Proline dehydrogenase contributes to pathogen defense in Arabidopsis

Oxidative stress induced in chloroplasts or mitochondria promotes proline accumulation in leaves of pea (Pisum sativum): another example of chloroplast-mitochondria interactions

Oxidative stress can occur in different parts of plant cells. We employed two oxidants that induce reactive oxygen species (ROS) in different intracellular compartments: methyl viologen (MV, in chloroplasts) and menadione (MD, in mitochondria). The responses of pea (Pisum sativum) leaf discs to MV or MD after 4-h incubation in dark or moderate (300 μE m-2 s-1) or high light (1200 μE m-2 s-1) were examined. Marked increase in ROS levels was observed, irrespective of compartment targeted. The levels of proline, a compatible solute, increased markedly much more than that of ascorbate or glutathione during oxidative/photo-oxidative stress, emphasizing the importance of proline. Further, the activities and transcripts of enzymes involved in biosynthesis or oxidation of proline were studied. An upregulation of biosynthesis and downregulation of oxidation was the basis of proline accumulation. Pyrroline-5-carboxylate synthetase (P5CS, involved in biosynthesis) and proline dehydrogenase (PDH, involved in oxidation) were the key enzymes regulated under oxidative stress. Since these two enzymes-P5CS and PDH-are located in chloroplasts and mitochondria, respectively, we suggest that proline metabolism can help to mediate inter-organelle interactions and achieve redox homeostasis under photo-oxidative stress.

Sweet Immunity: Inulin Boosts Resistance of Lettuce (Lactuca sativa) against Grey Mold (Botrytis cinerea) in an Ethylene-Dependent Manner

The concept of "Sweet Immunity" postulates that sugar metabolism and signaling influence plant immune networks. In this study, we tested the potential of commercially available inulin-type fructans to limit disease symptoms caused by Botrytis cinerea in lettuce. Spraying mature lettuce leaves, with inulin-type fructans derived from burdock or chicory was as effective in reducing grey mold disease symptoms caused by Botrytis cinerea as spraying with oligogalacturonides (OGs). OGs are well-known defense elicitors in several plant species. Spraying with inulin and OGs induced accumulation of hydrogen peroxide and levels further increased upon pathogen infection. Inulin and OGs were no longer able to limit Botrytis infection when plants were treated with the ethylene signaling inhibitor 1-methylcyclopropene (1-MCP), indicating that a functional ethylene signaling pathway is needed for the enhanced defense response. Soluble sugars accumulated in leaves primed with OGs, while 1-MCP treatment had an overall negative effect on the sucrose pool. Accumulation of γ-aminobutyric acid (GABA), a stress-associated non-proteinogenic amino acid and possible signaling compound, was observed in inulin-treated samples after infection and negatively affected by the 1-MCP treatment. We have demonstrated for the first time that commercially available inulin-type fructans and OGs can improve the defensive capacity of lettuce, an economically important species. We discuss our results in the context of a possible recognition of fructans as Damage or Microbe Associated Molecular Patterns.

Structural and Biochemical Characterization of Aldehyde Dehydrogenase 12, the Last Enzyme of Proline Catabolism in Plants

Heterokonts, Alveolata protists, green algae from Charophyta and Chlorophyta divisions, and all Embryophyta plants possess an aldehyde dehydrogenase (ALDH) gene named ALDH12. Here, we provide a biochemical characterization of two ALDH12 family members from the lower plant Physcomitrella patens and higher plant Zea mays. We show that ALDH12 encodes an NAD⁺-dependent glutamate γ-semialdehyde dehydrogenase (GSALDH), which irreversibly converts glutamate γ-semialdehyde (GSAL), a mitochondrial intermediate of the proline and arginine catabolism, to glutamate. Sedimentation equilibrium and small-angle X-ray scattering analyses reveal that in solution both plant GSALDHs exist as equilibrium between a domain-swapped dimer and the dimer-of-dimers tetramer. Plant GSALDHs share very low-sequence identity with bacterial, fungal, and animal GSALDHs (classified as ALDH4), which are the closest related ALDH superfamily members. Nevertheless, the crystal structure of ZmALDH12 at 2.2-Å resolution  shows that nearly all key residues involved in the recognition of GSAL are identical to those in ALDH4, indicating a close functional relationship with ALDH4. Phylogenetic analysis suggests that the transition from ALDH4 to ALDH12 occurred during the evolution of the endosymbiotic plant ancestor, prior to the evolution of green algae and land plants. Finally, ALDH12 expression in maize and moss is downregulated in response to salt and drought stresses, possibly to maintain proline levels. Taken together, these results provide molecular insight into the biological roles of the plant ALDH12 family.

Review: Functional linkages between amino acid transporters and plant responses to pathogens

Upon infection, plant pathogens become dependent on their hosts for nutrition. Therefore, the interaction between the two organisms is tightly linked to the availability and flux of nutrients in the plant. The plant's nitrogen metabolism is reprogrammed during pathogen attack, likely reflecting plant's response to invasion by the pathogen and active modification by the pathogen to promote feeding. Several lines of evidence indicate that plant-derived amino acids are an important source of nitrogen for diverse pathogens. Moreover, amino acid homeostasis is interconnected with the plant's immune signaling pathways. Here, we critically examine the knowns and unknowns about connections between plant-encoded amino acid transporters and resistance or susceptibility to pathogens and pests. We use recent insights into sugar transporters to frame a perspective with potential applicability to amino acids and other nutrients. We emphasize different approaches that have provided insight in this topic and we conclude with suggestions to fill gaps in foundational knowledge and explore new avenues for disease control.

Biological Roles of Ornithine Aminotransferase (OAT) in Plant Stress Tolerance: Present Progress and Future Perspectives

Plant tolerance to biotic and abiotic stresses is complicated by interactions between different stresses. Maintaining crop yield under abiotic stresses is the most daunting challenge for breeding resilient crop varieties. In response to environmental stresses, plants produce several metabolites, such as proline (Pro), polyamines (PAs), asparagine, serine, carbohydrates including glucose and fructose, and pools of antioxidant reactive oxygen species. Among these metabolites, Pro has long been known to accumulate in cells and to be closely related to drought, salt, and pathogen resistance. Pyrroline-5-carboxylate (P5C) is a common intermediate of Pro synthesis and metabolism that is produced by ornithine aminotransferase (OAT), an enzyme that functions in an alternative Pro metabolic pathway in the mitochondria under stress conditions. OAT is highly conserved and, to date, has been found in all prokaryotic and eukaryotic organisms. In addition, ornithine (Orn) and arginine (Arg) are both precursors of PAs, which confer plant resistance to drought and salt stresses. OAT is localized in the cytosol in prokaryotes and fungi, while OAT is localized in the mitochondria in higher plants. We have comprehensively reviewed the research on Orn, Arg, and Pro metabolism in plants, as all these compounds allow plants to tolerate different kinds of stresses.

Toxic effects of the fungicide tebuconazole on the root system of fusarium-infected wheat plants

The study investigates toxic effects of the fungicide tebuconazole (TEB) on Fusarium-infected wheat (Triticum aestivum) plants based on the morphological characteristics of root apices and changes in the integrated parameters of redox homeostasis, including the contents of free proline and products of peroxidation of proteins (carbonylated proteins, CP) and lipids (malondialdehyde, MDA) in roots. In two-day-old wheat sprouts infected by Fusarium graminearum, the levels of proline, CP, and border cells of root apices are higher than in roots of uninfected sprouts by a factor of 1.4, 8.0, and 3, respectively. The triazole fungicide tebuconazole (TEB) at the concentrations of 0.01, 0.10, and 1.00 μg ml^-1 of medium causes a dose-dependent decrease in the number of border cells. The study of the effects of TEB and fusarium infection on wheat plants in a 30-day experiment shows that the effect of the fungicide TEB on redox homeostasis in wheat roots varies depending on the plant growth stage and is significantly different in ecosystems with soil and plants infected by Fusarium phytopathogens. The study of the morphology of root apices shows that the toxic effects of TEB and fusarium infection are manifested in the destructive changes in root apices and the degradation of the root tip mantle.

Primed primary metabolism in systemic leaves: a functional systems analysis

Plants evolved mechanisms to counteract bacterial infection by preparing yet uninfected systemic tissues for an enhanced defense response, so-called systemic acquired resistance or priming responses. Primed leaves express a wide range of genes that enhance the defense response once an infection takes place. While hormone-driven defense signalling and defensive metabolites have been well studied, less focus has been set on the reorganization of primary metabolism in systemic leaves. Since primary metabolism plays an essential role during defense to provide energy and chemical building blocks, we investigated changes in primary metabolism at RNA and metabolite levels in systemic leaves of Arabidopsis thaliana plants that were locally infected with Pseudomonas syringae. Known defense genes were still activated 3–4 days after infection. Also primary metabolism was significantly altered. Nitrogen (N)-metabolism and content of amino acids and other N-containing metabolites were significantly reduced, whereas the organic acids fumarate and malate were strongly increased. We suggest that reduction of N-metabolites in systemic leaves primes defense against bacterial infection by reducing the nutritional value of systemic tissue. Increased organic acids serve as quickly available metabolic resources of energy and carbon-building blocks for the production of defense metabolites during subsequent secondary infections.

Proline Catabolism Modulates Innate Immunity in Caenorhabditis elegans

Metabolic pathways are regulated to fuel or instruct the immune responses to pathogen threats. However, the regulatory roles for amino acid metabolism in innate immune responses remains poorly understood. Here, we report that mitochondrial proline catabolism modulates innate immunity in Caenorhabditis elegans. Modulation of proline catabolic enzymes affects host susceptibility to bacterial pathogen Pseudomonas aeruginosa. Mechanistically, proline catabolism governs reactive oxygen species (ROS) homeostasis and subsequent activation of SKN-1, a critical transcription factor regulating xenobiotic stress response and pathogen defense. Intriguingly, proline catabolism-mediated activation of SKN-1 requires cell-membrane dual-oxidase Ce-Duox1/BLI-3, highlighting the importance of interaction between mitochondrial and cell-membrane components in host defense. Our findings reveal how animals utilize metabolism of a single amino acid to defend against a pathogen and identify proline catabolism as a component of innate immune signaling.

Climate Change, CO2, and Defense: The Metabolic, Redox, and Signaling Perspectives

Ongoing human-induced changes in the composition of the atmosphere continue to stimulate interest in the effects of high CO₂ on plants, but its potential impact on inducible plant defense pathways remains poorly defined. Recently, several studies have reported that growth at elevated CO₂ is sufficient to induce defenses such as the salicylic acid pathway, thereby increasing plant resistance to pathogens. These reports contrast with evidence that defense pathways can be promoted by photorespiration, which is inhibited at high CO₂. Here, we review signaling, metabolic, and redox processes modulated by CO₂ levels and discuss issues to be resolved in elucidating the relationships between primary metabolism, inducible defense, and biotic stress resistance.

Differential control and function of Arabidopsis ProDH1 and ProDH2 genes on infection with biotrophic and necrotrophic pathogens

Arabidopsis contains two proline dehydrogenase (ProDH) genes, ProDH1 and ProDH2, encoding for homologous and functional isoenzymes. Although ProDH1 has been studied extensively, especially under abiotic stress, ProDH2 has only started to be analysed in recent years. These genes display distinctive expression patterns and show weak transcriptional co-regulation, but are both activated in pathogen-infected tissues. We have demonstrated previously that Arabidopsis plants with silenced ProDH1/2 expression fail to trigger defences against the hemibiotrophic bacterial pathogen Pseudomonas syringae pv. tomato AvrRpm1 (Pst-AvrRpm1), and that ProDH1 and ProDH2 are differentially regulated by salicylic acid (SA). In the current work, we used prodh1 and prodh2 single-mutant plants to assess the particular contribution of each gene to resistance against Pst-AvrRpm1 and the necrotrophic fungal pathogen Botrytis cinerea. In addition, we studied the sensitivity of ProDH1 and ProDH2 to the jasmonic acid (JA) defence pathway. We found that ProDH1 and ProDH2 are both necessary to achieve maximum resistance against Pst-AvrRpm1 and B. cinerea. However, ProDH2 has a major effect on early restriction of B. cinerea growth. Interestingly, ProDH1 is up-regulated by SA and JA, whereas ProDH2 is only activated by JA, and both genes display transcriptional inter-regulation at basal and infection conditions. These studies provide the first evidence of the contribution of ProDH2 to disease resistance, and describe the differential regulation and non-redundant but complementary function of both enzyme isoforms in infected tissues, providing support for a fundamental role of ProDH in the control of biotrophic and necrotrophic pathogens.

Identification of Biomarkers for Defense Response to Plasmopara viticola in a Resistant Grape Variety

Downy mildew (Plasmopara viticola) is one of the most destructive diseases of the cultivated species Vitis vinifera. The use of resistant varieties, originally derived from backcrosses of North American Vitis spp., is a promising solution to reduce disease damage in the vineyards. To shed light on the type and the timing of pathogen-triggered resistance, this work aimed at discovering biomarkers for the defense response in the resistant variety Bianca, using leaf discs after inoculation with a suspension of P. viticola. We investigated primary and secondary metabolism at 12, 24, 48, and 96 h post-inoculation (hpi). We used methods of identification and quantification for lipids (LC-MS/MS), phenols (LC-MS/MS), primary compounds (GC-MS), and semi-quantification for volatile compounds (GC-MS). We were able to identify and quantify or semi-quantify 176 metabolites, among which 53 were modulated in response to pathogen infection. The earliest changes occurred in primary metabolism at 24-48 hpi and involved lipid compounds, specifically unsaturated fatty acid and ceramide; amino acids, in particular proline; and some acids and sugars. At 48 hpi, we also found changes in volatile compounds and accumulation of benzaldehyde, a promoter of salicylic acid-mediated defense. Secondary metabolism was strongly induced only at later stages. The classes of compounds that increased at 96 hpi included phenylpropanoids, flavonols, stilbenes, and stilbenoids. Among stilbenoids we found an accumulation of ampelopsin H + vaticanol C, pallidol, ampelopsin D + quadrangularin A, Z-miyabenol C, and α-viniferin in inoculated samples. Some of these compounds are known as phytoalexins, while others are novel biomarkers for the defense response in Bianca. This work highlighted some important aspects of the host response to P. viticola in a commercial variety under controlled conditions, providing biomarkers for a better understanding of the mechanism of plant defense and a potential application in field studies of resistant varieties.

The Reaumuria trigyna transcription factor RtWRKY1 confers tolerance to salt stress in transgenic Arabidopsis

Reaumuria trigyna (R. trigyna) is an endangered small shrub endemic to the Eastern Alxa-Western Ordos area in Inner Mongolia, China. Based on R. trigyna transcriptome data, the Group I WRKY transcription factor gene RtWRKY1 was cloned from R. trigyna. The full-length RtWRKY1 gene was 2100bp, including a 1261-bp open reading frame (ORF) encoding 573 amino acids. RtWRKY1 was mainly expressed in the stem and was induced by salt, cold stress, and ABA treatment. Overexpression of RtWRKY1 in Arabidopsis significantly enhanced the chlorophyll content, root length, and fresh weight of the transgenic lines under salt stress. RtWRKY1 transgenic Arabidopsis exhibited higher proline content, GSH-PX, POD, SOD, and CAT activities, and lower MDA content, Na⁺ content, and Na⁺/K⁺ ratio than wild-type Arabidopsis under salt stress conditions. Salt stress affected the expression of ion transport, proline biosynthesis, and antioxidant related genes, including AtAPX1, AtCAT1, AtSOD1, AtP5CS1, AtP5CS2, AtPRODH1, AtPRODH2, and AtSOS1 in transgenic lines. RtWRKY1 confers tolerance to salt stress in transgenic Arabidopsis by regulating plant growth, osmotic balance, Na⁺/K⁺ homeostasis, and the antioxidant system.

Durum wheat seedling responses to simultaneous high light and salinity involve a fine reconfiguration of amino acids and carbohydrate metabolism

Durum wheat plants are extremely sensitive to drought and salinity during seedling and early development stages. Their responses to stresses have been extensively studied to provide new metabolic targets and improving the tolerance to adverse environments. Most of these studies have been performed in growth chambers under low light [300-350 µmol m^-2 s^-1 photosynthetically active radiation (PAR), LL]. However, in nature plants have to face frequent fluctuations of light intensities that often exceed their photosynthetic capacity (900-2000 µmol m^-2 s^-1 ). In this study we investigated the physiological and metabolic changes potentially involved in osmotic adjustment and antioxidant defense in durum wheat seedlings under high light (HL) and salinity. The combined application of the two stresses decreased the water potential and stomatal conductance without reducing the photosynthetic efficiency of the plants. Glycine betaine (GB) synthesis was inhibited, proline and glutamate content decreased, while γ-aminobutyric acid (GABA), amides and minor amino acids increased. The expression level and enzymatic activities of Δ1-pyrroline-5-carboxylate synthetase, asparagine synthetase and glutamate decarboxylase, as well as other enzymatic activities of nitrogen and carbon metabolism, were analyzed. Antioxidant enzymes and metabolites were also considered. The results showed that the complex interplay seen in durum wheat plants under salinity at LL was simplified: GB and antioxidants did not play a main role. On the contrary, the fine tuning of few specific primary metabolites (GABA, amides, minor amino acids and hexoses) remodeled metabolism and defense processes, playing a key role in the response to simultaneous stresses.

Transcriptomic analysis reveals key early events of narciclasine signaling in Arabidopsis root apex

Histochemical staining and RNA-seq data demonstrated that the ROS- and plant hormone-regulated stress responses are the key early events of narciclasine signaling in Arabidopsis root cells. Narciclasine, an amaryllidaceae alkaloid isolated from Narcissus tazetta bulbs, employs a broad range of functions on plant development and growth. However, its molecular interactions that modulate these roles in plants are not fully understood. To elucidate the global responses of Arabidopsis roots to short-term narciclasine exposure, we first measured the accumulation of H₂O₂ and O₂^- with histochemical staining, and then profiled the gene expression pattern in Arabidopsis root tips treated with 0.5 µM narciclasine across different exposure times by RNA-seq. Physiological measurements showed a significant increase in H₂O₂ began at 30-60 min of narciclasine treatment and O₂^- accumulated by 120 min. Compared with controls, 236 genes were upregulated and 54 genes were downregulated with 2 h of narciclasine treatment, while 968 genes were upregulated and 835 genes were downregulated with 12 h of treatment. The Gene Ontology analysis revealed that the differentially expressed genes were highly enriched during oxidative stress, including those involved in the "regulation of transcription", "response to oxidative stress", "plant-pathogen interaction", "ribonucleotide binding", "plant cell wall organization", and "ribosome biogenesis". Moreover, Kyoto Encyclopedia of Genes and Genomes pathway enrichment statistics suggested that carbohydrate metabolism, amino acid metabolism, amino sugar and nucleotide sugar metabolism, and biosynthesis of phenylpropanoid and secondary metabolites were significantly inhibited by 12 h of narciclasine exposure. Hence, our results demonstrate that hormones and H₂O₂ are important regulators of narciclasine signaling and help to uncover the factors involved in the molecular interplay between narciclasine and phytohormones in Arabidopsis root cells.

Overexpression of Grain Amaranth (Amaranthus hypochondriacus) AhERF or AhDOF Transcription Factors in Arabidopsis thaliana Increases Water Deficit- and Salt-Stress Tolerance, Respectively, via Contrasting Stress-Amelioration Mechanisms

Two grain amaranth transcription factor (TF) genes were overexpressed in Arabidopsis plants. The first, coding for a group VII ethylene response factor TF (i.e., AhERF-VII) conferred tolerance to water-deficit stress (WS) in transgenic Arabidopsis without affecting vegetative or reproductive growth. A significantly lower water-loss rate in detached leaves coupled to a reduced stomatal opening in leaves of plants subjected to WS was associated with this trait. WS tolerance was also associated with an increased antioxidant enzyme activity and the accumulation of putative stress-related secondary metabolites. However, microarray and GO data did not indicate an obvious correlation between WS tolerance, stomatal closure, and abscisic acid (ABA)-related signaling. This scenario suggested that stomatal closure during WS in these plants involved ABA-independent mechanisms, possibly involving reactive oxygen species (ROS). WS tolerance may have also involved other protective processes, such as those employed for methyl glyoxal detoxification. The second, coding for a class A and cluster I DNA binding with one finger TF (i.e., AhDof-AI) provided salt-stress (SS) tolerance with no evident fitness penalties. The lack of an obvious development-related phenotype contrasted with microarray and GO data showing an enrichment of categories and genes related to developmental processes, particularly flowering. SS tolerance also correlated with increased superoxide dismutase activity but not with augmented stomatal closure. Additionally, microarray and GO data indicated that, contrary to AhERF-VII, SS tolerance conferred by AhDof-AI in Arabidopsis involved ABA-dependent and ABA-independent stress amelioration mechanisms.

Proline Coordination with Fatty Acid Synthesis and Redox Metabolism of Chloroplast and Mitochondria

Proline (Pro) accumulation is one of the most prominent changes in plant metabolism during drought and low water potential; however, the regulation and function of Pro metabolism remain unclear. We used a combination of forward genetic screening based on a Proline Dehydrogenase1 (PDH1) promoter-luciferase reporter (PDH1_pro:LUC2) and RNA sequencing of the Pro synthesis mutant p5cs1-4 to identify multiple loci affecting Pro accumulation in Arabidopsis (Arabidopsis thaliana). Two mutants having high PDH1_pro:LUC2 expression and increased Pro accumulation at low water potential were found to be alleles of Cytochrome P450, Family 86, Subfamily A, Polypeptide2 (CYP86A2) and Long Chain Acyl Synthetase2 (LACS2), which catalyze two successive steps in very-long-chain fatty acid (VLCFA) synthesis. Reverse genetic experiments found additional VLCFA and lipid metabolism-related mutants with increased Pro accumulation. Altered cellular redox status is a key factor in the coordination of Pro and VLCFA metabolism. The NADPH oxidase inhibitor diphenyleneiodonium (DPI) induced high levels of Pro accumulation and strongly repressed PDH1_pro:LUC2 expression. cyp86a2 and lacs2 mutants were hypersensitive to diphenyleneiodonium but could be reverted to wild-type Pro and PDH1_pro:LUC2 expression by reactive oxygen species scavengers. The coordination of Pro and redox metabolism also was indicated by the altered expression of chloroplast and mitochondria electron transport genes in p5cs1-4 These results show that Pro metabolism is both influenced by and influences cellular redox status via previously unknown coordination with several metabolic pathways. In particular, Pro and VLCFA synthesis share dual roles to help buffer cellular redox status while producing products useful for stress resistance, namely the compatible solute Pro and cuticle lipids.

Arabidopsis Proline Dehydrogenase Contributes to Flagellin-Mediated PAMP-Triggered Immunity by Affecting RBOHD

Plants activate different defense systems to counteract the attack of microbial pathogens. Among them, the recognition of conserved microbial- or pathogen-associated molecular patterns (MAMPs or PAMPs) by pattern-recognition receptors stimulates MAMP- or PAMP-triggered immunity (PTI). In recent years, the elicitors, receptors, and signaling pathways leading to PTI have been extensively studied. However, the contribution of organelles to this program deserves further characterization. Here, we studied how processes altering the mitochondrial electron transport chain (mETC) influence PTI establishment. With particular emphasis, we evaluated the effect of proline dehydrogenase (ProDH), an enzyme that can load electrons into the mETC and regulate the cellular redox state. We found that mETC uncouplers (antimycin or rotenone) and manganese superoxide dismutase deficiency impair flg22-induced responses such as accumulation of reactive oxygen species (ROS) and bacterial growth limitation. ProDH mutants also reduce these defenses, decreasing callose deposition as well. Using ProDH inhibitors and ProDH inducers (exogenous Pro treatment), we showed that this enzyme modulates the generation of ROS by the plasma membrane respiratory burst NADPH oxidase homolog D. In this way, we contribute to the understanding of mitochondrial activities influencing early and late PTI responses and the coordination of the redox-associated mitochondrial enzyme ProDH with defense events initiated at the plasma membrane.

Lysine acetylation in mitochondria: From inventory to function

Cellular signaling pathways are regulated in a highly dynamic fashion in order to quickly adapt to distinct environmental conditions. Acetylation of lysine residues represents a central process that orchestrates cellular metabolism and signaling. In mitochondria, acetylation seems to be the most prevalent post-translational modification, presumably linked to the compartmentation and high turnover of acetyl-CoA in this organelle. Similarly, the elevated pH and the higher concentration of metabolites in mitochondria seem to favor non-enzymatic lysine modifications, as well as other acylations. Hence, elucidating the mechanisms for metabolic control of protein acetylation is crucial for our understanding of cellular processes. Recent advances in mass spectrometry-based proteomics have considerably increased our knowledge of the regulatory scope of acetylation. Here, we review the current knowledge and functional impact of mitochondrial protein acetylation across species. We first cover the experimental approaches to identify and analyze lysine acetylation on a global scale, we then explore both commonalities and specific differences of plant and animal acetylomes and the evolutionary conservation of protein acetylation, as well as its particular impact on metabolism and diseases. Important future directions and technical challenges are discussed, and it is pointed out that the transfer of knowledge between species and diseases, both in technology and biology, is of particular importance for further advancements in this field.

The Roles of Mitochondrial Reactive Oxygen Species in Cellular Signaling and Stress Response in Plants

Mitochondria produce ATP via respiratory oxidation of organic acids and transfer of electrons to O2 via the mitochondrial electron transport chain. This process produces reactive oxygen species (ROS) at various rates that can impact respiratory and cellular function, affecting a variety of signaling processes in the cell. Roles in redox signaling, retrograde signaling, plant hormone action, programmed cell death, and defense against pathogens have been attributed to ROS generated in plant mitochondria (mtROS). The shortcomings of the black box-idea of mtROS are discussed in the context of mechanistic considerations and the measurement of mtROS The overall aim of this update is to better define our current understanding of mtROS and appraise their potential influence on cellular function in plants. Furthermore, directions for future research are provided, along with suggestions to increase reliability of mtROS measurements.

Comparative study of plant responses to carbon-based nanomaterials with different morphologies

The relationship between the morphology of carbon-based nanomaterials (CBNs) and the specific response of plants exposed to CBNs has not been studied systematically. Here, we prove that CBNs with different morphologies can activate cell growth, germination, and plant growth. A tobacco cell culture growth was found to increase by 22%-46% when CBNs such as helical multi-wall carbon nanotubes (MWCNTs), few-layered graphene, long MWCNTs, and short MWCNTs were added to the growth medium at a concentration of 50 μg ml(-1). The germination of exposed tomato seeds, as well as the growth of exposed tomato seedlings, were significantly enhanced by the addition of all tested CBNs. The presence of CBNs inside exposed seeds was confirmed by transmission electron microscopy and Raman spectroscopy. The effects of helical MWCNTs on gene expression in tomato seeds and seedlings were investigated by microarray technology and real time-PCR. Helical MWCNTs affected a number of genes involved in cellular and metabolic processes and response to stress factors. It was shown that the expression of the tomato water channel gene in tomato seeds exposed to helical MWCNTs was upregulated. These established findings demonstrate that CBNs with different morphologies can cause the same biological effects and share similar mechanisms in planta.

Transcriptome Analysis of Sunflower Genotypes with Contrasting Oxidative Stress Tolerance Reveals Individual- and Combined- Biotic and Abiotic Stress Tolerance Mechanisms

In nature plants are often simultaneously challenged by different biotic and abiotic stresses. Although the mechanisms underlying plant responses against single stress have been studied considerably, plant tolerance mechanisms under combined stress is not understood. Also, the mechanism used to combat independently and sequentially occurring many number of biotic and abiotic stresses has also not systematically studied. From this context, in this study, we attempted to explore the shared response of sunflower plants to many independent stresses by using meta-analysis of publically available transcriptome data and transcript profiling by quantitative PCR. Further, we have also analyzed the possible role of the genes so identified in contributing to combined stress tolerance. Meta-analysis of transcriptomic data from many abiotic and biotic stresses indicated the common representation of oxidative stress responsive genes. Further, menadione-mediated oxidative stress in sunflower seedlings showed similar pattern of changes in the oxidative stress related genes. Based on this a large scale screening of 55 sunflower genotypes was performed under menadione stress and those contrasting in oxidative stress tolerance were identified. Further to confirm the role of genes identified in individual and combined stress tolerance the contrasting genotypes were individually and simultaneously challenged with few abiotic and biotic stresses. The tolerant hybrid showed reduced levels of stress damage both under combined stress and few independent stresses. Transcript profiling of the genes identified from meta-analysis in the tolerant hybrid also indicated that the selected genes were up-regulated under individual and combined stresses. Our results indicate that menadione-based screening can identify genotypes not only tolerant to multiple number of individual biotic and abiotic stresses, but also the combined stresses.

Proteomic and functional analysis of proline dehydrogenase 1 link proline catabolism to mitochondrial electron transport in Arabidopsis thaliana

Proline accumulates in many plant species in response to environmental stresses. Upon relief from stress, proline is rapidly oxidized in mitochondria by proline dehydrogenase (ProDH) and then by pyrroline-5-carboxylate dehydrogenase (P5CDH). Two ProDH genes have been identified in the genome of the model plant Arabidopsis thaliana To gain a better understanding of ProDH1 functions in mitochondria, proteomic analysis was performed. ProDH1 polypeptides were identified in Arabidopsis mitochondria by immunoblotting gels after 2D blue native (BN)-SDS/PAGE, probing them with an anti-ProDH antibody and analysing protein spots by MS. The 2D gels showed that ProDH1 forms part of a low-molecular-mass (70-140 kDa) complex in the mitochondrial membrane. To evaluate the contribution of each isoform to proline oxidation, mitochondria were isolated from wild-type (WT) and prodh1, prodh2, prodh1prodh2 and p5cdh mutants. ProDH activity was high for genotypes in which ProDH, most likely ProDH1, was strongly induced by proline. Respiratory measurements indicate that ProDH1 has a role in oxidizing excess proline and transferring electrons to the respiratory chain.

Exogenous proteinogenic amino acids induce systemic resistance in rice

BACKGROUND

Plant immune responses can be induced by endogenous and exogenous signaling molecules. Recently, amino acids and their metabolites have been reported to affect the plant immune system. However, how amino acids act in plant defense responses has yet to be clarified. Here, we report that treatment of rice roots with amino acids such as glutamate (Glu) induced systemic disease resistance against rice blast in leaves.

RESULTS

Treatment of roots with Glu activated the transcription of a large variety of defense-related genes both in roots and leaves. In leaves, salicylic acid (SA)-responsive genes, rather than jasmonic acid (JA) or ethylene (ET)-responsive genes, were induced by this treatment. The Glu-induced blast resistance was partially impaired in rice plants deficient in SA signaling such as NahG plants expressing an SA hydroxylase, WRKY45-knockdown, and OsNPR1-knockdown plants. The JA-deficient mutant cpm2 exhibited full Glu-induced blast resistance.

CONCLUSIONS

Our results indicate that the amino acid-induced blast resistance partly depends on the SA pathway but an unknown SA-independent signaling pathway is also involved.

Overexpression of VaPAT1, a GRAS transcription factor from Vitis amurensis, confers abiotic stress tolerance in Arabidopsis

VaPAT1 functions as a stress-inducible GRAS gene and enhanced cold, drought and salt tolerance in transgenic Arabidopsis via modulation of the expression of a series of stress-related genes. The plant-specific GRAS transcription factor family regulates diverse processes involved in plant growth, development and stress responses. In this study, VaPAT1, a GRAS gene from Vitis amurensis was isolated and functionally characterized. Sequence alignment and phylogenetic analysis showed that VaPAT1 has a high sequence identity to CmsGRAS and OsCIGR1, which belong to PAT1 branch of GRAS family and function in stress resistance. The transcription of VaPAT1 was markedly induced by stress-related phytohormone abscisic acid (ABA) and various abiotic stress treatments such as cold, drought and high salinity, however, it was repressed by exogenous gibberellic acid (GA) application. Overexpression of VaPAT1 increased the cold, drought and high salinity tolerance in transgenic Arabidopsis. When compared with wild type (WT) seedlings, the VaPAT1-overexpression lines accumulated higher levels of proline and soluble sugar under these stress treatments. Moreover, stress-related genes such as AtSIZ1, AtCBF1, AtATR1/MYB34, AtMYC2, AtCOR15A, AtRD29A and AtRD29B showed higher expression levels in VaPAT1 transgenic lines than in WT Arabidopsis under normal growth conditions. Together, our results indicated that VaPAT1 functions as a positive transcriptional regulator involved in grapevine abiotic stress responses.

The synthetic cationic lipid diC14 activates a sector of the Arabidopsis defence network requiring endogenous signalling components

Natural and synthetic elicitors have contributed significantly to the study of plant immunity. Pathogen-derived proteins and carbohydrates that bind to immune receptors, allow the fine dissection of certain defence pathways. Lipids of a different nature that act as defence elicitors, have also been studied, but their specific effects have been less well characterized, and their receptors have not been identified. In animal cells, nanoliposomes of the synthetic cationic lipid 3-tetradecylamino-tert-butyl-N-tetradecylpropionamidine (diC14) activate the TLR4-dependent immune cascade. Here, we have investigated whether this lipid induces Arabidopsis defence responses. At the local level, diC14 activated early and late defence gene markers (FRK1, WRKY29, ICS1 and PR1), acting in a dose-dependent manner. This lipid induced the salicylic acid (SA)-dependent, but not jasmonic acid (JA)-dependent, pathway and protected plants against Pseudomonas syringae pv. tomato (Pst), but not Botrytis cinerea. diC14 was not toxic to plant or pathogen, and potentiated pathogen-induced callose deposition. At the systemic level, diC14 induced PR1 expression and conferred resistance against Pst. diC14-induced defence responses required the signalling protein EDS1, but not NDR1. Curiously, the lipid-induced defence gene expression was lower in the fls2/efr/cerk1 triple mutant, but still unchanged in the single mutants. The amidine headgroup and chain length were important for its activity. Given the robustness of the responses triggered by diC14, its specific action on a defence pathway and the requirement for well-known defence components, this synthetic lipid is emerging as a useful tool to investigate the initial events involved in plant innate immunity.

Proline affects the size of the root meristematic zone in Arabidopsis

BACKGROUND

We reported previously that root elongation in Arabidopsis is promoted by exogenous proline, raising the possibility that this amino acid may modulate root growth.

RESULTS

To evaluate this hypothesis we used a combination of genetic, pharmacological and molecular analyses, and showed that proline specifically affects root growth by modulating the size of the root meristem. The effects of proline on meristem size are parallel to, and independent from, hormonal pathways, and do not involve the expression of genes controlling cell differentiation at the transition zone. On the contrary, proline appears to control cell division in early stages of postembryonic root development, as shown by the expression of the G2/M-specific CYCLINB1;1 (CYCB1;1) gene.

CONCLUSIONS

The overall data suggest that proline can modulate the size of root meristematic zone in Arabidopsis likely controlling cell division and, in turn, the ratio between cell division and cell differentiation.

Arabidopsis AtbHLH112 regulates the expression of genes involved in abiotic stress tolerance by binding to their E-box and GCG-box motifs

Plant basic helix-loop-helix (bHLH) transcription factors play essential roles in abiotic stress tolerance. However, most bHLHs have not been functionally characterized. Here, we characterized the functional role of a bHLH transcription factor from Arabidopsis, AtbHLH112, in response to abiotic stress. AtbHLH112 is a nuclear-localized protein, and its nuclear localization is induced by salt, drought and abscisic acid (ABA). In addition, AtbHLH112 serves as a transcriptional activator, with the activation domain located at its N-terminus. In addition to binding to the E-box motifs of stress-responsive genes, AtbHLH112 binds to a novel motif with the sequence 'GG[GT]CC[GT][GA][TA]C' (GCG-box). Gain- and loss-of-function analyses showed that the transcript level of AtbHLH112 is positively correlated with salt and drought tolerance. AtbHLH112 mediates stress tolerance by increasing the expression of P5CS genes and reducing the expression of P5CDH and ProDH genes to increase proline levels. AtbHLH112 also increases the expression of POD and SOD genes to improve reactive oxygen species (ROS) scavenging ability. We present a model suggesting that AtbHLH112 is a transcriptional activator that regulates the expression of genes via binding to their GCG- or E-boxes to mediate physiological responses, including proline biosynthesis and ROS scavenging pathways, to enhance stress tolerance.

Connecting proline metabolism and signaling pathways in plant senescence

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

Bridging Between Proline Structure, Functions, Metabolism, and Involvement in Organism Physiology

Much is now known about proline multifunctionality and metabolism; some aspects of its biological functions are still unclear. Here, we discuss some cases in the proline, structure, definition, metabolism, compartmentalization, accumulation, plausible functions and also its implication in homeostasis and organism physiology. Indeed, we report the role of proline in cellular homeostasis, including redox balance and energy status and their implication as biocatalyst for aldolase activity. Proline can act as a signaling molecule to modulate mitochondrial functions, influence cell proliferation or cell death, and trigger specific gene expression, which can be essential for plant recovery from stresses. Although, the regulation and the function of proline accumulation, during abiotic stresses, are not yet completely understood. The engineering of proline metabolism could lead to new opportunities to improve plant tolerance against environmental stresses. This atypical amino acid has a potential role in the toxicity during growth of some microorganism, vegetal, and mammalian species. Furthermore, we note that the purpose through the work is to provide a rich, concise, and mostly cohesive source on proline, considered as a platform and an anchor between several disciplines and biological functions.

Expression and function of AtMBD4L, the single gene encoding the nuclear DNA glycosylase MBD4L in Arabidopsis

DNA glycosylases recognize and excise damaged or incorrect bases from DNA initiating the base excision repair (BER) pathway. Methyl-binding domain protein 4 (MBD4) is a member of the HhH-GPD DNA glycosylase superfamily, which has been well studied in mammals but not in plants. Our knowledge on the plant enzyme is limited to the activity of the Arabidopsis recombinant protein MBD4L in vitro. To start evaluating MBD4L in its biological context, we here characterized the structure, expression and effects of its gene, AtMBD4L. Phylogenetic analysis indicated that AtMBD4L belongs to one of the seven families of HhH-GPD DNA glycosylase genes existing in plants, and is unique on its family. Two AtMBD4L transcripts coding for active enzymes were detected in leaves and flowers. Transgenic plants expressing the AtMBD4L:GUS gene confined GUS activity to perivascular leaf tissues (usually adjacent to hydathodes), flowers (anthers at particular stages of development), and the apex of immature siliques. MBD4L-GFP fusion proteins showed nuclear localization in planta. Interestingly, overexpression of the full length MBD4L, but not a truncated enzyme lacking the DNA glycosylase domain, induced the BER gene LIG1 and enhanced tolerance to oxidative stress. These results suggest that endogenous MBD4L acts on particular tissues, is capable of activating BER, and may contribute to repair DNA damage caused by oxidative stress.

Mechanisms of induced susceptibility to Diplodia tip blight in drought-stressed Austrian pine

Plants experiencing drought stress are frequently more susceptible to pathogens, likely via alterations in physiology that create favorable conditions for pathogens. Common plant responses to drought include the production of reactive oxygen species (ROS) and the accumulation of free amino acids (AAs), particularly proline. These same phenomena also frequently occur during pathogenic attack. Therefore, drought-induced perturbations in AA and ROS metabolism could potentially contribute to the observed enhanced susceptibility. Furthermore, nitrogen (N) availability can influence AA accumulation and affect plant resistance, but its contributions to drought-induced susceptibility are largely unexplored. Here we show that drought induces accumulation of hydrogen peroxide (H2O2) in Austrian pine (Pinus nigra Arnold) shoots, but that shoot infection by the blight and canker pathogen Diplodia sapinea (Fr.) Fuckel leads to large reductions in H2O2 levels in droughted plants. In in vitro assays, H2O2 was toxic to D. sapinea, and the fungus responded to this oxidative stress by increasing catalase and peroxidase activities, resulting in substantial H2O2 degradation. Proline increased in response to drought and infection when examined independently, but unlike all other AAs, proline further increased in infected shoots of droughted trees. In the same tissues, the proline precursor, glutamate, decreased significantly. Proline was found to protect D. sapinea from H2O2 damage, while also serving as a preferred N source in vitro. Fertilization increased constitutive and drought-induced levels of some AAs, but did not affect plant resistance. A new model integrating interactions of proline and H2O2 metabolism with drought and fungal infection of plants is proposed.

ALD1 Regulates Basal Immune Components and Early Inducible Defense Responses in Arabidopsis

Robust immunity requires basal defense machinery to mediate timely responses and feedback cycles to amplify defenses against potentially spreading infections. AGD2-LIKE DEFENSE RESPONSE PROTEIN 1 (ALD1) is needed for the accumulation of the plant defense signal salicylic acid (SA) during the first hours after infection with the pathogen Pseudomonas syringae and is also upregulated by infection and SA. ALD1 is an aminotransferase with multiple substrates and products in vitro. Pipecolic acid (Pip) is an ALD1-dependent bioactive product induced by P. syringae. Here, we addressed roles of ALD1 in mediating defense amplification as well as the levels and responses of basal defense machinery. ALD1 needs immune components PAD4 and ICS1 (an SA synthesis enzyme) to confer disease resistance, possibly through a transcriptional amplification loop between them. Furthermore, ALD1 affects basal defense by controlling microbial-associated molecular pattern (MAMP) receptor levels and responsiveness. Vascular exudates from uninfected ALD1-overexpressing plants confer local immunity to the wild type and ald1 mutants yet are not enriched for Pip. We infer that, in addition to affecting Pip accumulation, ALD1 produces non-Pip metabolites that play roles in immunity. Thus, distinct metabolite signals controlled by the same enzyme affect basal and early defenses versus later defense responses, respectively.

Role of proline and pyrroline-5-carboxylate metabolism in plant defense against invading pathogens

Pyrroline-5-carboxylate (P5C) is an intermediate product of both proline biosynthesis and catabolism. Recent evidences indicate that proline-P5C metabolism is tightly regulated in plants, especially during pathogen infection and abiotic stress. However, role of P5C and its metabolism in plants has not yet been fully understood. Studies indicate that P5C synthesized in mitochondria has a role in both resistance (R)-gene-mediated and non-host resistance against invading pathogens. Proline dehydrogenase and delta-ornithine amino transferase-encoding genes, both involved in P5C synthesis in mitochondria are implicated in defense response of Nicotiana benthamiana and Arabidopsis thaliana against bacterial pathogens. Such defense response is proposed to involve salicylic acid-dependent pathway, reactive oxygen species (ROS) and hypersensitive response (HR)-associated cell death. Recently HR, a form of programmed cell death (PCD), has been proposed to be induced by changes in mitochondrial P5C synthesis or the increase in P5C levels per se in plants inoculated with either a host pathogen carrying suitable avirulent (Avr) gene or a non-host pathogen. Consistently, A. thaliana mutant plants deficient in P5C catabolism showed HR like cell death when grown in external P5C or proline supplemented medium. Similarly, yeast and plant cells under oxidative stress were shown to increase ROS production and PCD due to increase in P5C levels. Similar mechanism has also been reported as one of the triggers for apoptosis in mammalian cells. This review critically analyzes results from various studies and enumerates the pathways for regulation of P5C levels in the plant cell, especially in mitochondria, during pathogen infection. Further, mechanisms regulating P5C- mediated defense responses, namely HR are outlined. This review also provides new insights into the differential role of proline-P5C metabolism in plants exposed to pathogen infection.

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.

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

While intracellular proline accumulation in response to various stress conditions has been investigated in great detail, the biochemistry and physiological relevance of proline degradation in plants is much less understood. Moreover, the second and last step in proline catabolism, the oxidation of δ(1)-pyrroline-5-carboxylic acid (P5C) to glutamate, is shared with arginine catabolism. Little information is available to date concerning the regulatory mechanisms coordinating these two pathways. Expression of the gene coding for P5C dehydrogenase was analyzed in rice by real-time PCR either following the exogenous supply of amino acids of the glutamate family, or under hyperosmotic stress conditions. The rice enzyme was heterologously expressed in E. coli, and the affinity-purified protein was thoroughly characterized with respect to structural and functional properties. A tetrameric oligomerization state was observed in size exclusion chromatography, which suggests a structure of the plant enzyme different from that shown for the bacterial P5C dehydrogenases structurally characterized to date. Kinetic analysis accounted for a preferential use of NAD(+) as the electron acceptor. Cations were found to modulate enzyme activity, whereas anion effects were negligible. Several metal ions were inhibitory in the micromolar range. Interestingly, arginine also inhibited the enzyme at higher concentrations, with a mechanism of uncompetitive type with respect to P5C. This implies that millimolar levels of arginine would increase the affinity of P5C dehydrogenase toward its specific substrate. Results are discussed in view of the involvement of the enzyme in either proline or arginine catabolism.

New insights into plant salt acclimation: the roles of vesicle trafficking and reactive oxygen species signalling in mitochondria and the endomembrane system

In this study, we investigated the cellular and molecular mechanisms that regulate salt acclimation. The main objective was to obtain new insights into the molecular mechanisms that control salt acclimation. Therefore, we carried out a multidisciplinary study using proteomic, transcriptomic, subcellular and physiological techniques. We obtained a Nicotiana tabacum BY-2 cell line acclimated to be grown at 258 mM NaCl as a model for this study. The proteomic and transcriptomic data indicate that the molecular response to stress (chaperones, defence proteins, etc.) is highly induced in these salt-acclimated cells. The subcellular results show that salt induces sodium compartmentalization in the cell vacuoles and seems to be mediated by vesicle trafficking in tobacco salt-acclimated cells. Our results demonstrate that abscisic acid (ABA) and proline metabolism are crucial in the cellular signalling of salt acclimation, probably regulating reactive oxygen species (ROS) production in the mitochondria. ROS may act as a retrograde signal, regulating the cell response. The network of endoplasmic reticulum and Golgi apparatus is highly altered in salt-acclimated cells. The molecular and subcellular analysis suggests that the unfolded protein response is induced in salt-acclimated cells. Finally, we propose that this mechanism may mediate cell death in salt-acclimated cells.

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.

Evolution of proline biosynthesis: enzymology, bioinformatics, genetics, and transcriptional regulation

Proline is not only an essential component of proteins but it also has important roles in adaptation to osmotic and dehydration stresses, redox control, and apoptosis. Here, we review pathways of proline biosynthesis in the three domains of life. Pathway reconstruction from genome data for hundreds of eubacterial and dozens of archaeal and eukaryotic organisms revealed evolutionary conservation and variations of this pathway across different taxa. In the most prevalent pathway of proline synthesis, glutamate is phosphorylated to γ-glutamyl phosphate by γ-glutamyl kinase, reduced to γ-glutamyl semialdehyde by γ-glutamyl phosphate reductase, cyclized spontaneously to Δ(1)-pyrroline-5-carboxylate and reduced to proline by Δ(1)-pyrroline-5-carboxylate reductase. In higher plants and animals the first two steps are catalysed by a bi-functional Δ(1) -pyrroline-5-carboxylate synthase. Alternative pathways of proline formation use the initial steps of the arginine biosynthetic pathway to ornithine, which can be converted to Δ(1)-pyrroline-5-carboxylate by ornithine aminotransferase and then reduced to proline or converted directly to proline by ornithine cyclodeaminase. In some organisms, the latter pathways contribute to or could be fully responsible for the synthesis of proline. The conservation of proline biosynthetic enzymes and significance of specific residues for catalytic activity and allosteric regulation are analysed on the basis of protein structural data, multiple sequence alignments, and mutant studies, providing novel insights into proline biosynthesis in organisms. We also discuss the transcriptional control of the proline biosynthetic genes in bacteria and plants.

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.

Involvement of proline oxidase (PutA) in programmed cell death of Xanthomonas

Xanthomonas campestris strains have been reported to undergo programmed cell death (PCD) in a protein rich medium. Protein hydrolysates used in media such as nutrient broth comprise of casein digest with abundance of proline and glutamate. In the current study, X. campestris pv. campestris (Xcc) cells displayed PCD when grown in PCD inducing medium (PIM) containing casein tryptic digest. This PCD was also observed in PCD non-inducing carbohydrate rich medium (PNIM) fortified with either proline or proline along with glutamate. Surprisingly, no PCD was noticed in PNIM fortified with glutamate alone. Differential role of proline or glutamate in inducing PCD in Xcc cells growing in PNIM was studied. It was found that an intermediate product of this oxidation was involved in initiation of PCD. Proline oxidase also called as proline utilization A (PutA), catalyzes the two step oxidation of proline to glutamate. Interestingly, higher PutA activity was noticed in cells growing in PIM, and PCD was found to be inhibited by tetrahydro-2-furoic acid, a competitive inhibitor of this enzyme. Further, PCD was abolished in Xcc ΔputA strain generated using a pKNOCK suicide plasmid, and restored in Xcc ΔputA strain carrying functional PutA in a plasmid vector. Xanthomonas cells growing in PIM also displayed increased generation of ROS, as well as cell filamentation (a probable indication of SOS response). These filamented cells also displayed enhanced caspase-3-like activity during in situ labeling using a fluorescent tagged caspase-3 inhibitor (FITC-DEVD-FMK). The extent of PCD associated markers such as DNA damage, phosphatidylserine externalization and membrane depolarization were found to be significantly enhanced in wild type cells, but drastically reduced in Xcc ΔputA cells. These findings thus establish the role of PutA mediated proline oxidation in regulating death in stressed Xanthomonas cells.

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.

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.

Expression, Purification and Characterization of the Proline Dehydrogenase Domain of PutA from Pseudomonas putida POS-F84

The objective of the present work was to express a truncated form of Pseudomonas putida PutA that shows proline dehydrogenase (ProDH) activity. The putA gene encoding ProDH enzyme was cloned into pET23a vector and expressed in Escherichia coli strain BL-21 (DE3) plysS. The recombinant P. putida enzyme was biochemically characterized and its three dimensional structure was also predicted. ProDH encoding sequence showed an open reading frame of 1,035-bp encoding a 345 amino acid residues polypeptide chain. Purified His-tagged enzyme gave a single band with a molecular mass of 40 kDa on SDS-PAGE. The molecular mass of the isolated enzyme was found to be about 40 kDa by gel filtration. This suggested that the enzyme of interest consists of one subunit. The K m and V max values of recombinant P. putida ProDH were estimated to be 31 mM and 132 μmol/min, respectively. The optimum pH and temperature for the catalytic activity of the enzyme was about pH 8.5 and 30 °C. The modeling analysis of the three dimensional structure elucidated that Ser-165, Lys-195 and Ala-252 were key residues for the ProDH activity. This study provides data on the cloning, sequencing and recombinant expression of PutA ProDH domain from P. putida POS-F84.

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.

Regulation of primary plant metabolism during plant-pathogen interactions and its contribution to plant defense

Plants are constantly exposed to microorganisms in the environment and, as a result, have evolved intricate mechanisms to recognize and defend themselves against potential pathogens. One of these responses is the downregulation of photosynthesis and other processes associated with primary metabolism that are essential for plant growth. It has been suggested that the energy saved by downregulation of primary metabolism is diverted and used for defense responses. However, several studies have shown that upregulation of primary metabolism also occurs during plant-pathogen interactions. We propose that upregulation of primary metabolism modulates signal transduction cascades that lead to plant defense responses. In support of this thought, we here compile evidence from the literature to show that upon exposure to pathogens or elicitors, plants induce several genes associated with primary metabolic pathways, such as those involved in the synthesis or degradation of carbohydrates, amino acids and lipids. In addition, genetic studies have confirmed the involvement of these metabolic pathways in plant defense responses. This review provides a new perspective highlighting the relevance of primary metabolism in regulating plant defense against pathogens with the hope to stimulate further research in this area.