Proline Accumulation In Response Research Articles

Unraveling the evolutionary origin of the P5CS gene: a story of gene fusion and horizontal transfer.

The accumulation of proline in response to the most diverse types of stress is a widespread defense mechanism. In prokaryotes, fungi, and certain unicellular eukaryotes (green algae), the first two reactions of proline biosynthesis occur through two distinct enzymes, γ-glutamyl kinase (GK E.C. 2.7.2.11) and γ-glutamyl phosphate reductase (GPR E.C. 1.2.1.41), encoded by two different genes, ProB and ProA, respectively. Plants, animals, and a few unicellular eukaryotes carry out these reactions through a single bifunctional enzyme, the Δ1-pyrroline-5-carboxylate synthase (P5CS), which has the GK and GPR domains fused. To better understand the origin and diversification of the P5CS gene, we use a robust phylogenetic approach with a broad sampling of the P5CS, ProB and ProA genes, including species from all three domains of life. Our results suggest that the collected P5CS genes have arisen from a single fusion event between the ProA and ProB gene paralogs. A peculiar fusion event occurred in an ancestral eukaryotic lineage and was spread to other lineages through horizontal gene transfer. As for the diversification of this gene family, the phylogeny of the P5CS gene in plants shows that there have been multiple independent processes of duplication and loss of this gene, with the duplications being related to old polyploidy events.

Proline synthesis and catabolism-related genes synergistically regulate proline accumulation in response to abiotic stresses in grapevines

Proline is an important osmotic regulator for plants under abiotic stresses. Proline accumulation is regulated by various enzymes, while genes encoding these enzymes have not been fully investigated in grapevines. In this study, we identified four proline biosynthesis-related genes (P5CS1, P5CS2, P5CR and OAT) and two proline catabolism-related genes (PDH and P5CDH) from grapevines, and conducted bioinformatics analysis including protein characteristic, gene structure, phylogenetic relationship and cis-acting element of promoters. Promoter analysis found that cis-acting elements related to stress and phytohormone responsiveness were present in the promoters of these genes. Tissue-specific expression analysis of these genes indicated that P5CS2, OAT and PDH expressed highly in flower, and P5CS1, P5CR and P5CDH had highest expression levels in tendril, berry and leaf respectively. Expression patterns of these genes under abiotic stresses revealed that P5CS1, OAT and PDH could be significantly induced by salt, dehydration, PEG and H2O2 treatments, and P5CDH was significantly suppressed under these stress treatments. By measuring proline accumulation in grapevines, we found that proline content dramatically increased after treatments of salt, dehydration, PEG and H2O2. Our results revealed the molecular responses (especially P5CS1, OAT, PDH and P5CDH) in the proline metabolism pathway to osmotic and oxidative stresses in grapevines, demonstrating that proline synthesis and proline catabolism synergistically regulate proline accumulation in response to abiotic stresses.

Physiological responses of Dracocephalum kotschyi Boiss. to drought stress and bio-fertilizers

This study aimed to investigate the possible impact of the drought stress at three levels (80, 60 and 40% field capacity) and bio-fertilizers at four levels (Nitroxin, Super Nitro Plus, Biophosphorus, and non-biofertilizer) on Dracocephalum kotschyi. The experiments were conducted as factorial based on a completely randomized design, with 3 repetitions. The results showed that drought stress significantly enhanced electrolyte leakage (EL), reactive oxygen species (H2O2), and malondialdehyde content in the leaves. The proline content of the leaves increased corresponding to the drought stress progress. The application of bio-fertilizers intensified the proline accumulation in response to drought. The use of bio-fertilizers in 60 and 40% of FC treatments significantly increased the relative water content (RWC), while it reduced EL in D.kotschyi plants, as compared to non-inoculated plants. The highest levels of anthocyanins (82.37 mg g−1 FW) and flavonoid compounds (62.16 mg g−1 FW) were recorded in the plants grown under severe drought stress conditions (40% FC) and biophosphorus application. Furthermore, the results indicated that the activity of guaiacol peroxidase (GPX), superoxide dismutase (SOD), catalase (CAT), and ascorbate peroxidase (APX) were all increased under drought stress conditions. The use of bio-fertilizers alleviated the negative impacts of drought stress by modifying the antioxidant enzyme activity. Overall, the results revealed that the use of these bio-fertilizers, especially super nitroplus, improved most physiological parameters of D. kotschyi plants under drought stress conditions.

The dual-targeted prolyl aminopeptidase PAP1 is involved in proline accumulation in response to stress and during pollen development.

Plant endosymbiotic organelles such as mitochondria and chloroplasts harbour a wide array of biochemical reactions. As a part of protein homeostasis to maintain organellar activity and stability, unwanted proteins and peptides need to be completely degraded in a stepwise mechanism termed the processing pathway, where at the last stage single amino acids are released by aminopeptidases. Here, we determined the molecular and physiological functions of a prolyl aminopeptidase homologue PAP1 (At2g14260) that is able to release N-terminal proline. Transcript analyses demonstrate that an alternative transcription start site gives rise to two alternative transcripts, generating two in-frame proteins PAP1.1 and PAP1.2. Subcellular localization studies revealed that the longer isoform PAP1.1, which contains a 51 residue N-terminal extension, is exclusively targeted to chloroplasts, while the truncated isoform PAP1.2 is located in the cytosol. Distinct expression patterns in different tissues and developmental stages were observed. Investigations into the physiological role of PAP1 using loss-of-function mutants revealed that PAP1 activity may be involved in proline homeostasis and accumulation, required for pollen development and tolerance to osmotic stress. Enzymatic activity, subcellular location, and expression patterns of PAP1 suggest a role in the chloroplastic peptide processing pathway and proline homeostasis.

Salt stress alleviation in citrus plants by plant growth-promoting rhizobacteria Pseudomonas putida and Novosphingobium sp.

This work reveals the protective role of two rhizobacteria, Pseudomonas putida and Novosphingobium sp., on citrus plants subjected to salt stress conditions. Detrimental salt stress effects on crops are likely to increase due to climate change reducing the quality of irrigation water. Plant growth-promoting rhizobacteria (PGPRs) can mitigate stress-induced damage in plants cultivated under high salinity conditions. In this work, Citrus macrophylla (alemow) plants inoculated with the rhizobacteria Pseudomonas putida KT2440 or Novosphingobium sp. HR1a were subjected to salt stress for 30 days. Results showed that in absence of salt stress, Novosphingobium sp. HR1a induced a decrease of transpiration (E) and stomatal conductance (gs). Both rhizobacteria reduced salt stress-induced damage. Levels of abscisic acid (ABA) and salicylic acid (SA) were lower in inoculated plants under salt stress conditions. Similarly, under stress conditions maximum efficiency of photosystem II (Fv/Fm) in inoculated plants decreased to a lower extent than in non-inoculated ones. In stressed plants, Novosphingobium sp. HR1a also induced leaf accumulation of 3-indole acetic acid (IAA) and a delay in the decrease of quantum yield (ΦPSII). P. putida KT2440 inhibited root chloride and proline accumulation in response to salt stress. Although both bacterial species had beneficial effects on salt-stressed citrus plants, Novosphingobium sp. HR1a induced a better plant performance. Therefore, both strains could be candidates to be used as PGPRs in programs of inoculation for citrus protection against salt stress.

Changes in osmotic potential, protein content and proline accumulation in response to NaCl salinity and phosphorus in cucumber microshoots grown on proliferation medium

The aim of this work was to explore the contributions of phosphorus in mitigating the salinity impacts on osmotic potential, protein content and proline accumulation of cucumber grown in vitro on proliferation medium. Microshoots were grown on Murashige and Skoog (MS) proliferation media containing 1 mg/l kinetin and supplemented with a combination of NaCl at 0.0, 25, 50, 75, 100 mM with 0.5, 1.0 or 2.0 mM phosphorus using KH2PO4. Leaf osmotic potential was reduced with increasing NaCl concentration and within each of the 50, 75 and 100 mM NaCl concentrations, the incorporation of 2 mM KH2PO4 was needed to significantly increase leaf osmotic potential. Protein content of cucumber microshoots was shown to decrease with NaCl concentration, this reduction was lessened with increase in KH2PO4 level applied. In vitro-induced salinity caused profound proline accumulation in cucumber microshoots and combinations having 2 mM KH2PO4 showed lower proline accumulation especially at the highest two concentrations of NaCl.

Biochemical and transcriptomic analyses of drought stress responses of LY1306 tobacco strain

This study aimed to investigate drought resistance of the LY1306 tobacco strain. Seedlings of tobacco strains LY1306, ZhongYan 100 (ZY100) and Hong Hua Da Jin Yuan (HHDJY) were treated with polyethylene glycol (PEG)-6000 to induce osmotic stress. As validation, water-deficit-induced drought treatments, including mild drought (MD; watering 1.5 L/week) and severe drought (SD, without watering) were carried out. Changes in cell morphology, leaf water potential, antioxidant enzyme activity, as well as contents of malondialdehyde (MDA) and proline were determined for each treatment. Transcriptome sequencing was performed for the seedlings treated with 15% PEG-6000. No obvious changes were observed in morphology of LY1306 and ZY100 under osmotic or drought stress; whereas, visible wilting was observed in HHDJY. Superoxide dismutase and peroxidase activities of LY1036 and ZY100 under osmotic stress were significantly higher than those of HHDJY. Under SD, the MDA content of LY1306 was significantly lower and the proline content of LY1306 was significantly higher than that of HHDJY. Differential genes between LY1306, ZY100 and HHDJY were enriched in functions about alpha-linolenic acid, and arginine and proline metabolisms. LY1306 could increase its antioxidant enzyme activities and proline accumulation in response to drought stress, probably by regulating drought resistance-related pathways and genes.

PCA22 acts as a suppressor of atrzf1 to mediate proline accumulation in response to abiotic stress in Arabidopsis.

Proline metabolism is important for environmental responses, plant growth, and development. However, its precise roles in plant abiotic stress tolerance are not well understood. Mutants are valuable for the identification of new genes and for elucidating their roles in physiological mechanisms. We applied a suppressor mutation approach to identify novel genes involved in the regulation of proline metabolism in Arabidopsis. Using the atrzf1 (Arabidopsis thaliana ring zinc finger 1) mutant as a parental line for activation tagging mutagenesis, we selected several mutants with suppressed induction of proline accumulation under dehydration conditions. One of the selected mutants [proline content alterative 22 (pca22)] appeared to have reduced proline contents compared with the atrzf1 mutant under drought stress. Generally, pca22 mutant plants displayed suppressed atrzf1 insensitivity to dehydration and abscisic acid during early seedling growth. Additionally, the pca22 mutant exhibited shorter pollen tube length than wild-type (WT) and atrzf1 plants. Furthermore, PCA22-overexpressing plants were more sensitive to dehydration stress than the WT and RNAi lines. Green fluorescent protein-tagged PCA22 was localized to the cytoplasm of transgenic Arabidopsis cells. Collectively, these results suggest that pca22 acts as dominant suppressor mutant of atrzf1 in the abiotic stress response.

Effects of Salt Stress on Three Ecologically Distinct Plantago Species.

Comparative studies on the responses to salt stress of taxonomically related taxa should help to elucidate relevant mechanisms of stress tolerance in plants. We have applied this strategy to three Plantago species adapted to different natural habitats, P. crassifolia and P. coronopus–both halophytes–and P. major, considered as salt-sensitive since it is never found in natural saline habitats. Growth inhibition measurements in controlled salt treatments indicated, however, that P. major is quite resistant to salt stress, although less than its halophytic congeners. The contents of monovalent ions and specific osmolytes were determined in plant leaves after four-week salt treatments. Salt-treated plants of the three taxa accumulated Na+ and Cl- in response to increasing external NaCl concentrations, to a lesser extent in P. major than in the halophytes; the latter species also showed higher ion contents in the non-stressed plants. In the halophytes, K+ concentration decreased at moderate salinity levels, to increase again under high salt conditions, whereas in P. major K+ contents were reduced only above 400 mM NaCl. Sorbitol contents augmented in all plants, roughly in parallel with increasing salinity, but the relative increments and the absolute values reached did not differ much in the three taxa. On the contrary, a strong (relative) accumulation of proline in response to high salt concentrations (600–800 mM NaCl) was observed in the halophytes, but not in P. major. These results indicate that the responses to salt stress triggered specifically in the halophytes, and therefore the most relevant for tolerance in the genus Plantago are: a higher efficiency in the transport of toxic ions to the leaves, the capacity to use inorganic ions as osmotica, even under low salinity conditions, and the activation, in response to very high salt concentrations, of proline accumulation and K+ transport to the leaves of the plants.

Proline accumulation in response to high temperature in winter-acclimated shoots ofPrunus persica: a response associated with growth resumption or heat stress?

Our previous study revealed that proline (Pro) accumulated in peach trees when cold-acclimated shoots were exposed to higher temperatures. In this study, we determined whether such Pro accumulation...

Functional characterization and expression analysis of rice δ(1)-pyrroline-5-carboxylate dehydrogenase provide new insight into the regulation of proline and arginine catabolism.

While intracellular proline accumulation in response to various stress conditions has been investigated in great detail, the biochemistry and physiological relevance of proline degradation in plants is much less understood. Moreover, the second and last step in proline catabolism, the oxidation of δ1-pyrroline-5-carboxylic acid (P5C) to glutamate, is shared with arginine catabolism. Little information is available to date concerning the regulatory mechanisms coordinating these two pathways. Expression of the gene coding for P5C dehydrogenase was analyzed in rice by real-time PCR either following the exogenous supply of amino acids of the glutamate family, or under hyperosmotic stress conditions. The rice enzyme was heterologously expressed in E. coli, and the affinity-purified protein was thoroughly characterized with respect to structural and functional properties. A tetrameric oligomerization state was observed in size exclusion chromatography, which suggests a structure of the plant enzyme different from that shown for the bacterial P5C dehydrogenases structurally characterized to date. Kinetic analysis accounted for a preferential use of NAD+ as the electron acceptor. Cations were found to modulate enzyme activity, whereas anion effects were negligible. Several metal ions were inhibitory in the micromolar range. Interestingly, arginine also inhibited the enzyme at higher concentrations, with a mechanism of uncompetitive type with respect to P5C. This implies that millimolar levels of arginine would increase the affinity of P5C dehydrogenase toward its specific substrate. Results are discussed in view of the involvement of the enzyme in either proline or arginine catabolism.

Involvement of Phosphatidylinositol 3-kinase in the regulation of proline catabolism in Arabidopsis thaliana.

Plant adaptation to abiotic stresses such as drought and salinity involves complex regulatory processes. Deciphering the signaling components that are involved in stress signal transduction and cellular responses is of importance to understand how plants cope with salt stress. Accumulation of osmolytes such as proline is considered to participate in the osmotic adjustment of plant cells to salinity. Proline accumulation results from a tight regulation between its biosynthesis and catabolism. Lipid signal components such as phospholipases C and D have previously been shown to be involved in the regulation of proline metabolism in Arabidopsis thaliana. In this study, we demonstrate that proline metabolism is also regulated by class-III Phosphatidylinositol 3-kinase (PI3K), VPS34, which catalyses the formation of phosphatidylinositol 3-phosphate (PI3P) from phosphatidylinositol. Using pharmacological and biochemical approaches, we show that the PI3K inhibitor, LY294002, affects PI3P levels in vivo and that it triggers a decrease in proline accumulation in response to salt treatment of A. thaliana seedlings. The lower proline accumulation is correlated with a lower transcript level of Pyrroline-5-carboxylate synthetase 1 (P5CS1) biosynthetic enzyme and higher transcript and protein levels of Proline dehydrogenase 1 (ProDH1), a key-enzyme in proline catabolism. We also found that the ProDH1 expression is induced in a pi3k-hemizygous mutant, further demonstrating that PI3K is involved in the regulation of proline catabolism through transcriptional regulation of ProDH1. A broader metabolomic analysis indicates that LY294002 also reduced other metabolites, such as hydrophobic and aromatic amino acids and sugars like raffinose.

Salinity induced physiological and biochemical changes in chickpea (Cicer arietinum L.) genotypes

Plant growth and development are adversely affected by salinity- a major environmental stress that limits agricultural production. Chickpea (Cicer arietinum L.) is sensitive to salinity that affects its yield and there is need to identify the tolerant genotypes. The present study was conducted to evaluate the effect of salinity on chickpea genotypes with specific physiological and biochemical attributes contributing to their adaptability to salinity stress. Seven chickpea genotypes both desi (ICC8950, ICCV10, ICC15868, GL26054) and kabuli (BG1053, L550, L552) were evaluated for salinity tolerance. Maximum decrease in relative leaf water content and chlorophyll content was observed with ICC15868 and GL26054 among the desi and L552 from the kabuli genotypes. The photosynthetic pigments, activity of nitrate reductase and relative leaf water content was also reduced in response to salt application with effect being more pronounced in ICC15868, GL26054 and L552 as compared to ICC8950, ICCV10, BG1053 and L550. Lipid peroxidation increases with the increase in NaCl concentration, maximum increment was observed in genotypes ICC15868, GL26054 and L552. Accumulation of proline in response to environmental stresses seems to be widespread among plants. Higher protein fractions were observed with tolerant genotypes in contrast to sensitive genotypes. Salt imposed stress finally caused a higher decline in number of filled pods. On the basis of physiological and biochemical parameters genotypes ICC8950 and ICCV10 from the desi genotypes and BG1053 and L550 from kabuli were identified as the tolerant while ICC15868, GL26054 as the sensitive ones and L552 as the moderately tolerant genotypes. These genotypes could be used as a source of tolerance in breeding programme to develop salt tolerant genotypes.

INFLUENCE OF SEED PRIMING ON GROWTH AND SEED YIELD OF OKRA UNDER WATER STRESS

Seed priming is a technique that improves seed performance in the field and hence ameliorates subsequent germination, growth and seed yield. Moreover, seed priming is often implicated in improving the stress-tolerance of germinating seeds. This study was initiated to determine the seed yield and quality of a local genotype okra var. Sabahia in relation to seed priming and water stress. Two field experiments were conducted during the summer seasons 2012 and 2013 at Kaha (Kalyiobia Governorate), Horticulture Research Station, Egypt. The experimental design used is split plot with four replications. The main plots represented two irrigation regimes either normal or stress 20 days or 30 days intervals respectively, whereas the priming treatments represented the subplots. For priming treatments, seeds were soaked for 24 hours either in water (hydropriming) or in a 3% solution of Na2HPO4, MgSO4, or KCl (osmopriming), whereas dry seeds represented the control. Results indicated that all plant traits determined were reduced under water stress while this reduction was improved in the primed seeds. Using Na2HPO4 or MgSO4 seed priming treatments under both water regimes applied gave the highest and best results regarding plant growth, seed yield and quality. Obtained results also showed that seed priming with KCl gave lower positive effects in this concern and such results seemed to be higher than those obtained by hydropriming treatments. In addition, results clearly showed that okra plants could resist the applied water stresses treatments as a result of seed priming processes used which were found to cause accumulation of some osmolytes as proline. Data of the present work showed proline accumulation in response to water stress, which might assume that this osmolyte could be enhanced by seed priming. Generally, it could be concluded that seed priming results in improving growth and seed criteria of okra especially under water stress conditions.

Different modes of proline accumulation in response to saline-alkaline stress factors in rice (Oryza sativaL.)

Proline accumulation is a common physiological response to various abiotic stresses in plants. In this study, we show that rice seedlings accumulate proline to different levels in response to different saline-alkaline stress factors including osmotic, saline and alkaline stresses. Expression analysis of genes for proline biosynthesis (OsP5CS1, OsP5CS2 and OsP5CR) and catabolism (OsPDH1 and OsP5CDH) revealed up-regulation of the proline synthesis genes under osmotic and alkaline stresses. In comparison, under the salt stress, a decrease in catabolic enzyme gene expressions was observed. Our results suggest that rice seedlings employ different gene regulation in proline accumulation in response to different saline-alkaline stress factors.

DNA methylation involved in proline accumulation in response to osmotic stress in rice (Oryza sativa)

Proline accumulation is involved in plant osmotic stress tolerance. Given that DNA methylation is related to almost all metabolic processes through regulation of gene expression, we suspected that this epigenetic modification and proline biosynthesis are probably related. To test this, we investigated whether osmotic stress-induced proline accumulation is associated with DNA methylation modifications in rice. We assessed DNA methylation and expression of 3 key genes (P5CR, P5CS, and δ-OAT) involved in proline biosynthesis, and measured proline content under both osmotic stress (15% polyethylene glycol) and control conditions. After osmotic stress, selfed progenies of osmotic-stressed plants accumulated higher concentrations of proline in leaves under both normal conditions and under osmotic stress than the unstressed control plants. Concomitantly, under osmotic stress, the selfed progeny plants showed higher expression levels of P5CS and δ-OAT than the control. This up-regulated expression was stably inherited by the subsequent generation. Methylation-sensitive Southern blotting indicated that 2 of the 3 genes, P5CS and δ-OAT, had greater DNA demethylation in the selfed progenies than in the control. Apparently DNA demethylation facilitated proline accumulation by up-regulating expression of the P5CS and δ-OAT genes in response to osmotic stress.

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.

Mutations in the hyperosmotic stress-responsive mitochondrial BASIC AMINO ACID CARRIER2 enhance proline accumulation in Arabidopsis.

Mitochondrial carrier family proteins are diverse in their substrate specificity, organellar location, and gene expression. In Arabidopsis (Arabidopsis thaliana), 58 genes encode these six-transmembrane-domain proteins. We investigated the biological role of the basic amino acid carrier Basic Amino Acid Carrier2 (BAC2) from Arabidopsis that is structurally and functionally similar to ARG11, a yeast ornithine and arginine carrier, and to Arabidopsis BAC1. By studying the expression of BAC2 and the consequences of its mutation in Arabidopsis, we showed that BAC2 is a genuine mitochondrial protein and that Arabidopsis requires expression of the BAC2 gene in order to use arginine. The BAC2 gene is induced by hyperosmotic stress (with either 0.2 m NaCl or 0.4 m mannitol) and dark-induced senescence. The BAC2 promoter contains numerous stress-related cis-regulatory elements, and the transcriptional activity of BAC2:beta-glucuronidase is up-regulated by stress and senescence. Under hyperosmotic stress, bac2 mutants express the P5CS1 proline biosynthetic gene more strongly than the wild type, and this correlates with a greater accumulation of Pro. Our data suggest that BAC2 is a hyperosmotic stress-inducible transporter of basic amino acids that contributes to proline accumulation in response to hyperosmotic stress in Arabidopsis.

Proline antioxidant role in the common ice plant subjected to salinity and paraquat treatment inducing oxidative stress

Leaves of 4-week-old (juvenile) and 9-week-old (adult) plants of the halophyte Mesembryanthemum crystallinum L. (the common ice plant), cultured under controlled conditions in the phytotron, were treated with paraquat (0.1 μM), which produces superoxide radical, and (or) paraquat combined with introduction of NaCl (100 mM) or proline (5 mM) into nutrient medium. After a 20-h dark period (23°C), plants were transferred into light (4 h at 54.1 W/m2 of photosynthetically active radiation) for stimulation of O°2− formation in plastids. Activities of antioxidant enzymes, the contents of MDA, H2O2, chlorophyll, and free proline were measured in leaves. Plant responses in two age groups, which differed in the type of photosynthesis (juvenile plants had C3 type of photosynthesis, whereas adult plants were at the transition stage to Crassulacean Acid Metabolism (CAM) photosynthesis), differed in the levels of constitutive proline and proline, induced by NaCl and paraquat, as well as in activities of superoxide dismutase (SOD) and catalase. Changes in SOD activity and proline accumulation in response to paraquat treatment combined with NaCl revealed opposite dependence to accumulation of proline: the more proline accumulated in leaves, the lower activity of the enzyme. In response to paraquat treatment, the content of chlorophylls a and b most drastically declined in juvenile plants. Negative effect of salinity on the content of chlorophylls was lower than that of paraquat and was almost the same in plants of both age groups. Protective effect of exogenous proline was most profound in the case of paraquat treatment. Exogenous proline decreased the rate of lipid peroxidation, the content of superoxide radical and, consequently, SOD activity (almost fivefold), and increased the content of chlorophylls (a and b) in leaves of adult plants. The obtained data suggest that stress-induced accumulation of proline in the common ice plant has both osmoprotectory and antioxidant functions.

Proline metabolism in response to salt stress in common reed [Phragmites australis (Cav.) Trin. ex Steud

Abstract We studied effects of salt stress on proline accumulation and the activities of proline metabolic enzymes in reed, a salt-tolerant, cosmopolitan, aquatic grass. A concentration-dependent inhibition of plant growth occurred in the presence of NaCl, which was accompanied by high amounts of proline accumulation. Reed tolerance of salt stress reaches a limit at approximately 200 mm NaCl. Free proline content and pyrroline-5-carboxylate synthetase (P5CS) activity both increased with salt stress treatment, while ornithine aminotransferase (OAT) activity appeared to be unaffected, suggesting that the glutamate pathway, rather than the ornithine pathway, plays a vital role in proline accumulation during osmotic regulation. The activity of the proline degrading enzyme, proline dehydrogenase (PDH), decreased under salt stress, indicating that proline catabolism may also be responsible for proline accumulation in response to salt stress. Furthermore, high salinity ≥250 mm NaCl, which stops growth of reed, decreases proline accumulation and P5CS, OAT, and PDH activities. This suggests that changes in enzyme activities of proline metabolism coupled with increased proline accumulation are involved in osmotic regulation of reed plants only when they are grown at salinities ≤200 mm NaCl.