C Tracer Evidence for Synthesis of Choline and Betaine via Phosphoryl Base Intermediates in Salinized Sugarbeet Leaves

Glycine betaine capped ZnO NPs eliminate oxidative stress to coriander plants grown under NaCl presence

Salinity is one of the major abiotic stresses for sustainable agriculture. The use of mineral nutrients in form of nanoparticles can be a novel strategy to fight against abiotic stresses. An in vitro study has been conducted to investigate the effect of zinc oxide nanoparticles (ZnO NPs) capped with glycine betaine (ZnOBt) on coriander plants exposed to saline (NaCl) stress. SEM and XRD analysis revealed 14.73 nm and 17.34 nm size of ZnO and ZnOBt NPs, respectively with spherical to hexagonal structures. Coriander plant length and biomass increased by the application of ZnO and ZnOBt NPs. ZnOBt NPs depicted promising results at 100 mg/L where, shoot and root length increased up to 14 cm and 13 cm, respectively as compared to plants grown under saline stress. ZnOBt NPs also increased fresh and dry weight of shoots and roots as compared to other treatments. The results depict that ZnOBt NPs mitigated stress condition. This is evident from concentration of phenolic and flavonoid contents that decreased in both roots and shoots. Free radical scavenging activity, total antioxidant capacity and total reducing power also decreased in plants by ZnOBt NPs when applied with stress. The concentration of superoxide and peroxide dismutase also decreased by application of ZnOBt NPs to salt stress plants. Glycine betaine with ZnO NPs, in conclusion, can be an effective remedy for salinity-exposed plants. These nanoparticles can be encouraged as a viable technique to overcome the detrimental effects of saline stress on plants.

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.

A Methyltransferase Trio Essential for Phosphatidylcholine Biosynthesis and Growth

Phosphatidylcholine (PC) is a primary class of membrane lipids in most eukaryotes. In plants, the primary PC biosynthetic pathway and its role in plant growth and development remain elusive due to lack of a mutant model with substantially decreased PC content. Recently, a double mutant of Arabidopsis (Arabidopsis thaliana) PHOSPHO-BASE N-METHYLTRANSFERASE 1 (PMT1) and PMT3 was reported with reduced PC content and defective plant growth. However, residual PC content as well as the nonlethal phenotype of the mutant suggests an additional enzyme contributes to PC biosynthesis. In this article, we report on the role of three PMTs in PC biosynthesis and plant development, with a focus on PMT2. PMT2 had the highest expression level among the three PMTs, and it was highly expressed in roots. The pmt1 pmt2 double mutant enhanced the defects in root growth, cell viability, and PC content of pmt1, suggesting that PMT2 functions together with PMT1 in roots. Chemical inhibition of PMT activity in wild-type roots reproduced the short root phenotype observed in pmt1 pmt2, suggesting that PMT1 and PMT2 are the major PMT isoforms in roots. In shoots, pmt1 pmt2 pmt3 enhanced the phenotype of pmt1 pmt3, showing seedling lethality and further reduced PC content without detectable de novo PC biosynthesis. These results suggest that PMTs catalyze an essential reaction step in PC biosynthesis and that the three PMTs have differential tissue-specific functions in PC biosynthesis and plant growth.

A pair of phospho-base methyltransferases important for phosphatidylcholine biosynthesis in Arabidopsis

Phosphatidylcholine (PtdCho) is a predominant membrane lipid class in eukaryotes. Phospho-base N-methyltransferase (PMT) catalyzes a critical step in PtdCho biosynthesis. However, in Arabidopsis thaliana, the discovery of involvement of the specific PMT isoform in PtdCho biosynthesis remains elusive. Here, we show that PMT1 and PMT3 redundantly play an essential role in phosphocholine (PCho) biosynthesis, a prerequisite for PtdCho production. A pmt1 pmt3 double mutant was devoid of PCho, which affected PtdCho biosynthesis in vivo, showing severe growth defects in post-embryonic development. PMT1 and PMT3 were both highly expressed in the vasculature. The pmt1 pmt3 mutants had specifically affected leaf vein development and showed pale-green seedlings that were rescued by exogenous supplementation of PCho. We suggest that PMT1 and PMT3 are the primary enzymes for PCho biosynthesis and are involved in PtdCho biosynthesis and vascular development in Arabidopsis seedlings.

NMT1 and NMT3 N-Methyltransferase Activity Is Critical to Lipid Homeostasis, Morphogenesis, and Reproduction

Phosphatidylcholine (PC) is a major membrane phospholipid and a precursor for major signaling molecules. Understanding its synthesis is important for improving plant growth, nutritional value, and resistance to stress. PC synthesis is complex, involving several interconnected pathways, one of which proceeds from serine-derived phosphoethanolamine to form phosphocholine through three sequential phospho-base methylations catalyzed by phosphoethanolamine N-methyltransferases (PEAMTs). The contribution of this pathway to the production of PC and plant growth has been a matter of some debate. Although a handful of individual PEAMTs have been described, there has not been any in planta investigation of a PEAMT family. Here, we provide a comparative functional analysis of two Arabidopsis (Arabidopsis thaliana) PEAMTs, NMT1 and the little known NMT3. Analysis of loss-of-function mutants demonstrates that NMT1 and NMT3 synergistically regulate PC homeostasis, phase transition at the shoot apex, coordinated organ development, and fertility through overlapping but also specific functions. The nmt1 nmt3 double mutant shows extensive sterility, drastically reduced PC concentrations, and altered lipid profiles. These findings demonstrate that the phospho-base methylation pathway makes a major contribution to PC synthesis in Arabidopsis and that NMT1 and NMT3 play major roles in its catalysis and the regulation of PC homeostasis as well as in plant growth and reproduction.

A 13C isotope labeling method for the measurement of lignin metabolic flux in Arabidopsis stems

BACKGROUND

Metabolic fluxes represent the functional phenotypes of biochemical pathways and are essential to reveal the distribution of precursors among metabolic networks. Although analysis of metabolic fluxes, facilitated by stable isotope labeling and mass spectrometry detection, has been applied in the studies of plant metabolism, we lack experimental measurements for carbon flux towards lignin, one of the most abundant polymers in nature.

RESULTS

We developed a feeding strategy of excised Arabidopsis stems with 13C labeled phenylalanine (Phe) for the analysis of lignin biosynthetic flux. We optimized the feeding methods and found the stems continued to grow and lignify. Consistent with lignification profiles along the stems, higher levels of phenylpropanoids and activities of lignin biosynthetic enzymes were detected in the base of the stem. In the feeding experiments, 13C labeled Phe was quickly accumulated and used for the synthesis of phenylpropanoid intermediates and lignin. The intermediates displayed two different patterns of labeling kinetics during the feeding period. Analysis of lignin showed rapid incorporation of label into all three subunits in the polymers.

CONCLUSIONS

Our feeding results demonstrate the effectiveness of the stem feeding system and suggest a potential application for the investigations of other aspects in plant metabolism. The supply of exogenous Phe leading to a higher lignin deposition rate indicates the availability of Phe is a determining factor for lignification rates.

Evolution of the Phosphatidylcholine Biosynthesis Pathways in Green Algae: Combinatorial Diversity of Methyltransferases

Phosphatidylcholine (PC) is one of the most common phospholipids in eukaryotes, although some green algae such as Chlamydomonas reinhardtii are known to lack PC. Recently, we detected PC in four species in the genus Chlamydomonas: C. applanata NIES-2202, C. asymmetrica NIES-2207, C. debaryana NIES-2212, and C. sphaeroides NIES-2242. To reveal the PC biosynthesis pathways in green algae and the evolutionary scenario involved in their diversity, we analyzed the PC biosynthesis genes in these four algae using draft genome sequences. Homology searches suggested that PC in these species is synthesized by phosphoethanolamine-N-methyltransferase (PEAMT) and/or phosphatidylethanolamine-N-methyltransferase (PEMT), both of which are absent in C. reinhardtii. Recombinant PEAMTs from these algae showed methyltransferase activity for phosphoethanolamine but not for monomethyl phosphoethanolamine in vitro, in contrast to land plant PEAMT, which catalyzes the three methylations from phosphoethanolamine to phosphocholine. This suggested an involvement of other methyltransferases in PC biosynthesis. Here, we characterized the putative phospholipid-N-methyltransferase (PLMT) genes of these species by genetic and phylogenetic analysis. Complementation assays using a PC biosynthesis-deficient yeast suggested that the PLMTs of these algae can synthesize PC from phosphatidylethanolamine. These results indicated that the PC biosynthesis pathways in green algae differ from those of land plants, although the enzymes involved are homologous. Phylogenetic analysis suggested that the PEAMTs and PLMTs in these algae were inherited from the common ancestor of green algae. The absence of PC biosynthesis in many Chlamydomonas species is likely a result of parallel losses of PEAMT and PLMT in this genus.

Choline but not its derivative betaine blocks slow vacuolar channels in the halophyte Chenopodium quinoa: implications for salinity stress responses

Activity of tonoplast slow vacuolar (SV, or TPC1) channels has to be under a tight control, to avoid undesirable leak of cations stored in the vacuole. This is particularly important for salt-grown plants, to ensure efficient vacuolar Na(+) sequestration. In this study we show that choline, a cationic precursor of glycine betaine, efficiently blocks SV channels in leaf and root vacuoles of the two chenopods, Chenopodium quinoa (halophyte) and Beta vulgaris (glycophyte). At the same time, betaine and proline, two major cytosolic organic osmolytes, have no significant effect on SV channel activity. Physiological implications of these findings are discussed.

Frontiers in metabolic reconstruction and modeling of plant genomes

A major goal of post-genomic biology is to reconstruct and model in silico the metabolic networks of entire organisms. Work on bacteria is well advanced, and is now under way for plants and other eukaryotes. Genome-scale modelling in plants is much more challenging than in bacteria. The challenges come from features characteristic of higher organisms (subcellular compartmentation, tissue differentiation) and also from the particular severity in plants of a general problem: genome content whose functions remain undiscovered. This problem results in thousands of genes for which no function is known ('undiscovered genome content') and hundreds of enzymatic and transport functions for which no gene is yet identified. The severity of the undiscovered genome content problem in plants reflects their genome size and complexity. To bring the challenges of plant genome-scale modelling into focus, we first summarize the current status of plant genome-scale models. We then highlight the challenges - and ways to address them - in three areas: identifying genes for missing processes, modelling tissues as opposed to single cells, and finding metabolic functions encoded by undiscovered genome content. We also discuss the emerging view that a significant fraction of undiscovered genome content encodes functions that counter damage to metabolites inflicted by spontaneous chemical reactions or enzymatic mistakes.

Phosphoethanolamine methyltransferases in phosphocholine biosynthesis: functions and potential for antiparasite therapy

S-adenosyl-L-methionine (SAM)-dependent methyltransferases represent a diverse group of enzymes that catalyze the transfer of a methyl group from a methyl donor SAM to nitrogen, oxygen, sulfur or carbon atoms of a large number of biologically active large and small molecules. These modifications play a major role in the regulation of various biological functions such as gene expression, signaling, nuclear division and metabolism. The three-step SAM-dependent methylation of phosphoethanolamine to form phosphocholine catalyzed by phosphoethanolamine N-methyltransferases (PMTs) has emerged as an important biochemical step in the synthesis of the major phospholipid, phosphatidylcholine, in some eukaryotes. PMTs have been identified in nematodes, plants, African clawed frogs, zebrafish, the Florida lancelet, Proteobacteria and human malaria parasites. Data accumulated thus far suggest an important role for these enzymes in growth and development. This review summarizes published studies on the biochemical and genetic characterization of these enzymes, and discusses their evolution and their suitability as targets for the development of therapies against parasitic infections, as well as in bioengineering for the development of nutritional and stress-resistant plants.

Betaine synthesis in chenopods: Localization in chloroplasts

PLANTS FROM SEVERAL FAMILIES (CHENOPODIACEAE, GRAMINEAE, COMPOSITAE) ACCUMULATE BETAINE (GLYCINE BETAINE) IN RESPONSE TO SALT OR WATER STRESS VIA THE PATHWAY: choline --> betainal (betaine aldehyde) --> betaine. Betaine accumulation is probably a metabolic adaptation to stress. Intact protoplasts from leaves of spinach (Spinacia oleracea) oxidized [(14)C]choline to betainal and betaine, as did protoplast lysates. Upon differential centrifugation, the [(14)C]choline-oxidizing activity of lysates sedimented with chloroplasts. Chloroplasts purified from protoplast lysates by a Percoll cushion procedure retained strong [(14)C]choline-oxidizing activity (1-3 nmol/mg of chlorophyll per hr), although the proportion of the intermediate, [(14)C]betainal, in the reaction products was usually higher than for protoplasts. Isolated chloroplasts also readily oxidized [(14)C]betainal to betaine (20-100 nmol/mg of chlorophyll per hr). Light increased the oxidation of both [(14)C]choline and [(14)C]betainal by isolated chloroplasts approximately 3-fold; this light-stimulation was abolished by 3-(3,4-dichlorophenyl)-1,1-dimethylurea (DCMU). Similar results were obtained with another chenopod (Beta vulgaris) but not with pea (Pisum sativum), a species that accumulates no betaine. The chloroplast site for betaine synthesis in chenopods contrasts with the mitochondrial site in mammals.

Biochemical characterization of two wheat phosphoethanolamine N-methyltransferase isoforms with different sensitivities to inhibition by phosphatidic acid

In plants the triple methylation of phosphoethanolamine to phosphocholine catalyzed by phosphoethanolamine N-methyltransferase (PEAMT) is considered a rate-limiting step in the de novo synthesis of phosphatidylcholine. Besides being a major membrane phospholipid, phosphatidylcholine can be hydrolyzed into choline and phosphatidic acid. Phosphatidic acid is widely recognized as a second messenger in stress signaling, and choline can be oxidized within the chloroplast to yield the putative osmoprotectant glycine betaine. Here we describe the cloning and biochemical characterization of a second wheat PEAMT isoform that has a four times higher specific activity than the previously described WPEAMT/TaPEAMT1 enzyme and is less sensitive to product inhibition by S-adenosyl homocysteine, but more sensitive to inhibition by phosphocholine. Both enzymes follow a sequential random Bi Bi mechanism and show mixed-type product inhibition patterns with partial inhibition for TaPEAMT1 and a strong non-competitive component for TaPEAMT2. An induction of TaPEAMT protein expression and activity is observed after cold exposure, ahead of an increase in gene expression. Our results demonstrate direct repression of in vitro enzymatic activities by phosphatidic acid for both enzymes, with TaPEAMT1 being more sensitive than TaPEAMT2 in the physiological concentration range. Other lipid ligands identified in protein-lipid overlays are phosphoinositide mono- as well as some di-phosphates and cardiolipin. These results provide new insights into the complex regulatory circuits of phospholipid biosynthesis in plants and underline the importance of head group biosynthesis in adaptive stress responses.

Abscisic acid is involved in the water stress-induced betaine accumulation in pear leaves

ABA exogenously applied to the leaves of the whole plants of pear (Pyrus bretschneideri Redh. cv. Suly grafted on Pyrus betulaefolia Rehd.) significantly increased the betaine concentrations in the leaves when the plants were well watered. The plants subjected to 'drought plus ABA' treatment had significantly higher betaine concentrations in their leaves than those given drought treatment alone. The 'drought plus ABA' treatment increased the amount of betaine aldehyde dehydrogenase (BADH, EC 1.2.1.8) and its activity in the leaves more than did the drought treatment alone. The experiments with detached leaves showed that ABA treatment significantly increased the concentration of betaine, activity of BADH and apparent amount of BADH in non-dehydrated leaves, and enhanced the accumulation of betaine, activity of BADH and apparent amount of BADH in dehydrated leaves. These effects of ABA were both time- and dose-dependent. Two ABA isomers, (-)-cis, trans-ABA and 2-trans, 4-trans-ABA, had no effect on the betaine accumulation in the leaves, showing that the ABA-induced effects are specific. These data demonstrate that ABA is involved in the drought-induced betaine accumulation in the pear leaves.

Abscisic acid is involved in the water stress-induced betaine accumulation in pear leaves

ABA exogenously applied to the leaves of the whole plants of pear (Pyrus bretschneideri Redh. cv. Suly grafted on Pyrus betulaefolia Rehd.) significantly increased the betaine concentrations in the leaves when the plants were well watered. The plants subjected to 'drought plus ABA' treatment had significantly higher betaine concentrations in their leaves than those given drought treatment alone. The 'drought plus ABA' treatment increased the amount of betaine aldehyde dehydrogenase (BADH, EC 1.2.1.8) and its activity in the leaves more than did the drought treatment alone. The experiments with detached leaves showed that ABA treatment significantly increased the concentration of betaine, activity of BADH and apparent amount of BADH in non-dehydrated leaves, and enhanced the accumulation of betaine, activity of BADH and apparent amount of BADH in dehydrated leaves. These effects of ABA were both time- and dose-dependent. Two ABA isomers, (-)-cis, trans-ABA and 2-trans, 4-trans-ABA, had no effect on the betaine accumulation in the leaves, showing that the ABA-induced effects are specific. These data demonstrate that ABA is involved in the drought-induced betaine accumulation in the pear leaves.

Biosynthesis of phosphatidylcholine in bacteria

Phosphatidylcholine (PC) is the major membrane-forming phospholipid in eukaryotes and can be synthesized by either of two pathways, the methylation pathway or the CDP-choline pathway. Many prokaryotes lack PC, but it can be found in significant amounts in membranes of rather diverse bacteria and based on genomic data, we estimate that more than 10% of all bacteria possess PC. Enzymatic methylation of phosphatidylethanolamine via the methylation pathway was thought to be the only biosynthetic pathway to yield PC in bacteria. However, a choline-dependent pathway for PC biosynthesis has been discovered in Sinorhizobium meliloti. In this pathway, PC synthase, condenses choline directly with CDP-diacylglyceride to form PC in one step. A number of symbiotic (Rhizobium leguminosarum, Mesorhizobium loti) and pathogenic (Agrobacterium tumefaciens, Brucella melitensis, Pseudomonas aeruginosa, Borrelia burgdorferi and Legionella pneumophila) bacteria seem to possess the PC synthase pathway and we suggest that the respective eukaryotic host functions as the provider of choline for this pathway. Pathogens entering their hosts through epithelia (Streptococcus pneumoniae, Haemophilus influenzae) require phosphocholine substitutions on their cell surface components that are biosynthetically also derived from choline supplied by the host. However, the incorporation of choline in these latter cases proceeds via choline phosphate and CDP-choline as intermediates. The occurrence of two intermediates in prokaryotes usually found as intermediates in the eukaryotic CDP-choline pathway for PC biosynthesis raises the question whether some bacteria might form PC via a CDP-choline pathway.

Trimethylamine oxide accumulation in marine animals: relationship to acylglycerol storagej

Trimethylamine oxide (TMAO) is a common and compatible osmolyte in muscle tissues of marine organisms that is often credited with counteracting protein-destabilizing forces. However, the origin and synthetic pathways of TMAO are actively debated. Here, we examine the distribution of TMAO in marine animals and report a correlation between TMAO and acylglycerol storage. We put forward the hypothesis that TMAO is derived, at least in part, from the hydrolysis of phosphatidylcholine, endogenous or dietary, for storage as diacylglycerol ethers and triacylglycerols. TMAO is synthesized from the trimethylammonium moiety of choline, thus released, and is retained as a compatible solute in concentrations reflecting the amount of lipid stored in the body. A variation on this theme is proposed for sharks.

Mathematical modeling of plant metabolic pathways

The understanding of the control of metabolic flux in plants requires integrated mathematical formulations of gene and protein expression, enzyme kinetics, and developmental biology. Plants have a large number of metabolically active compartments, and non-steady-state conditions are frequently encountered. Consequently steady-state metabolic flux balance and isotopic flux balance modeling approaches have limited utility in probing plant metabolic systems. Transient isotopic flux analysis and kinetic modeling are powerful proven techniques for the quantification of metabolic fluxes in compartmentalized, dynamic metabolic systems. These tools are now widely used to address metabolic flux responses to environmental and genetic perturbations in plant metabolism. Continued developments in isotopic and kinetic modeling, quantifying metabolite exchange between compartments, and transcriptional and posttranscriptional regulatory mechanisms governing enzyme level and activity will enable simulation of large sections of plant metabolism under non-steady-state conditions. Metabolic control analysis will continue to make substantial contributions to the understanding of quantitative distribution of control of flux. From the synergy between mathematical models and experiments, creative methods for controlling the distribution of flux by genetic or environmental means will be discovered and rationally implemented.

Enhanced synthesis of choline and glycine betaine in transgenic tobacco plants that overexpress phosphoethanolamine N-methyltransferase

Choline (Cho) is the precursor of the osmoprotectant glycine betaine and is itself an essential nutrient for humans. Metabolic engineering of Cho biosynthesis in plants could therefore enhance both their resistance to osmotic stresses (drought and salinity) and their nutritional value. The key enzyme of the plant Cho-synthesis pathway is phosphoethanolamine N-methyltransferase, which catalyzes all three of the methylations required to convert phosphoethanolamine to phosphocholine. We show here that overexpressing this enzyme in transgenic tobacco increased the levels of phosphocholine by 5-fold and free Cho by 50-fold without affecting phosphatidylcholine content or growth. Moreover, the expanded Cho pool led to a 30-fold increase in synthesis of glycine betaine via an engineered glycine betaine pathway. Supplying the transgenics with the Cho precursor ethanolamine (EA) further enhanced Cho levels even though the supplied EA was extensively catabolized. These latter results establish that there is further scope for improving Cho synthesis by engineering an increased endogenous supply of EA and suggest that this could be achieved by enhancing EA synthesis and/or by suppressing its degradation.

Enzymes of choline synthesis in diverse plants: variation in phosphobase N-methyltransferase activities

S-adenosyl-L-methionine dependent phospho-base N-methyltransferases are involved in the sequential methylations of phosphoethanolamine [Formula: see text] phosphomethylethanolamine [Formula: see text] phosphodimethylethanolamine [Formula: see text] phosphocholine. Phosphocholine is a precursor for the ubiquitous phospholipid phosphatidylcholine and for free choline, which can be oxidized to produce the osmoprotectant glycine betaine. Despite the importance of these enzymes to growth and stress tolerance, their activities have been studied in comparatively few plant species. Phospho-base N-methylating activities were assayed in leaf extracts prepared from 17 diverse plant species. All plants tested can perform the first step ( N-methylation of phosphoethanolamine) with in vitro activity rates varying from 0.13 nmol·min–1·g–1 fresh weight for soybean (Glycine max (L.) Merr.) and pea (Pisum sativum L.) to 25 nmol·min–1·g–1 fresh weight for cotton (Gossypium hirsutum L.). Of the plant species surveyed, only soybean and pea showed no capacity to perform the two subsequent N-methylation steps. Exposing plants to prolonged dark periods led to decreased phosphoethanolamine N-methylating activity relative to light-exposed controls with the extent of decrease varying among the species from 30% (Limonium perezii (Stapf) F.T. Hubb) to over 90% (Spinacia oleracea L., Beta vulgaris L., and Amaranthus caudatus L.). Thus, light-responsive properties and levels of phosphobase methyltransferase activities vary among plants with a trend towards higher activities being found in plants that accumulate glycine betaine.Key words: glycine betaine, choline, phosphatidylcholine, phosphocholine.

Characterization of glycine sarcosine N-methyltransferase and sarcosine dimethylglycine N-methyltransferase

Glycine betaine is accumulated in cells living in high salt concentrations to balance the osmotic pressure. Glycine sarcosine N-methyltransferase (GSMT) and sarcosine dimethylglycine N-methyltransferase (SDMT) of Ectothiorhodospira halochloris catalyze the threefold methylation of glycine to betaine, with S-adenosylmethionine acting as the methyl group donor. These methyltransferases were expressed in Escherichia coli and purified, and some of their enzymatic properties were characterized. Both enzymes had high substrate specificities and pH optima near the physiological pH. No evidence of cofactors was found. The enzymes showed Michaelis-Menten kinetics for their substrates. The apparent K(m) and V(max) values were determined for all substrates when the other substrate was present in saturating concentrations. Both enzymes were strongly inhibited by the reaction product S-adenosylhomocysteine. Betaine inhibited the methylation reactions only at high concentrations.

Choline import into chloroplasts limits glycine betaine synthesis in tobacco: analysis of plants engineered with a chloroplastic or a cytosolic pathway

The biosynthesis of the osmoprotectant glycine betaine (GlyBet) is a target for metabolic engineering to enhance stress resistance in crops. Certain plants synthesize GlyBet in chloroplasts via a two-step oxidation of choline (Cho). In previous work, a chloroplastic GlyBet synthesis pathway was inserted into tobacco (which lacks GlyBet) by expressing spinach choline monooxygenase (CMO). The transformants had low CMO enzyme activity, and produced little GlyBet (less than or = 70 nmol g(-1) fresh wt). In this study, transformants with up to 100-fold higher CMO activity showed no further increase in GlyBet. In contrast, tobacco expressing a cytosolic GlyBet synthesis pathway accumulated significantly more GlyBet (430 nmol g(-1) fresh wt), suggesting that subcellular localization influences pathway flux. Modeling of the labeling kinetics of Cho metabolites observed when [14C]Cho was supplied to engineered plants demonstrated that Cho import into chloroplasts indeed limits the flux to GlyBet in the chloroplastic pathway. A high-activity Cho transporter in the chloroplast envelope may therefore be an integral part of the GlyBet synthesis pathway in species that accumulate GlyBet naturally, and hence a target for future engineering.

Metabolic modeling identifies key constraints on an engineered glycine betaine synthesis pathway in tobacco

Previous work has shown that tobacco (Nicotiana tabacum) plants engineered to express spinach choline monooxygenase in the chloroplast accumulate very little glycine betaine (GlyBet) unless supplied with choline (Cho). We therefore used metabolic modeling in conjunction with [(14)C]Cho labeling experiments and in vivo (31)P NMR analyses to define the constraints on GlyBet synthesis, and hence the processes likely to require further engineering. The [(14)C]Cho doses used were large enough to markedly perturb Cho and phosphocholine pool sizes, which enabled development and testing of models with rates dynamically responsive to pool sizes, permitting estimation of the kinetic properties of Cho metabolism enzymes and transport systems in vivo. This revealed that import of Cho into the chloroplast is a major constraint on GlyBet synthesis, the import rate being approximately 100-fold lower than the rates of Cho phosphorylation and transport into the vacuole, with which import competes. Simulation studies suggested that, were the chloroplast transport limitation corrected, additional engineering interventions would still be needed to achieve levels of GlyBet as high as those in plants that accumulate GlyBet naturally. This study reveals the rigidity of the Cho metabolism network and illustrates how computer modeling can help guide rational metabolic engineering design.

cDNA cloning of phosphoethanolamine N-methyltransferase from spinach by complementation in Schizosaccharomyces pombe and characterization of the recombinant enzyme

The N-methylation of phosphoethanolamine is the committing step in choline biogenesis in plants and is catalyzed by S-adenosyl-L-methionine:phosphoethanolamine N-methyltransferase (PEAMT, EC ). A spinach PEAMT cDNA was isolated by functional complementation of a Schizosaccharomyces pombe cho2(-) mutant and was shown to encode a protein with PEAMT activity and without ethanolamine- or phosphatidylethanolamine N-methyltransferase activity. The PEAMT cDNA specifies a 494-residue polypeptide comprising two similar, tandem methyltransferase domains, implying that PEAMT arose by gene duplication and fusion. Data base searches suggested that PEAMTs with the same tandem structure are widespread among flowering plants. Size exclusion chromatography of the recombinant enzyme indicates that it exists as a monomer. PEAMT catalyzes not only the first N-methylation of phosphoethanolamine but also the two subsequent N-methylations, yielding phosphocholine. Monomethyl- and dimethylphosphoethanolamine are detected as reaction intermediates. A truncated PEAMT lacking the C-terminal methyltransferase domain catalyzes only the first methylation. Phosphocholine inhibits both the wild type and the truncated enzyme, although the latter is less sensitive. Salinization of spinach plants increases PEAMT mRNA abundance and enzyme activity in leaves by about 10-fold, consistent with the high demand in stressed plants for choline to support glycine betaine synthesis.

Radiotracer and computer modeling evidence that phospho-base methylation is the main route of choline synthesis in tobacco

Among flowering plants, the synthesis of choline (Cho) from ethanolamine (EA) can potentially occur via three parallel, interconnected pathways involving methylation of free bases, phospho-bases, or phosphatidyl-bases. We investigated which pathways operate in tobacco (Nicotiana tabacum L.) because previous work has shown that the endogenous Cho supply limits accumulation of glycine betaine in transgenic tobacco plants engineered to convert Cho to glycine betaine. The kinetics of metabolite labeling were monitored in leaf discs supplied with [(33)P]phospho-EA, [(33)P]phospho-monomethylethanolamine, or [(14)C]formate, and the data were subjected to computer modeling. Because partial hydrolysis of phospho-bases occurred in the apoplast, modeling of phospho-base metabolism required consideration of the re-entry of [(33)P]phosphate into the network. Modeling of [(14)C]formate metabolism required consideration of the labeling of the EA and methyl moieties of Cho. Results supported the following conclusions: (a) The first methylation step occurs solely at the phospho-base level; (b) the second and third methylations occur mainly (83%-92% and 65%-85%, respectively) at the phospho-base level, with the remainder occurring at the phosphatidyl-base level; and (c) free Cho originates predominantly from phosphatidylcholine rather than from phospho-Cho. This study illustrates how computer modeling of radiotracer data, in conjunction with information on chemical pool sizes, can provide a coherent, quantitative picture of fluxes within a complex metabolic network.

Genetic engineering of glycinebetaine production toward enhancing stress tolerance in plants: metabolic limitations

Glycinebetaine (betaine) affords osmoprotection in bacteria, plants and animals, and protects cell components against harsh conditions in vitro. This and a compelling body of other evidence have encouraged the engineering of betaine production in plants lacking it. We have installed the metabolic step for oxidation of choline, a ubiquitous substance, to betaine in three diverse species, Arabidopsis, Brassica napus, and tobacco (Nicotiana tabacum), by constitutive expression of a bacterial choline oxidase gene. The highest levels of betaine in independent transgenics were 18.6, 12.8, and 13 micromol g(-1) dry weight, respectively, values 10- to 20-fold lower than the levels found in natural betaine producers. However, choline-fed transgenic plants synthesized substantially more betaine. Increasing the choline supplementation further enhanced betaine synthesis, up to 613 micromol g(-1) dry weight in Arabidopsis, 250 micromol g(-1) dry weight in B. napus, and 80 micromol g(-1) dry weight in tobacco. These studies demonstrate the need to enhance the endogenous choline supply to support accumulation of physiologically relevant amounts of betaine. A moderate stress tolerance was noted in some but not all betaine-producing transgenic lines based on relative shoot growth. Furthermore, the responses to stresses such as salinity, drought, and freezing were variable among the three species.

Salinity and hyperosmotic stress induce rapid increases in phosphatidylinositol 4,5-bisphosphate, diacylglycerol pyrophosphate, and phosphatidylcholine in Arabidopsis thaliana cells

In animal cells, phosphoinositides are key components of the inositol 1,4,5-trisphosphate/diacylglycerol-based signaling pathway, but also have many other cellular functions. These lipids are also believed to fulfill similar functions in plant cells, although many details concerning the components of a plant phosphoinositide system, and their regulation are still missing. Only recently have the different phosphoinositide isomers been unambiguously identified in plant cells. Another problem that hinders the study of the function of phosphoinositides and their derivatives, as well as the regulation of their metabolism, in plant cells is the need for a homogenous, easily obtainable material, from which the extraction and purification of phospholipids is relatively easy and quantitatively reproducible. We present here a thorough characterization of the phospholipids purified from [(32)P]orthophosphate- and myo-[2-(3)H]inositol-radiolabeled Arabidopsis thaliana suspension-cultured cells. We then show that NaCl treatment induces dramatic increases in the levels of phosphatidylinositol 4,5-bisphosphate and diacylglycerol pyrophosphate and also affects the turnover of phosphatidylcholine. The increase in phosphatidylinositol 4,5-bisphosphate was also observed with a non-ionic hyperosmotic shock. In contrast, the increase in diacylglycerol pyrophosphate and the turnover of phosphatidylcholine were relatively specific to salt treatments as only minor changes in the metabolism of these two phospholipids were detected when the cells were treated with sorbitol instead of NaCl.

The endogenous choline supply limits glycine betaine synthesis in transgenic tobacco expressing choline monooxygenase

Certain plants produce glycine betaine (GlyBet) in the chloroplast by a two-step oxidation of choline. Introducing GlyBet accumulation into plants that lack it is a well-established target for metabolic engineering because GlyBet can lessen damage from osmotic stress. The first step in GlyBet synthesis is catalyzed by choline mono-oxygenase (CMO), a stromal enzyme with a Rieske-type [2Fe-2S] center. The absence of CMO is the primary constraint on GlyBet production in GlyBet-deficient plants such as tobacco, but the endogenous choline supply is also potentially problematic. To investigate this, we constructed transgenic tobacco plants that constitutively express a spinach CMO cDNA. The CMO protein was correctly compartmented in chloroplasts and was enzymatically active, showing that its [2Fe-2S] cluster had been inserted. Salinization increased CMO protein levels, apparently via a post-transcriptional mechanism, to as high as 10% of that in salinized spinach. However, the GlyBet contents of CMO+ plants were very low (0.02-0.05 mumol g-1 fresh weight) in both unstressed and salinized conditions. Experiments with [14C]GlyBet demonstrated that this was not due to GlyBet catabolism. When CMO+ plants were supplied in culture with 5 mM choline or phosphocholine, their choline and GlyBet levels increased by at least 30-fold. The choline precursors mono- and dimethylethanolamine also enhanced choline and GlyBet levels but ethanolamine did not, pointing to a major constraint on flux to choline at the first methylation step in its synthesis. The extractable activity of the enzyme mediating this step in tobacco was only 3% that of spinach. We conclude that in GlyBet-deficient plants engineered with choline-oxidizing genes, the size of the free choline pool and the metabolic flux to choline need to be increased to attain GlyBet levels as high as those in natural accumulators.

Enzymes of Choline Synthesis in Spinach (Response of Phospho-Base N-Methyltransferase Activities to Light and Salinity)

In spinach (Spinacia oleracea L.), choline is synthesized by the sequential N-methylation of phosphoethanolamine -> phosphomono- -> phosphodi- -> phosphotrimethylethanolamine (i.e. phosphocholine) followed by hydrolysis to release choline. Differential centrifugation of spinach leaf extracts shows that enzymes catalyzing the three N-methylations are cytosolic. These enzymes were assayed in leaf extracts prepared from plants growing under various light/dark periods. Under a diurnal, 8-h light/16-h dark photoperiod, the activity of the enzyme catalyzing the N-methylation of phosphoethanolamine is highest at the end of the light period and lowest following the dark period. Prolonged dark periods (exceeding 16 h) lead to a further reduction in the activity of this enzyme, although activity is restored when plants are reexposed to light. In contrast, the activity of the enzyme(s) catalyzing the N-methylations of phosphomono- and phosphodimethylethanolamine does not undergo comparable changes in response to light/dark treatments. Salt shock of plants with 200 mM NaCl results in a 2-fold increase in all three N-methylation activities relative to nonsalinized controls but only in plants exposed to light. Thus, light is required for the salt-responsive up-regulation of choline synthesis in spinach.

Choline Synthesis in Spinach in Relation to Salt Stress

Choline metabolism was examined in spinach (Spinacia oleracea L.) plants growing under nonsaline and saline conditions. In spinach, choline is required for phosphatidylcholine synthesis and as a precursor for the compatible osmolyte glycine betaine (betaine). When control (nonsalinized) leaf discs were incubated for up to 2 h with [1,2-14C]ethanolamine, label appeared in the N-methylated derivatives of phosphoethanolamine including phosphomono-, phosphodi-, and phosphotri- (i.e. phosphocholine) methyl-ethanolamine, as well as in choline and betaine, whereas no radioactivity could be detected in the mono- and dimethylated derivatives of the free base ethanolamine. Leaf discs from salinized plants showed the same pattern of labeling, although the proportion of label that accumulated in betaine was almost 3-fold higher in the salinized leaf discs. Enzymes involved in choline metabolism were assayed in crude leaf extracts of plants. The activites of ethanolamine kinase and of the three S-adenosylmethionine:phospho-base N-methyltransferase enzymes responsible for N-methylating phosphoethanolamine to phosphocholine were all higher in extracts of plants salinized step-wise to 100, 200, or 300 mM NaCI compared with controls. In contrast, choline kinase, phosphocholine phosphatase, and cytidine 5[prime]-triphosphate: phosphocholine cytidylyltransferase activities showed little variation with salt stress. Thus, the increased diversion of choline to betaine in salt-stressed spinach appears to be mediated by the increased activity of several key enzymes involved in choline biosynthesis.

Phosphatidylcholine synthesis in castor bean endosperm : free bases as intermediates

The methylation steps in the biosynthesis of phosphatidylcholine by castor bean (Ricinus communis L.) endosperm have been studied by pulse-chase labeling. Endosperm halves were incubated with [methyl-(14)C]S-adenosyl-l-methionine, [2-(14)C]ethanolamine, [(14)C]ethanolamine phosphate, or [(14)C]serine phosphate. The kinetics of appearance were followed in the free, phospho-, and phosphatidyl-bases. The initial methylation utilized ethanolamine as a substrate to form methylethanolamine, which was then converted to dimethylethanolamine, choline, and phosphomethylethanolamine. Subsequent methylations occurred at the phospho-base and, to a lesser extent, the phosphatidyl-base levels, after which the radioactivity either remained constant or decreased in these compounds and accumulated in phosphatidylcholine. Although the precursors tested did support the synthesis of choline, the kinetics of the labeling make them unlikely to be the major sources of free choline to be utilized for the nucleotide pathway. A model with two pools of choline is proposed, and the implications of these results for the pathways leading to phosphatidylcholine biosynthesis are discussed.

Salt-inducible betaine aldehyde dehydrogenase from sugar beet: cDNA cloning and expression

Members of the Chenopodiaceae, such as sugar beet and spinach, accumulate glycine betaine in response to salinity or drought stress. The last enzyme in the glycine betaine biosynthetic pathway is betaine aldehyde dehydrogenase (BADH). In sugar beet the activity of BADH was found to increase two- to four-fold in both leaves and roots as the NaCl level in the irrigation solution was raised from 0 to 500 mM. This increase in BADH activity was paralleled by an increase in level of translatable BADH mRNA. Several cDNAs encoding BADH were cloned from a lambda gt10 library representing poly(A+) RNA from salinized leaves of sugar beet plants, by hybridization with a spinach BADH cDNA. Three nearly full-length cDNA clones were confirmed to encode BADH by their nucleotide and deduced amino acid sequence identity to spinach BADH; these clones showed minor nucleotide sequence differences consistent with their being of two different BADH alleles. The clones averaged 1.7 kb and contained an open reading frame predicting a polypeptide of 500 amino acids with 83% identity to spinach BADH. RNA gel blot analysis of total RNA showed that salinization to 500 mM NaCl increased BADH mRNA levels four-fold in leaves and three-fold in the taproot. DNA gel blot analyses indicated the presence of at least two copies of BADH in the haploid sugar beet genome.

Carbon-13 nuclear magnetic resonance analysis of [1-13C]glucose metabolism in Trypanosoma cruzi. Evidence of the presence of two alanine pools and of two CO2 fixation reactions

The non-invasive technique of 13C-nuclear magnetic resonance was applied to study glucose metabolism in vivo in Trypanosoma cruzi, the causative agent of American trypanosomiasis (Chagas' disease). It was found that under anaerobic conditions [1-13C]glucose undergoes a glycolytic pathway whose main metabolic products were identified as [3-13C]alanine, [2-13C]succinate and phosphoryl[1-13C]choline; [2-13C]alanine was also a minor metabolite. The addition of 70% 2H2O to the incubation mixture led to the formation of [3-13C, 3-2H]alanine derived from the prior incorporation of 2H+ into pyruvate. The existence of a [3-13C, 3-2H]pyruvate precursor, although not isolated, could be inferred from the formation of [2-13C, 2-2H]succinate in the same experiment. The latter derives from the CO2 fixation reaction on pyruvate or phosphoenolpyruvate to give malate, which is then converted to succinate through the fumarate intermediate step. The presence of [2-13C]alanine must be traced to a randomization of label at the malate-fumarate stage. Both [3-13C, 3-2H]alanine and [2-13C, 2-2H]succinate were excreted from the cells into the supernatant. When the cell pellet was lysed with perchloric acid it released [3-13C]alanine which was devoid of 2H+. Hence, T. cruzi has two alanine pools: one which incorporates 2H+ from the 2H2O present in the medium and excretes alanine into the latter, and another which is impervious to 2H+ exchange. The fixation of CO2 on a C3 precursor was confirmed by incubation of the T. cruzi cells with [1-13C]glucose and sodium [13C]bicarbonate which led to the formation of [1,2-13C2]succinate (Jcc = 51.8 Hz). Incubation with sodium [13C]bicarbonate and [13C]glucose led to the formation of [1-13C]succinate (182.5 ppm) derived from the 13CO2 fixation on the C3 precursor, and of phosphoryl[1-13C]choline (59.39 ppm) which revealed the presence in T. cruzi of a reductive pathway of CO2 which is independent of the CO2 fixation reaction. The formation of phosphoryl[1-13C]choline from [1-13C]glucose should be attributed to 13CO2 liberated from the former by glucose-6-phosphate dehydrogenase.

Phosphatidylcholine synthesis in the rat: the substrate for methylation and regulation by choline

Two lines of evidence led us to reexamine the possibility that methylation of phosphoethanolamine and its partially methylated derivatives, in addition to methylation of the corresponding phosphatidyl derivatives, plays a role in mammalian phosphatidylcholine biosynthesis: (a) Results obtained by Salerno and Beeler with rat [Salerno, D.M. and Beeler, D.A. (1973) Biochim. Biophys. Acta 326, 325-338] appear to strongly support such a role for methylation of phosphobases; (b) Such reactions have recently been shown to play major roles in phosphatidylcholine synthesis by higher plants [see Datko, A.H. and Mudd, S.H. (1988) Plant Physiol. 88, 854-861 and references therein]. We found that, following continuous labeling of rat liver with L-[methyl-3H]methionine for 10.4 min (intraperitoneal administration) or for 0.75 min (intraportal administration), virtually no 3H was detected in methylated derivatives of phosphoethanolamine, but readily detectable amounts of 3H were present in the base moiety of each methylated derivative of phosphatidylethanolamine. Thus, there was no indication that phospho-base methylation makes a significant contribution. Studies of cultured rat hepatoma cells showed definitively for the first time in a mammalian system that choline deprivation up-regulates the rate of flow of methyl groups originating in methionine into phosphatidylethanolamine and derivatives. Even under these conditions, methylation of phosphoethanolamine bases appeared to play a negligible role.

Development of two isogenic sweet corn hybrids differing for glycinebetaine content

A hybrid of sweet corn, Zea mays L. (;1720'; Rogers Brothers Seed Co.), was found to be comprised of glycinebetaine-positive and glycinebetaine-deficient individuals in a 1:1 mixture. This phenomenon was traced to segregation for a single, nuclear, dominant gene determining leaf glycinebetaine content within the female inbred parent of this hybrid. Selection for homozygous recessive (glycinebetaine-deficient) and homozygous dominant (glycinebetaine-positive) genotypes of the female inbred parent enabled production of two isogenic versions of hybrid ;1720' differing with respect to a single copy of the dominant allele, by mating these female parent selections with the common homozygous recessive (glycinebetaine-deficient) male parent. These two isogenic hybrids are shown to differ by a factor of 300- to 400-fold in glycinebetaine titer of young expanding leaves of salinized plants, but exhibit no striking differences in the levels of free amino acids or the level of N-methylnicotinic acid (nicotinic acid betaine; trigonelline). The only significant difference between the two hybrids in terms of amino acid composition was found to be in the level of alanine under nonsalinized conditions. The betaine-deficient hybrid exhibited a 14% lower alanine level than the betaine-positive hybrid. Betaine deficiency was not associated with altered stress-induced accumulation of amino acids such as proline, serine, and asparagine plus aspartate, attesting to the high specificity of the genetic difference between these isogenic hybrids with respect to betaine accumulation. This germplasm offers unique opportunities to test whether a single dominant allele determining stress-induced betaine accumulation capacity influences stress resistance in maize.

Synthesis of methylated ethanolamine moieties: regulation by choline in soybean and carrot

Cultured cell suspensions of both carrot (Daucus carota L.) and soybean (Glycine max) take up exogenous choline efficiently from their respective growth media. During sustained growth at a concentration near 50 micromolar choline, this compound was taken up at rates which exceeded those at which phosphatidylcholine, is synthesized by cells growing in standard (i.e. choline-free) media. In 50 micromolar choline, both types of cells metabolized this compound to phosphocholine and phosphatidylcholine, but not to other detected metabolites, and marked accumulations of phosphocholine and choline occurred relative to phosphatidylcholine. Pregrowth in 50 micromolar choline for several doublings decreased the rate at which carrot cells transferred (3)H from l-[(3)H(3)C] methionine into the network of all methylated derivatives of ethanolamine by some 98%. With soybean cells, a decrease of 77% was observed. In both cell types, transfer of (3)H into S-methylmethionine, pectin methyl esters, methylated nucleic acids, and nonpolar lipid continued unabated. Gel-filtered extracts of carrot cells pregrown in 50 micromolar choline had marked decreases in the specific activities of S-adenosylmethionine-dependent phosphoethanolamine, phosphomethylethanolamine, and phosphodimethylethanolamine N-methyltransferases; extracts of soybean cells had a similar decrease in phosphoethanolamine N-methyltransferase. The significance of these findings for regulation of the rate of synthesis of methylated ethanolamine moieties is discussed.

Evidence for a ferredoxin-dependent choline monooxygenase from spinach chloroplast stroma

Chenopods synthesize betaine in the chloroplast via a two-step oxidation of choline: choline --> betaine aldehyde --> betaine. Our previous experiments with intact chloroplasts, and in vivo(18)O(2) labeling studies, led us to propose that the first step is mediated by a monooxygenase which uses photosynthetically generated reducing power (C Lerma, AD Hanson, D Rhodes [1988] Plant Physiol 88: 695-702). Here, we report the detection of such an activity in vitro. In the presence of O(2) and reduced ferredoxin, the stromal fraction from spinach (Spinacia oleracea) chloroplasts converted choline to betaine aldehyde at rates similar to those in intact chloroplasts (20-50 nanomoles per hour per milligram protein). Incorporation of (18)O from (18)O(2) by the in vitro reaction was demonstrated by fast atom bombardment mass spectrometry. Ferredoxin could be reduced either with thylakoids in the light, or with NADPH plus ferredoxin-NADP reductase in darkness; NADPH alone could not substitute for ferredoxin. No choline-oxidizing activity was detected in the stromal fraction of pea (Pisum sativum L.), a species that does not accumulate betaine. The spinach choline-oxidizing enzyme was stimulated by 10 millimolar Mg(2+), had a pH optimum close to 8, and was insensitive to carbon monoxide. The specific activity was increased threefold in plants growing in 200 millimolar NaCl. Gel filtration experiments gave a molecular weight of 98 kilodaltons for the choline-oxidizing enzyme, and provided no evidence for other electron carriers which might mediate the reduction of the 98-kilodalton enzyme by ferredoxin.

Synthesis of methylated ethanolamine moieties: regulation by choline in lemna

The results of experiments in which intact plants of Lemna paucicostata were labeled with either l-[(3)H(3)C]methionine, l-[(14)CH(3)]methionine, or [1,2-(14)C]ethanolamine support the conclusion that growth in concentrations of choline of 3.0 micromolar or above brings about marked decreases in the rate of biosynthesis of methylated forms of ethanolamine (normally present chiefly as phosphatidylcholine, with lesser amounts of choline and phosphocholine). The in vivo locus of the block is at the committing step in the biosynthetic sequence at which phosphoethanolamine is methylated by S-adenosylmethionine to form phosphomethylethanolamine. The block is highly specific: flow of methyl groups originating in methionine continues into S-adenosylmethionine, S-methylmethionine, the methyl moieties of pectin methyl ester, and other methylated metabolites. When choline uptake is less than the total that would be synthesized by control plants, phosphoethanolamine methylation is down-regulated to balance the uptake; total plant content of choline and its derivatives remains essentially constant. At maximum down-regulation, phosphoethanolamine methylation continues at 5 to 10% of normal. A specific decrease in the total available activity of AdoMet: phosphoethanolamine N-methyltransferase, as well as feedback inhibition of this enzyme by phosphocholine, and prevention of accumulation of phosphoethanolamine by down-regulation of ethanolamine synthesis may each contribute to effective control of phosphoethanolamine methylation. This down-regulation may necessitate major changes in S-adenosylmethionine metabolism. Such changes are discussed.

Enzymes of phosphatidylcholine synthesis in lemna, soybean, and carrot

Cell-free extracts from Lemna and suspension cultured carrot (Daucus Carota L.) catalyze S-adenosylmethionine-dependent N-methylations of phosphoethanolamine, phosphomethylethanolamine, and phosphodimethylethanolamine; extracts of suspension cultured soybean (Glycine max), of phosphoethanolamine only. Material pelleted from each tissue between 15,000 and 100,000g catalyzes S-adenosylmethionine-dependent N-methylations of phosphatidylmethylethanolamine and phosphatidyl-dimethylethanolamine, but not phosphatidylethanolamine. Extracts from each tissue catalyze CTP-dependent cytidylyltransfers to each of the three methylated phosphoethanolamine derivatives, forming the corresponding CDP derivatives. Some of the properties of the activities investigated are reported. On the basis of in vivo labeling experiments, we have proposed (AH Datko, SH Mudd 1988 Plant Physiol 88: 854-861) differing pathways for phosphatidylcholine synthesis in which, after a common committing step, N-methylation of phosphoethanolamine, subsequent methylations occur in Lemna almost exclusively at the phospho-base level; in soybean, at the phosphatidyl-base level; and in carrot, at both levels. Thus, among the activities investigated, at least those required for the operation of the proposed pathways have been positively demonstrated. The extent to which the present results explain the differences between these pathways is discussed, and a speculation offered as to how these differences may have arisen phylogenetically.

Phosphatidylcholine synthesis: differing patterns in soybean and carrot

The methylation steps in the biosynthesis of phosphatidylcholine by tissue culture preparations of carrot (Daucus carota L.) and soybean (Glycine max), and by soybean leaf discs, have been studied. Preparations were incubated with tracer concentrations of l-[(3)H(3)C]methionine and the kinetics of appearance of radioactivity in phosphomethylethanolamine, phosphodimethylethanolamine, phosphocholine, phosphatidylmethylethanolamine, phosphatidyldimethylethanolamine, phosphatidylcholine, methylethanolamine, dimethylethanolamine, and choline followed at short incubation times. With soybean (tissue culture or leaves), an initial methylation utilizes phosphoethanolamine as substrate, forming phosphomethylethanolamine. The latter is converted to phosphatidylmethylethanolamine, which is successively methylated to phosphatidyldimethyethanolamine and to phosphatidylcholine. With carrot, again, an initial methylation is of phosphoethanolamine. Subsequent methylations occur at both the phospho-base and phosphatidyl-base levels. Both of these patterns differ qualitatively from that previously demonstrated in Lemna (SH Mudd, AH Datko 1986 Plant Physiol 82: 126-135) in which all three methylations occur at the phospho-base level. For soybean and carrot, some added contribution from initial methylation of phosphatidylethanolamine has not been excluded. These results, together with those from similar experiments carried out with water-stressed barley leaves (WD Hitz, D Rhodes, AD Hanson 1981 Plant Physiol 68: 814-822) and salinized sugarbeet leaves (AD Hanson, D Rhodes 1983 Plant Physiol 71: 692-700) suggest that in higher plants some, perhaps all, phosphatidylcholine synthesis occurs via a common committing step (conversion of phosphoethanolamine to phosphomethylethanolamine) followed by a methylation pattern which differs from plant to plant.

Phosphatidylcholine synthesis in castor bean endosperm: characteristics and reversibility of the choline kinase reaction

Choline kinase (EC 2.7.1.32) was measured in concentrated 100,000gav supernatants from castor bean endosperm (Ricinus communis L. var. Hale). Initial velocity analysis, along with competitive inhibitor (hemicholinium-3) and product inhibition (ADPMg2+) studies suggested that the forward reaction followed a sequentially ordered mechanism with ATPMg2+ binding to the enzyme first, followed by choline and then activation of the ternary complex by free Mg2+. The kinetic constants of the forward reaction are reported. A reverse reaction was measured which had a pH optimum of 6.5 and produced 1 mol of ATP for every mole of choline phosphate. The estimated maximum possible Keq at 7.25 was 5 X 10(-3) which suggested that this reaction is highly reversible in this tissue. The possible physiological significance of this is discussed.

Phosphatidylcholine Synthesis in Castor Bean Endosperm : I. Metabolism of l-Serine

Endosperm halves from 3-day-old castor bean (Ricinus communis var Hale) were incubated for 30 minutes with l-[(14)C]serine, after which label was observed in ethanolamine, choline, phosphatidylserine, phosphatidylethanolamine, phosphatidylcholine, ethanolaminephosphate, and CDPethanolamine, but not in cholinephosphate or CDPcholine. Only later did significant amounts of isotope become incorporated into cholinephosphate and CDPcholine. The choline kinase inhibitor hemicholinium-3 prevented the incorporation of label from serine into cholinephosphate and CDPcholine, reduced the incorporation of [(14)C]choline into phosphatidylcholine by 65%, but inhibited the incorporation of label into phosphatidylcholine from serine by only 15%. The inhibitor did not prevent the incorporation of labeled methyl groups from S-adenosyl-l-methionine into phosphatidyldimethylethanolamine plus phosphatidylcholine. The amount of incorporation of label from the methyl donor was only 8% of that from choline into phosphatidylcholine. The implications of these results for the pathway and regulation of phosphatidylcholine synthesis from the water-soluble precursors are discussed.

Phosphoethanolamine bases as intermediates in phosphatidylcholine synthesis by lemna

The pathway for synthesis of phosphatidylcholine, the dominant methyl-containing end product formed by Lemna paucicostata, has been investigated. Methyl groups originating in methionine are rapidly utilized by intact plants to methylate phosphoethanolamine successively to the mono-, di-, and tri-methyl (i.e. phosphocholine) phosphoethanolamine derivatives. With continued labeling, radioactivity initially builds up in these compounds, then passes on, accumulating chiefly in phosphatidylcholine (34% of the total radioactivity taken up by plants labeled to isotopic equilibrium with l-[(14)CH(3)]methionine), and in lesser amounts in soluble choline (6%). Radioactivity was detected in mono- and dimethyl derivatives of free ethanolamine or phosphatidylethanolamine only in trace amounts. Pulse-chase experiments with [(14)CH(3)]choline and [(3)H] ethanolamine confirmed that phosphoethanolamine is rapidly methylated and that phosphocholine is converted to phosphatidylcholine. Initial rates indicate that methylation of phosphoethanolamine predominates over methylation of either phosphatidylethanolamine or free ethanolamine at least 99:1. Although more studies are needed, it is suggested this pathway may well turn out to account for most phosphatidylcholine synthesis in higher plants. Phosphomethylethanolamine and phosphodimethylethanolamine are present in low quantities during steady-state growth (18% and 6%, respectively, of the amount of phosphocholine). Radioactivity was not detected in CDP-choline, probably due to the low steady-state concentration of this nucleotide.

Molecular Biology of Osmoregulation

The drought of 1983 resulted in some 10 billion dollars in agricultural losses and has focused attention on the vulnerability of our major crops to this devastating form of environmental stress. This article is concerned with the molecular biology of a new class of genes, called osm (osmotic tolerance) genes, that protect bacteria like Escherichia coli against osmotic stress and may work in a similar manner in plants and animals. Osm genes govern the production of a class of molecules, such as betaine and proline, that protect the cell and its constituents against dehydration. These osmoprotectant molecules have been known for many years to accumulate in plants but have only recently been shown to have potent antistress activity for bacteria.