Metabolic engineering of glycine betaine synthesis: plant betaine aldehyde dehydrogenases lacking typical transit peptides are targeted to tobacco chloroplasts where they confer betaine aldehyde resistance

Choline Chloride Mediates Salinity Tolerance in Cluster Bean (Cyamopsis tetragonoloba L.) by Improving Growth, Oxidative Defense, and Secondary Metabolism

Choline chloride (CC) application enhanced the tolerance of cluster bean (Cyamopsis tetragonoloba L.) against salinity stress. The aim of the study was to determine the protective role of CC on plant growth, photosynthesis, and biochemical indicators of oxidative stress. The seeds of BR-99 (tolerant) and BR-2017 (sensitive) were surface sterilized and sown in plastic pots containing river sandy soil. The design of the experiments was completely randomized with 4 replicates per treatment. Three weeks after germination, salinity (150 mM) was imposed. Then plants were sprayed with different concentrations of CC (3, 5, and 10 mM), while normal plants were sprayed with distilled water. Salinity decreased growth attributes, relative water contents, photosynthetic attributes, total soluble proteins, total free amino acids, phenolic, flavonoids, ascorbic acid, proline, and glycine betaine and increased the levels of oxidative stress indicators. However, the application of CC (particularly 5 mM) improved growth attributes, photosynthetic pigments, and activities of antioxidant compounds by reducing the levels of H2O2, malondialdehyde in salt-stressed plants in both cluster bean varieties. BR-99 variety showed more tolerance to salinity stress than that of BR-2017 in the form of greater oxidative defense and osmotic adjustment and clear from greater plant dry masses. Thus, our results showed that the application of CC (5 mM) is an efficient strategy for field use in the areas, where salt stress soils limit agriculture production.

Bona fide choline monoxygenases evolved in Amaranthaceae plants from oxygenases of unknown function: Evidence from phylogenetics, homology modeling and docking studies

Few land plants can synthesize and accumulate the osmoprotectant glycine betaine (GB) even though this metabolic trait has major adaptive importance given the prevalence of drought, hypersaline soils or cold. GB is synthesized from choline in two reactions catalyzed by choline monooxygenases (CMOs) and enzymes of the family 10 of aldehyde dehydrogenases (ALDH10s) that gained betaine aldehyde dehydrogenase activity (BADH). Homolog genes encoding CMO and ALDH10 enzymes are present in all known land plant genomes, but since GB-non-accumulators plants lack the BADH-type ALDH10 isozyme, they would be expected to also lack the CMO activity to avoid accumulation of the toxic betaine aldehyde. To explore CMOs substrate specificity, we performed amino acid sequence alignments, phylogenetic analysis, homology modeling and docking simulations. We found that plant CMOs form a monophyletic subfamily within the Rieske/mononuclear non-heme oxygenases family with two clades: CMO1 and CMO2, the latter diverging from CMO1 after gene duplication. CMO1 enzymes are present in all plants; CMO2s only in the Amaranthaceae high-GB-accumulators plants. CMO2s, and particularly their mononuclear non-heme iron domain where the active site is located, evolved at a faster rate than CMO1s, which suggests positive selection. The homology model and docking simulations of the spinach CMO2 enzyme showed at the active site three aromatic residues forming a box with which the trimethylammonium group of choline could interact through cation-π interactions, and a glutamate, which also may interact with the trimethylammonium group through a charge-charge interaction. The aromatic box and the carboxylate have been shown to be critical for choline binding in other proteins. Interestingly, these residues are conserved in CMO2 proteins but not in CMO1 proteins, where two of these aromatic residues are leucine and the glutamate is asparagine. These findings reinforce our proposal that the CMO1s physiological substrate is not choline but a still unknown metabolite.

Isolation and characterization of a salt stress-responsive betaine aldehyde dehydrogenase in Lycium ruthenicum Murr

As compatible solute, glycine betaine (GB) plays a significant role in salinity tolerance in GB accumulating plants. Solanaceous crops such as tomato (Solanum lycopersicum) and tobacco (Nicotiana tabacum) are salt sensitive and naturally GB non-accumulators. In Solanaceae, only the Lycium genus has been recorded as halophytes in China, and several Lycium species have been reported as GB accumulators. The last biosynthetic step of GB is catalyzed by aminoaldehyde dehydrogenase (AMADH) with betaine aldehyde dehydrogenase (BADH) activities. Failure of GB synthesis in tomato and tobacco was attributed to lack of BADH activity. Here, by comparing the BADH functional residues of AMADHs between the Lycium genus and solanaceous crops, we predict that all studied AMADH1s have low BADH activities while only LbAMADH2 from L. barbarum has high BADH activity. For two AMADHs in L. ruthenicum, results from substrate enzyme assays confirmed low BADH activity of LrAMADH1 and no BADH activity of LrAMADH2. Despite the very low GB contents in L. ruthenicum seedlings (< 0.5 μmol g^-1 fresh weight), GB contents in fruits are up to 150 μmol g^-1 FW, inferring fruits of L. ruthenicum as good GB sources. In NaCl treated seedlings, accompanied by elevated GB accumulation, expression of LrAMADH1 was up-regulated, indicating response of LrAMADH1 to salt stress in L. ruthenicum. Virus-induced silence of LrAMADH1 leads to less GB accumulation than control, revealing that LrAMADH1 participates in GB synthesis in planta. Collectively, our results show that LrAMADH1 is the bona fide BADH, which responds to salt stress in L. ruthenicum.

MdPIN1b encodes a putative auxin efflux carrier and has different expression patterns in BC and M9 apple rootstocks

Lower promoter activity is closely associated with lower MdPIN1b expression in the M9 interstem, which might contribute to the dwarfing effect in apple trees. Apple trees grafted onto dwarfing rootstock Malling 9 (M9) produce dwarfing tree architecture with high yield and widely applying in production. Previously, we have reported that in Malus 'Red Fuji' (RF) trees growing on M9 interstem and Baleng Crab (BC) rootstock, IAA content was relatively higher in bark tissue of M9 interstem than that in scion or rootstock. As IAA polar transportation largely depends on the PIN-FORMED (PIN) auxin efflux carrier. Herein, we identify two putative auxin efflux carrier genes in Malus genus, MdPIN1a and MdPIN1b, which were closely related to the AtPIN1. We found that MdPIN1b was expressed preferentially in BC and M9, and the expression of MdPIN1b was significantly lower in the phloem of M9 interstem than that in the scion and rootstock. The distinct expression of MdPIN1b and IAA content were concentrated in the cambium and adjacent xylem or phloem, and MdPIN1b protein was localized on cell plasma membrane in onion epidermal cells transiently expressing 35S:MdPIN1b-GFP fusion protein. Interestingly, an MdPIN1b mutant allele in the promoter region upstream of M9 exhibited decreased MdPIN1b expression compared to BC. MdPIN1b over-expressing interstem in tobacco exhibited increased polar auxin transport. It is proposed that natural allelic differences decreased promoter activity is closely associated with lower MdPIN1b expression in the M9 interstem, which might limit the basipetal transport of auxin, and in turn might contribute to the dwarfing effect. Taken together, these results reveal allelic variation underlying an important apple rootstock trait, and specifically a novel molecular genetic mechanism underlying dwarfing mechanism.

Identification of osmoadaptive strategies in the halophile, heterotrophic ciliate Schmidingerothrix salinarum

Hypersaline environments pose major challenges to their microbial residents. Microorganisms have to cope with increased osmotic pressure and low water activity and therefore require specific adaptation mechanisms. Although mechanisms have already been thoroughly investigated in the green alga Dunaliella salina and some halophilic yeasts, strategies for osmoadaptation in other protistan groups (especially heterotrophs) are neither as well known nor as deeply investigated as for their prokaryotic counterpart. This is not only due to the recent awareness of the high protistan diversity and ecological relevance in hypersaline systems, but also due to methodological shortcomings. We provide the first experimental study on haloadaptation in heterotrophic microeukaryotes, using the halophilic ciliate Schmidingerothrix salinarum as a model organism. We established three approaches to investigate fundamental adaptation strategies known from prokaryotes. First, proton nuclear magnetic resonance (1H-NMR) spectroscopy was used for the detection, identification, and quantification of intracellular compatible solutes. Second, ion-imaging with cation-specific fluorescent dyes was employed to analyze changes in the relative ion concentrations in intact cells. Third, the effect of salt concentrations on the catalytic performance of S. salinarum malate dehydrogenase (MDH) and isocitrate dehydrogenase (ICDH) was determined. 1H-NMR spectroscopy identified glycine betaine (GB) and ectoine (Ect) as the main compatible solutes in S. salinarum. Moreover, a significant positive correlation of intracellular GB and Ect concentrations and external salinity was observed. The addition of exogenous GB, Ect, and choline (Ch) stimulated the cell growth notably, indicating that S. salinarum accumulates the solutes from the external medium. Addition of external 13C2-Ch resulted in conversion to 13C2-GB, indicating biosynthesis of GB from Ch. An increase of external salinity up to 21% did not result in an increase in cytoplasmic sodium concentration in S. salinarum. This, together with the decrease in the catalytic activities of MDH and ICDH at high salt concentration, demonstrates that S. salinarum employs the salt-out strategy for haloadaptation.

Functional and expression analyses of two kinds of betaine aldehyde dehydrogenases in a glycinebetaine-hyperaccumulating graminaceous halophyte, Leymus chinensis

Glycinebetaine (GB) is an important compatible solute for salinity tolerance in many plants. In this study, we analyzed the enzymatic activity and the expression level of betaine aldehyde dehydrogenase (BADH), an important enzyme that catalyzes the last step in the GB synthesis in Leymus chinensis, a GB-hyperaccumulating graminaceous halophyte, and compared with those of barley, a graminaceous glycophyte. We have isolated cDNAs for two BADH genes, LcBADH1 and LcBADH2. LcBADH1 has a putative peroxisomal signal peptide (PTS1) at its C-terminus, while LcBADH2 does not have any typical signal peptide. Using immunofluorescent labeling, we showed that BADH proteins were localized to the cytosol and dot-shaped organelles in the mesophyll and bundle sheath cells of L.chinensis leaves. The affinity of recombinant LcBADH2 for betaine aldehyde was comparable to other plant BADHs, whereas recombinant LcBADH1 showed extremely low affinity for betaine aldehyde, indicating that LcBADH2 plays a major role in GB synthesis in L. chinensis. In addition, the recombinant LcBADH2 protein was tolerant to NaCl whereas LcBADH1 wasn't. The kinetics, subcellular and tissue localization of BADH proteins were comparable between L. chinensis and barley. The activity and expression level of BADH proteins were higher in L. chinensis compared with barley under both normal and salinized conditions, which may be related to the significant difference in the amount of GB accumulation between two plants.

Live-cell high resolution magic angle spinning magnetic resonance spectroscopy for in vivo analysis of Pseudomonas aeruginosa metabolomics

Pseudomonas aeruginosa (PA) is a pathogenic gram-negative bacterium that is widespread in nature, inhabiting soil, water, plants and animals. PA is a prevalent cause of deleterious human infections, particularly in patients whose host defense mechanisms have been compromised. Metabolomics is an important tool used to study host-pathogen interactions and to identify novel therapeutic targets and corresponding compounds. The aim of the present study was to report the metabolic profile of live PA bacteria using in vivo high-resolution magic angle spinning (HRMAS) nuclear magnetic resonance spectroscopy (NMR), in combination with 1- and 2-dimensional HRMAS NMR. This methodology provides a new and powerful technique to rapidly interrogate the metabolome of intact bacterial cells and has several advantages over traditional techniques that identify metabolome components from disrupted cells. Furthermore, application of multidimensional HRMAS NMR, in combination with the novel technique total through-Bond correlation Spectroscopy (TOBSY), is a promising approach that may be used to obtain in vivo metabolomics information from intact live bacterial cells and can mediate such analyses in a short period of time. Moreover, HRMAS ¹H NMR enables the investigation of the associations between metabolites and cell processes. In the present study, we detected and quantified several informative metabolic molecules in live PA cells, including N-acetyl, betaine, citrulline, alanine and glycine, which are important in peptidoglycan synthesis. The results provided a complete metabolic profile of PA for future studies of PA clinical isolates and mutants. In addition, this in vivo NMR biomedical approach might have clinical utility and should prove useful in gene function validation, the study of pathogenetic mechanisms, the classification of microbial strains into functional/clinical groups, the testing of anti-bacterial agents and the determination of metabolic profiles of bacterial mutants.

Physical linkage of metabolic genes in fungi is an adaptation against the accumulation of toxic intermediate compounds

Genomic analyses have proliferated without being tied to tangible phenotypes. For example, although coordination of both gene expression and genetic linkage have been offered as genetic mechanisms for the frequently observed clustering of genes participating in fungal metabolic pathways, elucidation of the phenotype(s) favored by selection, resulting in cluster formation and maintenance, has not been forthcoming. We noted that the cause of certain well-studied human metabolic disorders is the accumulation of toxic intermediate compounds (ICs), which occurs when the product of an enzyme is not used as a substrate by a downstream neighbor in the metabolic network. This raises the hypothesis that the phenotype favored by selection to drive gene clustering is the mitigation of IC toxicity. To test this, we examined 100 diverse fungal genomes for the simplest type of cluster, gene pairs that are both metabolic neighbors and chromosomal neighbors immediately adjacent to each other, which we refer to as "double neighbor gene pairs" (DNGPs). Examination of the toxicity of their corresponding ICs shows that, compared with chromosomally nonadjacent metabolic neighbors, DNGPs are enriched for ICs that have acutely toxic LD50 doses or reactive functional groups. Furthermore, DNGPs are significantly more likely to be divergently oriented on the chromosome; remarkably, ∼40% of these DNGPs have ICs known to be toxic. We submit that the structure of synteny in metabolic pathways of fungi is a signature of selection for protection against the accumulation of toxic metabolic intermediates.

Plastid genetic engineering in Solanaceae

Plastid genetic engineering has come of age, becoming today an attractive alternative approach for the expression of foreign genes, as it offers several advantages over nuclear transformants. Significant progress has been made in plastid genetic engineering in tobacco and other Solanaceae plants, through the use of improved regeneration procedures and transformation vectors with efficient promoters and untranslated regions. Many genes encoding for industrially important proteins and vaccines, as well as genes conferring important agronomic traits, have been stably integrated and expressed in the plastid genome. Despite these advances, it remains a challenge to achieve marked levels of plastid transgene expression in non-green tissues. In this review, we summarize the basic requirements of plastid genetic engineering and discuss the current status, limitations, and the potential of plastid transformation for expanding future studies relating to Solanaceae plants.

Expression and substrate specificity of betaine/proline transporters suggest a novel choline transport mechanism in sugar beet

Proline transporters (ProTs) originally described as highly selective transporters for proline, have been shown to also transport glycinebetaine (betaine). Here we examined and compared the transport properties of Bet/ProTs from betaine accumulating (sugar beet, Amaranthus, and Atriplex,) and non-accumulating (Arabidopsis) plants. Using a yeast mutant deficient for uptake of proline and betaine, it was shown that all these transporters exhibited higher affinity for betaine than proline. The uptake of betaine and proline was pH-dependent and inhibited by the proton uncoupler carbonylcyanide m-chlorophenylhydrazone (CCCP). We also investigated choline transport by using a choline transport-deficient yeast mutant. Results revealed that these transporters exhibited a higher affinity for choline uptake rather than betaine. Uptake of choline by sugar beet BvBet/ProT1 was independent of the proton gradient and the inhibition by CCCP was reduced compared with that for uptake of betaine, suggesting different proton binding properties between the transport of choline and betaine. Additionally, in situ hybridization experiments revealed the localization of sugar beet BvBet/ProT1 in phloem and xylem parenchyma cells.

Functional analysis of folate polyglutamylation and its essential role in plant metabolism and development

Cellular folates function as co-enzymes in one-carbon metabolism and are predominantly decorated with a polyglutamate tail that enhances co-enzyme affinity, subcellular compartmentation and stability. Polyglutamylation is catalysed by folylpolyglutamate synthetases (FPGSs) that are specified by three genes in Arabidopsis, FPGS1, 2 and 3, which reportedly encode plastidic, mitochondrial and cytosolic isoforms, respectively. A mutational approach was used to probe the functional importance of folate polyglutamylation in one-carbon metabolism and development. Biochemical analysis of single FPGS loss-of-function mutants established that folate polyglutamylation is essential for organellar and whole-plant folate homeostasis. However, polyglutamylated folates were still detectable, albeit at lower levels, in organelles isolated from the corresponding isozyme knockout lines, e.g. in plastids and mitochondria of the fpgs1 (plastidial) and fpgs2 (mitochondrial) mutants. This result is surprising given the purported single-compartment targeting of each FPGS isozyme. These results indicate redundancy in compartmentalised FPGS activity, which in turn explains the lack of anticipated phenotypic defects for the single FPGS mutants. In agreement with this hypothesis, fpgs1 fpgs2 double mutants were embryo-lethal, fpgs2 fpgs3 mutants exhibited seedling lethality, and fpgs1 fpgs3 mutants were dwarfed with reduced fertility. These phenotypic, metabolic and genetic observations are consistent with targeting of one or more FPGS isozymes to multiple organelles. These data confirm the importance of polyglutamylation in folate compartmentation, folate homeostasis and folate-dependent metabolic processes, including photorespiration, methionine and pantothenate biosynthesis.

OsBADH1 is possibly involved in acetaldehyde oxidation in rice plant peroxisomes

Although rice (Oryza sativa L.) produces little glycine betaine (GB), it has two betaine aldehyde dehydrogenase (BADH; EC 1.2.1.8) gene homologs (OsBADH1 and OsBADH2). We found that OsBADH1 catalyzes the oxidation of acetaldehyde efficiently, while the activity of OsBADH2 is extremely low. The accumulation of OsBADH1 mRNA decreases following submergence treatment, but quickly recovers after re-aeration. We confirmed that OsBADH1 localizes in peroxisomes. In this paper, a possible physiological function of OsBADH1 in the oxidation of acetaldehyde produced by catalase in rice plant peroxisomes is discussed.

Betaine aldehyde dehydrogenase in plants

Plant betaine aldehyde dehydrogenases (BADHs) have been the target of substantial research, especially during the last 20 years. Initial characterisation of BADH as an enzyme involved in the production of glycine betaine (GB) has led to detailed studies of the role of BADH in the response of plants to abiotic stress in vivo, and the potential for transgenic expression of BADH to improve abiotic stress tolerance. These studies have, in turn, yielded significant information regarding BADH and GB function. Recent research has identified the potential for BADH as an antibiotic-free marker for selection of transgenic plants, and a major role for BADH in 2-acetyl-1-pyrroline-based fragrance associated with jasmine and basmati style aromatic rice varieties.

Enzymatic characterization of peroxisomal and cytosolic betaine aldehyde dehydrogenases in barley

Betaine aldehyde dehydrogenase (BADH; EC 1.2.1.8) is an important enzyme that catalyzes the last step in the synthesis of glycine betaine, a compatible solute accumulated by many plants under various abiotic stresses. In barley (Hordeum vulgare L.), we reported previously the existence of two BADH genes (BBD1 and BBD2) and their corresponding proteins, peroxisomal BADH (BBD1) and cytosolic BADH (BBD2). To investigate their enzymatic properties, we expressed them in Escherichia coli and purified both proteins. Enzymatic analysis indicated that the affinity of BBD2 for betaine aldehyde was reasonable as other plant BADHs, but BBD1 showed extremely low affinity for betaine aldehyde with apparent K(m) of 18.9 microM and 19.9 mM, respectively. In addition, V(max)/K(m) with betaine aldehyde of BBD2 was about 2000-fold higher than that of BBD1, suggesting that BBD2 plays a main role in glycine betaine synthesis in barley plants. However, BBD1 catalyzed the oxidation of omega-aminoaldehydes such as 4-aminobutyraldehyde and 3-aminopropionaldehyde as efficiently as BBD2. We also found that both BBDs oxidized 4-N-trimethylaminobutyraldehyde and 3-N-trimethylaminopropionaldehyde.

Plastid-expressed choline monooxygenase gene improves salt and drought tolerance through accumulation of glycine betaine in tobacco

Glycine betaine (GlyBet), a quaternary ammonium compound, functions as an osmoprotectant in many organisms including plants. Previous research has shown that over-expression of enzymes for GlyBet biosynthesis in transgenic plants improved abiotic stress tolerance, but so far no study on the effects of plastid-expression of choline monooxygenase, the enzyme that catalyzes the conversion of choline into betaine aldehyde, has been reported. In the present study, tobacco (Nicotiana tabacum L. cv Wisconsin 38) plants were transformed with a gene for choline monooxygenase (BvCMO) from beet (Beta vulgaris) via plastid genetic engineering. Transplastomic plants constitutively expressing BvCMO under the control of the ribosomal RNA operon promoter and a synthetic T7 gene G10 leader were able to accumulate GlyBet in leaves, roots and seeds, and exhibited improved tolerance to toxic level of choline and to salt/drought stress when compared to wild type plants. Transplastomic plants also demonstrated higher net photosynthetic rate and apparent quantum yield of photosynthesis in the presence of 150 mM NaCl. Salt stress caused no significant change on the maximal efficiency of PSII photochemistry (Fv/Fm) in both wild type and transplastomic plants, but a decrease in the actual efficiency of PSII (PhiPSII) was observed, and such a decrease was much greater in wild type plants. Our results demonstrate the feasibility of improving salt and drought tolerance in plants through plastid transformation with BvCMO gene.

Safety and effectiveness of topical dry mouth products containing olive oil, betaine, and xylitol in reducing xerostomia for polypharmacy-induced dry mouth

Polypharmacy is a common cause of salivary hypofunction, producing symptoms of dry mouth or xerostomia, especially among older populations. As the number of older people continues to increase, polypharmacy-induced salivary hypofunction is becoming an increasing problem. Many over-the-counter products are available for relieving symptoms of dry mouth, but few have been tested in controlled clinical investigations. The purpose of this investigation was to evaluate the safety and efficacy of a group of topical dry mouth products (toothpaste, mouth rinse, mouth spray and gel) containing olive oil, betaine and xylitol. Forty adults were entered into this single-blinded, open-label, cross-over clinical study and 39 completed all the visits. Subjects were randomly assigned at baseline to using the novel topical dry mouth products daily for 1 week, or to maintain their normal dry mouth routine care. After 1 week, they were crossed over to the other dry mouth regimen. The results demonstrated that the use of the novel topical dry mouth products increased significantly unstimulated whole salivary flow rates, reduced complaints of xerostomia and improved xerostomia-associated quality of life. No clinically significant adverse events were observed. These data suggest that the daily use of topical dry mouth products containing olive oil, betaine and xylitol is safe and effective in relieving symptoms of dry mouth in a population with polypharmacy-induced xerostomia.

Heterologous signals allow efficient targeting of a nuclear-encoded fusion protein to plastids and endoplasmic reticulum in diverse plant species

Approximately 30% of plant nuclear genes appear to encode proteins targeted to the plastids or endoplasmic reticulum (ER). The signals that direct proteins into these compartments are diverse in sequence, but, on the basis of a limited number of tests in heterologous systems, they appear to be functionally conserved across species. To further test the generality of this conclusion, we tested the ability of two plastid transit peptides and an ER signal peptide to target green fluorescent protein (GFP) in 12 crops, including three monocots (barley, sugarcane, wheat) and nine dicots (Arabidopsis, broccoli, cabbage, carrot, cauliflower, lettuce, radish, tobacco, turnip). In all species, transient assays following microprojectile bombardment or vacuum infiltration using Agrobacterium showed that the plastid transit peptides from tomato DCL (defective chloroplast and leaves) and tobacco RbcS [ribulose bisphosphate carboxylase (Rubisco) small subunit] genes were effective in targeting GFP to the leaf plastids. GFP engineered as a fusion to the N-terminal ER signal peptide from Arabidopsis basic chitinase and a C-terminal HDEL signal for protein retention in the ER was accumulated in the ER of all species. The results in tobacco were confirmed in stably transformed cells. These signal sequences should be useful to direct proteins to the plastid stroma or ER lumen in diverse plant species of biotechnological interest for the accumulation of particular recombinant proteins or for the modification of particular metabolic streams.

Expression of bacterial L-aspartate-alpha-decarboxylase in tobacco increases beta-alanine and pantothenate levels and improves thermotolerance

L- Aspartate-alpha-decarboxylase catalyzes the decarboxylation of L -aspartate to generate Beta-alanine and carbon dioxide. This is an unusual pyruvoyl-dependent enzyme unique to prokaryotes that undergoes limited self-processing. The Escherichia coli pan D gene encoding L- aspartate-alpha-decarboxylase was expressed under a constitutive promoter in transgenic tobacco. Transgene expression was verified by assays based on RNA blots, immunoblots and enzyme activity in vitro. The pan D lines had increased levels of leaf Beta-alanine (1.2- to 4-fold), pantothenate (3.2- to 4.1-fold) and total free amino acids (up to 3.7-fold) compared to wild-type and vector controls. Growth of homozygous lines expressing E. coli L- aspartate-alpha-decarboxylase was less affected than that of the control lines when the plants were stressed for 1 week at 35 degrees C. When transferred from 35 to 30 degrees C for 3 weeks, the Pan D transgenic lines recovered significantly (P Collapse

Key Words Collapse

MESH Headings

• Acclimatization/genetics
• Acclimatization/physiology
• Biomass
• Escherichia coli/enzymology
• Escherichia coli/genetics
• Gene Expression
• Genes, Bacterial/genetics
• Germination/physiology
• Glutamate Decarboxylase/genetics
• Glutamate Decarboxylase/metabolism
• Pantothenic Acid/metabolism
• Phenotype
• Plant Leaves/metabolism
• Plants, Genetically Modified/genetics
• RNA, Messenger/genetics
• RNA, Messenger/metabolism
• Seeds/physiology
• Temperature
• Nicotiana/genetics
• Nicotiana/growth & development
• Nicotiana/metabolism
• Nicotiana/physiology
• beta-Alanine/metabolism

Collapse

Grants Collapse

Affiliation(s)

Walid M Fouad Plant Molecular and Cellular Biology Program, Horticultural Sciences Department, University of Florida, Gainesville, FL 32611-0690, USA
Bala Rathinasabapathi

Collapse

Functional characterization of sequence motifs in the transit peptide of Arabidopsis small subunit of rubisco

The transit peptides of nuclear-encoded chloroplast proteins are necessary and sufficient for targeting and import of proteins into chloroplasts. However, the sequence information encoded by transit peptides is not fully understood. In this study, we investigated sequence motifs in the transit peptide of the small subunit of the Rubisco complex by examining the ability of various mutant transit peptides to target green fluorescent protein reporter proteins to chloroplasts in Arabidopsis (Arabidopsis thaliana) leaf protoplasts. We divided the transit peptide into eight blocks (T1 through T8), each consisting of eight or 10 amino acids, and generated mutants that had alanine (Ala) substitutions or deletions, of one or two T blocks in the transit peptide. In addition, we generated mutants that had the original sequence partially restored in single- or double-T-block Ala (A) substitution mutants. Analysis of chloroplast import of these mutants revealed several interesting observations. Single-T-block mutations did not noticeably affect targeting efficiency, except in T1 and T4 mutations. However, double-T mutants, T2A/T4A, T3A/T6A, T3A/T7A, T4A/T6A, and T4A/T7A, caused a 50% to 100% loss in targeting ability. T3A/T6A and T4A/T6A mutants produced only precursor proteins, whereas T2A/T4A and T4A/T7A mutants produced only a 37-kD protein. Detailed analyses revealed that sequence motifs ML in T1, LKSSA in T3, FP and RK in T4, CMQVW in T6, and KKFET in T7 play important roles in chloroplast targeting. In T1, the hydrophobicity of ML is important for targeting. LKSSA in T3 is functionally equivalent to CMQVW in T6 and KKFET in T7. Furthermore, subcellular fractionation revealed that Ala substitution in T1, T3, and T6 produced soluble precursors, whereas Ala substitution in T4 and T7 produced intermediates that were tightly associated with membranes. These results demonstrate that the transit peptide contains multiple motifs and that some of them act in concert or synergistically.

Isolation of cDNA and enzymatic properties of betaine aldehyde dehydrogenase from Zoysia tenuifolia

We isolated cDNAs encoding betaine aldehyde dehydrogenase (BADH, EC 1.2.1.8) from the salt-tolerant Poaceae, Zoysia tenuifolia by polymerase chain reactions. Zoysia betaine aldehyde dehydrogenase 1 (ZBD1) is 1892bp long and codes for 507 amino acids. The deduced amino acid sequence of ZBD1 is 88% similar to the sequence of rice BADH. Ten cDNA clones were isolated from a cDNA Library of salt-treated Z. tenuifolia by using the ZBD1 fragment as a probe. The proteins coded in some clones were more homologous to BBD2, the cytosolic BADH of barley, than to ZBD1. To investigate their enzymatic properties, ZBD1 and spinach BADH were expressed in Escherichia coli and purified. The optimal pH of ZBD1 was 9.5, which was more alkaline than that of spinach BADH. ZBD1 was less tolerant to NaCl than spinach BADH. ZBD1 showed not only BADH activity but also aminoaldehyde dehydrogenase activity. The Km values of ZBD1 for betaine aldehyde, 4-aminobutyraldehyde (AB-ald), and 3-aminopropionaldehyde (AP-ald) were 291, 49, and 4.0 microM, respectively. ZBD1 showed higher specific activities for AB-ald and AP-ald than did spinach BADH.

Non-canonical transit peptide for import into the chloroplast

The large majority of plastid proteins are nuclear-encoded and, thus, must be imported within these organelles. Unlike most of the outer envelope proteins, targeting of proteins to all other plastid compartments (inner envelope membrane, stroma, and thylakoid) is strictly dependent on the presence of a cleavable transit sequence in the precursor N-terminal region. In this paper, we describe the identification of a new envelope protein component (ceQORH) and demonstrate that its subcellular localization is limited to the inner membrane of the chloroplast envelope. Immunopurification, microsequencing of the natural envelope protein and cloning of the corresponding full-length cDNA demonstrated that this protein is not processed in the N-terminal region during its targeting to the inner envelope membrane. Transient expression experiments in plant cells were performed with truncated forms of the ceQORH protein fused to the green fluorescent protein. These experiments suggest that neither the N-terminal nor the C-terminal are essential for chloroplastic localization of the ceQORH protein. These observations are discussed in the frame of the endosymbiotic theory of chloroplast evolution and suggest that a domain of the ceQORH bacterial ancestor may have evolved so as to exclude the general requirement of an N-terminal plastid transit sequence.

Enhanced stress tolerance in Escherichia coli and Nicotiana tabacum expressing a betaine aldehyde dehydrogenase/choline dehydrogenase fusion protein

In Escherichia coli the osmoprotective compound glycine betaine is produced from choline by two enzymes; choline dehydrogenase (CDH) oxidizes choline to betaine aldehyde and then further on to glycine betaine, while betaine aldehyde dehydrogenase (BADH) facilitates the conversion of betaine aldehyde to glycine betaine. To evaluate the importance of BADH, a BADH/CDH fusion enzyme was constructed and expressed in E. coli and in Nicotiana tabacum. The fusion enzyme displayed both enzyme activities, and a coupled reaction could be measured. The enzyme was characterized regarding molecular weight and the dependence of the enzyme activities on environmental factors (salt, pH, and poly(ethylene glycol) addition). At high choline concentrations, E. coli cells expressing BADH/CDH were able to grow to higher final densities and to accumulate more glycine betaine than cells expressing CDH only. The intracellular glycine betaine levels were almost 5-fold higher for BADH/CDH when product concentration was related to CDH activity. Also, after culturing the cells at high NaCl concentrations, more glycine betaine was accumulated. On medium containing 20 mM choline, transgenic tobacco plants expressing BADH/CDH grew considerably faster than vector-transformed control plants.

Metabolic engineering of osmoprotectant accumulation in plants

Drought and salinity are among the worst scourges of agriculture. One effective mechanism to reduce damage from these stresses is the accumulation of high intracellular levels of osmoprotectant compounds. These compounds include proline, ectoine, betaines, polyols, and trehalose and have evolved in many different organisms. Since some crop plants have low levels of these osmoprotectants or none at all, engineering osmoprotectant biosynthesis pathways is a potential way to improve stress tolerance. First-generation engineering work--much of it with single genes--has successfully introduced osmoprotectant pathways into plants that lack them naturally, and this has often improved stress tolerance. However, the engineered osmoprotectant levels are generally low and the increases in tolerance commensurately small. To get beyond trace levels of osmoprotectants and marginal tolerance increments we need to use flux measurements to diagnose what limits osmoprotectant levels in engineered plants and to use iterative cycles of engineering to overcome these limitations.

Novel ABA- and dehydration-inducible aldehyde dehydrogenase genes isolated from the resurrection plant Craterostigma plantagineum and Arabidopsis thaliana

In order to identify genes that are critical for the ABA-dependent stress response in the resurrection plant Craterostigma plantagineum, a gene was isolated with homology to class 3 variable substrate aldehyde dehydrogenases (ALDH). The C. plantagineum gene Cp-ALDH constitutes a novel class of plant ALDHs. In a search for corresponding genes from Arabidopsis thaliana, Ath-ALDH3 and Ath-ALDH4 were isolated, showing 70% and 80% similarity to Cp-ALDH. Phylogenetically, the Cp- and Ath-ALDH3 and -ALDH4 proteins are closely related to aldehyde dehydrogenases from bacteria and mammalian species and are separated from known plant ALDHs and betaine-aldehyde dehydrogenases (BADH). Cp-ALDH transcript and polypeptide are up-regulated in vegetative tissues and callus in response to dehydration or ABA-treatment. Ath-ALDH3 expression was induced in response to dehydration and ABA treatment, while Ath-ALDH4 is constitutively expressed at a low level. Recombinant Cp-ALDH protein oxidizes nonanal, propionaldehyde and acetaldehyde, with Km values of 2.2 microm, 0.27 mm and 3.23 mm, respectively, in an NAD-dependent manner. Immunogold electron microscopy shows that Cp-ALDH is localized in plastids.

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.

Antibiotic-free chloroplast genetic engineering - an environmentally friendly approach

Chloroplast genetic engineering offers several advantages over nuclear genetic engineering, including gene containment and hyperexpression. However, introducing thousands of copies of transgenes into the chloroplast genome amplifies the antibiotic resistance genes. Two recent articles report different and novel strategies to either remove antibiotic resistance genes or select chloroplast transformants without using these genes. This should eliminate their potential transfer to microorganisms or plants and ease public concerns about genetically modified crops.

Molecular characterization of tomato 3-dehydroquinate dehydratase-shikimate:NADP oxidoreductase

Analysis of cDNAs encoding the bifunctional 3-dehydroquinate dehydratase-shikimate:NADP oxidoreductase (DHQase-SORase) from tomato (Lycopersicon esculentum) revealed two classes of cDNAs that differed by 57 bp within the coding regions, but were otherwise identical. Comparison of these cDNA sequences with the sequence of the corresponding single gene unequivocally proved that the primary transcript is differentially spliced, potentially giving rise to two polypeptides that differ by 19 amino acids. Quantitative real-time polymerase chain reaction revealed that the longer transcript constitutes at most 1% to 2% of DHQase-SORase transcripts. Expression of the respective polypeptides in Escherichia coli mutants lacking the DHQase or the SORase activity gave functional complementation only in case of the shorter polypeptide, indicating that skipping of a potential exon is a prerequisite for the production of an enzymatically active protein. The deduced amino acid sequence revealed that the DHQase-SORase is most likely synthesized as a precursor with a very short (13-amino acid) plastid-specific transit peptide. Like other genes encoding enzymes of the prechorismate pathway in tomato, this gene is elicitor-inducible. Tissue-specific expression resembles the patterns obtained for 3-deoxy-D-arabino-heptulosonate 7-phosphate synthase 2 and dehydroquinate synthase genes. This work completes our studies of the prechorismate pathway in that cDNAs for all seven enzymes (including isozymes) of the prechorismate pathway from tomato have now been characterized.

Biotechnological applications of plant freezing associated proteins

Plants use a wide array of proteins to protect themselves against low temperature and freezing conditions. The identification of these freezing tolerance associated proteins and the elucidation of their cryoprotective functions will have important applications in several fields. Genes encoding structural proteins, osmolyte producing enzymes, oxidative stress scavenging enzymes, lipid desaturases and gene regulators have been used to produce transgenic plants. These studies have revealed the potential capacity of different genes to protect against temperature related stresses. In some cases, transgenic plants with significant cold tolerance have been produced. Furthermore, the biochemical characterization of the cold induced antifreeze proteins and dehydrins reveals many applications in the food and the medical industries. These proteins are being considered as food additives to improve the quality and shelf-life of frozen foods, as cryoprotective agents for organ and cell cryopreservation, and as chemical adjuvant in cancer cryosurgery.

Enhanced tolerance of transgenic Brassica juncea to choline confirms successful expression of the bacterial codA gene

Brassica juncea cv. Pusa Jaikisan was transformed with the codA gene for choline oxidase from Arthrobacter globiformis with an aim to introduce glycine betaine biosynthetic pathway, as it lacks any means to synthesize glycine betaine. Western blot analysis revealed the presence of choline oxidase in the protein extract from the codA transgenic lines, demonstrating that the bacterial codA gene had been successfully transcribed and translated in transgenic lines. Good activity of choline oxidase indicated its presence in fully functional form in the transformed lines. This was further confirmed by the presence of glycine betaine only in the transformed lines of B. juncea. The shoots of both wild type and transformed lines were exposed to various concentrations of choline in order to evaluate if the introduction of the codA gene in any way enhances the potential of B. juncea to tolerate high levels of choline. The growth (in terms of fresh weight and dry weight) of the shoots of transformed lines exposed to high levels of choline was significantly superior to those of wild type. Moreover, the loss in chlorophyll content and the activity of photosystem II in shoots of the transformed lines exposed to high concentration of choline were significantly lower than that observed in wild type. These results showed that shoots of B. juncea transformed with the codA gene, most probably had the potential to readily convert choline to glycine betaine. Therefore, choline tolerance can be used as an efficient marker for the identification of the lines transformed with the codA gene.

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.

PLANTCELLULAR ANDMOLECULARRESPONSES TOHIGHSALINITY

Plant responses to salinity stress are reviewed with emphasis on molecular mechanisms of signal transduction and on the physiological consequences of altered gene expression that affect biochemical reactions downstream of stress sensing. We make extensive use of comparisons with model organisms, halophytic plants, and yeast, which provide a paradigm for many responses to salinity exhibited by stress-sensitive plants. Among biochemical responses, we emphasize osmolyte biosynthesis and function, water flux control, and membrane transport of ions for maintenance and re-establishment of homeostasis. The advances in understanding the effectiveness of stress responses, and distinctions between pathology and adaptive advantage, are increasingly based on transgenic plant and mutant analyses, in particular the analysis of Arabidopsis mutants defective in elements of stress signal transduction pathways. We summarize evidence for plant stress signaling systems, some of which have components analogous to those that regulate osmotic stress responses of yeast. There is evidence also of signaling cascades that are not known to exist in the unicellular eukaryote, some that presumably function in intercellular coordination or regulation of effector genes in a cell-/tissue-specific context required for tolerance of plants. A complex set of stress-responsive transcription factors is emerging. The imminent availability of genomic DNA sequences and global and cell-specific transcript expression data, combined with determinant identification based on gain- and loss-of-function molecular genetics, will provide the infrastructure for functional physiological dissection of salt tolerance determinants in an organismal context. Furthermore, protein interaction analysis and evaluation of allelism, additivity, and epistasis allow determination of ordered relationships between stress signaling components. Finally, genetic activation and suppression screens will lead inevitably to an understanding of the interrelationships of the multiple signaling systems that control stress-adaptive responses in plants.

Genetic engineering of glycinebetaine synthesis in plants: current status and implications for enhancement of stress tolerance

Metabolic acclimation via the accumulation of compatible solutes is regarded as a basic strategy for the protection and survival of plants in extreme environments. Certain plants accumulate significant amounts of glycinebetaine (betaine), a compatible quaternary amine, in response to high salinity, cold and drought. It is likely that betaine is involved in the protection of macrocomponents of plant cells, such as protein complexes and membranes, under stress conditions. Genetic engineering of the biosynthesis of betaine from choline has been the focus of considerable attention as a potential strategy for increasing stress tolerance in stress-sensitive plants that are incapable of synthesizing this compatible/protective solute. Three distinct pathways for the synthesis of betaine have been identified in spinach, Escherichia coli and Arthrobacter globiformis, and various genes and cDNAs for the proteins involved are available. Moreover, each of the pathways has been exploited to a greater or lesser extent in efforts to convert betaine-deficient plants to betaine accumulators. In this review, the potential of several recent examples of transgenic approaches to the enhancement of stress tolerance in plants is summarized and discussed.

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.

Drought-induced responses in plant cells

Plants subjected to water stress undergo numerous physiological and metabolic changes. A general decrease in photosynthetic rate is among the most common responses. This is due to a programmed process involving the closure of stomata and reduction in the activity of photosynthetic enzymes. The plant hormone abscisic acid plays an important role in this process. Accumulation of compatible solutes, during water stress, is thought to be an adaptive response which has been developed by some plant species. Engineering the genes involved in the synthesis of these compounds, into nonaccumulating plants, has demonstrated promising results for genetic improvement of drought tolerance. Drought stress induces alteration of gene expression. A large number of genes which are upregulated by water stress have been isolated and characterized. Proteins encoded by some of these genes share several characteristics. The biochemical role of most of these gene products is unknown, but potential adaptive functions have been suggested. Abscisic acid is involved in the regulation of some of these genes.

Osmotic stress induces expression of choline monooxygenase in sugar beet and amaranth

Choline monooxygenase (CMO) catalyzes the committing step in the synthesis of glycine betaine, an osmoprotectant accumulated by many plants in response to salinity and drought. To investigate how these stresses affect CMO expression, a spinach (Spinacia oleracea L., Chenopodiaceae) probe was used to isolate CMO cDNAs from sugar beet (Beta vulgaris L., Chenopodiaceae), a salt- and drought-tolerant crop. The deduced beet CMO amino acid sequence comprised a transit peptide and a 381-residue mature peptide that was 84% identical (97% similar) to that of spinach and that showed the same consensus motif for coordinating a Rieske-type [2Fe-2S] cluster. A mononuclear Fe-binding motif was also present. When water was withheld, leaf relative water content declined to 59% and the levels of CMO mRNA, protein, and enzyme activity rose 3- to 5-fold; rewatering reversed these changes. After gradual salinization (NaCl:CaCl2 = 5.7:1, mol/mol), CMO mRNA, protein, and enzyme levels in leaves increased 3- to 7-fold at 400 mM salt, and returned to uninduced levels when salt was removed. Beet roots also expressed CMO, most strongly when salinized. Salt-inducible CMO mRNA, protein, and enzyme activity were readily detected in leaves of Amaranthus caudatus L. (Amaranthaceae). These data show that CMO most probably has a mononuclear Fe center, is inducibly expressed in roots as well as in leaves of Chenopodiaceae, and is not unique to this family.

Salinity promotes accumulation of 3-dimethylsulfoniopropionate and its precursor S-methylmethionine in chloroplasts

Wollastonia biflora (L.) DC. plants accumulate the osmoprotectant 3-dimethylsulfoniopropionate (DMSP), particularly when salinized. DMSP is known to be synthesized in the chloroplast from S-methylmethionine (SMM) imported from the cytosol, but the sizes of the chloroplastic and extrachloroplastic pools of these compounds are unknown. We therefore determined DMSP and SMM in mesophyll protoplasts and chloroplasts. Salinization with 30% (v/v) artificial seawater increased protoplast DMSP levels from 4.6 to 6.0 mumol mg-1 chlorophyll (Chl), and chloroplast levels from 0.9 to 1.9 mumol mg-1 Chl. The latter are minimum values because intact chloroplasts leaked DMSP during isolation. Correcting for this leakage, it was estimated that in vivo about one-half of the DMSP is chloroplastic and that stromal DMSP concentrations in control and salinized plants are about 60 and 130 mM, respectively. Such concentrations would contribute significantly to chloroplast osmoregulation and could protect photosynthetic processes from stress injury. SMM levels were measured using a novel mass-spectrometric method. About 40% of the SMM was located in the chloroplast in unsalinized W. biflora plants, as was about 80% in salinized plants; the chloroplastic pool in both cases was approximately 0.1 mumol mg-1 Chl. In contrast, > or = 85% of the SMM was extrachloroplastic in pea (Pisum sativum L.) and spinach (Spinacia oleracea L.), which lack DMSP. DMSP synthesis may be associated with enhanced accumulation of SMM in the chloroplasm.

Choline monooxygenase, an unusual iron-sulfur enzyme catalyzing the first step of glycine betaine synthesis in plants: prosthetic group characterization and cDNA cloning

Plants synthesize the osmoprotectant glycine betaine via the route choline --> betaine aldehyde --> glycine betaine. In spinach, the first step is catalyzed by choline monooxygenase (CMO), a ferredoxin-dependent stromal enzyme that has been hypothesized to be an oligomer of identical subunits and to be an Fe-S protein. Analysis by HPLC and matrix-assisted laser desorption ionization MS confirmed that native CMO contains only one type of subunit (Mr 42,864). Determination of acid-labile sulfur and nonheme iron demonstrated that there is one [2Fe-2S] cluster per subunit, and EPR spectral data indicated that this cluster is of the Rieske type--i.e., coordinated by two Cys and two His ligands. A full-length CMO cDNA (1,622 bp) was cloned from spinach using a probe generated by PCR amplification for which the primers were based on internal peptide sequences. The ORF encoded a 440-amino acid polypeptide that included a 60-residue transit peptide. The deduced amino acid sequence included two Cys-His pairs spaced 16 residues apart, a motif characteristic of Rieske-type Fe-S proteins. Larger regions that included this motif also showed some sequence similarity (approximately 40%) to Rieske-type proteins, particularly bacterial oxygenases. Otherwise there was very little similarity between CMO and proteins from plants or other organisms. RNA and immunoblot analyses showed that the expression of CMO in leaves increased several-fold during salinization. We conclude that CMO is a stress-inducible representative of a new class of plant oxygenases.

Synthesis of the osmoprotectant glycine betaine in Bacillus subtilis: characterization of the gbsAB genes

Synthesis of the osmoprotectant glycine betaine from the exogenously provided precursor choline or glycine betaine aldehyde confers considerable osmotic stress tolerance to Bacillus subtilis in high-osmolarity media. Using an Escherichia coli mutant (betBA) defective in the glycine betaine synthesis enzymes, we cloned by functional complementation the genes that are required for the synthesis of the osmoprotectant glycine betaine in B. subtilis. The DNA sequence of a 4.1-kb segment from the cloned chromosomal B. subtilis DNA was established, and two genes (gbsA and gbsB) whose products were essential for glycine betaine biosynthesis and osmoprotection were identified. The gbsA and gbsB genes are transcribed in the same direction, are separated by a short intergenic region, and are likely to form an operon. The deduced gbsA gene product exhibits strong sequence identity with members of a superfamily of specialized and nonspecialized aldehyde dehydrogenases. This superfamily comprises glycine betaine aldehyde dehydrogenases from bacteria and plants with known involvement in the cellular adaptation to high-osmolarity stress and drought. The deduced gbsB gene product shows significant similarity to the family of type III alcohol dehydrogenases. B. subtilis mutants with defects in the chromosomal gbsAB genes were constructed by marker replacement, and the growth properties of these mutant strains in high-osmolarity medium were analyzed. Deletion of the gbsAB genes destroyed the choline-glycine betaine synthesis pathway and abolished the ability of B. subtilis to deal effectively with high-osmolarity stress in choline- or glycine betaine aldehyde-containing medium. Uptake of radiolabelled choline was unaltered in the gbsAB mutant strain. The continued intracellular accumulation of choline or glycine betaine aldehyde in a strain lacking the glycine betaine-biosynthetic enzymes strongly interfered with the growth of B. subtilis, even in medium of moderate osmolarity. A single transcription initiation site for gbsAB was detected by high-resolution primer extension analysis. gbsAB transcription was initiated from a promoter with close homology to sigma A-dependent promoters and was stimulated by the presence of choline in the growth medium.