Evidence from engineering that decarboxylation of free serine is the major source of ethanolamine moieties in plants

Metabolome profiling of cacao (Theobroma cacao L.) callus under drought stress conditions induced by polyethylene glycol (PEG) as osmoticant

INTRODUCTION

The cacao tree (Theobroma cacao), a perennial crop that serves as a source of cacao beans, can suffer from drastic climate changes such as irregular rainfall and shorter rainy seasons. The search for hybrids which are capable of producing specific metabolites favoring adaptation in new climatic conditions is a challenge in cacao farming.

OBJECTIVES

We aimed to (1) analyze the metabolic changes in calli of three cacao genotypes during water deficit induced by incubation with polyethylene glycol and (2) assess their response to water deficit stress with regard to somatic embryo differentiation.

METHODS

Metabolic profiling was carried out using 1H-NMR spectroscopy and multivariate data analysis was applied to crude extracts of calli grown in non-stress or water deficit stress conditions.

RESULTS

Water deficit stress influences the capacity of calli to produce embryos. The SCA12 genotype exhibited the best conversion capacity under severe conditions and was considered as tolerant to drought, followed by the SCA6 genotype (mid-tolerant) and the MA12 genotype (sensitive). Fifty-four metabolites were identified in the three cacao genotypes and discriminant metabolites were identified. Metabolites involved in water stress tolerance such as fructose, trans-aconitic acid, leucine, and hydroxybenzene derivatives were observed in SCA12, the tolerant genotype.

CONCLUSION

These results demonstrate the utility of 1H-NMR metabolomics as an essential tool for the analysis of the drought tolerance characteristics of T. cacao.

The phospho-base N-methyltransferases PMT1 and PMT2 produce phosphocholine for leaf growth in phosphorus-starved Arabidopsis

Phosphorus (P) is an essential nutrient for plants. Membrane lipid remodeling is an adaptive mechanism for P-starved plants that replaces membrane phospholipids with non-P galactolipids, presumably to retrieve scarce P sources and maintain membrane integrity. Whereas metabolic pathways to convert phospholipids to galactolipids are well-established, the mechanism by which phospholipid biosynthesis is involved in this process remains elusive. Here, we report that phospho-base N-methyltransferases 1 and 2 (PMT1 and PMT2), which convert phosphoethanolamine to phosphocholine (PCho), are transcriptionally induced by P starvation. Shoots of seedlings of pmt1 pmt2 double mutant showed defective growth upon P starvation; however, membrane lipid profiles were unaffected. We found that P-starved pmt1 pmt2 with defective leaf growth had reduced PCho content, and the growth defect was rescued by exogenous supplementation of PCho. We propose that PMT1 and PMT2 are induced by P starvation to produce PCho mainly for leaf growth maintenance, rather than for phosphatidylcholine biosynthesis, in membrane lipid remodeling.

Glutamine Synthetases Play a Vital Role in High Accumulation of Theanine in Tender Shoots of Albino Tea Germplasm "Huabai 1"

Theanine (N-ethyl-γ-l-glutamine) is a special nonprotein amino acid that contributes to the umami taste and health function of tea. Although recent studies on tea breeding have focused on albino tea because of its umami taste, a factor of higher theanine concentration, the mechanism of biosynthesis of l-theanine is still unclear. In this study, four glutamine synthetase genes (CsGSs) were obtained and functionally characterized by overexpressing them in Arabidopsis. The enzyme activities of the purified CsGS proteins from Escherichia coli were detected. The results showed that CsGSs have a dual function in the synthesis of glutamine and theanine in vivo and in vitro. Interestingly, l-theanine was abundantly synthesized in the tender shoots of "Huabai 1". In the white tender shoots, the cytosol CsGS1.2 might exhibit increased expression to compensate for decreasing levels of chloroplast CsGS2, which plays a vital role in high accumulation of theanine in "Huabai 1". In addition, CsGS2 was most likely the key l-theanine synthases in green tissues of tea. The present findings will provide basis for and considerably broaden the scope of understanding the function of CsGSs and the mechanism of l-theanine accumulation in the tender shoots of "Huabai 1", and will be useful for breeding and screening tea with high l-theanine content.

At4g29530 is a phosphoethanolamine phosphatase homologous to PECP1 with a role in flowering time regulation

Phosphatidylcholine (PC) and phosphatidylethanolamine (PE) are the most abundant phospholipids in membranes. The biosynthesis of phospholipids occurs mainly via the Kennedy pathway. Recent studies have shown that through this pathway, choline (Cho) moieties are synthesized through the methylation of phosphoethanolamine (PEtn) to phosphocholine (PCho) by phospho-base N-methyltransferase. In Arabidopsis thaliana, the phosphoethanolamine/phosphocholine phosphatase1 (PECP1) is described as an enzyme that regulates the synthesis of PCho by decreasing the PEtn level during phosphate starvation to avoid the energy-consuming methylation step. By homology search, we identified a gene (At4g29530) encoding a putative PECP1 homolog from Arabidopsis with a currently unknown biological function in planta. We found that At4g29530 is not induced by phosphate starvation, and is mainly expressed in leaves and flowers. The analysis of null mutants and overexpression lines revealed that PEtn, rather than PCho, is the substrate in vivo, as in PECP1. Hydrophilic interaction chromatography-coupled mass spectrometry analysis of head group metabolites shows an increased PEtn level and decreased ethanolamine level in null mutants. At4g29530 null mutants have an early flowering phenotype, which is corroborated by a higher PC/PE ratio. Furthermore, we found an increased PCho level. The choline level was not changed, so the results corroborate that the PEtn-dependent pathway is the main route for the generation of Cho moieties. We assume that the PEtn-hydrolyzing enzyme participates in fine-tuning the metabolic pathway, and helps prevent the energy-consuming biosynthesis of PCho through the methylation pathway.

Metabolite Profiling in Arabidopsisthaliana with Moderately Impaired Photorespiration Reveals Novel Metabolic Links and Compensatory Mechanisms of Photorespiration

Photorespiration is an integral component of plant primary metabolism. Accordingly, it has been often observed that impairing the photorespiratory flux negatively impacts other cellular processes. In this study, the metabolic acclimation of the Arabidopsisthaliana wild type was compared with the hydroxypyruvate reductase 1 (HPR1; hpr1) mutant, displaying only a moderately reduced photorespiratory flux. Plants were analyzed during development and under varying photoperiods with a combination of non-targeted and targeted metabolome analysis, as well as ¹³C- and ¹⁴C-labeling approaches. The results showed that HPR1 deficiency is more critical for photorespiration during the vegetative compared to the regenerative growth phase. A shorter photoperiod seems to slowdown the photorespiratory metabolite conversion mostly at the glycerate kinase and glycine decarboxylase steps compared to long days. It is demonstrated that even a moderate impairment of photorespiration severely reduces the leaf-carbohydrate status and impacts on sulfur metabolism. Isotope labeling approaches revealed an increased CO₂ release from hpr1 leaves, most likely occurring from enhanced non-enzymatic 3-hydroxypyruvate decarboxylation and a higher flux from serine towards ethanolamine through serine decarboxylase. Collectively, the study provides evidence that the moderate hpr1 mutant is an excellent tool to unravel the underlying mechanisms governing the regulation of metabolic linkages of photorespiration with plant primary metabolism.

Non-canonical metabolic pathways in the malaria parasite detected by isotope-tracing metabolomics

The malaria parasite, Plasmodium falciparum, proliferates rapidly in human erythrocytes by actively scavenging multiple carbon sources and essential nutrients from its host cell. However, a global overview of the metabolic capacity of intraerythrocytic stages is missing. Using multiplex 13 C-labelling coupled with untargeted mass spectrometry and unsupervised isotopologue grouping, we have generated a draft metabolome of P. falciparum and its host erythrocyte consisting of 911 and 577 metabolites, respectively, corresponding to 41% of metabolites and over 70% of the metabolic reaction predicted from the parasite genome. An additional 89 metabolites and 92 reactions were identified that were not predicted from genomic reconstructions, with the largest group being associated with metabolite damage-repair systems. Validation of the draft metabolome revealed four previously uncharacterised enzymes which impact isoprenoid biosynthesis, lipid homeostasis and mitochondrial metabolism and are necessary for parasite development and proliferation. This study defines the metabolic fate of multiple carbon sources in P. falciparum, and highlights the activity of metabolite repair pathways in these rapidly growing parasite stages, opening new avenues for drug discovery.

Headgroup biosynthesis of phosphatidylcholine and phosphatidylethanolamine in seed plants

Phospholipid biosynthesis is crucial for plant growth and development. It involves attachment of fatty acids to a phospho-diacylglycerol backbone and modification of the phospho-group into an amino alcohol. The biochemistry and molecular biology of the former has been well established, but a number of enzymes responsible for the latter have only recently been cloned and functionally characterized in Arabidopsis and some other model plant species. The metabolism involving the polar head groups of phospholipids established by past biochemical studies can now be validated by available gene knockout models. Moreover, gene knockout studies have revealed emerging functions of phospholipids in regulating plant growth and development. This review aims to revisit the old questions of polar headgroup biosynthesis of plant phosphatidylcholine and phosphatidylethanolamine by giving an overview of recent advances in the field and beyond.

Exploring natural variation of photosynthesis in a site-specific manner: evolution, progress, and prospects

Site-specific changes of photosynthesis, a relatively new concept, can be used to improve the productivity of critical food crops to mitigate the foreseen food crisis. Global food security is threatened by an increasing population and the effects of climate change. Large yield improvements were achieved in major cereal crops between the 1950s and 1980s through the Green Revolution. However, we are currently experiencing a significant decline in yield progress. Of the many approaches to improved cereal yields, exploitation of the mode of photosynthesis has been intensely studied. Even though the C₄ pathway is considered the most efficient, mainly because of the carbon concentrating mechanisms around the enzyme ribulose-1,5-bisphosphate carboxylase/oxygenase, which minimize photorespiration, much is still unknown about the specific gene regulation of this mode of photosynthesis. Most of the critical cereal crops, including wheat and rice, are categorized as C₃ plants based on the photosynthesis of major photosynthetic organs. However, recent findings raise the possibility of different modes of photosynthesis occurring at different sites in the same plant and/or in plants grown in different habitats. That is, it seems possible that efficient photosynthetic traits may be expressed in specific organs, even though the major photosynthetic pathway is C₃. Knowledge of site-specific differences in photosynthesis, coupled with site-specific regulation of gene expression, may therefore hold a potential to enhance the yields of economically important C₃ crops.

Identification of a Novel Gene Encoding the Specialized Alanine Decarboxylase in Tea (Camellia sinensis) Plants

Theanine, a unique amino acid in Camellia sinensis, accounts for more than 50% of total free amino acids in tea and has a significant contribution to the quality of green tea. Previous research indicated that theanine is synthesized from glutamic acid (Glu) and ethylamine mainly in roots, and that theanine accumulation depends on the availability of ethylamine which is derived from alanine (Ala) decarboxylation catalyzed by alanine decarboxylase (AlaDC). However, the specific gene encoding AlaDC protein remains to be discovered in tea plants or in other species. To explore the gene of AlaDC in tea plants, the differences in theanine contents and gene expressions between pretreatment and posttreatment of long-time nitrogen starvation were analyzed in young roots of two tea cultivars. A novel gene annotated as serine decarboxylase (SDC) was noted for its expression levels, which showed high consistency with theanine content, and the expression was remarkably high in young roots under sufficient nitrogen condition. To verify its function, full-length complementary DNA (cDNA) of this candidate gene was cloned from young roots of tea seedlings, and the target protein was expressed and purified from Escherichia coli (E. coli). The enzymatic activity of the protein for Ala and Ser was measured in vitro using ultra-performance liquid chromatography coupled with mass spectrometry (UPLC-MS). The results illustrated that the target protein could catalyze the decarboxylation of Ala despite of its high similarity with SDC from other species. Therefore, this novel gene was identified as AlaDC and named CsAlaDC. Furthermore, the gene expression levels of CsAlaDC in different tissues of tea plants were also quantified with quantitative real-time PCR (qRT-PCR). The results suggest that transcription levels of CsAlaDC in root tissues are significantly higher than those in leaf tissues. That may explain why theanine biosynthesis preferentially occurs in the roots of tea plants. The expression of the gene was upregulated when nitrogen was present, suggesting that theanine biosynthesis is regulated by nitrogen supply and closely related to nitrogen metabolism for C. sinensis. The results of this study are significant supplements to the theanine biosynthetic pathway and provide evidence for the differential accumulation of theanine between C. sinensis and other species.

A plant-responsive bacterial-signaling system senses an ethanolamine derivative

Certain plant-associated Proteobacteria sense their host environment by detecting an unknown plant signal recognized by a member of a LuxR subfamily of transcription factors. This interkingdom communication is important for both mutualistic and pathogenic interactions. The Populus root endophyte Pseudomonas sp. GM79 possesses such a regulator, named PipR. In a previous study we reported that PipR activates an adjacent gene (pipA) coding for a proline iminopeptidase in response to Populus leaf macerates and peptides and that this activation is dependent on a putative ABC-type transporter [Schaefer AL, et al. (2016) mBio 7:e01101-16]. In this study we identify a chemical derived from ethanolamine that induces PipR activity at picomolar concentrations, and we present evidence that this is the active inducer present in plant leaf macerates. First, a screen of more than 750 compounds indicated ethanolamine was a potent inducer for the PipR-sensing system; however, ethanolamine failed to bind to the periplasmic-binding protein (PBP) required for the signal response. This led us to discover that a specific ethanolamine derivative, N-(2-hydroxyethyl)-2-(2-hydroxyethylamino) acetamide (HEHEAA), binds to the PBP and serves as a potent PipR-dependent inducer. We also show that a compound, which coelutes with HEHEAA in HPLC and induces pipA gene expression in a PipR-dependent manner, can be found in Populus leaf macerates. This work sheds light on how plant-associated bacteria can sense their environment and on the nature of inducers for a family of plant-responsive LuxR-like transcription factors found in plant-associated bacteria.

Pi starvation-dependent regulation of ethanolamine metabolism by phosphoethanolamine phosphatase PECP1 in Arabidopsis roots

A universal plant response to phosphorus deprivation is the up-regulation of a diverse array of phosphatases. As reported recently, the AtPECP1 gene encodes a phosphatase with in vitro substrate specificity for phosphoethanolamine and phosphocholine. The putative substrates suggested that AtPECP1 is related to phospholipid metabolism; however, the biological function of AtPECP1 is as yet not understood. In addition, whereas lipid remodelling processes as part of the phosphorus starvation response have been extensively studied, knowledge of the polar head group metabolism and its regulation is lacking. We found that AtPECP1 is expressed in the cytosol and exerts by far its strongest activity in roots of phosphate-starved plants. We established a novel LC-MS/MS-based method for the quantitative and simultaneous measurement of the head group metabolites. The analysis of Atpecp1 null mutants and overexpression lines revealed that phosphoethanolamine, but not phosphocholine is the substrate of AtPECP1 in vivo. The impact on head group metabolite levels is greatest in roots of both loss-of-function and gain-of-function transgenic lines, indicating that the biological role of AtPECP1 is mainly restricted to roots. We suggest that phosphoethanolamine hydrolysis by AtPECP1 during Pi starvation is required to down-regulate the energy-consuming biosynthesis of phosphocholine through the methylation pathway.

Arabidopsis Serine Decarboxylase 1 (SDC1) in Phospholipid and Amino Acid Metabolism

Arabidopsis thaliana serine decarboxylase 1 (SDC1) catalyzes conversion of serine to ethanolamine, the first reaction step of phosphatidylcholine and phosphatidylethanolamine biosynthesis. However, an involvement of SDC1 in amino acid metabolism remains elusive despite that serine is the substrate of SDC1. Here, we showed that SDC1 localizes in mitochondria although phosphatidylcholine and phosphatidylethanolamine are known to be produced in the endoplasmic reticulum (ER). Moreover, we found that overexpression of SDC1 decreased levels of amino acid compounds derived from mitochondrial tricarboxylic acid cycle. These results suggest that mitochondria-localized SDC1 plays an important role in both phospholipid and amino acid metabolism in A. thaliana.

The importance of SERINE DECARBOXYLASE1 (SDC1) and ethanolamine biosynthesis during embryogenesis of Arabidopsis thaliana

In plants, ethanolamine is considered a precursor for the synthesis of choline, which is an essential dietary nutrient for animals. An enzyme serine decarboxylase (SDC) has been identified and characterized in Arabidopsis, which directly converts serine to ethanolamine, a precursor to phosphorylethanolamine and its subsequent metabolites in plants. However, the importance of SDC and ethanolamine production in plant growth and development remains unclear. Here, we show that SDC is required for ethanolamine biosynthesis in vivo and essential in plant embryogenesis in Arabidopsis. The knockout of SDC1 caused an embryonic lethal defect due to the developmental arrest of the embryos at the heart stage. During embryo development, the expression was observed at the later stages, at which developmental defect occurred in the knockout mutant. Overexpression of SDC1 in planta increased levels of ethanolamine, phosphatidylethanolamine, and phosphatidylcholine both in leaves and siliques. These results suggest that SDC1 plays an essential role in ethanolamine biosynthesis during the embryogenesis in Arabidopsis.

Transcriptome Analysis of the Response to NaCl in Suaeda maritima Provides an Insight into Salt Tolerance Mechanisms in Halophytes

Although salt tolerance is a feature representative of halophytes, most studies on this topic in plants have been conducted on glycophytes. Transcriptome profiles are also available for only a limited number of halophytes. Hence, the present study was conducted to understand the molecular basis of salt tolerance through the transcriptome profiling of the halophyte Suaeda maritima, which is an emerging plant model for research on salt tolerance. Illumina sequencing revealed 72,588 clustered transcripts, including 27,434 that were annotated using BLASTX. Salt application resulted in the 2-fold or greater upregulation of 647 genes and downregulation of 735 genes. Of these, 391 proteins were homologous to proteins in the COGs (cluster of orthologous groups) database, and the majorities were grouped into the poorly characterized category. Approximately 50% of the genes assigned to MapMan pathways showed homology to S. maritima. The majority of such genes represented transcription factors. Several genes also contributed to cell wall and carbohydrate metabolism, ion relation, redox responses and G protein, phosphoinositide and hormone signaling. Real-time PCR was used to validate the results of the deep sequencing for the most of the genes. This study demonstrates the expression of protein kinase C, the target of diacylglycerol in phosphoinositide signaling, for the first time in plants. This study further reveals that the biochemical and molecular responses occurring at several levels are associated with salt tolerance in S. maritima. At the structural level, adaptations to high salinity levels include the remodeling of cell walls and the modification of membrane lipids. At the cellular level, the accumulation of glycinebetaine and the sequestration and exclusion of Na+ appear to be important. Moreover, this study also shows that the processes related to salt tolerance might be highly complex, as reflected by the salt-induced enhancement of transcription factor expression, including hormone-responsive factors, and that this process might be initially triggered by G protein and phosphoinositide signaling.

Evolutionary Trails of Plant Group II Pyridoxal Phosphate-Dependent Decarboxylase Genes

Type II pyridoxal phosphate-dependent decarboxylase (PLP_deC) enzymes play important metabolic roles during nitrogen metabolism. Recent evolutionary profiling of these genes revealed a sharp expansion of histidine decarboxylase genes in the members of Solanaceae family. In spite of the high sequence homology shared by PLP_deC orthologs, these enzymes display remarkable differences in their substrate specificities. Currently, limited information is available on the gene repertoires and substrate specificities of PLP_deCs which renders their precise annotation challenging and offers technical challenges in the immediate identification and biochemical characterization of their full gene complements in plants. Herein, we explored their evolutionary trails in a comprehensive manner by taking advantage of high-throughput data accessibility and computational approaches. We discussed the premise that has enabled an improved reconstruction of their evolutionary lineage and evaluated the factors offering constraints in their rapid functional characterization, till date. We envisage that the synthesized information herein would act as a catalyst for the rapid exploration of their biochemical specificity and physiological roles in more plant species.

Molecular dissection of pathway components unravel atisine biosynthesis in a non-toxic Aconitum species, A. heterophyllum Wall

Aconitum heterophyllum is an important component for various herbal drug formulations due to the occurrence of non-toxic aconites including marker compound, atisine. Despite huge pharmacological potential, the reprogramming of aconites production is limited due to lack of understanding on their biosynthesis. To address this problem, we have proposed here the complete atisine biosynthetic pathway for the first time connecting glycolysis, MVA/MEP, serine biosynthesis and diterpene biosynthetic pathways. The transcript profiling revealed phosphorylated pathway as a major contributor towards serine production in addition to repertoire of genes in glycolysis (G6PI, PFK, ALD and ENO), serine biosynthesis (PGDH and PSAT) and diterpene biosynthesis (KO and KH) sharing a similar pattern of expression (2-4-folds) in roots compared to shoots vis-à-vis atisine content (0-0.37 %). Quantification of steviol and comparative analysis of shortlisted genes between roots of high (0.37 %) vs low (0.14 %) atisine content accessions further confirmed the route of atisine biosynthesis. The results showed 6-fold increase in steviol content and 3-62-fold up-regulation of all the selected genes in roots of high content accession ascertaining their association towards atisine production. Moreover, significant positive correlations were observed between selected genes suggesting their co-expression and crucial role in atisine biosynthesis. This study, thus, offers unprecedented opportunities to explore the selected candidate genes for enhanced production of atisine in cultivated plant cells.

Engineering a novel biosynthetic pathway in Escherichia coli for production of renewable ethylene glycol

Ethylene glycol (EG) is an important commodity chemical with broad industrial applications. It is presently produced from petroleum or natural gas feedstocks in processes requiring consumption of significant quantities of non-renewable resources. Here, we report a novel pathway for biosynthesis of EG from the renewable sugar glucose in metabolically engineered Escherichia coli. Serine-to-EG conversion was first achieved through a pathway comprising serine decarboxylase, ethanolamine oxidase, and glycolaldehyde reductase. Serine provision in E. coli was then enhanced by overexpression of the serine-biosynthesis pathway. The integration of these two parts into the complete EG-biosynthesis pathway in E. coli allowed for production of 4.1 g/L EG at a cumulative yield of 0.14 g-EG/g-glucose, establishing a foundation for a promising biotechnology.

Effects of synthetic alkamides on Arabidopsis fatty acid amide hydrolase activity and plant development

Alkamides and N-acylethanolamines (NAEs) are bioactive, amide-linked lipids that influence plant development. Alkamides are restricted to several families of higher plants and some fungi, whereas NAEs are widespread signaling molecules in both plants and animals. Fatty acid amide hydrolase (FAAH) has been described as a key contributor to NAE hydrolysis; however, no enzyme has been associated with alkamide degradation in plants. Herein reported is synthesis of 12 compounds structurally similar to a naturally occurring alkamide (N-isobutyl-(2E,6Z,8E)decatrienamide or affinin) with different acyl compositions more similar to plant NAEs and various amino alkyl head groups. These "hybrid" synthetic alkamides were tested for activity toward recombinant Arabidopsis FAAH and for their effects on plant development (i.e., cotyledon expansion and primary root length). A substantial increase in FAAH activity was discovered toward NAEs in vitro in the presence of some of these synthetic alkamides, such as N-ethyllauroylamide (4). This "enhancement" effect was found to be due, at least in part, to relief from product inhibition of FAAH by ethanolamine, and not due to an alteration in the oligomerization state of the FAAH enzyme. For several of these alkamides, an inhibition of seedling growth was observed with greater results in FAAH knockouts and less in FAAH over-expressing plants, suggesting that these alkamides could be hydrolyzed by FAAH in planta. The tight regulation of NAE levels in vivo appears to be important for proper seedling establishment, and as such, some of these synthetic alkamides may be useful pharmacological tools to manipulate the effects of NAEs in situ.

New insights into photorespiration obtained from metabolomics

Photorespiration, one of the cornerstone pathways of primary metabolism, allows plant growth in a high oxygen-containing environment. While the oxygenase reaction of Rubisco directly influences photosynthesis per se, several other processes are also affected by photorespiration, including nitrogen assimilation, respiration, amino acid metabolism, 1-C metabolism and redox metabolism, cumulating to impose a severe impact across multiple signalling pathways. Accordingly, although the plant photorespiratory cycle is complex and highly compartmentalised, little is currently known about the participating transport proteins, and relatively few of them have been properly identified. Despite its centrality, uniqueness, and mystery, the biochemistry of photorespiration has historically been quite poorly understood, in part because at least some of its enzymes and intermediates tend to be labile and of low abundance. Fortunately, the integration of molecular and genetic approaches with biochemical ones, such as metabolite profiling, is now driving rapid advances in knowledge of the key metabolic roles and connections of the enzymes and genes of the photorespiratory pathway. While these experiments have revealed a surprising complexity in the response and established connections between photorespiration and other metabolic pathways, the mechanisms behind the observed responses have still to be fully elucidated. Here we review recent progress into photorespiration and its interaction with other metabolic processes, paying particular attention to data emanating from metabolic profiling studies.

Effects of fungicides on galanthamine and metabolite profiles in Narcissus bulbs

Large-scale plant cultivation usually involves the use of pesticides. Apart from eliminating the target organism, the external chemicals may affect the metabolism of the crop plant. This may have implications for plants cultivated for specific medicinal compounds. In this study the effects of diverse fungicides on the metabolism of Narcissus pseudonarcissus cv. Carlton bulbs were investigated. N. pseudonarcissus cv. Carlton is being cultivated for the extraction of the alkaloid galanthamine. Fungicides typically used in Narcissus cultivation were applied in a field experiment. The aim was to determine whether fungicide applications changed the concentration of galanthamine in the bulbs. (1)H NMR spectroscopy allowed quantitative analysis of galanthamine and other metabolites in bulb extracts. Multivariate data analysis revealed changes in bulb metabolite patterns caused by fungicides. Bulbs treated before planting generally had higher levels of alkaloids, while foliar field applications caused lower alkaloid levels but altered carbohydrate metabolism. Within these groups, certain fungicide treatments caused changes in specific metabolites. This study shows that the fungicides used in Narcissus cultivation can cause a change in the metabolome still detectable in the bulbs after harvest. The standard cultivation practices in terms of fungicide treatment were found suitable for the production of N. pseudonarcissus cv. Carlton as raw material for galanthamine extraction. In the cultivation of medicinal plants for secondary metabolites the potential effect of pesticides and other agrochemicals should be taken into account.

Arabidopsis serine decarboxylase mutants implicate the roles of ethanolamine in plant growth and development

Ethanolamine is important for synthesis of choline, phosphatidylethanolamine (PE) and phosphatidylcholine (PC) in plants. The latter two phospholipids are the major phospholipids in eukaryotic membranes. In plants, ethanolamine is mainly synthesized directly from serine by serine decarboxylase. Serine decarboxylase is unique to plants and was previously shown to have highly specific activity to l-serine. While serine decarboxylase was biochemically characterized, its functions and importance in plants were not biologically elucidated due to the lack of serine decarboxylase mutants. Here we characterized an Arabidopsis mutant defective in serine decarboxylase, named atsdc-1 (Arabidopsis thaliana serine decarboxylase-1). The atsdc-1 mutants showed necrotic lesions in leaves, multiple inflorescences, sterility in flower, and early flowering in short day conditions. These defects were rescued by ethanolamine application to atsdc-1, suggesting the roles of ethanolamine as well as serine decarboxylase in plant development. In addition, molecular analysis of serine decarboxylase suggests that Arabidopsis serine decarboxylase is cytosol-localized and expressed in all tissue.

The hydroxypyruvate-reducing system in Arabidopsis: multiple enzymes for the same end

Hydroxypyruvate (HP) is an intermediate of the photorespiratory pathway that originates in the oxygenase activity of the key enzyme of photosynthetic CO(2) assimilation, Rubisco. In course of this high-throughput pathway, a peroxisomal transamination reaction converts serine to HP, most of which is subsequently reduced to glycerate by the NADH-dependent peroxisomal enzyme HP reductase (HPR1). In addition, a NADPH-dependent cytosolic HPR2 provides an efficient extraperoxisomal bypass. The combined deletion of these two enzymes, however, does not result in a fully lethal photorespiratory phenotype, indicating even more redundancy in the photorespiratory HP-into-glycerate conversion. Here, we report on a third enzyme, HPR3 (At1g12550), in Arabidopsis (Arabidopsis thaliana), which also reduces HP to glycerate and shows even more activity with glyoxylate, a more upstream intermediate of the photorespiratory cycle. The deletion of HPR3 by T-DNA insertion mutagenesis results in slightly altered leaf concentrations of the photorespiratory intermediates HP, glycerate, and glycine, indicating a disrupted photorespiratory flux, but not in visible alteration of the phenotype. On the other hand, the combined deletion of HPR1, HPR2, and HPR3 causes increased growth retardation, decreased photochemical efficiency, and reduced oxygen-dependent gas exchange in comparison with the hpr1xhpr2 double mutant. Since in silico analysis and proteomic studies from other groups indicate targeting of HPR3 to the chloroplast, this enzyme could provide a compensatory bypass for the reduction of HP and glyoxylate within this compartment.

Rodent and nonrodent malaria parasites differ in their phospholipid metabolic pathways

Malaria, a disease affecting humans and other animals, is caused by a protist of the genus Plasmodium. At the intraerythrocytic stage, the parasite synthesizes a high amount of phospholipids through a bewildering number of pathways. In the human Plasmodium falciparum species, a plant-like pathway that relies on serine decarboxylase and phosphoethanolamine N-methyltransferase activities diverts host serine to provide additional phosphatidylcholine and phosphatidylethanolamine to the parasite. This feature of parasitic dependence toward its host was investigated in other Plasmodium species. In silico analyses led to the identification of phosphoethanolamine N-methyltransferase gene orthologs in primate and bird parasite genomes. However, the gene was not detected in the rodent P. berghei, P. yoelii, and P. chabaudi species. Biochemical experiments with labeled choline, ethanolamine, and serine showed marked differences in biosynthetic pathways when comparing rodent P. berghei and P. vinckei, and human P. falciparum species. Notably, in both rodent parasites, ethanolamine and serine were not significantly incorporated into phosphatidylcholine, indicating the absence of phosphoethanolamine N-methyltransferase activity. To our knowledge, this is the first study to highlight a crucial difference in phospholipid metabolism between Plasmodium species. The findings should facilitate efforts to develop more rational approaches to identify and evaluate new targets for antimalarial therapy.

A cytosolic pathway for the conversion of hydroxypyruvate to glycerate during photorespiration in Arabidopsis

Deletion of any of the core enzymes of the photorespiratory cycle, one of the major pathways of plant primary metabolism, results in severe air-sensitivity of the respective mutants. The peroxisomal enzyme hydroxypyruvate reductase (HPR1) represents the only exception to this rule. This indicates the presence of extraperoxisomal reactions of photorespiratory hydroxypyruvate metabolism. We have identified a second hydroxypyruvate reductase, HPR2, and present genetic and biochemical evidence that the enzyme provides a cytosolic bypass to the photorespiratory core cycle in Arabidopsis thaliana. Deletion of HPR2 results in elevated levels of hydroxypyruvate and other metabolites in leaves. Photosynthetic gas exchange is slightly altered, especially under long-day conditions. Otherwise, the mutant closely resembles wild-type plants. The combined deletion of both HPR1 and HPR2, however, results in distinct air-sensitivity and a dramatic reduction in photosynthetic performance. These results suggest that photorespiratory metabolism is not confined to chloroplasts, peroxisomes, and mitochondria but also extends to the cytosol. The extent to which cytosolic reactions contribute to the operation of the photorespiratory cycle in varying natural environments is not yet known, but it might be dynamically regulated by the availability of NADH in the context of peroxisomal redox homeostasis.

Phosphatidylserine and phosphatidylethanolamine in mammalian cells: two metabolically related aminophospholipids

Phosphatidylserine (PS) and phosphatidylethanolamine (PE) are two aminophospholipids whose metabolism is interrelated. Both phospholipids are components of mammalian cell membranes and play important roles in biological processes such as apoptosis and cell signaling. PS is synthesized in mammalian cells by base-exchange reactions in which polar head groups of preexisting phospholipids are replaced by serine. PS synthase activity resides primarily on mitochondria-associated membranes and is encoded by two distinct genes. Studies in mice in which each gene has been individually disrupted are beginning to elucidate the importance of these two synthases for biological functions in intact animals. PE is made in mammalian cells by two completely independent major pathways. In one pathway, PS is converted into PE by the mitochondrial enzyme PS decarboxylase. In addition, PE is made via the CDP-ethanolamine pathway, in which the final reaction occurs on the endoplasmic reticulum and nuclear envelope. The relative importance of these two pathways of PE synthesis has been investigated in knockout mice. Elimination of either pathway is embryonically lethal, despite the normal activity of the other pathway. PE can also be generated from a base-exchange reaction and by the acylation of lyso-PE. Cellular levels of PS and PE are tightly regulated by the implementation of multiple compensatory mechanisms.

Glycerolipid transfer for the building of membranes in plant cells

Membranes of plant organelles have specific glycerolipid compositions. Selective distribution of lipids at the levels of subcellular organelles, membrane leaflets and membrane domains reflects a complex and finely tuned lipid homeostasis. Glycerolipid neosynthesis occurs mainly in plastid envelope and endoplasmic reticulum membranes. Since most lipids are not only present in the membranes where they are synthesized, one cannot explain membrane specific lipid distribution by metabolic processes confined in each membrane compartment. In this review, we present our current understanding of glycerolipid trafficking in plant cells. We examine the potential mechanisms involved in lipid transport inside bilayers and from one membrane to another. We survey lipid transfers going through vesicular membrane flow and those dependent on lipid transfer proteins at membrane contact sites. By introducing recently described membrane lipid reorganization during phosphate deprivation and recent developments issued from mutant analyses, we detail the specific lipid transfers towards or outwards the chloroplast envelope.