO3˙): a kinetic and product study

Tertiary amides, such as in N-acylated proline or N-methyl glycine residues, react rapidly with nitrate radicals (NO₃˙) with absolute rate coefficients in the range of 4-7 × 10⁸ M^-1 s^-1 in acetonitrile. The major pathway proceeds through oxidative electron transfer (ET) at nitrogen, whereas hydrogen abstraction is only a minor contributor under these conditions. However, steric hindrance at the amide, for example by alkyl side chains at the α-carbon, lowers the rate coefficient by up to 75%, indicating that NO₃˙-induced oxidation of amide bonds proceeds through initial formation of a charge transfer complex. Furthermore, the rate of oxidative damage of proline and N-methyl glycine is significantly influenced by its position in a peptide. Thus, neighbouring peptide bonds, particularly in the N-direction, reduce the electron density at the tertiary amide, which slows down the rate of ET by up to one order of magnitude. The results from these model studies suggest that the susceptibility of proline residues in peptides to radical-induced oxidative damage should be considerably reduced, compared with the single amino acid.

Proline metabolic dynamics and implications in drought tolerance of peanut plants

Proline accumulation and metabolism are associated with mechanisms of abiotic stress avoidance in plants. Proline accumulation generally improves osmotic stress tolerance whereas proline metabolism can have varying effects from ATP generation to the formation of reactive oxygen species. To further understand the roles of proline in stress protection, two peanut cultivars with contrasting tolerance to drought were examined by transcriptional and biochemical analyses during water stress. Plants exposed to polyethylene glycol had diminished relative water content and increased proline content; while, only the drought sensitive plants, cultivar Granoleico, showed lipid oxidative damage (measured as thiobarbituric acid reactive substances). The expression of proline biosynthesis genes (P5CS1, P5CS2a, P5CS2b, P5CR) was increased in both cultivars upon exposure to water stress. However, the relative expression of proline catabolism genes (ProDH1, ProDH2) was increased only in the sensitive cultivar during stress. Exogenous addition of proline and the proline analogue thiazolidine-4-carboxylic acid (T4C), both substrates of proline dehydrogenase, was also used to exacerbate and identify plant responses. Pretreatment of plants with T4C induced unique changes in the drought tolerant EC-98 cultivar such as higher mRNA levels of proline biosynthetic and catabolic ProDH genes, even in the absence of water stress. The increased levels of ProDH gene expression, potentially associated with higher T4C conversion to cysteine, may contribute to the tolerant phenotype.

Photoinduced damage of AsLOV2 domain is accompanied by increased singlet oxygen production due to flavin dissociation

Flavin mononucleotide (FMN) belongs to the group of very efficient endogenous photosensitizers producing singlet oxygen, ¹O₂, but with limited ability to be targeted. On the other hand, in genetically-encoded photosensitizers, which can be targeted by means of various tags, the efficiency of FMN to produce ¹O₂ is significantly diminished due to its interactions with surrounding amino acid residues. Recently, an increase of ¹O₂ production yield by FMN buried in a protein matrix was achieved by a decrease of quenching of the cofactor excited states by weakening of the protein-FMN interactions while still forming a complex. Here, we suggest an alternative approach which relies on the blue light irradiation-induced dissociation of FMN to solvent. This dissociation unlocks the full capacity of FMN as ¹O₂ producer. Our suggestion is based on the study of an irradiation effect on two variants of the LOV2 domain from Avena sativa; wild type, AsLOV2 wt, and the variant with a replaced cysteine residue, AsLOV2 C450A. We detected irradiation-induced conformational changes as well as oxidation of several amino acids in both AsLOV2 variants. Detailed analysis of these observations indicates that irradiation-induced increase in ¹O₂ production is caused by a release of FMN from the protein. Moreover, an increased FMN dissociation from AsLOV2 wt in comparison with AsLOV2 C450A points to a role of C450 oxidation in repelling the cofactor from the protein.

Regulation of L-proline biosynthesis, signal transduction, transport, accumulation and its vital role in plants during variable environmental conditions

Background

In response to various environmental stresses, many plant species synthesize L-proline in the cytosol and accumulates in the chloroplasts. L-Proline accumulation in plants is a well-recognized physiological reaction to osmotic stress prompted by salinity, drought and other abiotic stresses. L-Proline plays several protective functions such as osmoprotectant, stabilizing cellular structures, enzymes, and scavenging reactive oxygen species (ROS), and keeps up redox balance in adverse situations. In addition, ample-studied osmoprotective capacity, L-proline has been also ensnared in the regulation of plant improvement, including flowering, pollen, embryo, and leaf enlargement.

Scope and conclusions

Albeit, ample is now well-known about L-proline metabolism, but certain characteristics of its biological roles are still indistinct. In the present review, we discuss the L-proline accumulation, metabolism, signaling, transport and regulation in the plants. We also discuss the effects of exogenous L-proline during different environmental conditions. L-Proline biosynthesis and catabolism are controlled by several cellular mechanisms, of which we identify only very fewer mechanisms. So, in the future, there is a requirement to identify such types of cellular mechanisms.

Silver nanoparticles and silver salt (AgNO3) elicits morphogenic and biochemical variations in callus cultures of sugarcane

The research work was arranged to check the role of AgNPs and silver ions on callus cells of sugarcane (Saccharum spp. cv CP-77,400). AgNPs were synthesized chemically and characterized by UV-Vis spectra, XRD and SEM. AgNPs and silver ions were applied in various concentrations (0, 20, 40, 60 ppm) to sugarcane calli and the induced stress was characterized by studying various morphological and biochemical parameters. AgNPs and silver ions treatments produced high levels of malondialdehyde, proline, proteins, TP and TF contents. Similarly, CAT, SOD and POX activity was also significant in both treatments. The lower concentration of AgNPs and silver ions (20 ppm) provided maximum intracellular GSH level. This work mainly showed effects of AgNPs and silver ions on sugarcane calli in terms of morphological aberrations and cell membrane damage due to severe oxidative stress and production of enhanced levels of enzymatic and non-enzymatic antioxidants as self-defence to tolerate oxidative stress by scavenging reactive oxygen species. These preliminary findings will provide the way to study ecotoxicity mechanism of the metal ions and NPs in medicine industry and in vitro toxicity research. Furthermore, silver ions alone and their chemically synthesised AgNPs can be used for various biomedical applications in future.

Diverse role of γ-aminobutyric acid in dynamic plant cell responses

Gamma-aminobutyric acid (GABA), a four-carbon non-protein amino acid, is found in most prokaryotic and eukaryotic organisms. Although, ample research into GABA has occurred in mammals as it is a major inhibitory neurotransmitter; in plants, a role for GABA has often been suggested as a metabolite that changes under stress rather than as a signal, as no receptor or motif for GABA binding was identified until recently and many aspects of its biological function (ranging from perception to function) remain to be answered. In this review, flexible properties of GABA in regulation of plant responses to various environmental biotic and abiotic stresses and its integration in plant growth and development either as a metabolite or a signaling molecule are discussed. We have elaborated on the role of GABA in stress adaptation (i.e., salinity, hypoxia/anoxia, drought, temperature, heavy metals, plant-insect interplay and ROS-related responses) and its contribution in non-stress-related biological pathways (i.e., involvement in plant-microbe interaction, contribution to the carbon and nitrogen metabolism and governing of signal transduction pathways). This review aims to represent the multifunctional contribution of GABA in various biological and physiological mechanisms under stress conditions; the objective is to review the current state of knowledge about GABA role beyond stress-related responses. Our effort is to place findings about GABA in an organized and broader context to highlight its shared metabolic and biologic functions in plants under variable conditions. This will provide potential modes of GABA crosstalk in dynamic plant cell responses.

Emissive Enhancement of the Singlet Oxygen Chemiluminescence Probe after Binding to Bovine Serum Albumin

A chemiluminescence probe for singlet oxygen ¹O₂ (SOCL) was investigated in phosphate buffer saline (PBS), either in the absence of proteins or containing bovine serum albumin (BSA). In the protein-free PBS, the reactivity of SOCL for methylene blue (MB)-photosensitized ¹O₂ was found to be moderate or low. The reaction yield increased with temperature and/or concentration of dissolved molecular oxygen. Unexpectedly, the presence of BSA boosted both the emissive nature and the thermal stability of the phenoxy-dioxetane intermediate formed in the chemiexcitation pathway. Isothermal titration calorimetry showed that SOCL has a moderate binding affinity for BSA and that entropy forces drive the formation of the SOCL-BSA complex. A model with two identical and independent binding sites was used to fit the binding isotherm data. Co-operative binding was observed when MB was present. Local viscosity factors and/or conformational restrictions of the BSA-bound SOCL phenoxy-dioxetane were proposed to contribute to the formation of the highly emissive benzoate ester during the chemically initiated electron exchange luminescence (CIEEL) process. These results led us to conclude that hydrophobic interactions of the SOCL with proteins can modify the emissive nature of its phenoxy-dioxetane, which should be taken into account when using SOCL or its cell-penetrating peptide derivative in living cells.

Linking Autophagy to Abiotic and Biotic Stress Responses

Autophagy is a process in which cellular components are delivered to lytic vacuoles to be recycled and has been demonstrated to promote abiotic/biotic stress tolerance. Here, we review how the responses triggered by stress conditions can affect autophagy and its signaling pathways. Besides the role of SNF-related kinase 1 (SnRK1) and TOR kinases in the regulation of autophagy, abscisic acid (ABA) and its signaling kinase SnRK2 have emerged as key players in the induction of autophagy under stress conditions. Furthermore, an interplay between reactive oxygen species (ROS) and autophagy is observed, ROS being able to induce autophagy and autophagy able to reduce ROS production. We also highlight the importance of osmotic adjustment for the successful performance of autophagy and discuss the potential role of GABA in plant survival and ethylene (ET)-induced autophagy.

Knock-Down of the Phosphoserine Phosphatase Gene Effects Rather N- Than S-Metabolism in Arabidopsis thaliana

The aim of present study was to elucidate the significance of the phosphorylated pathway of Ser production for Cys biosynthesis in leaves at day and night and upon cadmium (Cd) exposure. For this purpose, Arabidopsis wildtype plants as control and its psp mutant knocked-down in phosphoserine phosphatase (PSP) were used to test if (i) photorespiratory Ser is the dominant precursor of Cys synthesis in autotrophic tissue in the light, (ii) the phosphorylated pathway of Ser production can take over Ser biosynthesis in leaves at night, and (iii) Cd exposure stimulates Cys and glutathione (GSH) biosynthesis and effects the crosstalk of S and N metabolism, irrespective of the Ser source. Glycine (Gly) and Ser contents were not affected by reduction of the psp transcript level confirming that the photorespiratory pathway is the main route of Ser synthesis. The reduction of the PSP transcript level in the mutant did not affect day/night regulation of sulfur fluxes while day/night fluctuation of sulfur metabolite amounts were no longer observed, presumably due to slower turnover of sulfur metabolites in the mutant. Enhanced contents of non-protein thiols in both genotypes and of GSH only in the psp mutant were observed upon Cd treatment. Mutation of the phosphorylated pathway of Ser biosynthesis caused an accumulation of alanine, aspartate, lysine and a decrease of branched-chain amino acids. Knock-down of the PSP gene induced additional defense mechanisms against Cd toxicity that differ from those of WT plants.

The relationship between carbon and nitrogen metabolism in cucumber leaves acclimated to salt stress

The study examines the effect of acclimation on carbon and nitrogen metabolism in cucumber leaves subjected to moderate and severe NaCl stress. The levels of glucose, sucrose, NADH/NAD⁺-GDH, AspAT, AlaAT, NADP⁺-ICDH, G6PDH and 6GPDH activity were determined after 24 and 72 hour periods of salt stress in acclimated and non-acclimated plants. Although both groups of plants showed high Glc and Suc accumulation, they differed with regard to the range and time of accumulation. Acclimation to salinity decreased the activities of NADP⁺-ICDH and deaminating NAD⁺-GDH compared to controls; however, these enzymes, together with the other examined parameters, showed elevated values in the stressed plants. The acclimated plants showed higher G6PDH activity than the non-acclimated plants, whereas both groups demonstrated similar 6PGDH activity. The high activities of NADH-GDH, AlaAT and AspAT observed in the examined plants could be attributed to a high demand for glutamate. The observed changes may be required for the maintenance of correct TCA cycle activity, and acclimation appeared to positively influence these adaptive processes.

Title: Enhanced salt tolerance and photosynthetic performance: Implication of ɤ-amino butyric acid application in salt-exposed lettuce (Lactuca sativa L.) plants

Gamma-Amino Butyric Acid (GABA) is a substantial component of the free amino acid pool with low concentration in plant tissues. Enhanced GABA content occurs during plant growth and developmental processes like seed germination. GABA level, basically, alters in response to many endogenous and exogenous stimuli. In the current study, GABA effects were studied on germination, photosynthetic performance and oxidative damages in salt-exposed lettuce plants. Three NaCl (0, 40 and 80 mM) and two GABA (0 and 25 μM) concentrations were applied on lettuce during two different developmental (seed germination and seedlings growth) stages. Negative effects of salinity on germination and plant growth were removed by GABA application. GABA significantly reduced mean germination time (MGT) in salt-exposed lettuce seeds. Although, salinity caused a significant decline in maximum quantum yield of photosystem II (Fv/Fm) during distinct steps of plant growth, GABA application improved Fv/Fm particularly on high salinity level. GABA decreased specific energy fluxes per reaction center (RC) for energy absorption and dissipation, while enhanced-electron transport flux in photosynthetic apparatus of lettuce plants was observed in GABA-supplemented plants. Moreover, decline in non-photochemical quenching (NPQ) and quenching coefficients (qP, qL, qN) by salt stress were recovered by GABA application. Elevated electrolyte leakage considerably decreased by GABA exposure on salt-treated plants. Although, proline level increased by NaCl treatments in a concentration dependent manner, combined application of salt with GABA caused a significant reduction in proline content. Catalase; EC 1.11.1.6 (CAT), l-ascorbate peroxidase; EC 1.11.1.11 (APX), and superoxide dismutase; EC 1.15.1.1 (SOD) activities were increased by GABA exposure in salt-supplemented plants that resulted in regulated hydrogen peroxide level. In conclusion, a multifaceted role for GABA is suggested for minimizing detrimental effects of salinity on lettuce through improvement of photosynthetic functionality and regulation of oxidative stress.

Identification of Δ1-pyrroline 5-carboxylate synthase (P5CS) genes involved in the synthesis of proline in Lotus japonicus

Proline accumulation is a common response of plants to different biotic and abiotic stresses. In the model legume Lotus japonicus, osmotic stress-induced proline accumulation is one of the first responses of the plant, converting proline in a reliable stress marker. The main biosynthetic pathway of proline is from glutamate and the reaction catalyzed by the enzyme Δ1-pyrroline 5-carboxylate synthase (P5CS) is the rate limiting step. L. japonicus has been suggested to have three different P5CS genes. Here the predicted P5CS genes of L. japonicus were analyzed in silico and their expression under osmotic stress was determined. Contrary to previous suggestions this study demonstrated that L. japonicus has two different P5CS genes, as most dicotyledonous plants do. The gene that is inducible by osmotic stress and is located on chromosome 1, was called LjP5CS1, and the one located on chromosome 2 and not inducible by osmotic stress was called LjP5CS2.

Osmotic stress alters UV-based oxidative damage tolerance in a heterocyst forming cyanobacterium

Cyanobacteria are successful in diverse habitats due to their adaptation strategies. Their mechanisms to cope with individual stresses have been studied. However, the response to combined stress conditions as found in nature remains unclear. With this aim, we studied the dual effect of 24h-osmotic and 3h-UV irradiation on the cyanobacterium Calothrix BI22. Our approach included the study of redox homeostasis, oxidative damage, reactive oxygen species production-consumption processes and photosynthetic activity. Superoxide in vivo determination with confocal image processing showed the highest accumulation under UV. However, no lipoperoxidation occurred due to a high SOD activity. This cyanobacterium was less prepared to cope with the osmotic stress assayed. Under this condition, O₂ photoevolution decreased abruptly and oxidative damage was produced by reactive species other than superoxide. In this situation the cellular control of the amount of ROS failed to prevent oxidative damage and photosynthesis was seriously disturbed in spite of maximum quantum photosynthetic efficiency remained unchanged. Calothrix BI22 presented the more severe oxidative damage when both stressors were applied. The osmotic stress disentangled the mechanisms developed by this cyanobacterium to deal with 3h-UV irradiation alone.

Proline over-accumulation alleviates salt stress and protects photosynthetic and antioxidant enzyme activities in transgenic sorghum [Sorghum bicolor (L.) Moench]

Shoot-tip derived callus cultures of Sorghum bicolor were transformed by Agrobacterium tumefaciens as well as by bombardment methods with the mutated pyrroline-5-carboxylate synthetase (P5CSF129A) gene encoding the key enzyme for proline biosynthesis from glutamate. The transgenics were selfed for three generations and T4 plants were examined for 100 mM NaCl stress tolerance in pot conditions. The effect of salt stress on chlorophyll and carotenoid contents, photosynthetic rate, stomatal conductance, internal carbon dioxide concentration, transpiration rates, intrinsic transpiration and water use efficiencies, proline content, MDA levels, and antioxidant enzyme activities were evaluated in 40-day-old transgenic lines and the results were compared with untransformed control plants. The results show that chlorophyll content declines by 65% in untransformed controls compared to 30-38% loss (significant at P < 0.05) in transgenics but not carotenoid levels. Photosynthetic rate (PSII activity) was reduced in untransformed controls almost completely, while it declined by 62-88% in different transgenic lines. Salinity induced ca 100% stomatal closure in untransformed plants, while stomatal conductance was decreased only by 64-81% in transgenics after 4 days. The intercellular CO2 decreased by ca 30% in individual transgenic lines. Malondialdehyde (MDA) content was lower in transgenics compared to untransformed controls. The activities of superoxide dismutase (SOD; EC 1.15.1.1), catalase (CAT; EC 1.11.1.6) and glutathione reductase (GR; EC1.8.1.7) were quantified in leaves exposed to 100 mM NaCl stress and found higher in transgenics. The results suggest that transgenic lines were able to cope better with salt stress than untransformed controls by protecting photosynthetic and antioxidant enzyme activities.

Connecting proline metabolism and signaling pathways in plant senescence

The amino acid proline has a unique biological role in stress adaptation. Proline metabolism is manipulated under stress by multiple and complex regulatory pathways and can profoundly influence cell death and survival in microorganisms, plants, and animals. Though the effects of proline are mediated by diverse signaling pathways, a common theme appears to be the generation of reactive oxygen species (ROS) due to proline oxidation being coupled to the respiratory electron transport chain. Considerable research has been devoted to understand how plants exploit proline metabolism in response to abiotic and biotic stress. Here, we review potential mechanisms by which proline metabolism influences plant senescence, namely in the petal and leaf. Recent studies of petal senescence suggest proline content is manipulated to meet energy demands of senescing cells. In the flower and leaf, proline metabolism may influence ROS signaling pathways that delay senescence progression. Future studies focusing on the mechanisms by which proline metabolic shifts occur during senescence may lead to novel methods to rescue crops under stress and to preserve post-harvest agricultural products.

Connecting proline and γ-aminobutyric acid in stressed plants through non-enzymatic reactions

The accumulation of proline (Pro) in plants exposed to biotic/abiotic stress is a well-documented and conserved response in most vegetal species. Stress conditions induce the overproduction of reactive oxygen species which can lead to cellular damage. In vitro assays have shown that enzyme inactivation by hydroxyl radicals (^·OH) can be avoided in presence of Pro, suggesting that this amino acid could act as an ^·OH scavenger. We applied Density Functional Theory coupled with a polarizable continuum model to elucidate how Pro reacts with ^·OH. In this work we suggest that Pro reacts favourably with ^·OH by H–abstraction on the amine group. This reaction produces the spontaneous decarboxylation of Pro leading to the formation of pyrrolidin-1-yl. In turn, pyrrolidin-1-yl can easily be converted to Δ¹-pyrroline, the substrate of the enzyme Δ¹-pyrroline dehydrogenase, which produces γ-aminobutyric acid (GABA). GABA and Pro are frequently accumulated in stressed plants and several protective roles have been assigned to these molecules. Thereby we present an alternative non-enzymatic way to synthetize GABA under oxidative stress. Finally this work sheds light on a new beneficial role of Pro accumulation in the maintenance of photosynthetic activity.

How reactive oxygen species and proline face stress together

Reactive oxygen species (ROS) are continuously generated as a consequence of plant metabolic processes due to incomplete reduction of O2. Previously considered to be only toxic by-products of metabolism, ROS are now known to act as second messengers in intracellular signalling cascades to trigger tolerance of various abiotic and biotic stresses. The accumulation of proline is frequently observed during the exposure of plants to adverse environmental conditions. Interestingly proline metabolism may also contribute to ROS formation in mitochondria, which play notably a role in hypersensitive response in plants, life-span extension in worms and tumor suppression in animals. Here we review current knowledge about the regulation of proline metabolism in response to environmental constraints and highlight the key role of ROS in the regulation of this metabolism. The impact of proline on ROS generation is also investigated. Deciphering and integrating these relationships at the whole plant level will bring new perspectives on how plants adapt to environmental stresses.

Is proline accumulation per se correlated with stress tolerance or is proline homeostasis a more critical issue?

Proline has been recognized as a multi-functional molecule, accumulating in high concentrations in response to a variety of abiotic stresses. It is able to protect cells from damage by acting as both an osmotic agent and a radical scavenger. Proline accumulated during a stress episode is degraded to provide a supply of energy to drive growth once the stress is relieved. Proline homeostasis is important for actively dividing cells as it helps to maintain sustainable growth under long-term stress. It also underpins the importance of the expansion of the proline sink during the transition from vegetative to reproductive growth and the initiation of seed development. Its role in the reproductive tissue is to stabilize seed set and productivity. Thus, to cope with abiotic stress, it is important to develop strategies to increase the proline sink in the reproductive tissue. We give a holistic account of proline homeostasis, taking into account the regulation of proline synthesis, its catabolism, and intra- and intercellular transport, all of which are vital components of growth and development in plants challenged by stress.

ROS generation and proline metabolism in calli of halophyte Nitraria tangutorum Bobr. to sodium nitroprusside treatment

Nitric oxide (NO) is a stress factor or a signal molecule involved in various plant physiological and developmental processes. In the present study, the generation of reactive oxygen species and the metabolism of proline due to different sodium nitroprusside (SNP, an NO donor) concentrations were investigated in callus from halophyte Nitraria tangutorum Bobr. Treatment with SNP led to significant increases of hydrogen peroxide (H2O2) content and cell viability but notable reductions in hydrogen radical level and lipid peroxidation degree, and superoxide onion (O2 (-)) content also enhanced in 100 μM SNP-treated calli. Using a chemical inhibitor for plasma membrane (PM) NADPH oxidase diphenylene iodonium (DPI), we found low O2 (-) generation in untreated and 25 μM SNP-treated calli, whereas in those treated with 100 μM SNP O2 (-) level exhibited a very little alteration, comparable to the absence of DPI. These suggest a high activity of PM NADPH oxidase in untreated calli. H2O2 scavenging enzymes (catalase, peroxidase [POD] and ascorbate peroxidase) and H2O2 forming enzymes (superoxide dismutase [SOD], cell wall-POD and diamine oxidase [DAO]) stimulated significantly in calli treated with different SNP concentrations while glutathione reductase activity decreased. In addition, a reduction in proline content was observed in SNP-treated calli. Moreover, different SNP concentrations stimulated proline dehydrogenase (PDH) and ornithine δ-aminotransferase but inhibited r-glutamyl kinase (GK). In conclusion, our results suggest that the increasing H2O2 generation was associated with the stimulation of SOD, cell wall-POD and DAO, and that the reduction of proline content might be the consequence of increased PDH activity and decreased GK activity in N. tangutorum Bobr. calli under SNP treatment.

Molecular mechanisms for the reaction between (˙)OH radicals and proline: insights on the role as reactive oxygen species scavenger in plant stress

The accumulation of proline (Pro) and overproduction of reactive oxygen species (ROS) by plants exposed to stress is well-documented. In vitro assays show that enzyme inactivation by hydroxyl radicals ((•)OH) can be avoided in the presence of Pro, suggesting this amino acid might act as a (•)OH scavenger. Although production of hydroxyproline (Hyp) has been hypothesized in connection with such antioxidant activity, no evidence on the detailed mechanism of scavenging has been reported. To elucidate whether and how Hyp might be produced, we used density functional theory calculations coupled to a polarizable continuum model to explore 27 reaction channels including H-abstraction by (•)OH and (•)OH/H2O addition. The structure and energetics of stable species and transition states for each reaction channel were characterized at the PCM-(U)M06/6-31G(d,p) level in aqueous solution. Evidence is found for a main pathway in which Pro scavenges (•)OH by successive H-abstractions (ΔG(‡,298) = 4.1 and 7.5 kcal mol(-1)) to yield 3,4-Δ-Pro. A companion pathway with low barriers yielding Δ(1)-pyrroline-5-carboxylate (P5C) is also supported, linking with 5-Hyp through hydration. However, this connection remains unlikely in stressed plants because P5C would be efficiently recycled to Pro (contributing to its accumulation) by P5C reductase, hypothesis coined here as the "Pro-Pro cycle".

Calcium-induced proline accumulation contributes to amelioration of NaCl injury and expression of glutamine synthetase in greater duckweed (Spirodela polyrhiza L.)

The calcium-mediated proline accumulation is a critical response under NaCl stress and the function of the induced proline as a glutamine synthetase (GS) protectant in greater duckweed was investigated. The plants were treated with solutions containing 100mM NaCl, 200 mM NaCl, 200 mM NaCl plus 10mM CaCl2, or 10mM CaCl2 alone for 4 days. At the end of the experiment, the fronds of inoculum treated with 200 mM NaCl showed the chlorotic effect, higher glutamate dehydrogenase (NADH-GDH) activity and lower GS activity. At the lower salinity, the activities of GS and NADH-GDH were not altered markedly. A significant accumulation of proline was not found under either low or high salinity. The activity of Δ(1)-pyrroline-5-carboxylate reductase (P5CR) was enhanced only at 200 mM NaCl but remained unchanged at 100mM NaCl. The activity of Δ(1)-pyrroline-5-carboxylate synthetase (P5CS) did not change under salinity-stressed. Addition of CaCl2 to the salt stressed plants not only lowered NaCl injury but also showed an elevated level of proline contents in response to the salinity treatment. In addition, both GS activity and corresponding polypeptides were expressed close to the level of control. Exogenous proline protects GS2 and the 32 kDa protein in photosystem II reaction center (D1) from H2O2-induced redox degradation in the chloroplast lysates of duckweed. The results suggest that calcium-induced proline accumulation may play an important role as a GS protectant under NaCl exposure in S. polyrhiza.

Calcium-mediated responses and glutamine synthetase expression in greater duckweed (Spirodela polyrhiza L.) under diethyl phthalate-induced stress

This study was carried out to assess the influence of diethyl phthalate (DEP) alone or associated with calcium chloride (CaCl2) on greater duckweed plants, emphasizing the implications of calcium in amelioration of DEP-induced stress on plant growth. Greater duckweed were treated with DEP in variable concentrations, as 0, 0.25, 0.5, 1.0 and 2.0mM for 7 days, or treated with the same concentration either 2mM DEP or 2mM DEP plus 10mM CaCl2·2H2O in different duration 0-7 days. Treatment with 2mM DEP resulted in increasing proline content, protease activity, and ammonia accumulation in duckweed tissues. NADH-glutamate dehydrogenase (NADH-GDH; EC 1.4.1.2) and Δ(1)-pyrroline-5-carboxylate reductase (P5CR; EC 1.5.1.2), two key enzymes in the glutamate pathway of proline synthesis, showed increase in activity with DEP treatment and positively correlated with proline accumulation. No further increase in proline accumulation was observed with addition of calcium chloride to the DEP-treated cultures. However, supplementation of Ca(2+) can mitigate the adverse effect of DEP, at least in part to decrease the DEP-induced superoxide accumulation and increase in GDH activity for ammonia assimilation in duckweed fronds. In addition, effects of calcium on mitigation of DEP injury were also observed in glutamine synthetase (GS; EC 6.3.1.2) expression. Both GS1 and GS2 polypeptide accumulation and the level of total GS activity were nearly equivalent to the control. Exogenous proline protects GS2 from DEP-modulated redox damage in the chloroplast lysates but there is no remarkable protection effects on D1 (the 32kDa protein in photosystem II reaction center) degradation. In conclusion, the glutamate pathway of proline synthesis might be involved in mitigation of DEP-induced injury, and calcium plays an important role in increasing GDH, P5CR, and GS expression.

Antioxidant and photosystem II responses contribute to explain the drought-heat contrasting tolerance of two forage legumes

Identification of metabolic targets of environmental stress factors is critical to improve the stress tolerance of plants. Studying the biochemical and physiological responses of plants with different capacities to deal with stress is a valid approach to reach this objective. Lotus corniculatus (lotus) and Trifolium pratense (clover) are legumes with contrasting summer stress tolerances. In stress conditions, which are defined as drought, heat or a combination of both, we found that differential biochemical responses of leaves explain these behaviours. Lotus and clover showed differences in water loss control, proline accumulation and antioxidant enzymatic capacity. Drought and/or heat stress induced a large accumulation of proline in the tolerant species (lotus), whereas heat stress did not cause proline accumulation in the sensitive species (clover). In lotus, Mn-SOD and Fe-SOD were induced by drought, but in clover, the SOD-isoform profile was not affected by stress. Moreover, lotus has more SOD-isoforms and a higher total SOD activity than clover. The functionality and electrophoretic profile of photosystem II (PSII) proteins under stress also exhibited differences between the two species. In lotus, PSII activity was drastically affected by combined stress and, interestingly, was correlated with D2 protein degradation. Possible implications of this event as an adaption mechanism in tolerant species are discussed. We conclude that the stress-tolerant capability of lotus is related to its ability to respond to oxidative damage and adaption of the photosynthetic machinery. This reveals that these two aspects should be included in the evaluation of the tolerance of species to stress conditions.

Proline accumulation in leaves of Periploca sepium via both biosynthesis up-regulation and transport during recovery from severe drought

Drought resistance and recovery ability are two important requisites for plant adaptation to drought environments. Proline (Pro) metabolism has been a major concern in plant drought tolerance. However, roles of Pro metabolism in plant recovery ability from severe drought stress are largely unexplored. Periploca sepium Bunge has gained increasing attention for its adaptation to dry environments. Here, we investigated Pro metabolism in different tissues of P. sepium seedlings in the course of drought stress and recovery. We found that leaf Pro metabolism response during post-drought recovery was dependant on drought severity. Pro biosynthesis was down-regulated during recovery from -0.4 MPa but increased continually and notably during recovery from -1.0 MPa. Significant correlation between Pro concentration and Δ1-pyrroline-5-carboxylate synthetase activity indicates that Glutamate pathway is the predominant synthesis route during both drought and re-watering periods. Ornithine δ-aminotransferase activity was up-regulated significantly only during recovery from -1.0 MPa, suggesting positive contribution of ornithine pathway to improving plant recovery capacity from severe drought. In addition to up-regulation of biosynthesis, Pro transport from stems and roots also contributed to high Pro accumulation in leaves and new buds during recovery from -1.0 MPa, as indicated by the combined analysis of Pro concentration and its biosynthesis in stems, roots and new buds. Except its known roles as energy, carbon and nitrogen sources for plant rapid recovery, significant positive correlation between Pro concentration and total antioxidant activity indicates that Pro accumulation can also promote plant damage repair ability by up-regulating antioxidant activity during recovery from severe drought stress.