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

Integrating transcriptomics and metabolomics to characterise the response of Astragalus membranaceus Bge. var. mongolicus (Bge.) to progressive drought stress

BACKGROUND

Astragalus membranaceus Bge. var. mongolicus (Bge.) Hsiao (A. mongolicus) is an important traditional Chinese herb that is cultivated on a large scale in northwestern China. Understanding plant responses to drought has important effects on ecological environment recovery and local economic development. Here, we combined transcriptomics (Illumina Hiseq 2000) and metabolomics ((1)H-NMR) to investigate how the roots of two-year-old A. mongolicus responded to 14 days of progressive drought stress.

RESULTS

The dried soil reduced the relative water content (RWC) of the leaves and biomass, induced the differential expression of a large fraction of the transcriptome and significantly altered the metabolic processes. PCA analysis demonstrated that the sucrose, proline, and malate metabolites contributed greatly to the separation. Strikingly, proline was increased by almost 60-fold under severe stress compared to the control. Some backbone pathways, including glycolysis, tricarboxylic acid (TCA) cycle, glutamate-mediated proline biosynthesis, aspartate family metabolism and starch and sucrose metabolism, were significantly affected by drought. An integrated analysis of the interaction between key genes and the altered metabolites involved in these pathways was performed.

CONCLUSIONS

Our findings demonstrated that the expression of drought-responsive genes showed a strong stress-dose dependency. Analysis of backbone pathways of the transcriptome and metabolome revealed specific genotypic responses to different levels of drought. The variation in molecular strategies to the drought may play an important role in how A. mongolicus and other legume crops adapt to drought stress.

Physiological and metabolomic analysis of Punica granatum (L.) under drought stress

Punica granatum has a noticeable adaptation to drought stress. The levels of the green leaf volatile trans-2-hexenal increased in response to drought stress suggesting a possible role of this compound in drought stress response in pomegranate. Punica granatum (L.) is a highly valued fruit crop for its health-promoting effects and it is mainly cultivated in semi-arid areas. Thus, understanding the response mechanisms to drought stress is of great importance. In the present research, a metabolomics analysis was performed to evaluate the effects of drought stress on volatile organic compounds extracted from the leaves of pomegranate plants grown under water shortage conditions. The time course experiment (7 days of water deprivation and 24-h recovery) consisted of three treatments (control, drought stress, and rehydration of drought-stressed plants). Plant weights were recorded and control plants were irrigated daily at pot capacity to provide the lost water. Fraction of transpirable soil water has been evaluated as indicator of soil water availability in stressed plants. The levels of proline, hydrogen peroxide and lipid peroxidation as well as of the photosynthetic parameters such as photosynthesis rate (A), stomatal conductance (g s), photosynthetic efficiency of photosystem II, and photochemical quenching were monitored after the imposition of drought stress and recovery as markers of plant stress. Constitutive carbon volatile components were analyzed in the leaf of control and drought-stressed leaves using Head Space Solid Phase Micro Extraction sampling coupled with Gas Chromatography Mass Spectrometry. A total of 12 volatile compounds were found in pomegranate leaf profiles, mainly aldehydes, alcohols, and organic acids. Among them, the trans-2-hexenal showed a significant increase in water-stressed and recovered leaves respect to the well-watered ones. These data evidence a possible role of the oxylipin pathway in the response to water stress in pomegranate plants.

Physiological and biochemical characterisation of watered and drought-stressed barley mutants in the HvDWARF gene encoding C6-oxidase involved in brassinosteroid biosynthesis

Brassinosteroids (BR) are plant steroid hormones that were discovered more than thirty years ago, but their physiological function has yet to be fully explained. The aim of the study was to answer the question of whether/how disturbances in the production of BR in barley affects the plant's metabolism and development under conditions of optimal watering and drought. Mutants with an impaired production of BR are one of the best tools in research aimed at understanding the mechanisms of action of these hormones. The study used barley cultivars with a normal BR synthesis (wild type) and semi-dwarf allelic mutants with an impaired activity of C6-oxidase (mutation in HvDWARF), which resulted in a decreased BR synthesis. Half of the plants were subjected to drought stress in the seedling stage and the other half were watered optimally. Plants with impaired BR production were characterised by a lower height and developmental retardation. Under both optimal watering and drought, BR synthesis disorders caused the reduced production of ABA and cytokinins, but not auxins. The BR mutants also produced less osmoprotectant (proline). The optimally watered and drought-stressed mutants accumulated less sucrose, which was accompanied by changes in the production of other soluble sugars. The increased content of fructooligosaccharide (kestose) in optimally watered mutants would suggest that BR is a negative regulator of kestose production. The decreased level of nystose in the drought-stressed mutants also suggests BR involvement in the regulation of the production of this fructooligosaccharide. The accumulation of the transcripts of genes associated with stress response (hsp90) was lower in the watered and drought-stressed BR-deficient mutants. In turn, the lower efficiency of photosystem II and the net photosynthetic rate in mutants was revealed only under drought conditions. The presented research allows for the physiological and biochemical traits of two BR-barley mutants to be characterised, which helps BR function to be understood. The knowledge can also be a good starting point for some breeding companies that are interested in introducing new semi-dwarf barley cultivars.

Effect of salt stress on ion concentration, proline content, antioxidant enzyme activities and gene expression in tomato cultivars

Salinity is a constraint limiting plant growth and productivity of crops throughout the world. Understanding the mechanism underlying plant response to salinity provides new insights into the improvement of salt tolerance-crops of importance. In the present study, we report on the responses of twenty cultivars of tomato. We have clustered genotypes into scale classes according to their response to increased NaCl levels. Three local tomato genotypes, representative of different saline scale classes, were selected for further investigation. During early (0 h, 6 h and 12 h) and later (7 days) stages of the response to salt treatment, ion concentrations (Na⁺, K⁺ and Ca²⁺), proline content, enzyme activities (catalase, ascorbate peroxidase and guiacol peroxidase) were recorded. qPCR analysis of candidate genes WRKY (8, 31and 39), ERF (9, 16 and 80), LeNHX (1, 3 and 4) and HKT (class I) were performed. A high K⁺, Ca^2 +and proline accumulation as well as a decrease of Na⁺ concentration-mediated salt tolerance. Concomitant with a pattern of high-antioxidant enzyme activities, tolerant genotypes also displayed differential patterns of gene expression during the response to salt stress.

Divergent Regulation of CBF Regulon on Cold Tolerance and Plant Phenotype in Cassava Overexpressing Arabidopsis CBF3 Gene

Cassava is a tropical origin plant that is sensitive to chilling stress. In order to understand the CBF cold response pathway, a well-recognized regulatory mechanism in temperate plants, in cassava, overexpression of an Arabidopsis CBF3 gene is studied. This gene renders cassava increasingly tolerant to cold and drought stresses but is associated with retarded plant growth, leaf curling, reduced storage root yield, and reduced anthocyanin accumulation in a transcript abundance-dependent manner. Physiological analysis revealed that the transgenic cassava increased proline accumulation, reduced malondialdehyde production, and electrolyte leakage under cold stress. These transgenic lines also showed high relative water content when faced with drought. The expression of partial CBF-targeted genes in response to cold displayed temporal and spatial variations in the wild-type and transgenic plants: highly inducible in leaves and less altered in apical buds. In addition, anthocyanin accumulation was inhibited by downregulating the expression of genes involved in its biosynthesis and by interplaying between the CBF3 and the endogenous transcription factors. Thus, the heterologous CBF3 modulates the expression of stress-related genes and carries out a series of physiological adjustments under stressful conditions, showing a varied regulation pattern of CBF regulon from that of cassava CBFs.

Hydrogen peroxide produced by NADPH oxidases increases proline accumulation during salt or mannitol stress in Arabidopsis thaliana

Many plants accumulate proline, a compatible osmolyte, in response to various environmental stresses such as water deficit and salinity. In some stress responses, plants generate hydrogen peroxide (H2 O2 ) that mediates numerous physiological and biochemical processes. The aim was to study the relationship between stress-induced proline accumulation and H2 O2 production. Using pharmacological and reverse genetic approaches in Arabidopsis thaliana, we investigated the role of NADPH oxidases, Respiratory burst oxidase homologues (Rboh), in the induction of proline accumulation was investigated in response to stress induced by either 200 mM NaCl or 400 mM mannitol. Stress from NaCl or mannitol resulted in a transient increase in H2 O2 content accompanied by accumulation of proline. Dimethylthiourea, a scavenger of H2 O2 , and diphenylene iodonium (DPI), an inhibitor of H2 O2 production by NADPH oxidase, were found to significantly inhibit proline accumulation in these stress conditions. DPI also reduced the expression level of Δ(1) -pyrroline-5-carboxylate synthetase, the key enzyme involved in the biosynthesis of proline. Similarly, less proline accumulated in knockout mutants lacking either AtRbohD or AtRbohF than in wild-type plants in response to the same stresses. Our data demonstrate that AtRbohs (A. thaliana Rbohs) contribute to H2 O2 production in response to NaCl or mannitol stress to increase proline accumulation in this plant.

Increased glutathione contributes to stress tolerance and global translational changes in Arabidopsis

Although glutathione is well known for its reactive oxygen species (ROS) scavenging function and plays a protective role in biotic stress, its regulatory function in abiotic stress still remains to be elucidated. Our previous study showed that exogenously applied reduced glutathione (GSH) could improve abiotic stress tolerance in Arabidopsis. Here, we report that endogenously increased GSH also conferred tolerance to drought and salt stress in Arabidopsis. Moreover, both exogenous and endogenous GSH delayed senescence and flowering time. Polysomal profiling results showed that global translation was enhanced after GSH treatment and by the induced increase of GSH level by salt stress. By performing transcriptomic analyses of steady-state and polysome-bound mRNAs in GSH-treated plants, we reveal that GSH has a substantial impact on translation. Translational changes induced by GSH treatment target numerous hormones and stress signaling molecules, which might contribute to the enhanced stress tolerance in GSH-treated plants. Our translatome analysis also revealed that abscisic acid (ABA), auxin and jasmonic acid (JA) biosynthesis, as well as signaling genes, were activated during GSH treatment, which has not been reported in previously published transcriptomic data. Together, our data suggest that the increased glutathione level results in stress tolerance and global translational changes.

Ethylene Response Factors: A Key Regulatory Hub in Hormone and Stress Signaling

Ethylene is essential for many developmental processes and a key mediator of biotic and abiotic stress responses in plants. The ethylene signaling and response pathway includes Ethylene Response Factors (ERFs), which belong to the transcription factor family APETALA2/ERF. It is well known that ERFs regulate molecular response to pathogen attack by binding to sequences containing AGCCGCC motifs (the GCC box), a cis-acting element. However, recent studies suggest that several ERFs also bind to dehydration-responsive elements and act as a key regulatory hub in plant responses to abiotic stresses. Here, we review some of the recent advances in our understanding of the ethylene signaling and response pathway, with emphasis on ERFs and their role in hormone cross talk and redox signaling under abiotic stresses. We conclude that ERFs act as a key regulatory hub, integrating ethylene, abscisic acid, jasmonate, and redox signaling in the plant response to a number of abiotic stresses.

Coping with drought: stress and adaptive responses in potato and perspectives for improvement

Potato (Solanum tuberosum L.) is often considered as a drought sensitive crop and its sustainable production is threatened due to frequent drought episodes. There has been much research aiming to understand the physiological, biochemical, and genetic basis of drought tolerance in potato as a basis for improving production under drought conditions. The complex phenotypic response of potato plants to drought is conditioned by the interactive effects of the plant's genotypic potential, developmental stage, and environment. Effective crop improvement for drought tolerance will require the pyramiding of many disparate characters, with different combinations being appropriate for different growing environments. An understanding of the interaction between below ground water uptake by the roots and above ground water loss from the shoot system is essential. The development of high throughput precision phenotyping platforms is providing an exciting new tool for precision screening, which, with the incorporation of innovative screening strategies, can aid the selection and pyramiding of drought-related genes appropriate for specific environments. Outcomes from genomics, proteomics, metabolomics, and bioengineering advances will undoubtedly compliment conventional breeding strategies and presents an alternative route toward development of drought tolerant potatoes. This review presents an overview of past research activity, highlighting recent advances with examples from other crops and suggesting future research directions.

Network Candidate Genes in Breeding for Drought Tolerant Crops

Climate change leading to increased periods of low water availability as well as increasing demands for food in the coming years makes breeding for drought tolerant crops a high priority. Plants have developed diverse strategies and mechanisms to survive drought stress. However, most of these represent drought escape or avoidance strategies like early flowering or low stomatal conductance that are not applicable in breeding for crops with high yields under drought conditions. Even though a great deal of research is ongoing, especially in cereals, in this regard, not all mechanisms involved in drought tolerance are yet understood. The identification of candidate genes for drought tolerance that have a high potential to be used for breeding drought tolerant crops represents a challenge. Breeding for drought tolerant crops has to focus on acceptable yields under water-limited conditions and not on survival. However, as more and more knowledge about the complex networks and the cross talk during drought is available, more options are revealed. In addition, it has to be considered that conditioning a crop for drought tolerance might require the production of metabolites and might cost the plants energy and resources that cannot be used in terms of yield. Recent research indicates that yield penalty exists and efficient breeding for drought tolerant crops with acceptable yields under well-watered and drought conditions might require uncoupling yield penalty from drought tolerance.

Stress physiology functions of the Arabidopsis histidine kinase cytokinin receptors

Cytokinin signaling has complex effects on abiotic stress responses that remain to be fully elucidated. The Arabidopsis histidine kinases (AHKs), AHK2, AHK3 and CRE1 (cytokinin response1/AHK4) are the principle cytokinin receptors of Arabidopsis. Using a set of ahk mutants, we found dramatic differences in response to low water potential and salt stress among the AHKs. ahk3-3 mutants had increased root elongation after transfer to low water potential media. Conversely ahk2-2 was hypersensitive to salt stress in terms of root growth and fresh weight and accumulated higher than wild-type levels of proline specifically under salt stress. Strongly reduced proline accumulation in ahk double mutants after low water potential treatment indicated a more general role of cytokinin signaling in proline metabolism. Reduced P5CS1 (Δ(1) -pyrroline-5-carboxylate synthetase1) gene expression may have contributed to this reduced proline accumulation. Low water potential phenotypes of ahk mutants were not caused by altered abscisic acid (ABA) accumulation as all ahk mutants had wild-type ABA levels, despite the observation that ahk double mutants had reduced NCED3 (9-cis-epoxycartenoid dioxygenase3) expression when exposed to low water potential. No difference in osmoregulatory solute accumulation was detected in any of the ahk mutants indicating that they do not affect drought responsive osmotic adjustment. Overall, our examination of ahk mutants found specific phenotypes associated with AHK2 and AHK3 as well as a general function of cytokinin signaling in proline accumulation and low water potential induction of P5CS1 and NCED3 expression. These results show the stress physiology function of AHKs at a new level of detail.

Overexpression of ZmMAPK1 enhances drought and heat stress in transgenic Arabidopsis thaliana

Mitogen-activated protein kinase (MAPK) signal transduction cascades play a crucial role in the response to extracellular stimuli in eukaryotes. A number of MAPK family genes have been isolated in plants, but the maize MAPK genes have been little studied. Here, we studied the role of maize MAP kinase 1 (ZmMAPK1) using gene expression, protein subcellular localization, transformation in Arabidopsis, expression patterns of the stress-responsive genes and physiological parameter analysis. Our physiological parameter analysis suggested that over-expression ZmMAPK1 can increase proline content and decrease malondialdehyde content under drought, and prevent chlorophyll loss and the production of scavenger reactive oxygen species under heat stress. The resistance characteristics of the over-expression of ZmMAPK1 were associated with a significant increase in survival rate. These results suggest that ZmMAPK1 plays a positive role in response to drought and heat stress in Arabidopsis, and provide new insights into the mechanisms of action of MAPK in response to abiotic stress in plants.

Variation in primary metabolites in parental and near-isogenic lines of the QTL qDTY12.1 : altered roots and flag leaves but similar spikelets of rice under drought

There is a widespread consensus that drought will mostly affect present and future agriculture negatively. Generating drought-tolerant crops is thus a high priority. However complicated the underlying genetic and regulatory networks for differences in plant performance under stress are, they would be reflected in straightforward differences in primary metabolites. This is because primary metabolites such as amino acids and sugars form the building blocks of all pathways and processes for growth, development, reproduction, and environmental responses. Comparison of such differences was undertaken between the parental line and a near-isogenic line of qDTY12.1 , a QTL for rice yield under drought. The comparison was informative regarding the effect of the QTL in three genetic backgrounds: donor, recipient, and improved recipient, thus illustrating the gene × gene (G × G) interactions. Such a comparison when extended to well-watered and drought conditions illustrated the gene × environment (G × E) interactions. Assessment of such G × G and G × E responses in roots, flag leaves, and spikelets added a yet more informative dimension of tissue-specific responses to drought, mediated by qDTY12.1 . Data on variation in primary metabolites subjected to ANOVA, Tukey's test, Welch's t test, and PCA underscored the importance of the roots and demonstrated concordance between variation in metabolites and morpho-physiological responses to drought. Results suggested that for gainful insights into rice yield under drought, rather than vegetative stage drought tolerance, multiple tissues and genotypes must be assessed at the reproductive stage to avoid misleading conclusions about using particular metabolites or related genes and proteins as candidates or markers for drought tolerance.

The metabolomics of oxidative stress

Oxidative stress resulting from increased availability of reactive oxygen species (ROS) is a key component of many responses of plants to challenging environmental conditions. The consequences for plant metabolism are complex and manifold. We review data on small compounds involved in oxidative stress, including ROS themselves and antioxidants and redox buffers in the membrane and soluble phases, and we discuss the wider consequences for plant primary and secondary metabolism. While metabolomics has been exploited in many studies on stress, there have been relatively few non-targeted studies focused on how metabolite signatures respond specifically to oxidative stress. As part of the discussion, we present results and reanalyze published datasets on metabolite profiles in catalase-deficient plants, which can be considered to be model oxidative stress systems. We emphasize the roles of ROS-triggered changes in metabolites as potential oxidative signals, and discuss responses that might be useful as markers for oxidative stress. Particular attention is paid to lipid-derived compounds, the status of antioxidants and antioxidant breakdown products, altered metabolism of amino acids, and the roles of phytohormone pathways.

Exploiting Differential Gene Expression and Epistasis to Discover Candidate Genes for Drought-Associated QTLs in Arabidopsis thaliana

Soil water availability represents one of the most important selective agents for plants in nature and the single greatest abiotic determinant of agricultural productivity, yet the genetic bases of drought acclimation responses remain poorly understood. Here, we developed a systems-genetic approach to characterize quantitative trait loci (QTLs), physiological traits and genes that affect responses to soil moisture deficit in the TSUxKAS mapping population of Arabidopsis thaliana. To determine the effects of candidate genes underlying QTLs, we analyzed gene expression as a covariate within the QTL model in an effort to mechanistically link markers, RNA expression, and the phenotype. This strategy produced ranked lists of candidate genes for several drought-associated traits, including water use efficiency, growth, abscisic acid concentration (ABA), and proline concentration. As a proof of concept, we recovered known causal loci for several QTLs. For other traits, including ABA, we identified novel loci not previously associated with drought. Furthermore, we documented natural variation at two key steps in proline metabolism and demonstrated that the mitochondrial genome differentially affects genomic QTLs to influence proline accumulation. These findings demonstrate that linking genome, transcriptome, and phenotype data holds great promise to extend the utility of genetic mapping, even when QTL effects are modest or complex.

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

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

Functional properties and structural characterization of rice δ(1)-pyrroline-5-carboxylate reductase

The majority of plant species accumulate high intracellular levels of proline to cope with hyperosmotic stress conditions. Proline synthesis from glutamate is tightly regulated at both the transcriptional and the translational levels, yet little is known about the mechanisms for post-translational regulation of the enzymatic activities involved. The gene coding in rice (Oryza sativa L.) for δ(1)-pyrroline-5-carboxylate (P5C) reductase, the enzyme that catalyzes the second and final step in this pathway, was isolated and expressed in Escherichia coli. The structural and functional properties of the affinity-purified protein were characterized. As for most species, rice P5C reductase was able to use in vitro either NADH or NADPH as the electron donor. However, strikingly different effects of cations and anions were found depending on the pyridine nucleotide used, namely inhibition of NADH-dependent activity and stimulation of NADPH-dependent activity. Moreover, physiological concentrations of proline and NADP(+) were strongly inhibitory for the NADH-dependent reaction, whereas the NADPH-dependent activity was mildly affected. Our results suggest that only NADPH may be used in vivo and that stress-dependent variations in ion homeostasis and NADPH/NADP(+) ratio could modulate enzyme activity, being functional in promoting proline accumulation and potentially also adjusting NADPH consumption during the defense against hyperosmotic stress. The apparent molecular weight of the native protein observed in size exclusion chromatography indicated a high oligomerization state. We also report the first crystal structure of a plant P5C reductase at 3.40-Å resolution, showing a decameric quaternary assembly. Based on the structure, it was possible to identify dynamic structural differences among rice, human, and bacterial enzymes.

P5CDH affects the pathways contributing to Pro synthesis after ProDH activation by biotic and abiotic stress conditions

Plants facing adverse conditions usually alter proline (Pro) metabolism, generating changes that help restore the cellular homeostasis. These organisms synthesize Pro from glutamate (Glu) or ornithine (Orn) by two-step reactions that share Δ(1) pyrroline-5-carboxylate (P5C) as intermediate. In the catabolic process, Pro is converted back to Glu using a different pathway that involves Pro dehydrogenase (ProDH), P5C dehydrogenase (P5CDH), and P5C as intermediate. Little is known about the coordination of the catabolic and biosynthetic routes under stress. To address this issue, we analyzed how P5CDH affects the activation of Pro synthesis, in Arabidopsis tissues that increase ProDH activity by transient exposure to exogenous Pro, or infection with Pseudomonas syringae pv. tomato. Wild-type (Col-0) and p5cdh mutant plants subjected to these treatments were used to monitor the Pro, Glu, and Orn levels, as well as the expression of genes from Pro metabolism. Col-0 and p5cdh tissues consecutively activated ProDH and Pro biosynthetic genes under both conditions. However, they manifested a different coordination between these routes. When external Pro supply was interrupted, wild-type leaves degraded Pro to basal levels at which point Pro synthesis, mainly via Glu, became activated. Under the same condition, p5cdh leaves sustained ProDH induction without reducing the Pro content but rather increasing it, apparently by stimulating the Orn pathway. In response to pathogen infection, both genotypes showed similar trends. While Col-0 plants seemed to induce both Pro biosynthetic routes, p5cdh mutant plants may primarily activate the Orn route. Our study contributes to the functional characterization of P5CDH in biotic and abiotic stress conditions, by revealing its capacity to modulate the fate of P5C, and prevalence of Orn or Glu as Pro precursors in tissues that initially consumed Pro.

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.

Mechanisms of salt tolerance in habanero pepper plants (Capsicum chinense Jacq.): Proline accumulation, ions dynamics and sodium root-shoot partition and compartmentation

Despite its economic relevance, little is known about salt tolerance mechanisms in pepper plants. To address this question, we compared differences in responses to NaCl in two Capsicum chinense varieties: Rex (tolerant) and Chichen-Itza (sensitive). Under salt stress (150 mM NaCl over 7 days) roots of Rex variety accumulated 50 times more compatible solutes such as proline compared to Chichen-Itza. Mineral analysis indicated that Na(+) is restricted to roots by preventing its transport to leaves. Fluorescence analysis suggested an efficient Na(+) compartmentalization in vacuole-like structures and in small intracellular compartments in roots of Rex variety. At the same time, Na(+) in Chichen-Itza plants was compartmentalized in the apoplast, suggesting substantial Na(+) extrusion. Rex variety was found to retain more K(+) in its roots under salt stress according to a mineral analysis and microelectrode ion flux estimation (MIFE). Vanadate-sensitive H(+) efflux was higher in Chichen-Itza variety plants, suggesting a higher activity of the plasma membrane H(+)-ATPase, which fuels the extrusion of Na(+), and, possibly, also the re-uptake of K(+). Our results suggest a combination of stress tolerance mechanisms, in order to alleviate the salt-induced injury. Furthermore, Na(+) extrusion to apoplast does not appear to be an efficient strategy for salt tolerance in pepper plants.

Proline metabolism increases katG expression and oxidative stress resistance in Escherichia coli

The oxidation of l-proline to glutamate in Gram-negative bacteria is catalyzed by the proline utilization A (PutA) flavoenzyme, which contains proline dehydrogenase (PRODH) and Δ(1)-pyrroline-5-carboxylate (P5C) dehydrogenase domains in a single polypeptide. Previous studies have suggested that aside from providing energy, proline metabolism influences oxidative stress resistance in different organisms. To explore this potential role and the mechanism, we characterized the oxidative stress resistance of wild-type and putA mutant strains of Escherichia coli. Initial stress assays revealed that the putA mutant strain was significantly more sensitive to oxidative stress than the parental wild-type strain. Expression of PutA in the putA mutant strain restored oxidative stress resistance, confirming that depletion of PutA was responsible for the oxidative stress phenotype. Treatment of wild-type cells with proline significantly increased hydroperoxidase I (encoded by katG) expression and activity. Furthermore, the ΔkatG strain failed to respond to proline, indicating a critical role for hydroperoxidase I in the mechanism of proline protection. The global regulator OxyR activates the expression of katG along with several other genes involved in oxidative stress defense. In addition to katG, proline increased the expression of grxA (glutaredoxin 1) and trxC (thioredoxin 2) of the OxyR regulon, implicating OxyR in proline protection. Proline oxidative metabolism was shown to generate hydrogen peroxide, indicating that proline increases oxidative stress tolerance in E. coli via a preadaptive effect involving endogenous hydrogen peroxide production and enhanced catalase-peroxidase activity.

A class-information-based penalized matrix decomposition for identifying plants core genes responding to abiotic stresses

In terms of making genes expression data more interpretable and comprehensible, there exists a significant superiority on sparse methods. Many sparse methods, such as penalized matrix decomposition (PMD) and sparse principal component analysis (SPCA), have been applied to extract plants core genes. Supervised algorithms, especially the support vector machine-recursive feature elimination (SVM-RFE) method, always have good performance in gene selection. In this paper, we draw into class information via the total scatter matrix and put forward a class-information-based penalized matrix decomposition (CIPMD) method to improve the gene identification performance of PMD-based method. Firstly, the total scatter matrix is obtained based on different samples of the gene expression data. Secondly, a new data matrix is constructed by decomposing the total scatter matrix. Thirdly, the new data matrix is decomposed by PMD to obtain the sparse eigensamples. Finally, the core genes are identified according to the nonzero entries in eigensamples. The results on simulation data show that CIPMD method can reach higher identification accuracies than the conventional gene identification methods. Moreover, the results on real gene expression data demonstrate that CIPMD method can identify more core genes closely related to the abiotic stresses than the other methods.

A step towards understanding plant responses to multiple environmental stresses: a genome-wide study

In natural habitats, especially in arid areas, plants are often simultaneously exposed to multiple abiotic stresses, such as salt, osmotic and heat stresses. However, most analyses of gene expression in stress responses examine individual stresses. In this report, we compare gene expression in individual and combined stresses. We show that combined stress treatments with salt, mannitol and heat induce a unique pattern of gene expression that is not a simple merge of the individual stress responses. Under multiple stress conditions, expression of most heat and salt stress-responsive genes increased to levels similar to or higher than those measured in single stress conditions, but osmotic stress-responsive genes increased to lower levels. Genes up-regulated to higher levels under multiple stress condition than single stress conditions include genes for heat shock proteins, heat shock regulators and late embryogenesis abundant proteins (LEAs), which protect other proteins from damage caused by stresses, suggesting their importance in multiple stress condition. Based on this analysis, we identify candidate genes for engineering crop plants tolerant to multiple stresses.

How reactive oxygen species and proline face stress together

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

Δ1-Pyrroline-5-carboxylate reductase from Arabidopsis thaliana: stimulation or inhibition by chloride ions and feedback regulation by proline depend on whether NADPH or NADH acts as co-substrate

Δ(1)-pyrroline-5-carboxylate (P5C) reductase (P5CR) catalyses the final step of proline synthesis in plants. In Arabidopsis thaliana, protein levels are correlated neither to the corresponding mRNA copy numbers, nor to intracellular proline concentrations. The occurrence of post-translational regulatory mechanisms has therefore been hypothesized, but never assessed. The purification of A. thaliana P5CR was achieved through either a six-step protocol from cultured cells, or heterologous expression of AtP5CR in Escherichia coli. The protein was characterized with respect to structural, kinetic, and biochemical properties. P5CR was able to use either NADPH or NADH as the electron donor, with contrasting affinities and maximum reaction rates. The presence of equimolar concentrations of NADP(+) completely suppressed the NADH-dependent activity, whereas the NADPH-dependent reaction was mildly affected. Proline inhibited only the NADH-dependent reaction. At physiological values, increasing concentrations of salt progressively inhibited the NADH-dependent activity, but were stimulatory of the NADPH-dependent reaction. The biochemical properties of A. thaliana P5CR suggest a complex regulation of enzyme activity by the redox status of the pyridine nucleotide pools, and the concentrations of proline and chloride in the cytosol. Data support a to date underestimated role of P5CR in controlling stress-induced proline accumulation.

The roles of reactive oxygen metabolism in drought: not so cut and dried

Drought is considered to cause oxidative stress, but the roles of oxidant-induced modifications in plant responses to water deficit remain obscure. Key unknowns are the roles of reactive oxygen species (ROS) produced at specific intracellular or apoplastic sites and the interactions between the complex, networking antioxidative systems in restricting ROS accumulation or in redox signal transmission. This Update discusses the physiological aspects of ROS production during drought, and analyzes the relationship between oxidative stress and drought from different but complementary perspectives. We ask to what extent redox changes are involved in plant drought responses and discuss the roles that different ROS-generating processes may play. Our discussion emphasizes the complexity and the specificity of antioxidant systems, and the likely importance of thiol systems in drought-induced redox signaling. We identify candidate drought-responsive redox-associated genes and analyze the potential importance of different metabolic pathways in drought-associated oxidative stress signaling.

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

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

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.

Context of action of proline dehydrogenase (ProDH) in the Hypersensitive Response of Arabidopsis

BACKGROUND

Proline (Pro) dehydrogenase (ProDH) potentiates the oxidative burst and cell death of the plant Hypersensitive Response (HR) by mechanisms not yet elucidated. ProDH converts Pro into ∆1 pyrroline-5-carboxylate (P5C) and can act together with P5C dehydrogenase (P5CDH) to produce Glu, or with P5C reductase (P5CR) to regenerate Pro and thus stimulate the Pro/P5C cycle. To better understand the effects of ProDH in HR, we studied the enzyme at three stages of the defense response differing in their ROS and cell death levels. In addition, we tested if ProDH requires P5CDH to potentiate HR.

RESULTS

Control and infected leaves of wild type and p5cdh plants were used to monitor ProDH activity, in vivo Pro catabolism, amino acid content, and gene expression. Wild type plants activated ProDH at all HR stages. They did not consume Pro during maximal ROS accumulation, and maintained almost basal P5C levels at all conditions. p5cdh mutants activated ProDH as wild type plants. They achieved maximum oxidative burst and cell death levels producing normal HR lesions, but evidenced premature defense activation.

CONCLUSION

ProDH activation has different effects on HR. Before the oxidative burst it leads to Pro consumption involving the action of P5CDH. During the oxidative burst, ProDH becomes functionally uncoupled to P5CDH and apparently works with P5CR. The absence of P5CDH does not reduce ROS, cell death, or pathogen resistance, indicating this enzyme is not accompanying ProDH in the potentiation of these defense responses. In contrast, p5cdh infected plants displayed increased ROS burst and earlier initiation of HR cell death. In turn, our results suggest that ProDH may sustain HR by participating in the Pro/P5C cycle, whose action on HR must be formally evaluated in a future.

Genome-wide association mapping combined with reverse genetics identifies new effectors of low water potential-induced proline accumulation in Arabidopsis

Arabidopsis (Arabidopsis thaliana) exhibits natural genetic variation in drought response, including varying levels of proline (Pro) accumulation under low water potential. As Pro accumulation is potentially important for stress tolerance and cellular redox control, we conducted a genome-wide association (GWAS) study of low water potential-induced Pro accumulation using a panel of natural accessions and publicly available single-nucleotide polymorphism (SNP) data sets. Candidate genomic regions were prioritized for subsequent study using metrics considering both the strength and spatial clustering of the association signal. These analyses found many candidate regions likely containing gene(s) influencing Pro accumulation. Reverse genetic analysis of several candidates identified new Pro effector genes, including thioredoxins and several genes encoding Universal Stress Protein A domain proteins. These new Pro effector genes further link Pro accumulation to cellular redox and energy status. Additional new Pro effector genes found include the mitochondrial protease LON1, ribosomal protein RPL24A, protein phosphatase 2A subunit A3, a MADS box protein, and a nucleoside triphosphate hydrolase. Several of these new Pro effector genes were from regions with multiple SNPs, each having moderate association with Pro accumulation. This pattern supports the use of summary approaches that incorporate clusters of SNP associations in addition to consideration of individual SNP probability values. Further GWAS-guided reverse genetics promises to find additional effectors of Pro accumulation. The combination of GWAS and reverse genetics to efficiently identify new effector genes may be especially applicable for traits difficult to analyze by other genetic screening methods.

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

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

Functional characterization of an ornithine cyclodeaminase-like protein of Arabidopsis thaliana

BACKGROUND

In plants, proline synthesis occurs by two enzymatic steps starting from glutamate as a precursor. Some bacteria, including bacteria such as Agrobacterium rhizogenes have an Ornithine Cyclodeaminase (OCD) which can synthesize proline in a single step by deamination of ornithine. In A. rhizogenes, OCD is one of the genes transferred to the plant genome during the transformation process and plants expressing A. rhizogenes OCD have developmental phenotypes. One nuclear encoded gene of Arabidopsis thaliana has recently been annotated as an OCD (OCD-like; referred to here as AtOCD) but nothing is known of its function. As proline metabolism contributes to tolerance of low water potential during drought, it is of interest to determine if AtOCD affects proline accumulation or low water potential tolerance.

RESULTS

Expression of AtOCD was induced by low water potential stress and by exogenous proline, but not by the putative substrate ornithine. The AtOCD protein was plastid localized. T-DNA mutants of atocd and AtOCD RNAi plants had approximately 15% higher proline accumulation at low water potential while p5cs1-4/atocd double mutants had 40% higher proline than p5cs1 at low water potential but no change in proline metabolism gene expression which could directly explain the higher proline level. AtOCD overexpression did not affect proline accumulation. Enzymatic assays with bacterially expressed AtOCD or AtOCD purified from AtOCD:Flag transgenic plants did not detect any activity using ornithine, proline or several other amino acids as substrates. Moreover, AtOCD mutant or over-expression lines had normal morphology and no difference in root elongation or flowering time, in contrast to previous report of transgenic plants expressing A. rhizogenes OCD. Metabolite analysis found few differences between AtOCD mutants and overexpression lines.

CONCLUSIONS

The Arabidopsis OCD-like protein (AtOCD) may not catalyze ornithine to proline conversion and this is consistent with observation that three residues critical for activity of bacterial OCDs are not conserved in AtOCD. AtOCD was, however, stress and proline induced and lack of AtOCD expression increased proline accumulation by an unknown mechanism which did not require expression of P5CS1, the main enzyme responsible for stress-induced proline synthesis from glutamate. The results suggest that AtOCD may have function distinct from bacterial OCDs.

Transcriptome sequencing and whole genome expression profiling of chrysanthemum under dehydration stress

Background

Chrysanthemum is one of the most important ornamental crops in the world and drought stress seriously limits its production and distribution. In order to generate a functional genomics resource and obtain a deeper understanding of the molecular mechanisms regarding chrysanthemum responses to dehydration stress, we performed large-scale transcriptome sequencing of chrysanthemum plants under dehydration stress using the Illumina sequencing technology.

Results

Two cDNA libraries constructed from mRNAs of control and dehydration-treated seedlings were sequenced by Illumina technology. A total of more than 100 million reads were generated and de novo assembled into 98,180 unique transcripts which were further extensively annotated by comparing their sequencing to different protein databases. Biochemical pathways were predicted from these transcript sequences. Furthermore, we performed gene expression profiling analysis upon dehydration treatment in chrysanthemum and identified 8,558 dehydration-responsive unique transcripts, including 307 transcription factors and 229 protein kinases and many well-known stress responsive genes. Gene ontology (GO) term enrichment and biochemical pathway analyses showed that dehydration stress caused changes in hormone response, secondary and amino acid metabolism, and light and photoperiod response. These findings suggest that drought tolerance of chrysanthemum plants may be related to the regulation of hormone biosynthesis and signaling, reduction of oxidative damage, stabilization of cell proteins and structures, and maintenance of energy and carbon supply.

Conclusions

Our transcriptome sequences can provide a valuable resource for chrysanthemum breeding and research and novel insights into chrysanthemum responses to dehydration stress and offer candidate genes or markers that can be used to guide future studies attempting to breed drought tolerant chrysanthemum cultivars.

Proline mechanisms of stress survival

SIGNIFICANCE

The imino acid proline is utilized by different organisms to offset cellular imbalances caused by environmental stress. The wide use in nature of proline as a stress adaptor molecule indicates that proline has a fundamental biological role in stress response. Understanding the mechanisms by which proline enhances abiotic/biotic stress response will facilitate agricultural crop research and improve human health.

RECENT ADVANCES

It is now recognized that proline metabolism propels cellular signaling processes that promote cellular apoptosis or survival. Studies have shown that proline metabolism influences signaling pathways by increasing reactive oxygen species (ROS) formation in the mitochondria via the electron transport chain. Enhanced ROS production due to proline metabolism has been implicated in the hypersensitive response in plants, lifespan extension in worms, and apoptosis, tumor suppression, and cell survival in animals.

CRITICAL ISSUES

The ability of proline to influence disparate cellular outcomes may be governed by ROS levels generated in the mitochondria. Defining the threshold at which proline metabolic enzyme expression switches from inducing survival pathways to cellular apoptosis would provide molecular insights into cellular redox regulation by proline. Are ROS the only mediators of proline metabolic signaling or are other factors involved?

FUTURE DIRECTIONS

New evidence suggests that proline biosynthesis enzymes interact with redox proteins such as thioredoxin. An important future pursuit will be to identify other interacting partners of proline metabolic enzymes to uncover novel regulatory and signaling networks of cellular stress response.

The agony of choice: how plants balance growth and survival under water-limiting conditions

When confronted with water limitation, plants actively reprogram their metabolism and growth. Recently, it has become clear that growing tissues show specific and highly dynamic responses to drought, which differ from the well-studied responses in mature tissues. Here, we provide an overview of recent advances in understanding shoot growth regulation in water-limiting conditions. Of special interest is the balance between maintained growth and competitiveness on the one hand and ensured survival on the other hand. A number of master regulators controlling this balance have been identified, such as DELLAs and APETALA2/ETHYLENE RESPONSE FACTOR-type transcription factors. The possibilities of engineering or breeding crops that maintain growth in periods of mild drought, while still being able to activate protective tolerance mechanisms, are discussed.

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

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

The Arabidopsis ETHYLENE RESPONSE FACTOR1 regulates abiotic stress-responsive gene expression by binding to different cis-acting elements in response to different stress signals

ETHYLENE RESPONSE FACTOR1 (ERF1) is an upstream component in both jasmonate (JA) and ethylene (ET) signaling and is involved in pathogen resistance. Accumulating evidence suggests that ERF1 might be related to the salt stress response through ethylene signaling. However, the specific role of ERF1 in abiotic stress and the molecular mechanism underlying the signaling cross talk still need to be elucidated. Here, we report that ERF1 was highly induced by high salinity and drought stress in Arabidopsis (Arabidopsis thaliana). The salt stress induction required both JA and ET signaling but was inhibited by abscisic acid. ERF1-overexpressing lines (35S:ERF1) were more tolerant to drought and salt stress. They also displayed constitutively smaller stomatal aperture and less transpirational water loss. Surprisingly, 35S:ERF1 also showed enhanced heat tolerance and up-regulation of heat tolerance genes compared with the wild type. Several suites of genes activated by JA, drought, salt, and heat were found in microarray analysis of 35S:ERF1. Chromatin immunoprecipitation assays found that ERF1 up-regulates specific suites of genes in response to different abiotic stresses by stress-specific binding to GCC or DRE/CRT. In response to biotic stress, ERF1 bound to GCC boxes but not DRE elements; conversely, under abiotic stress, we observed specific binding of ERF1 to DRE elements. Furthermore, ERF1 bound preferentially to only one among several GCC box or DRE/CRT elements in the promoter region of its target genes. ERF1 plays a positive role in salt, drought, and heat stress tolerance by stress-specific gene regulation, which integrates JA, ET, and abscisic acid signals.

Divergent low water potential response in Arabidopsis thaliana accessions Landsberg erecta and Shahdara

The Arabidopsis thaliana accession Shahdara (Sha) differs from Landsberg erecta (Ler) and other accessions in its responses to drought and low water potential including lower levels of proline accumulation. However, Sha maintained greater seedling root elongation at low water potential and a higher NADP/NADPH ratio than Ler. Profiling of major amino acids and organic acids found that Sha had reduced levels of all glutamate family amino acids metabolically related to proline, but increased levels of aspartate-derived amino acids (particularly isoleucine), leucine and valine at low water potential. Although Sha is known for its different abiotic stress response, RNA sequencing and co-expression clustering found that Sha differed most from Ler in defence/immune response and reactive oxygen-related gene expression. HVA22B and Osmotin34 were two of the relatively few abiotic stress-associated genes differentially expressed between Ler and Sha. Insensitivity to exogenous glutamine and a different expression profile of glutamate receptors were further factors that may underlie the differing metabolism and low water potential phenotypes of Sha. These data define the unique environmental adaptation and differing metabolism of Sha including differences in defence gene expression, and will facilitate further analysis of Sha natural variation to understand metabolic regulation and abiotic/biotic stress interaction.

Cold tolerance in Osmotin transgenic tomato (Solanum lycopersicum L.) is associated with modulation in transcript abundance of stress responsive genes

Our containment trials have established cold tolerance in Nicotiana tabacum osmotin (Nt Osm) transgenic tomato (Solanum lycopersicum L. cv. Pusa Ruby). Though, the stress tolerance mechanisms have been studied at physio-biochemical levels, molecular mechanisms underlying the tolerant response are still not well studied. Therefore, quantitative transcript expression of Osmotin and other stress responsive genes (CBF1, P5CS and APX) was studied in response to cold (4°C; 2 and 24 h) treatment in the transgenic and wild type tomato plants. The expression analysis revealed differential transcript regulation in the transgenic and wild type plants on the cold exposure. In general, the genes were either earlier induced or the extent of fold change in transcript expression over the respective untreated controls was higher in transgenic than in the wild type plants on cold exposure. The transcript expression data also supported the metabolite analysis on free Proline and ascorbate content. The results thus suggest that constitutive over expression of the Osmotin modulate transcript abundance and functional expression products of the other stress responsive genes thereby, imparting cold tolerance in the transgenic tomato plants.

Water stress induces a differential and spatially distributed nitro-oxidative stress response in roots and leaves of Lotus japonicus

Water stress is one of the most severe problems for plant growth and productivity. Using the legume Lotus japonicus exposed to water stress, a comparative analysis of key components in metabolism of reactive nitrogen and oxygen species (RNS and ROS, respectively) were made. After water stress treatment plants accumulated proline 23 and 10-fold in roots and leaves respectively, compared with well-watered plants. Significant changes in metabolism of RNS and ROS were observed, with an increase in both protein tyrosine nitration and lipid peroxidation, which indicate that water stress induces a nitro-oxidative stress. In roots, ·NO content was increased and S-nitrosoglutathione reductase activity was reduced by 23%, wherein a specific protein nitration pattern was observed. As part of this response, activity of NADPH-generating dehydrogenases was also affected in roots resulting in an increase of the NADPH/NADP(+) ratio. Our results suggest that in comparison with leaves, roots are significantly affected by water stress inducing an increase in proline and NO content which could highlight multiple functions for these metabolites in water stress adaptation, recovery and signaling. Thus, it is proposed that water stress generates a spatial distribution of nitro-oxidative stress with the oxidative stress component being higher in leaves whereas the nitrosative stress component is higher in roots.

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

Plants are commonly subjected to several environmental stresses that lead to an overproduction of reactive oxygen species (ROS). As plants accumulate proline in response to stress conditions, some authors have proposed that proline could act as a non-enzymatic antioxidant against ROS. One type of ROS aimed to be quenched by proline is singlet oxygen ((1)O(2))-molecular oxygen in its lowest energy electronically excited state-constitutively generated in oxygenic, photosynthetic organisms. In this study we clearly prove that proline cannot quench (1)O(2) in aqueous buffer, giving rise to a rethinking about the antioxidant role of proline against (1)O(2).

[Proline, a multifunctional amino-acid involved in plant adaptation to environmental constraints]

In addition to its role in primary metabolism as a component of proteins, proline is one of the most widely distributed compatible solutes that accumulates in plants during adverse environmental constraints and plays an important role in plant stress tolerance. Proline was proposed to act as stabilizer for proteins and macromolecular complexes, scavenger of free radicals and regulator of cellular redox potential. Intracellular proline concentration depends on a tight regulation between its biosynthesis and catabolism. However the exact role of proline and the signaling pathways involved in the regulation of its metabolism are not completely known yet. Investigation of proline metabolism in model plants would allow to acquire information about the diversity of the mechanisms developed by plants to overcome environmental constraints and to establish some reliable tools for the improvement of crop tolerance.

Role of the putative osmosensor Arabidopsis histidine kinase1 in dehydration avoidance and low-water-potential response

The molecular basis of plant osmosensing remains unknown. Arabidopsis (Arabidopsis thaliana) Histidine Kinase1 (AHK1) can complement the osmosensitivity of yeast (Saccharomyces cerevisiae) osmosensor mutants lacking Synthetic Lethal of N-end rule1 and SH3-containing Osmosensor and has been proposed to act as a plant osmosensor. We found that ahk1 mutants in either the Arabidopsis Nossen-0 or Columbia-0 background had increased stomatal density and stomatal index consistent with greater transpirational water loss. However, the growth of ahk1 mutants was not more sensitive to controlled moderate low water potential (ψ(w)) or to salt stress. Also, ahk1 mutants had increased, rather than reduced, solute accumulation across a range of low ψ(w) severities. ahk1 mutants had reduced low ψ(w) induction of Δ(1)-Pyrroline-5-Carboxylate Synthetase1 (P5CS1) and 9-cis-Epoxycarotenoid Dioxygenase3, which encode rate-limiting enzymes in proline and abscisic acid (ABA) synthesis, respectively. However, neither Pro nor ABA accumulation was reduced in ahk1 mutants at low ψ(w). P5CS1 protein level was not reduced in ahk1 mutants. This indicated that proline accumulation was regulated in part by posttranscriptional control of P5CS1 that was not affected by AHK1. Expression of AHK1 itself was reduced by low ψ(w), in contrast to previous reports. These results define a role of AHK1 in controlling stomatal density and the transcription of stress-responsive genes. These phenotypes may be mediated in part by reduced ABA sensitivity. More rapid transpiration and water depletion can also explain the previously reported sensitivity of ahk1 to uncontrolled soil drying. The unimpaired growth, ABA, proline, and solute accumulation of ahk1 mutants at low ψ(w) suggest that AHK1 may not be the main plant osmosensor required for low ψ(w) tolerance.

Requirement of proline synthesis during Arabidopsis reproductive development

BACKGROUND

Gamete and embryo development are crucial for successful reproduction and seed set in plants, which is often the determining factor for crop yield. Proline accumulation was largely viewed as a specific reaction to overcome stress conditions, while recent studies suggested important functions of proline metabolism also in reproductive development. Both the level of free proline and proline metabolism were proposed to influence the transition to flowering, as well as pollen and embryo development.

RESULTS

In this study, we performed a detailed analysis of the contribution of individual proline biosynthetic enzymes to vegetative development and reproductive success in Arabidopsis. In contrast to previous reports, we found that pyrroline-5-carboxylate (P5C) synthetase 2 (P5CS2) is not essential for sexual reproduction although p5cs2 mutant plants were retarded in vegetative development and displayed reduced fertility under long-day conditions. Single mutant plants devoid of P5CS1 did not show any developmental defects. Simultaneous absence of both P5CS isoforms resulted in pollen sterility, while fertile egg cells could still be produced. Expression of P5C reductase (P5CR) was indispensable for embryo development but surprisingly not needed for pollen or egg cell fertility. The latter observation could be explained by an extreme stability of P5CR activity, which had a half-life time of greater than 3 weeks in vitro. Expression of P5CR-GFP under the control of the endogenous P5CR promoter was able to restore growth of homozygous p5cr mutant embryos. The analysis of P5CR-GFP-fluorescence in planta supported an exclusively cytoplasmatic localisation of P5CR.

CONCLUSIONS

Our results demonstrate that potential alternative pathways for proline synthesis or inter-generation transfer of proline are not sufficient to overcome a defect in proline biosynthesis from glutamate during pollen development. Proline biosynthesis through P5CS2 and P5CR is limiting for vegetative and reproductive development in Arabidopsis, whereas disruption of P5CS1 alone does not affect development of non-stressed plants.

Cold tolerance in thiourea primed capsicum seedlings is associated with transcript regulation of stress responsive genes

Benefits of seed priming in seedling establishment and tolerance to subsequent stress exposure are well reported. However, the molecular mechanisms underlying the priming mediated benefits are not much discovered. Results of our earlier experiments established that thiourea (TU) seed priming imparts cold tolerance to capsicum seedlings. Therefore, to understand molecular mechanisms underlying priming mediated cold stress tolerance, quantitative transcript expression of stress responsive genes involved in transcript regulation (CaCBF1A, CaCBF1B, Zinc Finger protein, CaWRKY30), osmotic adjustment (PROX1, P5CS, Osmotin), antioxidant defence (CAT2, APX, GST, GR1, Cu/Zn SOD, Mn SOD, Fe SOD), signaling (Annexin), movement of solutes and water (CaPIP1), and metabolite biosynthesis through phenylpropanoid pathway (CAH) was studied in response to cold (4 °C; 4 and 24 h) stress in seedlings grown from the TU primed, hydroprimed and unsoaked seeds. The transcript expression of CaWRKY30, PROX1, Osmotin, Cu/Zn SOD and CAH genes was either higher or induced earlier on cold exposure in thiourea priming than that of hydroprimed and unsoaked over the respective unstressed controls. The results thus suggest that the TU priming modulate expression of these genes thereby imparting cold tolerance in capsicum seedlings.

Unique drought resistance functions of the highly ABA-induced clade A protein phosphatase 2Cs

Six Arabidopsis (Arabidopsis thaliana) clade A protein phosphatase 2Cs (PP2Cs) have established abscisic acid (ABA) signaling roles; however, phenotypic roles of the remaining three "HAI" PP2Cs, Highly ABA-Induced1 (HAI1), AKT1-Interacting PP2C1/HAI2, and HAI3, have remained unclear. HAI PP2C mutants had enhanced proline and osmoregulatory solute accumulation at low water potential, while mutants of other clade A PP2Cs had no or lesser effect on these drought resistance traits. hai1-2 also had increased expression of abiotic stress-associated genes, including dehydrins and late embryogenesis abundant proteins, but decreased expression of several defense-related genes. Conversely, the HAI PP2Cs had relatively less impact on several ABA sensitivity phenotypes. HAI PP2C single mutants were unaffected in ABA sensitivity, while double and triple mutants were moderately hypersensitive in postgermination ABA response but ABA insensitive in germination. The HAI PP2Cs interacted most strongly with PYL5 and PYL7 to -10 of the PYL/RCAR ABA receptor family, with PYL7 to -10 interactions being relatively little affected by ABA in yeast two-hybrid assays. HAI1 had especially limited PYL interaction. Reduced expression of the main HAI1-interacting PYLs at low water potential when HAI1 expression was strongly induced also suggests limited PYL regulation and a role of HAI1 activity in negatively regulating specific drought resistance phenotypes. Overall, the HAI PP2Cs had greatest effect on ABA-independent low water potential phenotypes and lesser effect on classical ABA sensitivity phenotypes. Both this and their distinct PYL interaction demonstrate a new level of functional differentiation among the clade A PP2Cs and a point of cross talk between ABA-dependent and ABA-independent drought-associated signaling.

Nitrogen availability impacts oilseed rape (Brassica napus L.) plant water status and proline production efficiency under water-limited conditions

Large amounts of nitrogen (N) fertilizers are used in the production of oilseed rape. However, as low-input methods of crop management are introduced crops will need to withstand temporary N deficiency. In temperate areas, oilseed rape will also be affected by frequent drought periods. Here we evaluated the physiological and metabolic impact of nitrate limitation on the oilseed rape response to water deprivation. Different amounts of N fertilizer were applied to plants at the vegetative stage, which were then deprived of water and rehydrated. Both water and N depletion accelerated leaf senescence and reduced leaf development. N-deprived plants exhibited less pronounced symptoms of wilting during drought, probably because leaves were smaller and stomata were partially closed. Efficiency of proline production, a major stress-induced diversion of nitrogen metabolism, was assessed at different positions along the whole plant axis and related to leaf developmental stage and water status indices. Proline accumulation, preferentially in younger leaves, accounted for 25-85% of the free amino acid pool. This was mainly due to a better capacity for proline synthesis in fully N-supplied plants whether they were subjected to drought or not, as deduced from the expression patterns of the proline metabolism BnP5CS and BnPDH genes. Although less proline accumulated in the oldest leaves, a significant amount was transported from senescing to emerging leaves. Moreover, during rehydration proline was readily recycled. Our results therefore suggest that proline plays a significant role in leaf N remobilization and in N use efficiency in oilseed rape.

Intron-mediated alternative splicing of Arabidopsis P5CS1 and its association with natural variation in proline and climate adaptation

Drought-induced proline accumulation is widely observed in plants but its regulation and adaptive value are not as well understood. Proline accumulation of the Arabidopsis accession Shakdara (Sha) was threefold less than that of Landsberg erecta (Ler) and quantitative trait loci mapping identified a reduced function allele of the proline synthesis enzyme Δ(1)-pyrroline-5-carboxylate synthetase1 (P5CS1) as a basis for the lower proline of Sha. Sha P5CS1 had additional TA repeats in intron 2 and a G-to-T transversion in intron 3 that were sufficient to promote alternative splicing and production of a nonfunctional transcript lacking exon 3 (exon 3-skip P5CS1). In Sha, and additional accessions with the same intron polymorphisms, the nonfunctional exon 3-skip P5CS1 splice variant constituted as much as half of the total P5CS1 transcript. In a larger panel of Arabidopsis accessions, low water potential-induced proline accumulation varied by 10-fold and variable production of exon 3-skip P5CS1 among accessions was an important, but not the sole, factor underlying variation in proline accumulation. Population genetic analyses suggest that P5CS1 may have evolved under positive selection, and more extensive correlation of exon 3-skip P5CS1 production than proline abundance with climate conditions of natural accessions also suggest a role of P5CS1 in local adaptation to the environment. These data identify a unique source of alternative splicing in plants, demonstrate a role of exon 3-skip P5CS1 in natural variation of proline metabolism, and suggest an association of P5CS1 and its alternative splicing with environmental adaptation.