Responsive modes of Medicago sativa proline dehydrogenase genes during salt stress and recovery dictate free proline accumulation

The nonphototrophic hypocotyl 3 (NPH3) domain protein NRL5 is a trafficking-associated GTPase essential for drought resistance

The mechanisms of plant drought resistance are unclear but may involve membrane trafficking and metabolic reprogramming, including proline accumulation. Forward genetic screening using a proline dehydrogenase 1 (ProDH1) promoter:reporter identified a drought hypersensitive mutant with a single-amino acid substitution (P335L) in the nonphototrophic hypocotyl 3 (NPH3) domain of NPH3/root phototropism 2-like 5 (NRL5)/naked pins in Yucca 8 (NPY8). Further experiments found that NRL5 and other NPH3 domain proteins are guanosine triphosphatases (GTPases). NRL5, but not NRL5P335L, interacted with the RABE1c and RABH1b GTPases and the soluble N-ethylmaleimide-sensitive factor attachment protein receptors (SNAREs) Vesicle-Associated Membrane Protein (VAMP)721/722. These proteins controlled NRL5 localization and connection to trafficking while also being genetically downstream of, and potentially regulated by, NRL5. These data demonstrate that NRL5-mediated restraint of proline catabolism is required for drought resistance and also reveal unexpected functions of the NPH3 domain such that the role of NPH3 domain proteins in signaling, trafficking, and cellular polarity can be critically reevaluated.

Bioinformatics and expression analysis of proline metabolism-related gene families in alfalfa under saline-alkali stress

Regulation of the proline metabolic pathway is essential for the accumulation of proline under abiotic stress and for the amelioration of plant stress resistance. Δ1-pyrroline-5-carboxylate synthase (P5CS), pyrroline-5-carboxylate reductase (P5CR), ornithine transaminase (δ-OAT), proline dehydrogenase (PDH), pyrroline-5-carboxylate dehydrogenase (P5CDH), and proline transporter (ProT) are the key enzymes in the proline metabolic pathway. However, the gene families responsible for proline metabolism have not yet been identified or reported in alfalfa. In this study, a total of 12 MsP5CSs, 4 MsP5CRs, 3 MsOATs, 6 MsPDHs, 2 MsP5CDHs, and 5 MsProTs were identified in the genome of alfalfa, and the members of the same subfamily had similar gene structures and conserved motifs. Analysis of cis-regulatory elements revealed the presence of light-responsive, hormone-regulated, and stress-responsive elements in the promoter regions of alfalfa proline metabolism-related genes. Following treatment with saline-alkali, the expression of MsP5CSs, MsP5CRs, MsOATs, and MsProTs was significantly upregulated, whereas the expression of MsPDH1.1, MsPDH1.3, and MsP5CDH was significantly downregulated. The proline content and enzyme activity of P5CS gradually increased, whereas the enzyme activity of PDH gradually decreased as the duration of stress increased. Root growth rates decreased upon MsP5CS1a suppression (MsP5CS1a-RNAi) in the hairy roots of alfalfa compared to the empty vector line under saline-alkali stress. These results show that proline metabolism-related genes play an important role in the saline-alkali stress tolerance of alfalfa and provide a theoretical basis for further research on the functions of proline metabolism-related genes in alfalfa in response to saline-alkali stress.

Burning questions for a warming and changing world: 15 unknowns in plant abiotic stress

We present unresolved questions in plant abiotic stress biology as posed by 15 research groups with expertise spanning eco-physiology to cell and molecular biology. Common themes of these questions include the need to better understand how plants detect water availability, temperature, salinity, and rising carbon dioxide (CO2) levels; how environmental signals interface with endogenous signaling and development (e.g. circadian clock and flowering time); and how this integrated signaling controls downstream responses (e.g. stomatal regulation, proline metabolism, and growth versus defense balance). The plasma membrane comes up frequently as a site of key signaling and transport events (e.g. mechanosensing and lipid-derived signaling, aquaporins). Adaptation to water extremes and rising CO2 affects hydraulic architecture and transpiration, as well as root and shoot growth and morphology, in ways not fully understood. Environmental adaptation involves tradeoffs that limit ecological distribution and crop resilience in the face of changing and increasingly unpredictable environments. Exploration of plant diversity within and among species can help us know which of these tradeoffs represent fundamental limits and which ones can be circumvented by bringing new trait combinations together. Better defining what constitutes beneficial stress resistance in different contexts and making connections between genes and phenotypes, and between laboratory and field observations, are overarching challenges.

Comparative physiological and root transcriptome analysis of two annual ryegrass cultivars under drought stress

Annual ryegrass is a widely cultivated forage grass with rapid growth and high productivity. However, drought is one of the abiotic stresses affecting ryegrass growth and quality. In this study, we compared the physiological and transcriptome responses of Chuansi No.1 (drought-tolerant, DT) and Double Barrel (drought-sensitive, DS) under drought stress simulated by PEG-6000 for 7 days. The results showed that Chuansi No. 1 had stronger physiological and biochemical parameters such as root properties, water content, osmotic adjustment ability and antioxidant ability. In addition, RNA-seq was used to elucidate the molecular mechanism of root drought resistance. We identified 8588 differentially expressed genes related to drought tolerance in root, which were mainly enriched in oxidation-reduction process, carbohydrate metabolic process, apoplast, arginine and proline metabolism, and phenylpropanoid biosynthesis pathways. The expression levels of DEGs were consistent with physiological changes of ryegrass under drought stress. We found that genes related to sucrose and starch synthesis, root development, osmotic adjustment, ABA signal regulation and specifically up-regulated transcription factors such as WRKY41, WRKY51, ERF7, ERF109, ERF110, NAC43, NAC68, bHLH162 and bHLH148 in Chuansi No. 1 may be the reason for its higher drought tolerance. This study revealed the underlying physiological and molecular mechanisms of root response to drought stress in ryegrass and provided some new candidate genes for breeding rye drought tolerant varieties.

Proline Concentration and Its Metabolism Are Regulated in a Leaf Age Dependent Manner But Not by Abscisic Acid in Pea Plants Exposed to Cadmium Stress

The accumulation of proline is one of the defense mechanisms of plants against the harmful effects of adverse environmental conditions; however, when pea plants were treated for 12 h with CdCl2, the proline concentration decreased in the youngest A (not expanded) and B1 (expanded) leaves, and did not change significantly in the B2 (mature, expanded) or C (the oldest) leaves. After 24 h of cadmium (Cd) stress, the proline concentration remained low in A and B1 leaves, while in B2 and C leaves, it increased, and after 48 h, an increase in the proline concentration in the leaves at each stage of development was observed. The role of proline in the different phases of plant response to the Cd treatment is discussed. Changes in proline accumulation corresponded closely with changes in the transcript levels of PsP5CS2, a gene encoding D1-pyrroline-5-carboxylate synthetase involved in proline synthesis, and PsPDH1, a gene encoding proline dehydrogenase engaged in proline degradation. CdCl2 application induced the expression of PsProT1 and PsProT2, genes encoding proline transporters, especially during the first 12 h of treatment in A and B1 leaves. When the time courses of abscisic acid (ABA) and proline accumulation were compared, it was concluded that an increase in the proline concentration in the leaves of Cd-treated pea plants was more related to a decrease in chlorophyll concentration (leaves B2 and C) and an increase in the malondialdehyde level (A and B1 leaves) than with an increase in ABA concentration alone. Exogenous application of ABA (0.5, 5, 50 µM) significantly increased the proline concentration in the A leaves of pea plants only, and was accompanied by an elevated and repressed expression of PsP5CS2 and PsPDH1 in these leaves, respectively. The presented results suggest that under Cd stress, the accumulation of proline in leaves of pea plants may take place independently of the ABA signaling.

ASCORBATE PEROXIDASE6 delays the onset of age-dependent leaf senescence

Age-dependent changes in reactive oxygen species (ROS) levels are critical in leaf senescence. While H2O2-reducing enzymes such as catalases and cytosolic ASCORBATE PEROXIDASE1 (APX1) tightly control the oxidative load during senescence, their regulation and function are not specific to senescence. Previously, we identified the role of ASCORBATE PEROXIDASE6 (APX6) during seed maturation in Arabidopsis (Arabidopsis thaliana). Here, we show that APX6 is a bona fide senescence-associated gene. APX6 expression is specifically induced in aging leaves and in response to senescence-promoting stimuli such as abscisic acid (ABA), extended darkness, and osmotic stress. apx6 mutants showed early developmental senescence and increased sensitivity to dark stress. Reduced APX activity, increased H2O2 level, and altered redox state of the ascorbate pool in mature pre-senescing green leaves of the apx6 mutants correlated with the early onset of senescence. Using transient expression assays in Nicotiana benthamiana leaves, we unraveled the age-dependent post-transcriptional regulation of APX6. We then identified the coding sequence of APX6 as a potential target of miR398, which is a key regulator of copper redistribution. Furthermore, we showed that mutants of SQUAMOSA PROMOTER BINDING PROTEIN-LIKE7 (SPL7), the master regulator of copper homeostasis and miR398 expression, have a higher APX6 level compared with the wild type, which further increased under copper deficiency. Our study suggests that APX6 is a modulator of ROS/redox homeostasis and signaling in aging leaves that plays an important role in developmental- and stress-induced senescence programs.

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.

How Does Proline Treatment Promote Salt Stress Tolerance During Crop Plant Development?

Soil salinity is one of the major abiotic stresses restricting the use of land for agriculture because it limits the growth and development of most crop plants. Improving productivity under these physiologically stressful conditions is a major scientific challenge because salinity has different effects at different developmental stages in different crops. When supplied exogenously, proline has improved salt stress tolerance in various plant species. Under high-salt conditions, proline application enhances plant growth with increases in seed germination, biomass, photosynthesis, gas exchange, and grain yield. These positive effects are mainly driven by better nutrient acquisition, water uptake, and biological nitrogen fixation. Exogenous proline also alleviates salt stress by improving antioxidant activities and reducing Na⁺ and Cl^- uptake and translocation while enhancing K⁺ assimilation by plants. However, which of these mechanisms operate at any one time varies according to the proline concentration, how it is applied, the plant species, and the specific stress conditions as well as the developmental stage. To position salt stress tolerance studies in the context of a crop plant growing in the field, here we discuss the beneficial effects of exogenous proline on plants exposed to salt stress through well-known and more recently described examples in more than twenty crop species in order to appreciate both the diversity and commonality of the responses. Proposed mechanisms by which exogenous proline mitigates the detrimental effects of salt stress during crop plant growth are thus highlighted and critically assessed.

Transcriptional, metabolic and DNA methylation changes underpinning the response of Arundo donax ecotypes to NaCl excess

Arundo donax ecotypes react differently to salinity, partly due to differences in constitutive defences and methylome plasticity. Arundo donax L. is a C3 fast-growing grass that yields high biomass under stress. To elucidate its ability to produce biomass under high salinity, we investigated short/long-term NaCl responses of three ecotypes through transcriptional, metabolic and DNA methylation profiling of leaves and roots. Prolonged salt treatment discriminated the sensitive ecotype 'Cercola' from the tolerant 'Domitiana' and 'Canneto' in terms of biomass. Transcriptional and metabolic responses to NaCl differed between the ecotypes. In roots, constitutive expression of ion transporter and stress-related transcription factors' genes was higher in 'Canneto' and 'Domitiana' than 'Cercola' and 21-day NaCl drove strong up-regulation in all ecotypes. In leaves, unstressed 'Domitiana' confirmed higher expression of the above genes, whose transcription was repressed in 'Domitiana' but induced in 'Cercola' following NaCl treatment. In all ecotypes, salinity increased proline, ABA and leaf antioxidants, paralleled by up-regulation of antioxidant genes in 'Canneto' and 'Cercola' but not in 'Domitiana', which tolerated a higher level of oxidative damage. Changes in DNA methylation patterns highlighted a marked capacity of the tolerant 'Domitiana' ecotype to adjust DNA methylation to salt stress. The reduced salt sensitivity of 'Domitiana' and, to a lesser extent, 'Canneto' appears to rely on a complex set of constitutively activated defences, possibly due to the environmental conditions of the site of origin, and on higher plasticity of the methylome. Our findings provide insights into the mechanisms of adaptability of A. donax ecotypes to salinity, offering new perspectives for the improvement of this species for cultivation in limiting environments.

Low CO2 induces urea cycle intermediate accumulation in Arabidopsis thaliana

The non-proteinogenic amino acid ornithine links several stress response pathways. From a previous study we know that ornithine accumulates in response to low CO2. To investigate ornithine accumulation in plants, we shifted plants to either low CO2 or low light. Both conditions increased carbon limitation, but only low CO2 also increased the rate of photorespiration. Changes in metabolite profiles of light- and CO2-limited plants were quite similar. Several amino acids that are known markers of senescence accumulated strongly under both conditions. However, urea cycle intermediates respond differently between the two treatments. While the levels of both ornithine and citrulline were much higher in plants shifted to 100 ppm CO2 compared to those kept in 400 ppm CO2, their metabolite abundance did not significantly change in response to a light limitation. Furthermore, both ornithine and citrulline accumulation is independent from sugar starvation. Exogenous supplied sugar did not significantly change the accumulation of the two metabolites in low CO2-stressed plants, while the accumulation of other amino acids was reduced by about 50%. Gene expression measurements showed a reduction of the entire arginine biosynthetic pathway in response to low CO2. Genes in both proline biosynthesis and degradation were induced. Hence, proline did not accumulate in response to low CO2 like observed for many other stresses. We propose that excess of nitrogen re-fixed during photorespiration can be alternatively stored in ornithine and citrulline under low CO2 conditions. Furthermore, ornithine is converted to pyrroline-5-carboxylate by the action of δOAT.

SELENOPROTEIN O is a chloroplast protein involved in ROS scavenging and its absence increases dehydration tolerance in Arabidopsis thaliana

The evolutionary conserved family of Selenoproteins performs redox-regulatory functions in bacteria, archaea and eukaryotes. Among them, members of the SELENOPROTEIN O (SELO) subfamily are located in mammalian and yeast mitochondria, but their functions are thus far enigmatic. Screening of T-DNA knockout mutants for resistance to the proline analogue thioproline (T4C), identified mutant alleles of the plant SELO homologue in Arabidopsis thaliana. Absence of SELO resulted in a stress-induced transcriptional activation instead of silencing of mitochondrial proline dehydrogenase, and also high elevation of Δ(1)-pyrroline-5-carboxylate dehydrogenase involved in degradation of proline, thereby alleviating T4C inhibition and lessening drought-induced proline accumulation. Unlike its animal homologues, SELO was localized to chloroplasts of plants ectopically expressing SELO-GFP. The protein was co-fractionated with thylakoid membrane complexes, and co-immunoprecipitated with FNR, PGRL1 and STN7, all involved in regulating PSI and downstream electron flow. The selo mutants displayed extended survival under dehydration, accompanied by longer photosynthetic activity, compared with wild-type plants. Enhanced expression of genes encoding ROS scavenging enzymes in the unstressed selo mutant correlated with higher oxidant scavenging capacity and reduced methyl viologen damage. The study elucidates SELO as a PSI-related component involved in regulating ROS levels and stress responses.

Comparative analysis of alfalfa (Medicago sativa L.) leaf transcriptomes reveals genotype-specific salt tolerance mechanisms

BACKGROUND

Soil salinity is an important factor affecting growth, development, and productivity of almost all land plants, including the forage crop alfalfa (Medicago sativa). However, little is known about how alfalfa responds and adapts to salt stress, particularly among different salt-tolerant cultivars.

RESULTS

Among seven alfalfa cultivars, we found that Zhongmu-1 (ZM) is relatively salt-tolerant and Xingjiang Daye (XJ) is salt-sensitive. Compared to XJ, ZM showed slower growth under low-salt conditions, but exhibited stronger tolerance to salt stress. RNA-seq analysis revealed 2237 and 1125 differentially expressed genes (DEGs) between ZM and XJ in the presence and absence of salt stress, among which many genes are involved in stress-related pathways. After salt treatment, compared with the controls, the number of DEGs in XJ (19373) was about four times of that in ZM (4833). We also detected specific differential gene expression patterns: In response to salt stress, compared with XJ, ZM maintained relatively more stable expression levels of genes related to the ROS and Ca²⁺ pathways, phytohormone biosynthesis, and Na⁺/K⁺ transport. Notably, several salt resistance-associated genes always showed greater levels of expression in ZM than in XJ, including a transcription factor. Consistent with the suppression of plant growth resulting from salt stress, the expression of numerous photosynthesis- and growth hormone-related genes decreased more dramatically in XJ than in ZM. By contrast, the expression levels of photosynthetic genes were lower in ZM under low-salt conditions.

CONCLUSIONS

Compared with XJ, ZM is a salt-tolerant alfalfa cultivar possessing specific regulatory mechanisms conferring exceptional salt tolerance, likely by maintaining high transcript levels of abiotic and biotic stress resistance-related genes. Our results suggest that maintaining this specific physiological status and/or plant adaptation to salt stress most likely arises by inhibition of plant growth in ZM through plant hormone interactions. This study identifies new candidate genes that may regulate alfalfa tolerance to salt stress and increases the understanding of the genetic basis for salt tolerance.

Approaches in modulating proline metabolism in plants for salt and drought stress tolerance: Phytohormones, mineral nutrients and transgenics

Major abiotic stress factors such as salt and drought adversely affect important physiological processes and biochemical mechanisms and cause severe loss in crop productivity worldwide. Plants develop various strategies to stand healthy against these stress factors. The accumulation of proline (Pro) is one of the striking metabolic responses of plants to salt and drought stress. Pro biosynthesis and signalling contribute to the redox balance of cell under normal and stressful conditions. However, literature is meager on the sustainable strategies potentially fit for modulating Pro biosynthesis and production in stressed plants. Considering the recent literature, this paper in its first part overviews Pro biosynthesis and transport in plants and also briefly highlights the significance of Pro in plant responses to salt and drought stress. Secondly, this paper discusses mechanisms underlying the regulation of Pro metabolism in salt and drought-exposed plant via phytohormones, mineral nutrients and transgenic approaches. The outcome of the studies may give new opportunities in modulating Pro metabolism for improving plant tolerance to salt and drought stress and benefit sustainable agriculture.

Responses of photosynthesis, nitrogen and proline metabolism to salinity stress in Solanum lycopersicum under different levels of nitrogen supplementation

In the present study, effect of different levels of nitrogen (N₀, deprived; N₂₅, sub-optimum; N₇₅, optimum and N₁₅₀, supra-optimum) in Solanum lycopersicum L. seedlings under NaCl (NaCl₁, 0.3 g kg^-1 sand and NaCl₂, 0.5 g kg^-1sand) stress was investigated. Biomass accumulation, pigments, K⁺ concentration, nitrate and nitrite contents were declined by NaCl in dose dependent manner. As compared to control (N₇₅ without NaCl), fresh weight declined by 4% and 11%, and dry weight by 7 and 13% when seedlings were grown under N₇₅+NaCl₁ and N₇₅+NaCl₂ combinations, respectively. Furthermore, fluorescence parameters (JIP-test): the size and number of active reaction centres of photosynthetic apparatus (F_v/F₀), efficiency of water splitting complex (F₀/F_v), quantum yield of primary photochemistry (φP₀ or Phi_P₀), yield of electron transport per trapped excitation (Ψ₀ or Psi_0), the quantum yield of electron transport (φE₀), and performance index of PS II (PI_ABS) and parameters related to energy fluxes per reaction centre (ABS/RC, TR₀/RC, ET₀/RC and DI₀/RC) were also affected by NaCl. However, toxic effect of NaCl on photosystem II photochemistry was ameliorated by N. The lower dose (NaCl₁) of NaCl exerts damaging effect on oxidation side of PS II, while higher dose (NaCl₂) damages PS II reaction centre and its reduction side. Moreover, control seedlings (N₇₅ without NaCl) when exposed to NaCl₁ and NaCl₂ exhibited a significant enhancement in respiration rate by 6 and 16%, Na⁺ accumulation by 111 and 169% in shoot, and 141 and 223% in root and ammonium contents by 19 and 34% respectively. Nitrate and ammonium assimilating enzymes such as nitrate reductase (NR), nitrite reductase (NiR), glutamine synthetase (GS) and glutamate synthase (GOGAT) were adversely affected by NaCl stress while glutamate dehydrogenase (GDH) showed reverse trend. N addition caused further enhancement in free proline, and activity of Δ₁-pyrroline-5-carboxylate synthetase (P5CS), while activity of proline dehydrogenase (ProDH) decreased. The results indicate that different levels of N significantly modulated NaCl-induced damaging effects in tomato seedlings. Furthermore, the results suggest that after N addition Na⁺, nitrite, nitrate, ammonium contents, nitrogen metabolic enzymes, proline content, and activity of P5CS are favourably regulated, which might be associated with mitigation of NaCl stress and effect was more pronounced with supra-optimum level of N (N₁₅₀).

Water deficit stress-induced changes in carbon and nitrogen partitioning in Chenopodium quinoa Willd

Water deficit stress followed by re-watering during grain filling resulted in the induction of the ornithine pathway and in changes in Quinoa grain quality. The genetic diversity of Chenopodium quinoa Willd. (Quinoa) is accompanied by an outstanding environmental adaptability and high nutritional properties of the grains. However, little is known about the biochemical and physiological mechanisms associated with the abiotic stress tolerance of Quinoa. Here, we characterized carbon and nitrogen metabolic changes in Quinoa leaves and grains in response to water deficit stress analyzing their impact on the grain quality of two lowland ecotypes (Faro and BO78). Differences in the stress recovery response were found between genotypes including changes in the activity of nitrogen assimilation-associated enzymes that resulted in differences in grain quality. Both genotypes showed a common strategy to overcome water stress including the stress-induced synthesis of reactive oxygen species scavengers and osmolytes. Particularly, water deficit stress induced the stimulation of the ornithine and raffinose pathways. Our results would suggest that the regulation of C- and N partitioning in Quinoa during grain filling could be used for the improvement of the grain quality without altering grain yields.

Proline accumulation and metabolism-related genes expression profiles in Kosteletzkya virginica seedlings under salt stress

Proline accumulation is a common response to salt stress in many plants. Salt stress also increased proline concentration in roots, stems, and leaves of Kosteletzkya virginica seedling treated with 300 mM NaCl for 24 h and reached 3.75-, 4.76-, and 6.83-fold higher than controls. Further study on proline content in leaves under salt stress showed that proline content increased with increasing NaCl concentrations or time. The proline level peaked at 300 mM NaCl for 24 h and reached more than sixfold higher than control, but at 400 mM NaCl for 24 h proline content fell back slightly along with wilting symptom. To explore the cause behind proline accumulation, we first cloned full length genes related to proline metabolism including KvP5CS1, KvOAT, KvPDH, and KvProT from K. virginica and investigated their expression profiles. The results revealed that the expressions of KvP5CS1 and KvProT were sharply up-regulated by salt stress and the expression of KvOAT showed a slight increase with increasing salt concentrations or time, while the expression of KvPDH was not changed much and slightly decreased before 12 h and then returned to the original level. As the key enzyme genes for proline biosynthesis, the up-regulated expression of KvP5CS1 played a more important role than KvOAT for proline accumulation in leaves under salt stress. The low expression of KvPDH for proline catabolism also made a contribution to proline accumulation before 12 h.

Dynamic proline metabolism: importance and regulation in water limited environments

Drought-induced proline accumulation observed in many plant species has led to the hypothesis that further increases in proline accumulation would promote drought tolerance. Here we discuss both previous and new data showing that proline metabolism and turnover, rather than just proline accumulation, functions to maintain growth during water limitation. Mutants of Δ (1)-Pyrroline-5-Carboxylate Synthetase1 (P5CS1) and Proline Dehydrogenase1 (PDH1), key enzymes in proline synthesis and catabolism respectively, both have similar reductions in growth during controlled soil drying. Such results are consistent with patterns of natural variation in proline accumulation and with evidence that turnover of proline can act to buffer cellular redox status during drought. Proline synthesis and catabolism are regulated by multiple cellular mechanisms, of which we know only a few. An example of this is immunoblot detection of P5CS1 and PDH1 showing that the Highly ABA-induced (HAI) protein phosphatase 2Cs (PP2Cs) have different effects on P5CS1 and PDH1 protein levels despite having similar increases in proline accumulation. Immunoblot data also indicate that both P5CS1 and PDH1 are subjected to unknown post-translational modifications.

Plastid osmotic stress activates cellular stress responses in Arabidopsis

Little is known about cytoplasmic osmoregulatory mechanisms in plants, and even less is understood about how the osmotic properties of the cytoplasm and organelles are coordinately regulated. We have previously shown that Arabidopsis (Arabidopsis thaliana) plants lacking functional versions of the plastid-localized mechanosensitive ion channels Mechanosensitive Channel of Small Conductance-Like2 (MSL2) and MSL3 contain leaf epidermal plastids under hypoosmotic stress, even during normal growth and development. Here, we use the msl2 msl3 mutant as a model to investigate the cellular response to constitutive plastid osmotic stress. Under unstressed conditions, msl2 msl3 seedlings exhibited several hallmarks of drought or environmental osmotic stress, including solute accumulation, elevated levels of the compatible osmolyte proline (Pro), and accumulation of the stress hormone abscisic acid (ABA). Furthermore, msl2 msl3 mutants expressed Pro and ABA metabolism genes in a pattern normally seen under drought or osmotic stress. Pro accumulation in the msl2 msl3 mutant was suppressed by conditions that reduce plastid osmotic stress or inhibition of ABA biosynthesis. Finally, treatment of unstressed msl2 msl3 plants with exogenous ABA elicited a much greater Pro accumulation response than in the wild type, similar to that observed in plants under drought or osmotic stress. These results suggest that osmotic imbalance across the plastid envelope can elicit a response similar to that elicited by osmotic imbalance across the plasma membrane and provide evidence for the integration of the osmotic state of an organelle into that of the cell in which it resides.

Isolation and expression analysis of proline metabolism-related genes in Chrysanthemum lavandulifolium

Proline plays a significant role in plant resistance to abiotic stresses, and its level is determined by a combination of synthesis, catabolism and transport. The primary proteins involved are Δ(1)-pyrroline-5-carboxylate synthetase (P5CS), proline dehydrogenase (PDH) and proline transporter (ProT). To utilise proline metabolism to improve the stress resistance of Chrysanthemum×morifolium, we isolated two P5CS-homologous genes (ClP5CS1 and ClP5CS2), one PDH gene (ClPDH) and four ProT-homologous genes (ClProT1-4) (GenBANK accession numbers: KF743136-KF743142) from Chrysanthemum lavandulifolium, which is closely related to chrysanthemums and exhibits strong resistance to stresses. Expression analysis of these genes in different organs and under various stresses indicated that ClP5CSs showed substantial constitutive expression, while ClPDH was only strongly expressed in the capitulum and was inhibited under most stresses. The expression patterns of four ClProT genes presented characteristics of organ specificity and disparity under stresses. Above all, the expression of ClProT2 was restricted to above-ground organs, especially strong in the capitulum and could be obviously induced by various stress conditions. Promoters of ClPDH and ClProTs contained many cis-acting regulatory elements involved in stress responses and plant growth and development. High levels of free proline were found in flower buds, the capitulum under the non-stress condition and later periods of stress conditions except cold treatment. Interestingly, organ specificity and disparity also exist in the level of free proline under different stress conditions. Our study indicates that ClProTs play significant roles in proline accumulation and stress responses, and that ClProT2 could be used to genetically modify the stress resistance of chrysanthemums. In addition, proline metabolism might be closely related to plant flowering and floral development.

Proline and its metabolism enzymes in cucumber cell cultures during acclimation to salinity

Proline is an important osmolyte appearing as the result of salt stress response of plants. In the present study, we measured the proline concentration, activities of pyrroline-5-carboxylate synthetase (P5CS), pyrroline-5-carboxylate reductase (P5CR), and proline dehydrogenase (PDH) key regulatory enzymes in the biosynthesis and degradation of proline in the acclimated (AC20) and the non-acclimated (NAC) cucumber cell suspension cultures subjected to moderate (150 mM NaCl; AC20-150, NAC-150, respectively) and severe (200 mM NaCl; AC20-200, NAC-200, respectively) salt stress. The data showed that salt stress brought about a linear increase in proline content in both types of cultures. However, in the acclimated culture proline accumulation was observed earlier, in third hour after stress. Only in the acclimated culture moderate and severe stresses up-regulated P5CS activity throughout the experiment, whereas the activity of P5CR grew in response to both NaCl concentrations only in 24th and 48th hour. The severe salt stress resulted in decrease in P5CR in NAC-200 cultures. In response to salt stress, both types of cell suspension cultures reacted with decline in PDH activity below the spectrophotometrically detected level. Cell cultures vigor correlated with salt concentration and time of exposure to the stress factor. Both NaCl concentrations caused linear decline in vigor of the non-acclimated culture up to 80-90 % at the end of the experiment, whereas in the acclimated culture significant decrease by about 30-40 % was reached in 24th hour after stress. The presented data suggest that acclimation to salt stress up-regulated proline synthesis enzyme activity and caused intensive accumulations of proline by inhibiting its oxidation.

Salt stress encourages proline accumulation by regulating proline biosynthesis and degradation in Jerusalem artichoke plantlets

Proline accumulation is an important mechanism for osmotic regulation under salt stress. In this study, we evaluated proline accumulation profiles in roots, stems and leaves of Jerusalem artichoke (Helianthus tuberosus L.) plantlets under NaCl stress. We also examined HtP5CS, HtOAT and HtPDH enzyme activities and gene expression patterns of putative HtP5CS1, HtP5CS2, HtOAT, HtPDH1, and HtPDH2 genes. The objective of our study was to characterize the proline regulation mechanisms of Jerusalem artichoke, a moderately salt tolerant species, under NaCl stress. Jerusalem artichoke plantlets were observed to accumulate proline in roots, stems and leaves during salt stress. HtP5CS enzyme activities were increased under NaCl stress, while HtOAT and HtPDH activities generally repressed. Transcript levels of HtP5CS2 increased while transcript levels of HtOAT, HtPDH1 and HtPDH2 generally decreased in response to NaCl stress. Our results supports that for Jerusalem artichoke, proline synthesis under salt stress is mainly through the Glu pathway, and HtP5CS2 is predominant in this process while HtOAT plays a less important role. Both HtPDH genes may function in proline degradation.

The accumulation of endogenous proline induces changes in gene expression of several antioxidant enzymes in leaves of transgenic Swingle citrumelo

Plant exposure to abiotic stresses leads to an accumulation of reactive oxygen species with the concomitant increase in antioxidant defense mechanisms. Previous studies showed that exogenous application of proline mitigate the deleterious effects caused by oxidative stress due to its ability to increase the activity of antioxidant enzymes. However, there are no reports of the effects of high endogenous accumulation of proline in the transcriptional pattern of antioxidant enzymes genes under normal conditions of water supply or in response to water deficit. Here, we show that isoforms of four antioxidant enzymes genes (Ascorbate peroxidase-APX, Catalase-CAT, Superoxide dismutase-SOD and Glutathione reductase-GR) were differentially regulated in leaves of Swingle citrumelo transgenic plants with high endogenous proline accumulation submitted to water deficits and also under normal water supply condition. Proline per se caused a two-fold change in the transcription activity of APX1, APXcl, CAT2 and Cu/ZnSOD2, while during water deficit proline influenced mRNAs levels in APXs and Cu/ZnSODs isoforms, MnSODmit and GRcl. This study adds new information on the role of proline during drought conditions and, more important, without the potential confounding effects imposed by water deficiency. We showed that, in addition to its known effects on diverse plant physiological and biochemical processes, high endogenous proline can also acts as a regulatory/signalling molecule capable of altering the transcript levels of stress-related genes.

Exogenous proline and trehalose promote recovery of rice seedlings from salt-stress and differentially modulate antioxidant enzymes and expression of related genes

Proline (Pro) and Trehalose (Tre) function as compatible solutes and are upregulated in plants under abiotic stress. They play an osmoprotective role in physiological responses, enabling the plants to better tolerate the adverse effects of abiotic stress. We investigated the effect of exogenous Pro and Tre (10 mM) in seedlings of Thai aromatic rice (cv. KDML105; salt-sensitive) during salt stress and subsequent recovery. Salt stress (S, NaCl) resulted in growth reduction, increase in the Na(+)/K(+) ratio, increase in Pro level and up-regulation of Pro synthesis genes (pyrroline-5-carboxylatesynthetase, P5CS; pyrroline-5-carboxylate reductase, P5CR) as well as accumulation of hydrogen peroxide (H(2)O(2)), increased activity of antioxidative enzymes (superoxide dismutase, SOD; peroxidase, POX; ascorbate peroxidase, APX; catalase, CAT) and transcript up-regulation of genes encoding antioxidant enzymes (Cu/ZnSOD, MnSOD, CytAPX, CatC). Under salt stress, exogenous Pro (PS; Pro+NaCl) reduced the Na(+)/K(+) ratio, further increased endogenous Pro and transcript levels of P5CS and P5CR, but decreased the activity of the four antioxidant enzymes. The transcription of genes encoding several antioxidant enzymes was upregulated. Exogenous Tre (TS; Tre+NaCl) also reduced the Na(+)/K(+) ratio and strongly decreased endogenous Pro. Transcription of P5CS and P5CR was upregulated, the activities of SOD and POX decreased, the activity of APX increased and the transcription of all antioxidant enzyme genes upregulated. Although exogenous osmoprotectants did not alleviate growth inhibition during salt stress, they exhibited a pronounced beneficial effect during recovery period showing higher percentage of growth recovery in PS (162.38%) and TS (98.43%) compared with S (3.68%). During recovery, plants treated with PS showed a much greater reduction in endogenous Pro than NaCl-treated (S) or Tre-treated plants (TS). Increase in CAT activity was most related to significant reduction in H(2)O(2), particularly in the case of PS-treated plants. Advantageous effects of Pro were also associated with increase in APX activity during recovery.

Tobacco leaves and roots differ in the expression of proline metabolism-related genes in the course of drought stress and subsequent recovery

In plants, members of gene families differ in function and mode of regulation. Fine-tuning of the expression of individual genes helps plants to cope with a variable environment. Genes encoding proline dehydrogenase (PDH), the key enzyme in proline degradation, and the proline biosynthetic enzyme, Δ(1)-pyrroline-5-carboxylate synthetase (P5CS), play an important role in responses to osmotic and drought stresses. We compared the expression patterns of three PDH and two putative P5CS genes during drought stress progression and subsequent recovery. Whereas the NtPDH1 gene was affected little by dehydration or rehydration, the NtPDH2 gene responded rapidly to both conditions, and was down-regulated under drought. The CIG1 gene, encoding cytokinin-inducible PDH, exhibited an intermediate transcription pattern. Whereas P5CS B was not affected by the stress conditions, the P5CS A gene was highly up-regulated during drought stress. CIG1 and NtPDH1 transcription was not activated, and P5CS A was only partially reduced in leaves within 24-h after rehydration, a re-watering period sufficient for large physiological changes to occur. The lack of activation of tobacco PDH genes and incomplete reduction of the P5CS A gene in leaves within 24-h of rehydration may reflect the need for the protection of plants to potential subsequent stresses. The data indicate that recovery is a specific physiological process following different patterns in leaves and roots.

Essential role of tissue-specific proline synthesis and catabolism in growth and redox balance at low water potential

To better define the still unclear role of proline (Pro) metabolism in drought resistance, we analyzed Arabidopsis (Arabidopsis thaliana) Δ(1)-pyrroline-5-carboxylate synthetase1 (p5cs1) mutants deficient in stress-induced Pro synthesis as well as proline dehydrogenase (pdh1) mutants blocked in Pro catabolism and found that both Pro synthesis and catabolism were required for optimal growth at low water potential (ψ(w)). The abscisic acid (ABA)-deficient mutant aba2-1 had similar reduction in root elongation as p5cs1 and p5cs1/aba2-1 double mutants. However, the reduced growth of aba2-1 but not p5cs1/aba2-1 could be complemented by exogenous ABA, indicating that Pro metabolism was required for ABA-mediated growth protection at low ψ(w). PDH1 maintained high expression in the root apex and shoot meristem at low ψ(w) rather than being repressed, as in the bulk of the shoot tissue. This, plus a reduced oxygen consumption and buildup of Pro in the root apex of pdh1-2, indicated that active Pro catabolism was needed to sustain growth at low ψ(w). Conversely, P5CS1 expression was most highly induced in shoot tissue. Both p5cs1-4 and pdh1-2 had a more reduced NADP/NADPH ratio than the wild type at low ψ(w). These results indicate a new model of Pro metabolism at low ψ(w) whereby Pro synthesis in the photosynthetic tissue regenerates NADP while Pro catabolism in meristematic and expanding cells is needed to sustain growth. Tissue-specific differences in Pro metabolism and function in maintaining a favorable NADP/NADPH ratio are relevant to understanding metabolic adaptations to drought and efforts to enhance drought resistance.

Elevation of free proline and proline-rich protein levels by simultaneous manipulations of proline biosynthesis and degradation in plants

Proline-rich proteins (PRP) are cell wall and plasma membrane-anchored factors involved in cell wall maintenance and its stress-induced fortification. Here we compare the synthesis of P5C as the proline (Pro) precursor in the cytosol and chloroplast by an introduced alien system and evaluate correlation between PRP synthesis and free Pro accumulation in plants. We developed a Pro over-producing system by generating transgenic tobacco plants overexpressing E. coli P5C biosynthetic enzymes; Pro-indifferent gamma-glutamyl kinase 74 (GK74) and gamma-glutamylphosphate reductase (GPR), as well as antisensing proline dehydrogenase (ProDH) transcription. GK74 and GPR enzymes were targeted either to the cytosol or plastids. Molecular analyses indicated that the two bacterial enzymes are efficiently expressed in plant cells, correctly targeted to the cytosol or chloroplasts, and processed to active enzymatic complexes in the two compartments. Maximal Pro increase is obtained when GK74 and GPR are active in chloroplasts, and ProDH mRNA level is reduced by anti-sense silencing, resulting in more than 50-fold higher Pro content compared to that of wild type tobacco plants. The Pro over-producing system efficiently works in tobacco and Arabidopsis. The elevation of Pro levels promotes accumulation of ectopically expressed Cell Wall Linker Protein (AtCWLP), a membrane protein with an external Pro-rich domain. These results suggest that the Pro-generating system can support endogenous or alien PRP production in plants.

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.

Mercury-induced oxidative stress and impact on antioxidant enzymes in Chlamydomonas reinhardtii

Investigation of mercury toxicology in green algae is of great importance from ecological point of view, because mercury has become a major contaminant in recent years. In higher plants, accumulation of mercury modifies many aspects of cellular functions. However, the process that mercury exerts detrimental effects on green algae is largely unknown. In this study, we performed an experiment focusing on the biological responses of Chlamydomonas reinhardtii, a unicellular model organism, to Hg(2+)-induced toxicity. C. reinhardtii was exposed to 0, 1, 2, 4, 6, and 8 μM Hg in media. Concentrations of Hg were negatively correlated with the cell growth. Treatment with Hg induced accumulation of reactive oxygen species and peroxidative products. Endogenous proline levels increased in Hg-exposed algae. Hg exposure activated superoxide dismutase (SOD), catalase (CAT) and ascorbate peroxidase (APX). To get insights into the molecular response, a RT-PCR-based assay was performed to analyze the transcript abundance of Mn-SOD, CAT and APX. Our analysis revealed that expression of the genes was up-regulated by Hg exposure, with a pattern similar to the enzyme activities. Additional investigation was undertaken on the effect of Hg on the transcript amount of ∆(1)-pyrroline-5-carboxylate synthetase, a key enzyme of proline biosynthesis and on that of heme oxygenase-1 (HO-1), an enzyme regulating heavy metal tolerance. Expressions of both P5CS and HO-1 were up-regulated by Hg. These data indicate that Hg-induced oxidative stress was responsible for the disturbance of the growth and antioxidant defensive systems in C. reinhardtii.

Salicylic acid reduces napropamide toxicity by preventing its accumulation in rapeseed (Brassica napus L.)

Napropamide is a widely used herbicide for controlling weeds in crop production. However, extensive use of the herbicide has led to its accumulation in ecosystems, thus causing toxicity to crops and reducing crop production and quality. Salicylic acid (SA) plays multiple roles in regulating plant adaptive responses to biotic and environmental stresses. However, whether SA regulates plant response to herbicides (or pesticides) was unknown. In this study, we investigated the effect of SA on herbicide napropamide accumulation and biological processes in rapeseed (Brassica napus). Plants exposed to 8 mg kg(-1) napropamide showed growth stunt and oxidative damage. Treatment with 0.1 mM SA improved growth and reduced napropamide levels in plants. Treatment with SA also decreased the abundance of O (2) (-.) and H(2)O(2) as well as activities of superoxide dismutase (SOD), catalase (CAT), and ascorbate peroxidase (APX), and increased activities of guaiacol peroxidase (POD) and glutathione-S-transferase (GST) in napropamide-exposed plants. Analysis of SOD, CAT, and POD activities using nondenaturing polyacrylamide gel electrophoresis (PAGE) confirmed the results. These results may help to understand how SA regulates plant response to organic contaminants and provide a basis to control herbicide/pesticide contamination in crop production.

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.

Unraveling delta1-pyrroline-5-carboxylate-proline cycle in plants by uncoupled expression of proline oxidation enzymes

The two-step oxidation of proline in all eukaryotes is performed at the inner mitochondrial membrane by the consecutive action of proline dehydrogenase (ProDH) that produces Delta(1)-pyrroline-5-carboxylate (P5C) and P5C dehydrogenase (P5CDH) that oxidizes P5C to glutamate. This catabolic route is down-regulated in plants during osmotic stress, allowing free Pro accumulation. We show here that overexpression of MsProDH in tobacco and Arabidopsis or impairment of P5C oxidation in the Arabidopsis p5cdh mutant did not change the cellular Pro to P5C ratio under ambient and osmotic stress conditions, indicating that P5C excess was reduced to Pro in a mitochondrial-cytosolic cycle. This cycle, involving ProDH and P5C reductase, exists in animal cells and now demonstrated in plants. As a part of the cycle, Pro oxidation by the ProDH-FAD complex delivers electrons to the electron transport chain. Hyperactivity of the cycle, e.g. when an excess of exogenous l-Pro is provided, generates mitochondrial reactive oxygen species (ROS) by delivering electrons to O(2), as demonstrated by the mitochondria-specific MitoSox staining of superoxide ions. Lack of P5CDH activity led to higher ROS production under dark and light conditions in the presence of Pro excess, as well as rendered plants hypersensitive to heat stress. Balancing mitochondrial ROS production during increased Pro oxidation is therefore critical for avoiding Pro-related toxic effects. Hence, normal oxidation of P5C to Glu by P5CDH is key to prevent P5C-Pro intensive cycling and avoid ROS production from electron run-off.

Proline accumulation and transcriptional regulation of proline biosynthesis and degradation in Brassica napus

To understand the molecular mechanism underlying proline accumulation in Brassica napus, cDNAs encoding Delta(1)-pyrroline-5-carboxylate synthetase (BnP5CS), ornithine delta-aminotransferase (BnOAT) and proline dehydrogenase (BnPDH) were isolated and characterized. Southern blot analysis of BnP5CSs in B. napus and its diploid ancestors suggested a gene loss may have occurred during evolution. The expression of BnP5CS1 and BnP5CS2 was induced, while the expression of BnPDH was inhibited under salt stress, ABA treatment and dehydration, prior to proline accumulation. The upregulation of BnOAT expression was only detected during prolonged severe osmotic stress. Our results indicate that stress-induced proline accumulation in B. napus results from the reciprocal action of activated biosynthesis and inhibited proline degradation. Whether the ornithine pathway is activated depends on the severity of stress. During development, proline content was high in reproductive organs and was accompanied by markedly high expression of BnP5CS and BnPDH, suggesting possible roles of proline during flower development. [BMB reports 2009; 42(1): 28-34].

Copper-induced proline synthesis is associated with nitric oxide generation in Chlamydomonas reinhardtii

Excess copper affects the growth and metabolism of plants and green algae. However, the physiological processes under Cu stress are largely unknown. In this study, we investigated Cu-induced nitric oxide (NO) generation and its relationship to proline synthesis in Chlamydomonas reinhardtii. The test alga accumulated a large amount of proline after exposure to relatively low Cu concentrations (2.5 and 5.0 microM Cu2+). A concomitant increase in the intracellular NO level was observed with increasing concentrations of Cu applied. Data analysis revealed that the endogenous NO generated was positively associated with the proline level in Cu-stressed algae. The involvement of NO in Cu-induced proline accumulation was confirmed by using an NO-specific donor, sodium nitroprusside (SNP), and an NO scavenger cPTIO [2-(4-carboxy-2-phenyl)-4,4,5,5-tetramethylinidazoline-1-oxyl-3-oxide]. Pre-treatment with 10 microM SNP increased the proline accumulation in Cu-treated cells by about 1.5-fold, while this effect could be blocked by addition of 10 microM cPTIO. We further investigated the effect of Cu and NO on the activity and transcript amount of Delta(1)-pyrroline-5-carboxylate synthetase (P5CS, EC 2.7.2.11), the key enzyme of proline biosynthesis, and observed that application of SNP was able to stimulate the P5CS activity and up-regulate the expression of P5CS in the Cu-treated algae. These results indicate that Cu-responsive proline synthesis is closely related to NO generation in C. reinhardtii, suggesting the regulatory function of NO in proline metabolism under heavy metal stress.

Eco-physiological characteristics of alfalfa seedlings in response to various mixed salt-alkaline stresses

Soil salinization and alkalization frequently co-occur in nature, but little is known about the mixed effects of salt-alkaline stresses on plants. An experiment with mixed salts (NaCl, Na(2)SO(4), NaHCO(3) and Na(2)CO(3)) and 30 salt-alkaline combinations (salinity 24-120 mmol/L and pH 7.03-10.32) treating Medicago sativa seedlings was conducted. The results demonstrated that salinity and alkalinity significantly affected total biomass and biomass components of seedlings. There were interactive effects of salt composition and concentration on biomass (P<or= 0.001). The interactions between salinity and alkalinity stresses led to changes in the root activity along the salinity gradient (P<or= 0.001). The effects of alkalinity on seedling survival rate were more significant than those of salinity, and the seedlings demonstrated some physiological responses (leaf electrolyte leakage rate and proline content) in order to adapt to mixed salt-alkaline stresses. It was concluded that the mixed salt-alkaline stresses, which differ from either salt or alkali stress, emphasize the significant interaction between salt concentration (salinity) and salt component (alkalinity). Further, the effects of the interaction between high alkalinity and salinity are more severe than those of either salt or alkali stress, and such a cooperative interaction results in more sensitive responses of ecological and physiological characteristics in plants.

Identification of regulatory pathways involved in the reacquisition of root growth after salt stress in Medicago truncatula

Root growth and function are determined by the action of environmental stresses through specific genes that adapt root development to these restrictive conditions. We have defined in vitro conditions affecting the growth and recovery of Medicago truncatula roots after a salt stress. A dedicated macroarray containing 384 genes, based on a large-scale subtractive hybridization approach, was constructed and used to analyze gene expression during salt stress and recovery of root growth from this stress. Several potential regulatory genes were identified as being linked to this recovery process: a novel RNA-binding protein, a small G-protein homologous to ROP9, a receptor-like kinase, two TF IIIA-like and an AP2-like transcription factors (TF), MtZpt2-1, MtZpt2-2 and MtAp2, and a histidine kinase associated with cytokinin transduction pathways. The two ZPT2-type TFs were also rapidly induced by cold stress in roots. By analyzing transgenic M. truncatula plants showing reduced expression levels of both TFs and affected in their capacity to recover root growth after a salt stress, we identified potential target genes that were either activated or repressed in these plants. Overexpression of MtZpt2-1 in roots conferred salt tolerance and affected the expression of three putative targets in the predicted manner: a cold-regulated A (CORA) homolog, a flower-promoting factor (FPF1) homolog and an auxin-induced proline-rich protein (PRP) gene. Hence, regulatory networks depending on TFIIIA-like transcription factors are involved in the control of root adaptation to salt stress.

Two tobacco proline dehydrogenases are differentially regulated and play a role in early plant development

Proline dehydrogenase is the rate-limiting enzyme in proline degradation and serves important functions in the stress responses and development of plants. We isolated two tobacco proline dehydrogenases, NtPDH1 and NtPDH2, in the course of screening for genes upregulated in stressed tobacco (Nicotiana tabacum) microspores. Expression analysis revealed that the two genes are differentially regulated. Under unstressed conditions, their steady-state transcript levels were similar in mature pollen and apical meristems, whereas NtPDH2 was expressed predominantly in vegetative organs, styles, and ovules. The expression of NtPDH1 was maintained at a constant low level during 24 h of dehydration, whereas NtPDH2 was upregulated within 1 h after the onset of stress and subsequently downregulated to undetectable levels. Differential and sustained expression was also found for the two enzymatic isoforms of Arabidopsis thaliana AtPDH. Silencing of the NtPDH genes by RNA interference using the CaMV 35S promoter led to increased proline contents, decreased seed set, delayed seed germination and retarded seedling development pointing towards an important function of at least one of the two NtPDH genes during plant reproductive development.

Water and salinity stress in grapevines: early and late changes in transcript and metabolite profiles

Grapes are grown in semiarid environments, where drought and salinity are common problems. Microarray transcript profiling, quantitative reverse transcription-PCR, and metabolite profiling were used to define genes and metabolic pathways in Vitis vinifera cv. Cabernet Sauvignon with shared and divergent responses to a gradually applied and long-term (16 days) water-deficit stress and equivalent salinity stress. In this first-of-a-kind study, distinct differences between water deficit and salinity were revealed. Water deficit caused more rapid and greater inhibition of shoot growth than did salinity at equivalent stem water potentials. One of the earliest responses to water deficit was an increase in the transcript abundance of RuBisCo activase (day 4), but this increase occurred much later in salt-stressed plants (day 12). As water deficit progressed, a greater number of affected transcripts were involved in metabolism, transport, and the biogenesis of cellular components than did salinity. Salinity affected a higher percentage of transcripts involved in transcription, protein synthesis, and protein fate than did water deficit. Metabolite profiling revealed that there were higher concentrations of glucose, malate, and proline in water-deficit-treated plants as compared to salinized plants. The metabolite differences were linked to differences in transcript abundance of many genes involved in energy metabolism and nitrogen assimilation, particularly photosynthesis, gluconeogenesis, and photorespiration. Water-deficit-treated plants appear to have a higher demand than salinized plants to adjust osmotically, detoxify free radicals (reactive oxygen species), and cope with photoinhibition.

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.