Glutamate decarboxylase-1 is essential for efficient acclimation of Arabidopsis thaliana to nutritional phosphorus deprivation

Glutamate decarboxylase (GAD) is a Ca2+ -calmodulin-activated, cytosolic enzyme that produces γ-aminobutyrate (GABA) as the committed step of the GABA shunt. This pathway bypasses the 2-oxoglutarate to succinate reactions of the tricarboxylic acid (TCA) cycle. GABA also accumulates during many plant stresses. We tested the hypothesis that AtGAD1 (At5G17330) facilitates Arabidopsis acclimation to Pi deprivation. Quantitative RT-PCR and immunoblotting revealed that AtGAD1 transcript and protein expression is primarily root-specific, but inducible at lower levels in shoots of Pi-deprived (-Pi) plants. Pi deprivation reduced levels of the 2-oxoglutarate dehydrogenase (2-OGDH) cofactor thiamine diphosphate (ThDP) in shoots and roots by > 50%. Growth of -Pi atgad1 T-DNA mutants was significantly attenuated relative to wild-type plants. This was accompanied by: (i) an > 60% increase in shoot and root GABA levels of -Pi wild-type, but not atgad1 plants, and (ii) markedly elevated anthocyanin and reduced free and total Pi levels in leaves of -Pi atgad1 plants. Treatment with 10 mM GABA reversed the deleterious development of -Pi atgad1 plants. Our results indicate that AtGAD1 mediates GABA shunt upregulation during Pi deprivation. This bypass is hypothesized to circumvent ThDP-limited 2-OGDH activity to facilitate TCA cycle flux and respiration by -Pi Arabidopsis.

Improving Boron and Molybdenum Use Efficiencies in Contrasting Cultivars of Subirrigated Greenhouse-Grown Pot Chrysanthemums

Fertilizer boron (B) and molybdenum (Mo) were provided to contrasting cultivars of subirrigated pot chrysanthemums at approximately 6-100% of current industry standards in an otherwise balanced nutrient solution during vegetative growth, and then all nutrients were removed during reproductive growth. Two experiments were conducted for each nutrient in a naturally lit greenhouse using a randomized complete block split-plot design. Boron (0.313-5.00 µmol L^-1) or Mo (0.031-0.500 µmol L^-1) was the main plot, and cultivar was the sub-plot. Petal quilling was observed with leaf-B of 11.3-19.4 mg kg^-1 dry mass (DM), whereas Mo deficiency was not observed with leaf-Mo of 1.0-3.7 mg kg^-1 DM. Optimized supplies resulted in leaf tissue levels of 48.8-72.5 mg B kg^-1 DM and 1.9-4.8 mg Mo kg^-1 DM. Boron uptake efficiency was more important than B utilization efficiency in sustaining plant/inflorescence growth with decreasing B supply, whereas Mo uptake and utilization efficiencies appeared to have similar importance in sustaining plant/inflorescence growth with decreasing Mo supply. This research contributes to the development of a sustainable low-input nutrient delivery strategy for floricultural operations, wherein nutrient supply is interrupted during reproductive growth and optimized during vegetative growth.

γ-Aminobutyrate Improves the Postharvest Marketability of Horticultural Commodities: Advances and Prospects

Postharvest deterioration can result in qualitative and quantitative changes in the marketability of horticultural commodities, as well as considerable economic loss to the industry. Low temperature and controlled atmosphere conditions (low O2 and elevated CO2) are extensively employed to prolong the postharvest life of these commodities. Nevertheless, they may suffer from chilling injury and other physiological disorders, as well as excessive water loss and bacterial/fungal decay. Research on the postharvest physiological, biochemical, and molecular responses of horticultural commodities indicates that low temperature/controlled atmosphere storage is associated with the promotion of γ-aminobutyrate (GABA) pathway activity, with or without the accumulation of GABA, delaying senescence, preserving quality and ameliorating chilling injury. Regardless of whether apple fruits are stored under low temperature/controlled atmosphere conditions or room temperature, elevated endogenous GABA or exogenous GABA maintains their quality by stimulating the activity of the GABA shunt (glutamate GABA succinic semialdehyde succinate) and the synthesis of malate, and delaying fruit ripening. This outcome is associated with changes in the genetic and biochemical regulation of key GABA pathway reactions. Flux estimates suggest that the GABA pool is derived primarily from glutamate, rather than polyamines, and that succinic semialdehyde is converted mainly to succinate, rather than γ-hydroxybutyrate. Exogenous GABA is a promising strategy for promoting the level of endogenous GABA and the activity of the GABA shunt in both intact and fresh-cut commodities, which increases carbon flux through respiratory pathways, restores or partially restores redox and energy levels, and improves postharvest marketability. The precise mechanisms whereby GABA interacts with other signaling molecules such as Ca2+, H2O2, polyamines, salicylic acid, nitric oxide and melatonin, or with phytohormones such as ethylene, abscisic acid and auxin remain unknown. The occurrence of the aluminum-activated malate transporter and the glutamate/aspartate/GABA exchanger in the tonoplast, respectively, offers prospects for reducing transpirational water in cut flowers and immature green fruit, and for altering the development, flavor and biotic resistance of apple fruits.

From plant biology research to technology transfer and knowledge extension: improving food quality and mitigating environmental impacts

Academic scientists face an unpredictable path from plant biology research to real-life application. Fundamental studies of γ-aminobutyrate and carotenoid metabolism, control of Botrytis infection, and the uptake and distribution of mineral nutrients illustrate that most academic research in plant biology could lead to innovative solutions for food, agriculture, and the environment. The time to application depends on various factors such as the fundamental nature of the scientific questions, the development of enabling technologies, the research priorities of funding agencies, the existence of competitive research, the willingness of researchers to become engaged in commercial activities, and ultimately the insight and creativity of the researchers. Applied research is likely to be adopted more rapidly by industry than basic research, so academic scientists engaged in basic research are less likely to participate in science commercialization. It is argued that the merit of Discovery Grant applications to the Natural Sciences and Engineering Research Council (NSERC) of Canada should not be evaluated for their potential impact on policy and (or) technology. Matching industry funds in Canada rarely support the search for knowledge. Therefore, NSERC Discovery Grants should fund basic research in its entirety.

γ-Aminobutyrate (GABA) Regulated Plant Defense: Mechanisms and Opportunities

Global climate change and associated adverse abiotic and biotic stress conditions affect plant growth and development, and agricultural sustainability in general. Abiotic and biotic stresses reduce respiration and associated energy generation in mitochondria, resulting in the elevated production of reactive oxygen species (ROS), which are employed to transmit cellular signaling information in response to the changing conditions. Excessive ROS accumulation can contribute to cell damage and death. Production of the non-protein amino acid γ-aminobutyrate (GABA) is also stimulated, resulting in partial restoration of respiratory processes and energy production. Accumulated GABA can bind directly to the aluminum-activated malate transporter and the guard cell outward rectifying K+ channel, thereby improving drought and hypoxia tolerance, respectively. Genetic manipulation of GABA metabolism and receptors, respectively, reveal positive relationships between GABA levels and abiotic/biotic stress tolerance, and between malate efflux from the root and heavy metal tolerance. The application of exogenous GABA is associated with lower ROS levels, enhanced membrane stability, changes in the levels of non-enzymatic and enzymatic antioxidants, and crosstalk among phytohormones. Exogenous GABA may be an effective and sustainable tolerance strategy against multiple stresses under field conditions.

Does the GABA Shunt Regulate Cytosolic GABA?

The GABA shunt has long been known as a metabolic pathway that produces GABA in, and removes GABA from, the cytosol. There is no consensus regarding its function. The hypothesis presented here is that the GABA shunt regulates cytosolic GABA levels and GABA signaling.

Spermine Is a Potent Plant Defense Activator Against Gray Mold Disease on Solanum lycopersicum, Phaseolus vulgaris, and Arabidopsis thaliana

Polyamines (PAs) are ubiquitous aliphatic amines that play important roles in growth, development, and environmental stress responses in plants. In this study, we report that exogenous application of spermine (Spm) is effective in the induction of resistance to gray mold disease, which is caused by the necrotrophic fungal pathogen Botrytis cinerea, on tomato (Solanum lycopersicum), bean (Phaseolus vulgaris), and Arabidopsis thaliana. High throughput transcriptome analysis revealed a priming role for the Spm molecule in the genus Arabidopsis, resulting in strong upregulation of several important defense-associated genes, particularly those involved in systemic-acquired resistance. Microscopic analysis confirmed that Spm application potentiates endogenous defense responses in tomato leaves through the generation of reactive oxygen species and the hypersensitive response, which effectively contained B. cinerea growth within the inoculated area. Moreover, co-application of Spm and salicylic acid resulted in a synergistic effect against the pathogen, leading to higher levels of resistance than those induced by separate applications of the two compounds. The Spm plus salicylic acid treatment also reduced infection in systemic nontreated leaves of tomato plants. Our findings suggest that Spm, particularly when applied in combination with salicylic acid, functions as a potent plant defense activator that leads to effective local and systemic resistance against B. cinerea.

Spermine Differentially Refines Plant Defense Responses Against Biotic and Abiotic Stresses

Roles of the major polyamines (mPA), putrescine, spermidine, and spermine (Spm), in various developmental and physiological processes in plants have been well documented. Recently, there has been increasing focus on the link between mPA metabolism and defense response during plant-stress interactions. Empirical evidence is available for a unique role of Spm, distinct from the other mPA, in eliciting an effective defense response to (a)biotic stresses. Our understanding of the precise molecular mechanism(s) by which Spm modulates these defense mechanisms is limited. Further analysis of recent studies indicates that plant Spm functions differently during biotic and abiotic interactions in the regulation of oxidative homeostasis and phytohormone signaling. Here, we summarize and integrate current knowledge about Spm-mediated modulation of plant defense responses to (a)biotic stresses, highlighting the importance of Spm as a potent plant defense activator with broad-spectrum protective effects. A model is proposed to explain how Spm refines defense mechanisms to tailor an optimal resistance response.

Metabolic Alterations in Postharvest Pear Fruit As Influenced by 1-Methylcyclopropene and Controlled Atmosphere Storage

This study assessed the impact of 1-methylcyclopropene (1-MCP) and controlled atmosphere (CA) on the metabolism of targeted amino acids, organic acids, and antioxidants in stored 'AC Harrow Crisp' pears and their relationships to storage disorders. Pears were treated with 0 or 300 nL L^-1 1-MCP and stored at 0 °C under ambient air or CA. Spectrophotometric assays demonstrated that glutathione levels fluctuated with storage and were most preserved by 1-MCP under ambient air. HPLC analysis revealed that ascorbate concentrations declined with storage and were little affected by 1-MCP and CA. Citrate, lactate, and fumarate accumulated with storage but were differentially affected by 1-MCP. Aspartate and glutamate concentrations were greater with 1-MCP; γ-aminobutyrate accumulated in disordered fruit. Principal component analysis demonstrated that alterations in citrate and fumarate were, respectively, correlated with internal breakdown and senescent scald. γ-Aminobutyrate and alanine were associated with internal cavities. All disorders were associated with antioxidant depletion.

Targeted quantitative profiling of metabolites and gene transcripts associated with 4-aminobutyrate (GABA) in apple fruit stored under multiple abiotic stresses

4-Aminobutyrate accumulates in plants under abiotic stress. Here, targeted quantitative profiling of metabolites and transcripts was conducted to monitor glutamate- and polyamine-derived 4-aminobutyrate production and its subsequent catabolism to succinate or 4-hydroxybutyrate in apple (Malus x domestica Borkh.) fruit stored at 0 °C with 2.5 kPa O₂ and 0.03 or 5 kPa CO₂ for 16 weeks. Low-temperature-induced protein hydrolysis appeared to be responsible for the enhanced availability of amino acids during early storage, and the resulting higher glutamate level stimulated 4-aminobutyrate levels more than polyamines. Elevated CO₂ increased the levels of polyamines, as well as succinate and 4-hydroxybutyrate, during early storage, and 4-aminobutyrate and 4-hydroxybutyrate over the longer term. Expression of all of the genes likely involved in 4-aminobutyrate metabolism from glutamate/polyamines to succinate/4-hydroxybutyrate was induced in a co-ordinated manner. CO₂-regulated expression of apple GLUTAMATE DECARBOXYLASE 2, AMINE OXIDASE 1, ALDEHYDE DEHYDROGENASE 10A8 and POLYAMINE OXIDASE 2 was evident with longer term storage. Evidence suggested that respiratory activities were restricted by the elevated CO₂/O₂ environment, and that decreasing NAD⁺ availability and increasing NADPH and NADPH/NADP⁺, respectively, played key roles in the regulation of succinate and 4-hydroxybutyate accumulation. Together, these findings suggest that both transcriptional and biochemical mechanisms are associated with 4-aminobutyrate and 4-hydroxybutyrate metabolism in apple fruit stored under multiple abiotic stresses.

Plant Glyoxylate/Succinic Semialdehyde Reductases: Comparative Biochemical Properties, Function during Chilling Stress, and Subcellular Localization

Plant NADPH-dependent glyoxylate/succinic semialdehyde reductases 1 and 2 (cytosolic GLYR1 and plastidial/mitochondrial GLYR2) are considered to be of particular importance under abiotic stress conditions. Here, the apple (Malus × domestica Borkh.) and rice (Oryza sativa L.) GLYR1s and GLYR2s were characterized and their kinetic properties were compared to those of previously characterized GLYRs from Arabidopsis thaliana [L.] Heynh. The purified recombinant GLYRs had an affinity for glyoxylate and succinic semialdehyde, respectively, in the low micromolar and millimolar ranges, and were inhibited by NADP+. Comparison of the GLYR activity in cell-free extracts from wild-type Arabidopsis and a glyr1 knockout mutant revealed that approximately 85 and 15% of the cellular GLYR activity is cytosolic and plastidial/mitochondrial, respectively. Recovery of GLYR activity in purified mitochondria from the Arabidopsis glyr1 mutant, free from cytosolic GLYR1 or plastidial GLYR2 contamination, provided additional support for the targeting of GLYR2 to mitochondria, as well as plastids. The growth of plantlets or roots of various Arabidopsis lines with altered GLYR activity responded differentially to succinic semialdehyde or glyoxylate under chilling conditions. Taken together, these findings highlight the potential regulation of highly conserved plant GLYRs by NADPH/NADP+ ratios in planta, and their roles in the reduction of toxic aldehydes in plants subjected to chilling stress.

Subcellular compartmentation of 4-aminobutyrate (GABA) metabolism in arabidopsis: An update

This addendum discusses the compartmentation of γ-aminobutyrate (GABA) metabolism, highlighting recent progress with Arabidopsis thaliana and raising new questions about the roles of mitochondria, plastids and peroxisomes in abiotic stress tolerance.

Ancient Plant Glyoxylate/Succinic Semialdehyde Reductases: GLYR1s Are Cytosolic, Whereas GLYR2s Are Localized to Both Mitochondria and Plastids

Plant NADPH-dependent glyoxylate/succinic semialdehyde reductases 1 and 2 (GLYR1 and GLYR2) are considered to be involved in detoxifying harmful aldehydes, thereby preserving plant health during exposure to various abiotic stresses. Phylogenetic analysis revealed that the two GLYR isoforms appeared in the plant lineage prior to the divergence of the Chlorophyta and Streptophyta, which occurred approximately 750 million years ago. Green fluorescent protein fusions of apple (Malus x domestica Borkh.), rice (Oryza sativa L.) and Arabidopsis thaliana [L.] Heynh GLYRs were transiently expressed in tobacco (Nicotiana tabaccum L.) suspension cells or Arabidopsis protoplasts, as well in methoxyfenozide-induced, stably transformed Arabidopsis seedlings. The localization of apple GLYR1 confirmed that this isoform is cytosolic, whereas apple, rice and Arabidopsis GLYR2s were localized to both mitochondria and plastids. These findings highlight the potential involvement of GLYRs within distinct compartments of the plant cell.

Plant GABA: Not Just a Metabolite

γ-Aminobutyric acid (GABA) accumulates rapidly when plants are exposed to stress. Whether GABA accumulation represents the regulation of metabolism in response to stress or an adaptive response to mitigate stress is unknown. Genetic manipulation of GABA levels has revealed that GABA accumulation functions in defense against drought and insect herbivory.

Oxidative metabolism is associated with physiological disorders in fruits stored under multiple environmental stresses

In combination with low temperature, controlled atmosphere storage and 1-methylcyclopropene (ethylene antagonist) application are used to delay senescence of many fruits and vegetables. Controlled atmosphere consists of low O2 and elevated CO2. When sub-optimal partial pressures are used, these practices represent multiple abiotic stresses that can promote the development of physiological disorders in pome fruit, including flesh browning and cavities, although there is some evidence for genetic differences in susceptibility. In the absence of surface disorders, fruit with flesh injuries are not easily distinguished from asymptomatic fruit until these are consumed. Oxidative stress metabolites tend to accumulate (e.g., γ-aminobutyrate) or rapidly decline (e.g., ascorbate and glutathione) in vegetative tissues exposed to hypoxic and/or elevated CO2 environments. Moreover, these phenomena can be associated with altered energy and redox status. Biochemical investigations of Arabidopsis and tomato plants with genetically-altered levels of enzymes associated with the γ-aminobutyrate shunt and the ascorbate-glutathione pathway indicate that these metabolic processes are functionally related and critical for dampening the oxidative burst in vegetative and fruit tissues, respectively. Here, we hypothesize that γ-aminobutyrate accumulation, as well energy and antioxidant depletion are associated with the development of physiological injury in pome fruit under multiple environmental stresses. An improved understanding of this relationship could assist in maintaining the quality of stored fruit.

NAD(+)-aminoaldehyde dehydrogenase candidates for 4-aminobutyrate (GABA) and β-alanine production during terminal oxidation of polyamines in apple fruit

The last step of polyamine catabolism involves the oxidation of 3-aminopropanal or 4-aminobutanal via aminoaldehyde dehydrogenase. In this study, two apple (Malus x domestica) AMADH genes were selected (MdAMADH1 and MdAMADH2) as candidates for encoding 4-aminobutanal dehydrogenase activity. Maximal activity and catalytic efficiency were obtained with NAD(+) and 3-aminopropanal, followed by 4-aminobutanal, at pH 9.8. NAD(+) reduction was accompanied by the production of GABA and β-alanine, respectively, when 4-aminobutanal and 3-aminopropanal were utilized as substrates. MdAMADH2 was peroxisomal and MdAMADH1 cytosolic. These findings shed light on the potential role of apple AMADHs in 4-aminobutyrate and β-alanine production.

Apple fruit copper amine oxidase isoforms: peroxisomal MdAO1 prefers diamines as substrates, whereas extracellular MdAO2 exclusively utilizes monoamines

4-Aminobutyrate (GABA) accumulates in apple fruit during controlled atmosphere storage. A potential source of GABA is the polyamine putrescine, which can be oxidized via copper-containing amine oxidase (CuAO), resulting in the production 4-aminobutanal/Δ(1)-pyrroline, with the consumption of O2 and release of H2O2 and ammonia. Five putative CuAO genes (MdAO genes) were cloned from apple (Malus domestica Borkh. cv. Empire) fruit, and the deduced amino acid sequences found to contain the active sites typically conserved in CuAOs. Genes encoding two of these enzymes, MdAO1 and MdAO2, were highly expressed in apple fruit and selected for further analysis. Amino acid sequence analysis predicted the presence of a C-terminal peroxisomal targeting signal 1 tripeptide in MdAO1 and an N-terminal signal peptide and N-glycosylation site in MdAO2. Transient expression of green fluorescent fusion proteins in Arabidopsis protoplasts or onion epidermal cells revealed a peroxisomal localization for MdAO1 and an extracellular localization for MdAO2. The enzymatic activities of purified recombinant MdAO1 and MdAO2 were measured continuously as H2O2 production using a coupled reaction. MdAO1 did not use monoamines or polyamines and displayed high catalytic efficiency for 1,3-diaminopropane, putrescine and cadaverine, whereas MdAO2 exclusively utilized aliphatic and aromatic monoamines, including 2-phenylethylamine and tyramine. Together, these results indicate that MdAO1 may contribute to GABA production via putrescine oxidation in the peroxisome of apple fruit under controlled atmosphere conditions. MdAO2 seems to be involved in deamination of 2-phenylethylamine, which is a step in the biosynthesis of 2-phenylethanol, a contributor to fruit flavor and flower fragrance.

Impact of 1-methylcyclopropene and controlled atmosphere storage on polyamine and 4-aminobutyrate levels in "Empire" apple fruit

1-Methylcyclopropene (1-MCP) delays ethylene-meditated ripening of apple (Malus domestica Borkh.) fruit during controlled atmosphere (CA) storage. Here, we tested the hypothesis that 1-MCP and CA storage enhances the levels of polyamines (PAs) and 4-aminobutyrate (GABA) in apple fruit. A 46-week experiment was conducted with "Empire" apple using a split-plot design with four treatment replicates and 3°C, 2.5 kPa O2, and 0.03 or 2.5 kPa CO2 with or without 1 μL L(-1) 1-MCP. Total PA levels were not elevated by the 1-MCP treatment. Examination of the individual PAs revealed that: (i) total putrescine levels tended to be lower with 1-MCP regardless of the CO2 level, and while this was mostly at the expense of free putrescine, large transient increases in soluble conjugated putrescine were also evident; (ii) total spermidine levels tended to be lower with 1-MCP, particularly at 2.5 kPa CO2, and this was mostly at the expense of soluble conjugated spermidine; (iii) total spermine levels at 2.5 kPa CO2 tended to be lower with 1-MCP, and this was mostly at the expense of both soluble and insoluble conjugated spermine; and (iv) total spermidine and spermine levels at 0.03 kPa were relatively unaffected, compared to 2.5 kPa CO2, but transient increases in free spermidine and spermine were evident. These findings might be due to changes in the conversion of putrescine into higher PAs and the interconversion of free and conjugated forms in apple fruit, rather than altered S-adenosylmethionine availability. Regardless of 1-MCP and CO2 treatments, the availability of glutamate showed a transient peak initially, probably due to protein degradation, and this was followed by a steady decline over the remainder of the storage period which coincided with linear accumulation of GABA. This pattern has been attributed to the stimulation of glutamate decarboxylase activity and inhibition of GABA catabolism, rather than a contribution of PAs to GABA production.

Functional analyses of NSF1 in wine yeast using interconnected correlation clustering and molecular analyses

Analyzing time-course expression data captured in microarray datasets is a complex undertaking as the vast and complex data space is represented by a relatively low number of samples as compared to thousands of available genes. Here, we developed the Interdependent Correlation Clustering (ICC) method to analyze relationships that exist among genes conditioned on the expression of a specific target gene in microarray data. Based on Correlation Clustering, the ICC method analyzes a large set of correlation values related to gene expression profiles extracted from given microarray datasets. ICC can be applied to any microarray dataset and any target gene. We applied this method to microarray data generated from wine fermentations and selected NSF1, which encodes a C₂H₂ zinc finger-type transcription factor, as the target gene. The validity of the method was verified by accurate identifications of the previously known functional roles of NSF1. In addition, we identified and verified potential new functions for this gene; specifically, NSF1 is a negative regulator for the expression of sulfur metabolism genes, the nuclear localization of Nsf1 protein (Nsf1p) is controlled in a sulfur-dependent manner, and the transcription of NSF1 is regulated by Met4p, an important transcriptional activator of sulfur metabolism genes. The inter-disciplinary approach adopted here highlighted the accuracy and relevancy of the ICC method in mining for novel gene functions using complex microarray datasets with a limited number of samples.

Calmodulin-dependent and calmodulin-independent glutamate decarboxylases in apple fruit

BACKGROUND

The ubiquitous, non-proteinaceous amino acid GABA (γ-aminobutyrate) accumulates in plants subjected to abiotic stresses such as chilling, O2 deficiency and elevated CO2. Recent evidence indicates that controlled atmosphere storage causes the accumulation of GABA in apple (Malus x domestica Borkh.) fruit, and now there is increasing interest in the biochemical mechanisms responsible for this phenomenon. Here, we investigated whether this phenomenon could be mediated via Ca(2+)/calmodulin (CaM) activation of glutamate decarboxylase (GAD) activity.

RESULTS

GAD activity in cell-free extracts of apple fruit was stimulated by Ca(2+)/CaM at physiological pH, but not at the acidic pH optimum. Based on bioinformatics analysis of the apple genome, three apple GAD genes were identified and their expression determined in various apple organs, including fruit. Like recombinant Arabidopsis GAD1, the activity and spectral properties of recombinant MdGAD1 and MdGAD2 were regulated by Ca(2+)/CaM at physiological pH and both enzymes possessed a highly conserved CaM-binding domain that was autoinhibitory. In contrast, the activity and spectral properties of recombinant MdGAD3 were not affected by Ca(2+)/CaM and they were much less sensitive to pH than MdGAD1, MdGAD2 and Arabidopsis GAD1; furthermore, the C-terminal region neither bound CaM nor functioned as an autoinhibitory domain.

CONCLUSIONS

Plant GADs typically differ from microbial and animal GAD enzymes in possessing a C-terminal 30-50 amino acid residue CaM-binding domain. To date, rice GAD2 is the only exception to this generalization; notably, the C-terminal region of this enzyme still functions as an autoinhibitory domain. In the present study, apple fruit were found to contain two CaM-dependent GADs, as well as a novel CaM-independent GAD that does not possess a C-terminal autoinhibitory domain.

Identification of catalytically important amino acid residues for enzymatic reduction of glyoxylate in plants

NADPH-dependent glyoxylate reductases from Arabidopsis thaliana (AtGLYR) convert both glyoxylate and succinic semialdehyde into their corresponding hydroxyacid equivalents. The primary sequence of cytosolic AtGLYR1 reveals several sequence elements that are consistent with the β-HAD (β-hydroxyacid dehydrogenase) protein family, whose members include 3-hydroxyisobutyrate dehydrogenase, tartronate semialdehyde reductase and 6-phosphogluconate dehydrogenase. Here, site-directed mutagenesis was utilized to identify catalytically important amino acid residues for glyoxylate reduction in AtGLYR1. Kinetic studies and binding assays established that Lys170 is essential for catalysis, Phe231, Asp239, Ser121 and Thr95 are more important in substrate binding than in catalysis, and Asn174 is more important in catalysis. The low activity of the mutant enzymes precluded kinetic studies with succinic semialdehyde. The crystal structure of AtGLYR1 in the absence of substrate was solved to 2.1Å by molecular replacement using a previously unrecognized member of the β-HAD family, cytokine-like nuclear factor, thereby enabling the 3-D structure of the protein to be modeled with substrate and co-factor. Structural alignment of AtGLYR1 with β-HAD family members provided support for the essentiality of Lys170, Phe173, Asp239, Ser121, Asn174 and Thr95 in the active site and preliminary support for an acid/base catalytic mechanism involving Lys170 as the general acid and a conserved active-site water molecule. This information established that AtGLYR1 is a member of the β-HAD protein family. Sequence and activity comparisons indicated that AtGLYR1 and the plastidial AtGLYR2 possess structural features that are absent in Arabidopsis hydroxypyruvate reductases and probably account for their stronger preference for glyoxylate over hydroxypyruvate.

Hypothesis/review: contribution of putrescine to 4-aminobutyrate (GABA) production in response to abiotic stress

4-Aminobutyrate (GABA) accumulates in various plant parts, including bulky fruits such as apples, in response to abiotic stress. It is generally believed that the GABA is derived from glutamate, although a contribution from polyamines is possible. Putrescine, but not spermidine and spermine, generally accumulates in response to the genetic manipulation of polyamine biosynthetic enzymes and abiotic stress. However, the GABA levels in stressed plants are influenced by processes other than putrescine availability. It is hypothesized that the catabolism of putrescine to GABA is regulated by a combination of gene-dependent and -independent processes. The expression of several putative diamine oxidase genes is weak, but highly stress-inducible in certain tissues of Arabidopsis. In contrast, candidate genes that encode 4-aminobutyraldehyde dehydrogenase are highly constitutive, but not stress inducible. Changes in O(2) availability and cellular redox balance due to stress may directly influence the activities of diamine oxidase and 4-aminobutyraldehyde dehydrogenase, thereby restricting GABA formation. Apple fruit is known to accumulate GABA under controlled atmosphere storage and therefore could serve as a model system for investigating the relative contribution of putrescine and glutamate to GABA production.

Glyoxylate reductase isoform 1 is localized in the cytosol and not peroxisomes in plant cells

Glyoxylate reductase (GLYR) is a key enzyme in plant metabolism which catalyzes the detoxification of both photorespiratory glyoxylate and succinic semialdehdye, an intermediate of the γ-aminobutyrate (GABA) pathway. Two isoforms of GLYR exist in plants, GLYR1 and GLYR2, and while GLYR2 is known to be localized in plastids, GLYR1 has been reported to be localized in either peroxisomes or the cytosol. Here, we reappraised the intracellular localization of GLYR1 in Arabidopsis thaliana L. Heynh (ecotype Lansberg erecta) using both transiently-transformed suspension cells and stably-transformed plants, in combination with fluorescence microscopy. The results indicate that GLYR1 is localized exclusively to the cytosol regardless of the species, tissue and/or cell type, or exposure of plants to environmental stresses that would increase flux through the GABA pathway. Moreover, the C-terminal tripeptide sequence of GLYR1, -SRE, despite its resemblance to a type 1 peroxisomal targeting signal, is not sufficient for targeting to peroxisomes. Collectively, these results define the cytosol as the intracellular location of GLYR1 and provide not only important insight to the metabolic roles of GLYR1 and the compartmentation of the GABA and photorespiratory pathways in plant cells, but also serve as a useful reference for future studies of proteins proposed to be localized to peroxisomes and/or the cytosol.

Compartmentation of GABA metabolism raises intriguing questions

This synopsis covers the compartmentation of γ-aminobutyrate (GABA) metabolism, highlighting recent progress with Arabidopsis (Arabidopsis thaliana) and raising questions about mitochondrial GABA and succinic semialdehyde (SSA) transport, the fate of succinic semialdehyde once it exits mitochondria, and biochemical interactions between GABA metabolism and related processes such as photorespiration.

Heterosis for carotenoid concentration and profile in maize hybrids

Production of high-lutein maize grain is of particular interest as a value-added feed source to produce high-lutein eggs. In this paper, it is demonstrated that heterosis for total carotenoid concentration and for the ratio of lutein to zeaxanthin (L:Z ratio), or profile type, exists infrequently in yellow dent crosses. However, yellow dent inbred maize lines A619 and CG102, both possessing high-lutein profiles, produce F1 seed with a classic overdominant expression of lutein levels (i.e., 49 µg/g dry weight (DW) above the high-parent value). Reciprocal crosses of A619 and CG102 with one another and with two high-zeaxanthin (i.e., low lutein), high-carotenoid lines both suggest that the A619 and CG102 high-lutein phenotypes are achieved by different and complementary genotypes. The contribution of CG102 to the heterotic response was examined using a QTL-based approach that involved phenotyping the mapping population in a testcross to A619. Significant QTL were found at loci known to be involved in the carotenoid pathway but also at loci proximate to, but separate from, known carotenoid pathway steps. Exploiting an overdominant heterotic response for lutein and total carotenoids should be given strong consideration as a viable method of producing high-carotenoid hybrid maize lines.

Reappraisal of nitrogen use efficiency in rice overexpressing glutamine synthetase1

Cytosolic glutamine synthetase (GS1) is responsible for the primary assimilation of ammonia, and a role in nitrogen (N) remobilization is implicated from its vascular localization and enhanced expression during senescence. This paper tested the hypothesis that overexpression (OX) of GS1 in rice improves utilization N use efficiency (UtE = spikelet yield/shoot N content). Three GS1 OX lines were identified using activity assays and quantitative polymerase chain reaction. Physiological analysis of the OX lines, as well as azygous and wild-type (Wt) controls, was conducted with mature plants after growth under varying nitrate conditions (non-limiting N, limiting N, transfer from non-limiting N to limiting N at panicle emergence) and growth environments (growth chamber vs greenhouse). Overall, OX lines did not differ from azygous controls in vegetative yield or shoot N content. In two of the three growth trials (i.e. the growth chamber trials) harvest index, N harvest index (spikelet N content/shoot N content) and UtE were generally enhanced in the OX lines relative to their azygous controls. These characteristics were highly correlated with percent spikelets filled and spikelet number. Thus, N partitioning in rice during grain filling could be altered by GS1 OX, resulting in improved UtE. Unfortunately, GS OX did not result in more efficient use of N under limiting N than under non-limiting N, and is therefore unlikely to result in the use of less N under field conditions. Transformation effects significantly hindered the productivity of the OX lines, but backcrossing to the Wt should overcome this.

Impact of postharvest handling on carotenoid concentration and composition in high-carotenoid maize (Zea mays L.) kernels

High carotenoid maize is an ideal source of high value dietary carotenoids, especially lutein and zeaxanthin, in human and animal feed and has been proposed as a feedstock for high carotenoid egg production. A modified analytical method was demonstrated to have reliability, reproducibility, and improved run-time and separation of xanthophylls. This method was used to confirm the localization of carotenoids in endosperm and to determine the effects of drying and storage on carotenoid levels in maize grain. A preliminary trial using room temperature drying indicated that while carotenoid profiles remain stable during storage, carotenoid levels decrease significantly from initial levels between 3 and 6 months of storage, but then remain stable for another year. A more rigorous trial using three drying and storage regimes (freeze-drying and storage at -80 degrees C; room temperature drying and storage; 90 degrees C drying and room temperature storage) indicated that extreme caution is needed to maintain carotenoid levels in maize during handling and storage, but in situations where freeze-drying is not possible, high heat drying is no more detrimental than low heat drying.

Increased nitrogen-use efficiency in transgenic rice plants over-expressing a nitrogen-responsive early nodulin gene identified from rice expression profiling

Development of genetic varieties with improved nitrogen-use efficiency (NUE) is essential for sustainable agriculture. In this study, we developed a growth system for rice wherein N was the growth-limiting factor, and identified N-responsive genes by a whole genome transcriptional profiling approach. Some genes were selected to test their functionality in NUE by a transgenic approach. One such example with positive effects on NUE is an early nodulin gene OsENOD93-1. This OsENOD93-1 gene responded significantly to both N induction and N reduction. Transgenic rice plants over-expressing the OsENOD93-1 gene had increased shoot dry biomass and seed yield. This OsENOD93-1 gene was expressed at high levels in roots of wild-type (WT) plants, and its protein product was localized in mitochondria. Transgenic plants accumulated higher concentrations of total amino acids and total N in roots. A higher concentration of amino acids in xylem sap was detected in transgenic plants, especially under N stress. In situ hybridization revealed that OsENOD93-1 is expressed in vascular bundles, as well as in epidermis and endodermis. This work demonstrates that transcriptional profiling, coupled with a transgenic validation approach, is an effective strategy for gene discovery. The knowledge gained from this study could be applied to other important crops.

Subcellular localization and expression of multiple tomato gamma-aminobutyrate transaminases that utilize both pyruvate and glyoxylate

Gamma-aminobutyric acid transaminase (GABA-T) catalyses the breakdown of GABA to succinic semialdehyde. In this report, three GABA-T isoforms were identified in the tomato (Solanum lycopersicum L.) plant. The deduced amino acid sequences of the three isoforms are highly similar over most of their coding regions with the exception of their N-terminal regions. Transient expression of the individual full-length GABA-T isoforms fused to the green fluorescent protein in tobacco suspension-cultured cells revealed their distinct subcellular localizations to the mitochondrion, plastid or cytosol, and that the specific targeting of the mitochondrion- and plastid-localized isoforms is mediated by their predicted N-terminal presequences. Removal of the N-terminal targeting presequences from the mitochondrion and plastid GABA-T isoforms yielded good recovery of the soluble recombinant proteins in Escherichia coli when they were co-expressed with the GroES/EL molecular chaperone complex. Activity assays indicated that all three recombinant isoforms possess both pyruvate- and glyoxylate-dependent GABA-T activities, although the mitochondrial enzyme has a specific activity that is significantly higher than that of its plastid and cytosolic counterparts. Finally, differential expression patterns of the three GABA-T isoforms in reproductive tissues, but not vegetative tissues, suggest unique roles for each enzyme in developmental processes. Overall, these findings, together with recent information about rice and pepper GABA-Ts, indicate that the subcellular distribution of GABA-T in the plant kingdom is highly variable.

Biochemical characterization, mitochondrial localization, expression, and potential functions for an Arabidopsis gamma-aminobutyrate transaminase that utilizes both pyruvate and glyoxylate

Gamma-aminobutyrate transaminase (GABA-T) catalyses the breakdown of GABA to succinic semialdehyde. In this report, the previously identified Arabidopsis thaliana (L.) Heyhn GABA-T (AtGABA-T) was characterized in more detail. Full-length AtGABA-T contains an N-terminal 36 amino acid long targeting pre-sequence (36 amino acids) that is both sufficient and necessary for targeting the enzyme to mitochondria. Removal of the pre-sequence encoding this N-terminal targeting domain and co-expression of the resulting truncated AtGABA-T cDNA with the GroES/EL molecular chaperone complex in Escherichia coli yielded good recovery of the soluble recombinant proteins. Activity assays indicated that purified recombinant GABA-T has both pyruvate- and glyoxylate-dependent activities, but cannot utilize 2-oxoglutarate as amino acceptor. Kinetic parameters for glyoxylate- and pyruvate-dependent GABA-T activities were similar, with physiologically relevant affinities. Assays of GABA-T activity in cell-free leaf extracts from wild-type Arabidopsis and two knockout mutants in different genetic backgrounds confirmed that the native enzyme possesses both pyruvate- and glyoxylate-dependent activities. The GABA-T transcript was present throughout the plant, but its expression was highest in roots and increased as a function of leaf development. A GABA-T with dual functions suggests the potential for interaction between GABA metabolism and photorespiratory glyoxylate production.

Gamma-hydroxybutyrate accumulation in Arabidopsis and tobacco plants is a general response to abiotic stress: putative regulation by redox balance and glyoxylate reductase isoforms

Enzymes that reduce the aldehyde chemical grouping (i.e. H-C=O) to its corresponding alcohol are probably crucial in maintaining plant health during stress. Succinic semialdehyde (SSA) is a mitochondrially-generated intermediate in the metabolism of gamma-aminobutyrate (GABA), which accumulates in response to a variety of biotic and abiotic stresses. SSA can be reduced to gamma-hydroxybutyrate (GHB) under oxygen deficiency and high light conditions. Recent evidence indicates that distinct cytosolic and plastidial glyoxylate reductase isoforms from Arabidopsis (designated herein after as AtGR1 and AtGR2, respectively) catalyse the in vitro conversion of SSA to GHB, as well as glyoxylate to glycolate, via NADPH-dependent reactions. In the present report, the responses of GHB and related amino acids, as well as NADP(+) and NADPH, were monitored in leaves from Arabidopsis or tobacco plants subjected to various abiotic stresses (i.e. Arabidopsis during exposure to salinity, drought, submergence, cold, or heat; tobacco during exposure to, and recovery from, submergence). Time-course experiments revealed that GHB accumulated in both Arabidopsis and tobacco plants subjected to stress, and that this accumulation was generally accompanied by higher GABA and alanine levels, higher NADPH/NADP(+) ratio, and lower glutamate levels. Furthermore, the analysis of gene expression in Arabidopsis revealed that the relative abundance of GR1 (salinity, drought, submergence, cold, and heat) and GR2 (cold and heat) transcripts was enhanced by the stress tested. Thus, GHB accumulation in plants is a general response to abiotic stress and appears to be regulated by both biochemical and transcriptional processes.

Identification and characterization of a plastid-localized Arabidopsis glyoxylate reductase isoform: comparison with a cytosolic isoform and implications for cellular redox homeostasis and aldehyde detoxification

Enzymes that reduce the aldehyde chemical grouping (i.e. H-C=O) to its corresponding alcohol could be crucial in maintaining plant health. Recently, recombinant expression of a cytosolic enzyme from Arabidopsis thaliana (L.) Heynh (designated as glyoxylate reductase 1 or AtGR1) revealed that it effectively catalyses the in vitro reduction of both glyoxylate and succinic semialdehyde (SSA). In this paper, web-based bioinformatics tools revealed a second putative GR cDNA (GenBank Accession No. AAP42747; designated herein as AtGR2) that is 57% identical on an amino acid basis to GR1. Sequence encoding a putative targeting signal (N-terminal 43 amino acids) was deleted from the full-length GR2 cDNA and the resulting truncated gene was co-expressed with the molecular chaperones GroES/EL in Escherichia coli, enabling production and purification of soluble recombinant protein. Kinetic analysis revealed that recombinant GR2 catalysed the conversion of glyoxylate to glycolate (K(m) glyoxylate=34 microM), and SSA to gamma-hydroxybutyrate (K(m) SSA=8.96 mM) via an essentially irreversible, NADPH-based mechanism. GR2 had a 350-fold higher preference for glyoxylate than SSA, based on the performance constants (k(cat)/K(m)). Fluorescence microscopic analysis of tobacco (Nicotiana tabacum L.) suspension cells transiently transformed with GR1 linked to the green fluorescent protein (GFP) revealed that GR1 was localized to the cytosol, whereas GR2-GFP was localized to plastids via targeting information contained within its N-terminal 45 amino acids. The identification and characterization of distinct plastidial and cytosolic glyoxylate reductase isoforms is discussed with respect to aldehyde detoxification and the plant stress response.

Characteristics of anArabidopsisglyoxylate reductase: general biochemical properties and substrate specificity for the recombinant protein, and developmental expression and implications for glyoxylate and succinic semialdehyde metabolism in planta

Constitutive expression of an Arabidopsis thaliana (L.) Heynh cDNA (GenBank accession No. AY044183 ) in a succinic semialdehyde (SSA) dehydrogenase-deficient yeast ( Saccharomyces cerevisiae Hansen) mutant enables growth on γ-aminobutyrate and significantly enhances the accumulation of γ-hydroxybutyrate. In this report, the cDNA (designated hereinafter as AtGR1) was functionally expressed in Escherichia coli , and the recombinant protein purified to homogeneity. Kinetic analysis of substrate specificity revealed that the enzyme catalyzed the conversion of glyoxylate to glycolate (Km,glyoxylate= 4.5 μmol·L–1) as well as SSA to γ-hydroxybutyrate (Km, SSA= 0.87 mmol·L–1) via an essentially irreversible, NADPH-based mechanism. The enzyme had a 250-fold higher preference for glyoxylate than SSA based on the performance constants (kcat/Km), and with the exception of 4-carboxybenzaldehyde, at least a 100-fold higher preference for SSA than all other substrates tested (formaldehyde, acetaldehyde, butyraldehyde, 2-carboxybenzaldehyde, glyoxal, methylglyoxal, phenylglyoxal, phenylglyoxylate). In vitro assays of SSA reductase activity in cell-free extracts from Arabidopisis revealed its presence throughout the plant, although its specific activity was considerably higher in leaves at all developmental stages and in reproductive parts than in roots. It is proposed that the enzyme functions in redox homeostasis and the detoxification of both glyoxylate and SSA, in planta, resulting in the production of glycolate and γ-hydroxybutyrate, respectively.

Kinetic mechanism of a recombinantArabidopsisglyoxylate reductase: studies of initial velocity, dead-end inhibition and product inhibition

Kinetic analysis of substrate specificity revealed that a recombinant Arabidopsis protein catalyzes the conversion of glyoxylate to glycolate (Km,glyoxylate= 4.5 μmol·L–1) and succinic semialdehyde (SSA) to γ-hydroxybutyrate (Km, SSA= 0.87 mmol·L–1) via an essentially irreversible, NADPH-based mechanism. In this report, the enzyme was further characterized via initial-velocity, dead-end inhibition and product inhibition studies. The kinetic mechanism was ordered Bi Bi, involving the complexation of NADPH to the enzyme before glyoxylate or SSA, and the release of NADP+before glycolate or γ-hydroxybutyrate, respectively. It can be concluded that the enzyme functions as a NADPH-dependent glyoxylate reductase (EC 1.1.1.79) or possibly an aldehyde reductase (EC 1.1.1.2), and the kinetic mechanism involved is consistent with that found in members of both the aldo-keto reductase and 3-hydroxyisobutyrate dehydrogenase-related superfamilies of enzymes. Since NADP+was an effective competitive inhibitor with respect to NADPH (Ki= 1–3 µmol·L–1), it is proposed that the ratio of NADPH/NADP+regulates enzymatic activity in planta.

Identification of the full-length Hs1pro-1 coding sequence and preliminary evaluation of soybean cyst nematode resistance in soybean transformed with Hs1pro-1 cDNA

The Hs1pro-1 gene reportedly confers resistance to the beet cyst nematode in wild beet and sugar beet. Here, we tested the hypothesis that Hs1pro-1 confers resistance in soybean against the soybean cyst nematode (SCN). The full-length Hs1pro-1 coding sequence, which encodes a predicted polypeptide of 490 amino acids, was first acquired then expressed in ‘Westag’ soybean using a constitutive octopine synthase – mannopine synthase promoter. Thirty T0 lines that successfully expressed the Hs1pro-1 gene, as indicated by both polymerase chain reaction and reverse transcriptase – polymerase chain reaction analyses, were generated. Bioassay of the T1 progeny from these lines revealed that only five T0 lines grew normally and exhibited a high degree of SCN resistance. On average, these T1 transgenic progeny were about 70% more resistant to SCN than susceptible control cultivars. These preliminary data suggest that Hs1pro-1 is a promising candidate for genetically engineering SCN resistance in elite, locally adapted soybean cultivars.

QTL associated with horizontal resistance to soybean cyst nematode in Glycine soja PI464925B

Soybean cyst nematode (Heterodera glycines Ichinohe; SCN) is the primary disease responsible for yield loss of soybean [Glycine max (L.) Merr.]. Resistant cultivars are an effective management tool; however, the sources currently available have common resistant genes. Glycine soja Sieb. and Zucc., the wild ancestor of domesticated soybean, represents a diverse germplasm pool with known SCN resistance. The objectives of this research were to: (1) determine the genetic variation and inheritance of SCN resistance in a G. max ('S08-80') x G. soja (PI464925B) F (4:5) recombinant inbred line (RIL) population; and (2) identify and evaluate quantitative trait loci (QTL) associated with SCN resistance. Transgressive segregation for resistance was observed, although neither parent was resistant to the Chatham and Ruthven SCN isolates. Broad sense heritability was 0.81 for the Ruthven and 0.91 for the Chatham isolate. Root dry weight was a significant covariate that influenced cyst counts. One RIL [female index (FI) = 5.2 +/- 1.11] was identified as resistant to the Chatham isolate (FI < 10). Seventeen and three RILs infected with Chatham and Ruthven isolates, respectively, had mean adjusted cyst counts of zero. Unique and novel QTL, which derived resistance from G. soja, were identified on linkage groups I, K, and O, and individually explained 8, 7 and 5% (LOD = 2.1-2.7) of the total phenotypic variation, respectively. Significant epistatic interactions were found between pairs of SSR markers that individually may or may not have been associated with SCN resistance, which explained between 10 and 15% of the total phenotypic variation. Best-fit regression models explained 21 and 31% of the total phenotypic variation in the RIL population to the Chatham and Ruthven isolates, respectively. The results of this study help to improve the understanding of the genetic control of SCN resistance in soybean caused by minor genes resulting in horizontal resistance. The incorporation of the novel resistance QTL from G. soja could increase the durability of SCN-resistance in soybean cultivars, especially if major gene resistance breaks down.

Gamma-aminobutyrate: defense against invertebrate pests?

Gamma-aminobutyrate (GABA) is a ubiquitous four-carbon, non-protein amino acid. In plants, stress-induced GABA accumulation is well documented. However, the role(s) of GABA accumulation is contentious. In this Opinion article, we argue that wounding due to herbivory and crawling by insect larvae causes rapid GABA accumulation via the disruption of cellular compartmentation and the release of the acidic vacuolar contents to the cytosol. The activity of glutamate decarboxylase, the cytosolic enzyme responsible for GABA synthesis, has an acidic pH optimum. Subsequent GABA ingestion has a plant defense function by directly acting on GABA-regulated invertebrate neuromuscular junctions. Plants with an enhanced GABA-producing capacity reduce herbivory by invertebrate pests. These findings suggest that GABA accumulation is a rapidly deployed, local resistance mechanism that constitutes a first line of defense in deterring herbivory.

Fluctuations of γ-aminobutyrate, γ-hydroxybutyrate, and related amino acids in Arabidopsis leaves as a function of the light–dark cycle, leaf age, and N stressEditorial decisions for this paper were made by Robert Ireland, Associate Editor, Canadian Journal of Botany

To gain further insight into the metabolic role of γ-aminobutyrate (GABA), we determined the pool sizes of GABA and its catabolic products, alanine and γ-hydroxybutyrate (GHB), as well as key amino acids (Glu, Gln, Asp, Asn, Pro, Gly, Ser), in Arabidopsis leaves as a function of the light–dark cycle, leaf age (old versus young), and N stress (continuous versus interrupted N supply). Regardless of time of day and leaf age, there was a close relationship among Glu, GABA, and GHB when N was supplied continuously, indicating that GABA and GHB were probably derived exclusively from Glu and GABA, respectively. Ala was also closely linked to GABA in young leaves, but not in old leaves, a result consistent with the existence of multiple sources of Ala. The nature of the responses of GABA and GHB to an interrupted N supply depended on leaf age, and differed from responses exhibited by Glu, Gln, and Asn. Overall fluctuations in primary amino acids under both continuous and interrupted N supply, as well as those associated with photorespiration, aging, and stress, suggest that the old and young leaves chosen for study here function in Arabidopsis as source and sink leaves, respectively.

GABA controls the level of quorum-sensing signal in Agrobacterium tumefaciens

The concentration of GABA increases rapidly in wounded plant tissues, but the implication of this GABA pulse for plant-bacteria interactions is not known. Here we reveal that GABA stimulated the inactivation of the N-(3-oxooctanoyl)homoserine lactone (OC8-HSL) quorum-sensing signal (or "quormone") by the Agrobacterium lactonase AttM. GABA induced the expression of the attKLM operon, which was correlated to a decrease in OC8-HSL concentration in Agrobacterium tumefaciens cultures. The Agrobacterium GABA transporter Bra was required for this GABA-signaling pathway. Furthermore, transgenic tobacco plants with elevated GABA levels were less sensitive to A. tumefaciens C58 infection than were wild-type plants. These findings indicate that plant GABA may modulate quorum sensing in A. tumefaciens, thereby affecting its virulence on plants. Whereas GABA is an essential cell-to-cell signal in eukaryotes, here we provide evidence of GABA acting as a signal between eukaryotes and pathogenic bacteria. The GABA signal represents a potential target for the development of a strategy to control the virulence of bacterial pathogens.

Internalization of Escherichia coli O157:H7 following biological and mechanical disruption of growing spinach plants

The internalization and persistence of a bioluminescent Escherichia coli O157:H7 Ph1 was investigated in growing spinach plants that had been either biologically or mechanically damaged. In control (undamaged) plants cultivated in soil microcosms inoculated with E. coli O157:H7 Phl, the bacterium was recovered from surface-sterilized root tissue but not from leaves. Mechanical disruption of the seminal root and root hairs of the plants did not result in the internalization of the pathogen into the aerial leaf tissue. When imprints of the root tissue were made on plates of tryptic soy agar plus ampicillin, no colonies of E. coli O157:H7 were observed around damaged tissue. The roots of growing plants were exposed to the northern root-knot nematode, Meloidogyne hapla, in the presence of E. coli O157:H7. Although this treatment caused knot formation on the roots, it did not enhance the internalization of the bacterium into the plant vascular system. Coinoculation of intact leaves with E. coli O157:H7 and the phytopathogen Pseudomonas syringae DC3000 resulted in localized necrosis, but the persistence of the human pathogen was not affected. The mechanical disruption of roots does not result in the internalization of E. coli O157:H7 into the aerial tissue of spinach, and there does not appear to be any effect of P. syringae in terms of enhancing the persistence of E. coli O157:H7 in spinach leaves.

A novel gamma-hydroxybutyrate dehydrogenase: identification and expression of an Arabidopsis cDNA and potential role under oxygen deficiency

In plants, gamma-aminobutyrate (GABA), a non-protein amino acid, accumulates rapidly in response to a variety of abiotic stresses such as oxygen deficiency. Under normoxia, GABA is catabolized to succinic semialdehyde and then to succinate with the latter reaction being catalyzed by succinic semialdehyde dehydrogenase (SSADH). Complementation of an SSADH-deficient yeast mutant with an Arabidopsis cDNA library enabled the identification of a novel cDNA (designated as AtGH-BDH for Arabidopsis thaliana gamma-hydroxybutyrate dehydrogenase), which encodes a 289-amino acid polypeptide containing an NADP-binding domain. Constitutive expression of AtGHBDH in the mutant yeast enabled growth on 20 mm GABA and significantly enhanced the cellular concentrations of gamma-hydroxybutyrate, the product of the GHDBH reaction. These data confirm that the cDNA encodes a polypeptide with GHBDH activity. Arabidopsis plants subjected to flooding-induced oxygen deficiency for up to 4 h possessed elevated concentrations of gamma-hydroxybutyrate as well as GABA and alanine. RNA expression analysis revealed that GHBDH transcription was not up-regulated by oxygen deficiency. These findings suggest that GHBDH activity is regulated by the supply of succinic semialdehyde or by redox balance. It is proposed that GHBDH and SSADH activities in plants are regulated in a complementary fashion and that GHBDH and gamma-hydroxybutyrate function in oxidative stress tolerance.

Overexpression of glutamate decarboxylase in transgenic tobacco plants deters feeding by phytophagous insect larvae

Gamma-aminobutyrate (GABA) is a ubiquitous four-carbon, non-protein amino acid synthesized by glutamate decarboxylase. Previous research suggests that the endogenous synthesis of GABA, a naturally occurring inhibitory neurotransmitter at neuromuscular junctions, serves as a plant resistance mechanism against invertebrate pests. In this study, two homozygous transgenic tobacco lines constitutively overexpressing a single copy of a full-length chimeric glutamate decarboxylase cDNA and possessing enhanced capacity for GABA accumulation (GAD plants), a homozygous transgenic line lacking the gene insert, and wild-type tobacco were employed. Tobacco budworm larvae were presented with plant attached wild type and transgenic leaves for 4 hr in a feeding preference study. Larvae consumed six to twelve times more leaf tissue from wild-type plants than from GAD plants. These results suggest that leaf GABA accumulation, which is known to occur in response to insect larval walking and feeding, represents a rapidly deployed local resistance mechanism.

Calcium/calmodulin activation of two divergent glutamate decarboxylases from tobacco

Glutamate decarboxylase (GAD, EC 4.1.1.15) catalyses the alpha-decarboxylation of glutamate to produce gamma-aminobutyrate (GABA). The nucleotide sequences of two divergent GADs (designated GAD1 and GAD3) were isolated from a Nicotiana tabacum L. cv. Samsun NN leaf cDNA library. Open reading frames indicated that GAD1 encodes a polypeptide of 496 amino acids and has greater than 99% identity with known tobacco GADs, whereas GAD3 encodes a polypeptide of 491 amino acids and has about 14% divergence from known tobacco GADs. Genomic DNA analysis suggested that there are at least four tobacco GAD genes, existing in pairs of highly identical genes. An in vitro assay at pH 7.3 revealed that activities of the recombinant proteins, after isolation from Escherichia coli and partial purification by nickel-affinity chromatography, are 57-133 times the control levels in the presence of 0.5 mM calcium and 0.2 micro M bovine calmodulin.

Plant pyruvate-dependent gamma-aminobutyrate transaminase: identification of anArabidopsiscDNA and its expression inEscherichia coli

Both pyruvate- and 2-oxoglutarate-dependent gamma-aminobutyrate transaminase (GABA-T) activities are present in crude tobacco (Nicotiana tabacum L.) leaf extracts. In this study, GABA:pyruvate-T activity was partially purified using mitochondrial isolation and protein solubilization in 3-[3-cholamidopropyl)dimethylammonio]-1-propanesulfonate, and a combination of chromatographic and electrophoretic procedures. A partial amino acid sequence of the putative 55-kDa GABA-T subunit enabled identification of a predicted Arabidopsis thaliana (L.) Heynh. GABA:pyruvate-T expressed sequence tag and subsequent amplification of a 1515 bp open reading frame encoding a 504-amino acid polypeptide. Computer analysis using web-based tools revealed the presence of a putative mitochondrial signal sequence and a pyridoxal-5-phosphate binding domain in the polypeptide. Functional expression of the GABA-T cDNA in Escherichia coli revealed that the recombinant protein uses pyruvate but not 2-oxoglutarate. The Arabidopsis GABA:pyruvate-T cDNA could form the basis for identification of multiple GABA-T isoforms and generation of GABA-T mutants for determining the fate of GABA nitrogen and elucidating the physiological function of GABA in plants.Key words: amino acceptor, gamma-aminobutyrate, gamma-aminobutyrate transaminase, protein purification, heterologous expression, recombinant protein.

Metabolism and functions of gamma-aminobutyric acid

Gamma-aminobutyric acid (GABA), a four-carbon non-protein amino acid, is a significant component of the free amino acid pool in most prokaryotic and eukaryotic organisms. In plants, stress initiates a signal-transduction pathway, in which increased cytosolic Ca2+ activates Ca2+/calmodulin-dependent glutamate decarboxylase activity and GABA synthesis. Elevated H+ and substrate levels can also stimulate glutamate decarboxylase activity. GABA accumulation probably is mediated primarily by glutamate decarboxylase. However, more information is needed concerning the control of the catabolic mitochondrial enzymes (GABA transaminase and succinic semialdehyde dehydrogenase) and the intracellular and intercellular transport of GABA. Experimental evidence supports the involvement of GABA synthesis in pH regulation, nitrogen storage, plant development and defence, as well as a compatible osmolyte and an alternative pathway for glutamate utilization. There is a need to identify the genes of enzymes involved in GABA metabolism, and to generate mutants with which to elucidate the physiological function(s) of GABA in plants.

Identification and characterization of GABA, proline and quaternary ammonium compound transporters from Arabidopsis thaliana

Arabidopsis thaliana grows efficiently on GABA as the sole nitrogen source, thereby providing evidence for the existence of GABA transporters in plants. Heterologous complementation of a GABA uptake-deficient yeast mutant identified two previously known plant amino acid transporters, AAP3 and ProT2, as GABA transporters with Michaelis constants of 12.9 +/- 1.7 and 1.7 +/- 0.3 mM at pH 4, respectively. The simultaneous transport of [1-14C]GABA and [2,3-3H]proline by ProT2 as a function of pH, provided evidence that the zwitterionic state of GABA is an important parameter in substrate recognition. ProT2-mediated [1-14C]GABA transport was inhibited by proline and quaternary ammonium compounds.

Accumulation of γ-aminobutyric acid in nodulated soybean in response to drought stress

Nitrogen fixation and nodule permeability to O₂ diffusion are decreased by drought stress. Since γ-aminobutyric acid (GABA) synthesis is rapidly stimulated by a variety of stress conditions including hypoxia, it was hypothesized that decreased O₂ availability in nodules stimulates glutamate decarboxylase (GAD) activity (EC 4.1.1.15), thereby resulting in GABA accumulation. First, the amino acid composition of xylem sap was determined in plants subjected to soil water deficits. While the xylem sap concentration of several amino acids increased when the plant was subjected to a water deficit, the greatest increase was in GABA. GABA accumulation was examined in response to stress induced by hypoxia or the addition of polyethylene glycol (PEG) to the nutrient solution. The exposure of soybean nodules to hypoxia for 6 h enhanced the GABA concentration by 6-fold, but there was no change in GABA concentration in response to the PEG treatment. No major changes in the in vitro GAD activity were measured in nodule cytosol or bacteroids. The present data do not support the hypothesis that decreased nodule O₂ permeability and a resulting O₂ deprivation inside nodules may stimulate in vitro GAD activity and thus GABA accumulation. However, the data could indicate a possible effect of hypoxia and drought stress on the in vivo activity of GAD.

The simultaneous measurement of low rates of CO2 and O2 exchange in biological systems

An instrument for measuring low rates of biological O2 exchange using an open-flow gas analysis system is described. A novel differential O2 sensor that is capable of measuring as little as 0.4 Pa O2 against a back-ground of ambient air (20,900 Pa O2), yet has a dynamic range of +/- 2000 Pa O2 (i.e., +/- ca. 2% O2) is described. Baseline drift was typically less than 0.025 Pa min-1. The differential O2 sensor was incorporated into a respiratory quotient/photosynthetic quotient analyzer that contained other environmental sensors for atmospheric pressure, absolute O2 and CO2 concentration, temperature of the differential O2 sensor block, and differential pressure between reference and sample streams. Protocols for how these sensors can be used to calibrate the differential O2 sensor and to improve its stability with time are described. Together, the differential O2 sensor, the environmental sensors, and the simple calibration techniques allow for simultaneous, noninvasive, and accurate measurements of O2 and CO2 exchange in tissues with metabolic rates as low as about 0.1 mumol O2 or CO2 h-1. Example data are provided in which O2 differentials of 3 to 41 Pa O2 were measured in an open-flow system.