Peroxisomal metabolism of propionic acid and isobutyric acid in plants

Growth inhibition, oxidative stress and characterisation of mortality in green algae under the influence of beta-blockers and non-steroidal anti-inflammatory drugs

Bisoprolol and ketoprofen are widely used pharmaceuticals in medical treatment hence these substances are occurring in wastewaters and in water environment. This research investigated the toxic effects of bisoprolol and ketoprofen on two microalgae taxa, Chlorella vulgaris and Desmodesmus armatus. The results showed that both drugs inhibited the growth of the species tested and induced a decrease in chlorophyll a content compared to controls. Ketoprofen turned out to be harmful to algae as the half maximal effective concentration (EC50) values (14 days) were 37.69 mg L-1 for C. vulgaris and 40.93 mg L-1 for D. armatus. On the other hand, for bisoprolol, the EC50 values were greater than the established NOEC, 100 mg L-1. Bisoprolol and ketoprofen induced oxidative stress in the tested microorganisms, as indicated by changes in the activities of antioxidant enzymes. Exposure to 100 mg L-1 of drugs significantly increased the activity of catalase, peroxidase and superoxide dismutase. Fluorescence microscopy showed that both medicaments changed the cells' morphology. There was atrophy of chlorophyll in the cells, moreover, dying multinuclear cells and cells without nuclei were observed. In addition, there were atrophic cells, namely cells that lacked nuclei and chlorophyll. Profile area analyses showed that bisoprolol and ketoprofen treated C. vulgaris cells were approximately 4 and 2 times greater compared to control ones. Our experimental findings highlight the ecotoxicological threats for aquatic primary producers from bisoprolol and ketoprofen and provide insight into the characteristics of their death.

Mapping the castor bean endosperm proteome revealed a metabolic interaction between plastid, mitochondria, and peroxisomes to optimize seedling growth

In this work, we studied castor-oil plant Ricinus communis as a classical system for endosperm reserve breakdown. The seeds of castor beans consist of a centrally located embryo with the two thin cotyledons surrounded by the endosperm. The endosperm functions as major storage tissue and is packed with nutritional reserves, such as oil, proteins, and starch. Upon germination, mobilization of the storage reserves requires inter-organellar interplay of plastids, mitochondria, and peroxisomes to optimize growth for the developing seedling. To understand their metabolic interactions, we performed a large-scale organellar proteomic study on castor bean endosperm. Organelles from endosperm of etiolated seedlings were isolated and subjected to liquid chromatography-tandem mass spectrometry (LC-MS/MS). Computer-assisted deconvolution algorithms were applied to reliably assign the identified proteins to their correct subcellular localization and to determine the abundance of the different organelles in the heterogeneous protein samples. The data obtained were used to build a comprehensive metabolic model for plastids, mitochondria, and peroxisomes during storage reserve mobilization in castor bean endosperm.

Multi-Omics Analysis of Vicia cracca Responses to Chronic Radiation Exposure in the Chernobyl Exclusion Zone

Our understanding of the long-term consequences of chronic ionising radiation for living organisms remains scarce. Modern molecular biology techniques are helpful tools for researching pollutant effects on biota. To reveal the molecular phenotype of plants growing under chronic radiation exposure, we sampled Vicia cracca L. plants in the Chernobyl exclusion zone and areas with normal radiation backgrounds. We performed a detailed analysis of soil and gene expression patterns and conducted coordinated multi-omics analyses of plant samples, including transcriptomics, proteomics, and metabolomics. Plants growing under chronic radiation exposure showed complex and multidirectional biological effects, including significant alterations in the metabolism and gene expression patterns of irradiated plants. We revealed profound changes in carbon metabolism, nitrogen reallocation, and photosynthesis. These plants showed signs of DNA damage, redox imbalance, and stress responses. The upregulation of histones, chaperones, peroxidases, and secondary metabolism was noted.

Frankia alni Carbonic Anhydrase Regulates Cytoplasmic pH of Nitrogen-Fixing Vesicles

A phyloprofile of Frankia genomes was carried out to identify those genes present in symbiotic strains of clusters 1, 1c, 2 and 3 and absent in non-infective strains of cluster 4. At a threshold of 50% AA identity, 108 genes were retrieved. Among these were known symbiosis-associated genes such as nif (nitrogenase), and genes which are not know as symbiosis-associated genes such as can (carbonic anhydrase, CAN). The role of CAN, which supplies carbonate ions necessary for carboxylases and acidifies the cytoplasm, was thus analyzed by staining cells with pH-responsive dyes; assaying for CO₂ levels in N-fixing propionate-fed cells (that require a propionate-CoA carboxylase to yield succinate-CoA), fumarate-fed cells and N-replete propionate-fed cells; conducting proteomics on N-fixing fumarate and propionate-fed cells and direct measurement of organic acids in nodules and in roots. The interiors of both in vitro and nodular vesicles were found to be at a lower pH than that of hyphae. CO₂ levels in N₂-fixing propionate-fed cultures were lower than in N-replete ones. Proteomics of propionate-fed cells showed carbamoyl-phosphate synthase (CPS) as the most overabundant enzyme relative to fumarate-fed cells. CPS combines carbonate and ammonium in the first step of the citrulline pathway, something which would help manage acidity and NH₄⁺. Nodules were found to have sizeable amounts of pyruvate and acetate in addition to TCA intermediates. This points to CAN reducing the vesicles' pH to prevent the escape of NH₃ and to control ammonium assimilation by GS and GOGAT, two enzymes that work in different ways in vesicles and hyphae. Genes with related functions (carboxylases, biotin operon and citrulline-aspartate ligase) appear to have undergone decay in non-symbiotic lineages.

Comprehensive profiling of aroma imparting biomolecules in foliar extract of Hibiscus fragrans Roxburgh: a metabologenesis perspective

Plants possess numerous secondary metabolites imparting flavor and aroma. However, fragrance inducing natural biomolecules and their potential sources are yet to be thoroughly explored. GC-MS analysis of a sweetly scented Malvacean liana; Hibiscus fragrans Roxburgh was conducted to explore and characterize the concerned aroma fingerprints with sound insights on anticipated array of biosynthetic pathways. Leaf extract of the plant was analyzed by Gas Chromatography-Mass Spectrometry (GC-MS) technique. Biosynthetic pathways of signature aroma compounds were deduced utilizing bioinformatic databases and reviewing literatures. A rare fragrant biomolecule '2-n-Heptylcyclopentanone' and 22 other aroma impacting biomolecules were detected and functional attributes were deliberately scrutinized. Interactive biosynthetic pathway schemes for all the 23 aromatic metabolomes including proposal for probable origin of 2-n-Heptylcyclopentanone and six other biomolecules (Pentadecanal; Cis-9-Hexadecenal; 14-Heptadecenal; Octadecanal; Undecane and 1-Decyne) with no previous biosynthesis report; out of a total of 47 GC-MS revealed metabolites were designed. Increased production of fragrant molecules in controlled surroundings availing biotechnological administration through metabolic bioengineering and in vitro tissue culture techniques may offer exciting dimensions to fragrance research.Communicated by Ramaswamy H. Sarma.

Alkaloids of the Genus Datura: Review of a Rich Resource for Natural Product Discovery

The genus Datura (Solanaceae) contains nine species of medicinal plants that have held both curative utility and cultural significance throughout history. This genus’ particular bioactivity results from the enormous diversity of alkaloids it contains, making it a valuable study organism for many disciplines. Although Datura contains mostly tropane alkaloids (such as hyoscyamine and scopolamine), indole, beta-carboline, and pyrrolidine alkaloids have also been identified. The tools available to explore specialized metabolism in plants have undergone remarkable advances over the past couple of decades and provide renewed opportunities for discoveries of new compounds and the genetic basis for their biosynthesis. This review provides a comprehensive overview of studies on the alkaloids of Datura that focuses on three questions: How do we find and identify alkaloids? Where do alkaloids come from? What factors affect their presence and abundance? We also address pitfalls and relevant questions applicable to natural products and metabolomics researchers. With both careful perspectives and new advances in instrumentation, the pace of alkaloid discovery—from not just Datura—has the potential to accelerate dramatically in the near future.

Transcriptomic reprogramming of the oceanic diatom Skeletonema dohrnii under warming ocean and acidification

Under ocean warming and acidification, diatoms use a unique acclimation and adaptation strategy by saving energy and utilizing it for other cellular processes. However, the molecular mechanisms that underlie this reprogramming of energy utilization are currently unknown. Here, we investigate the metabolic reprogramming of the ecologically important diatom Skeletonema dohrnii grown under two different temperature (21°C and 25°C) and pCO₂ (400 and 1000 ppm) levels, utilizing global transcriptomic analysis. We find that evolutionary changes in the baseline gene expression, which we termed transcriptional up- and downregulation, is the primary mechanism used by diatoms to acclimate to the combined conditions of ocean warming and acidification. This transcriptional regulation shows that under higher temperature and pCO₂ conditions, photosynthesis, electron transport and carboxylation were modified with increasing abundances of genes encoding ATP, NADPH and carbon gaining for the carbon-dioxide-concentrating mechanisms (CCMs). Our results also indicate that changes in the transcriptional regulation of CCMs led to a decrease in the metabolic cost to save energy by promoting amino acid synthesis and nitrogen assimilation for the active protein processing machinery to adapt to warming and ocean acidification. This study generated unique metabolic insights into diatoms and suggests that future climate change conditions will cause evolutionary changes in oceanic diatoms that will facilitate their acclimation strategy.

A streamlined and predominantly diploid genome in the tiny marine green alga Chloropicon primus

Tiny marine green algae issued from two deep branches of the Chlorophyta, the Mamiellophyceae and Chloropicophyceae, dominate different regions of the oceans and play key roles in planktonic communities. Considering that the Mamiellophyceae is the sole lineage of prasinophyte algae that has been intensively investigated, the extent to which these two algal groups differ in their metabolic capacities and cellular processes is currently unknown. To address this gap of knowledge, we investigate here the nuclear genome sequence of a member of the Chloropicophyceae, Chloropicon primus. Among the main biological insights that emerge from this 17.4 Mb genome, we find an unexpected diploid structure for most chromosomes and a propionate detoxification pathway in green algae. Our results support the notion that separate events of genome minimization, which entailed differential losses of genes/pathways, have occurred in the Chloropicophyceae and Mamiellophyceae, suggesting different strategies of adaptation to oceanic environments.

Metabolic Profiling of Water-Soluble Compounds from the Extracts of Dark Septate Endophytic Fungi (DSE) Isolated from Scots Pine (Pinus sylvestris L.) Seedlings Using UPLC-Orbitrap-MS

Endophytes are microorganisms living inside plant hosts and are known to be beneficial for the host plant vitality. In this study, we isolated three endophytic fungus species from the roots of Scots pine seedlings growing on Finnish drained peatland setting. The isolated fungi belonged to dark septate endophytes (DSE). The metabolic profiles of the hot water extracts of the fungi were investigated using Ultrahigh Performance Liquid Chromatography with Diode Array Detection and Electron Spray Ionization source Mass Spectrometry with Orbitrap analyzer (UPLC-DAD-ESI-MS-Orbitrap). Out of 318 metabolites, we were able to identify 220, of which a majority was amino acids and peptides. Additionally, opine amino acids, amino acid quinones, Amadori compounds, cholines, nucleobases, nucleosides, nucleotides, siderophores, sugars, sugar alcohols and disaccharides were found, as well as other previously reported metabolites from plants or endophytes. Some differences of the metabolic profiles, regarding the amount and identity of the found metabolites, were observed even though the fungi were isolated from the same host. Many of the discovered metabolites have been described possessing biological activities and properties, which may make a favorable contribution to the host plant nutrient availability or abiotic and biotic stress tolerance.

3-Hydroxyisobutyryl-CoA hydrolase involved in isoleucine catabolism regulates triacylglycerol accumulation in Phaeodactylum tricornutum

Since methylmalonyl-CoA epimerase appears to be absent in the majority of photosynthetic organisms, including diatoms, (S)-methylmalonyl-CoA, the intermediate of isoleucine (Ile) catabolism, cannot be metabolized to (R)-methylmalonyl-CoA then to succinyl-CoA. In this study, propionyl-CoA carboxylase (PCC) RNAi silenced strains and 3-hydroxyisobutyryl-CoA hydrolase (HIBCH) overexpression strains were constructed to elucidate the Ile degradation pathway and its influence on lipid accumulation in Phaeodactylum tricornutum based on growth, neutral lipid content and metabolite profile analysis. Knockdown of PCC disturbed the metabolism of Ile through propionyl-CoA to methylmalonyl-CoA, as illustrated by much higher Ile content at day 6. However, Ile decreased to comparable levels to the wild-type at day 10. PCC silencing redirected propionyl-CoA to acetyl-CoA via a modified β-oxidation pathway, and transcript levels for some branched-chain amino acid (BCAA) degradation-related genes, especially HIBCH, significantly upregulated in the PCC mutant, which enhanced the BCAA degradations and thus resulted in higher triacylglycerol (TAG) content. Overexpression of HIBCH accelerates Ile degradation and results in a lowered Ile content in the overexpression strains, thus enhancing carbon skeletons to the tricarboxylic acid cycle and giving rise to increasing TAG accumulation. Our study provides a good strategy to obtain high-lipid-yield transgenic diatoms by modifying the propionyl-CoA metabolism.This article is part of the themed issue 'The peculiar carbon metabolism in diatoms'.

Disruptions in valine degradation affect seed development and germination in Arabidopsis

We have functionally characterized the role of two putative mitochondrial enzymes in valine degradation using insertional mutants. Prior to this study, the relationship between branched-chain amino acid degradation (named for leucine, valine and isoleucine) and seed development was limited to leucine catabolism. Using a reverse genetics approach, we show that disruptions in the mitochondrial valine degradation pathway affect seed development and germination in Arabidopsis thaliana. A null mutant of 3-hydroxyisobutyryl-CoA hydrolase (CHY4, At4g31810) resulted in an embryo lethal phenotype, while a null mutant of methylmalonate semialdehyde dehydrogenase (MMSD, At2g14170) resulted in seeds with wrinkled coats, decreased storage reserves, elevated valine and leucine, and reduced germination rates. These data highlight the unique contributions CHY4 and MMSD make to the overall growth and viability of plants. It also increases our knowledge of the role branched-chain amino acid catabolism plays in seed development and amino acid homeostasis.

The Roles of β-Oxidation and Cofactor Homeostasis in Peroxisome Distribution and Function in Arabidopsis thaliana

Key steps of essential metabolic pathways are housed in plant peroxisomes. We conducted a microscopy-based screen for anomalous distribution of peroxisomally targeted fluorescence in Arabidopsis thaliana This screen uncovered 34 novel alleles in 15 genes affecting oil body mobilization, fatty acid β-oxidation, the glyoxylate cycle, peroxisome fission, and pexophagy. Partial loss-of-function of lipid-mobilization enzymes conferred peroxisomes clustered around retained oil bodies without other notable defects, suggesting that this microscopy-based approach was sensitive to minor perturbations, and that fatty acid β-oxidation rates in wild type are higher than required for normal growth. We recovered three mutants defective in PECTIN METHYLESTERASE31, revealing an unanticipated role in lipid mobilization for this cytosolic enzyme. Whereas mutations reducing fatty acid import had peroxisomes of wild-type size, mutations impairing fatty acid β-oxidation displayed enlarged peroxisomes, possibly caused by excess fatty acid β-oxidation intermediates in the peroxisome. Several fatty acid β-oxidation mutants also displayed defects in peroxisomal matrix protein import. Impairing fatty acid import reduced the large size of peroxisomes in a mutant defective in the PEROXISOMAL NAD⁺ TRANSPORTER (PXN), supporting the hypothesis that fatty acid accumulation causes pxn peroxisome enlargement. The diverse mutants isolated in this screen will aid future investigations of the roles of β-oxidation and peroxisomal cofactor homeostasis in plant development.

Amino Acid Catabolism in Plants

Amino acids have various prominent functions in plants. Besides their usage during protein biosynthesis, they also represent building blocks for several other biosynthesis pathways and play pivotal roles during signaling processes as well as in plant stress response. In general, pool sizes of the 20 amino acids differ strongly and change dynamically depending on the developmental and physiological state of the plant cell. Besides amino acid biosynthesis, which has already been investigated in great detail, the catabolism of amino acids is of central importance for adjusting their pool sizes but so far has drawn much less attention. The degradation of amino acids can also contribute substantially to the energy state of plant cells under certain physiological conditions, e.g. carbon starvation. In this review, we discuss the biological role of amino acid catabolism and summarize current knowledge on amino acid degradation pathways and their regulation in the context of plant cell physiology.

Proposed guidelines for the diagnosis and management of methylmalonic and propionic acidemia

Methylmalonic and propionic acidemia (MMA/PA) are inborn errors of metabolism characterized by accumulation of propionic acid and/or methylmalonic acid due to deficiency of methylmalonyl-CoA mutase (MUT) or propionyl-CoA carboxylase (PCC). MMA has an estimated incidence of ~ 1: 50,000 and PA of ~ 1:100’000 -150,000. Patients present either shortly after birth with acute deterioration, metabolic acidosis and hyperammonemia or later at any age with a more heterogeneous clinical picture, leading to early death or to severe neurological handicap in many survivors. Mental outcome tends to be worse in PA and late complications include chronic kidney disease almost exclusively in MMA and cardiomyopathy mainly in PA. Except for vitamin B₁₂ responsive forms of MMA the outcome remains poor despite the existence of apparently effective therapy with a low protein diet and carnitine. This may be related to under recognition and delayed diagnosis due to nonspecific clinical presentation and insufficient awareness of health care professionals because of disease rarity.

These guidelines aim to provide a trans-European consensus to guide practitioners, set standards of care and to help to raise awareness. To achieve these goals, the guidelines were developed using the SIGN methodology by having professionals on MMA/PA across twelve European countries and the U.S. gather all the existing evidence, score it according to the SIGN evidence level system and make a series of conclusive statements supported by an associated level of evidence. Although the degree of evidence rarely exceeds level C (evidence from non-analytical studies like case reports and series), the guideline should provide a firm and critical basis to guide practice on both acute and chronic presentations, and to address diagnosis, management, monitoring, outcomes, and psychosocial and ethical issues. Furthermore, these guidelines highlight gaps in knowledge that must be filled by future research. We consider that these guidelines will help to harmonize practice, set common standards and spread good practices, with a positive impact on the outcomes of MMA/PA patients.

Methylcrotonyl-CoA Carboxylase Regulates Triacylglycerol Accumulation in the Model Diatom Phaeodactylum tricornutum

The model marine diatom Phaeodactylum tricornutum can accumulate high levels of triacylglycerols (TAGs) under nitrogen depletion and has attracted increasing attention as a potential system for biofuel production. However, the molecular mechanisms involved in TAG accumulation in diatoms are largely unknown. Here, we employed a label-free quantitative proteomics approach to estimate differences in protein abundance before and after TAG accumulation. We identified a total of 1193 proteins, 258 of which were significantly altered during TAG accumulation. Data analysis revealed major changes in proteins involved in branched-chain amino acid (BCAA) catabolic processes, glycolysis, and lipid metabolic processes. Subsequent quantitative RT-PCR and protein gel blot analysis confirmed that four genes associated with BCAA degradation were significantly upregulated at both the mRNA and protein levels during TAG accumulation. The most significantly upregulated gene, encoding the β-subunit of methylcrotonyl-CoA carboxylase (MCC2), was selected for further functional studies. Inhibition of MCC2 expression by RNA interference disturbed the flux of carbon (mainly in the form of leucine) toward BCAA degradation, resulting in decreased TAG accumulation. MCC2 inhibition also gave rise to incomplete utilization of nitrogen, thus lowering biomass during the stationary growth phase. These findings help elucidate the molecular and metabolic mechanisms leading to increased lipid production in diatoms.

Candida albicans utilizes a modified β-oxidation pathway for the degradation of toxic propionyl-CoA

Propionyl-CoA arises as a metabolic intermediate from the degradation of propionate, odd-chain fatty acids, and some amino acids. Thus, pathways for catabolism of this intermediate have evolved in all kingdoms of life, preventing the accumulation of toxic propionyl-CoA concentrations. Previous studies have shown that fungi generally use the methyl citrate cycle for propionyl-CoA degradation. Here, we show that this is not the case for the pathogenic fungus Candida albicans despite its ability to use propionate and valerate as carbon sources. Comparative proteome analyses suggested the presence of a modified β-oxidation pathway with the key intermediate 3-hydroxypropionate. Gene deletion analyses confirmed that the enoyl-CoA hydratase/dehydrogenase Fox2p, the putative 3-hydroxypropionyl-CoA hydrolase Ehd3p, the 3-hydroxypropionate dehydrogenase Hpd1p, and the putative malonate semialdehyde dehydrogenase Ald6p essentially contribute to propionyl-CoA degradation and its conversion to acetyl-CoA. The function of Hpd1p was further supported by the detection of accumulating 3-hydroxypropionate in the hpd1 mutant on propionyl-CoA-generating nutrients. Substrate specificity of Hpd1p was determined from recombinant purified enzyme, which revealed a preference for 3-hydroxypropionate, although serine and 3-hydroxyisobutyrate could also serve as substrates. Finally, virulence studies in a murine sepsis model revealed attenuated virulence of the hpd1 mutant, which indicates generation of propionyl-CoA from host-provided nutrients during infection.

Asparagusic acid

Asparagusic acid (1,2-dithiolane-4-carboxylic acid) is a simple sulphur-containing 5-membered heterocyclic compound that appears unique to asparagus, though other dithiolane derivatives have been identified in non-food species. This molecule, apparently innocuous toxicologically to man, is the most probable culprit responsible for the curious excretion of odorous urine following asparagus ingestion. The presence of the two adjacent sulphur atoms leads to an enhanced chemical reactivity, endowing it with biological properties including the ability to substitute potentially for α-lipoic acid in α-keto-acid oxidation systems. This brief review collects the scattered data available in the literature concerning asparagusic acid and highlights its properties, intermediary metabolism and exploratory applications.

Different stress responsive strategies to drought and heat in two durum wheat cultivars with contrasting water use efficiency

Background

Durum wheat often faces water scarcity and high temperatures, two events that usually occur simultaneously in the fields. Here we report on the stress responsive strategy of two durum wheat cultivars, characterized by different water use efficiency, subjected to drought, heat and a combination of both stresses.

Results

The cv Ofanto (lower water use efficiency) activated a large set of well-known drought-related genes after drought treatment, while Cappelli (higher water use efficiency) showed the constitutive expression of several genes induced by drought in Ofanto and a modulation of a limited number of genes in response to stress. At molecular level the two cvs differed for the activation of molecular messengers, genes involved in the regulation of chromatin condensation, nuclear speckles and stomatal closure. Noteworthy, the heat response in Cappelli involved also the up-regulation of genes belonging to fatty acid β-oxidation pathway, glyoxylate cycle and senescence, suggesting an early activation of senescence in this cv. A gene of unknown function having the greatest expression difference between the two cultivars was selected and used for expression QTL analysis, the corresponding QTL was mapped on chromosome 6B.

Conclusion

Ofanto and Cappelli are characterized by two opposite stress-responsive strategies. In Ofanto the combination of drought and heat stress led to an increased number of modulated genes, exceeding the simple cumulative effects of the two single stresses, whereas in Cappelli the same treatment triggered a number of differentially expressed genes lower than those altered in response to heat stress alone. This work provides clear evidences that the genetic system based on Cappelli and Ofanto represents an ideal tool for the genetic dissection of the molecular response to drought and other abiotic stresses.

Electronic supplementary material

The online version of this article (doi:10.1186/1471-2164-14-821) contains supplementary material, which is available to authorized users.

Metabolite transporters of the plant peroxisomal membrane: known and unknown

Tremendous progress in plant peroxisome research has revealed unexpected metabolic functions for plant peroxisomes. Besides photorespiration and lipid metabolism, plant peroxisomes play a key role in many metabolic and signaling pathways, such as biosynthesis of phytohormones, pathogen defense, senescence-associated processes, biosynthesis of biotin and isoprenoids, and metabolism of urate, polyamines, sulfite, phylloquinone, volatile benzenoids, and branched chain amino acids. These peroxisomal pathways require an interplay with other cellular compartments, including plastids, mitochondria, and the cytosol. Consequently, a considerable number of substrates, intermediates, end products, and cofactors have to shuttle across peroxisome membranes. However, our knowledge of their membrane passage is still quite limited. This review describes the solute transport processes required to connect peroxisomes with other cell compartments. Furthermore, we discuss the known and yet-to-be-defined transport proteins that mediate these metabolic exchanges across the peroxisomal bilayer.

Peroxisomal Acyl-CoA oxidase 4 activity differs between Arabidopsis accessions

In plants, peroxisomes are the primary site of fatty acid β-oxidation. Following substrate activation, fatty acids are oxidized by Acyl-CoA Oxidase (ACX) enzymes. Arabidopsis has six ACX genes, although ACX6 is not expressed. Biochemical characterization has revealed that each ACX enzyme acts on specific chain-length targets, but in a partially overlapping manner, indicating a degree of functional redundancy. Genetic analysis of acx single and double mutants in the Columbia (Col-0) accession revealed only minor phenotypes, but an acx3acx4 double mutant from Wassileskija (Ws) is embryo lethal. In this study, we show that acx3acx4(Col) and acx1acx3acx4(Col) mutants are viable and that enzyme activity in these mutants is significantly reduced on a range of substrates compared to wild type. However, the triple mutant displays only minor defects in seed-storage mobilization, seedling development, and adult growth. Although the triple mutant is defective in the three most active and highly-expressed ACX proteins, increases in ACX2 expression may support partial β-oxidation activity. Comparison of acx mutant alleles in the Col-0 and Ws accessions reveals independent phenotypes; the Ws acx4 mutant uniquely shows increased sensitivity to propionate, whereas the Col-0 acx4 allele has sucrose-dependent growth in the light. To dissect the issues between Col-0 and Ws, we generated mixed background mutants. Although alleles with the Col-0 acx4 mutant were viable, we were unable to isolate an acx3acx4 line using the Ws acx4 allele. Reducing ACX4 expression in several Arabidopsis backgrounds showed a split response, suggesting that the ACX4 gene and/or protein functions differently in Arabidopsis accessions.

Peroxisomal polyhydroxyalkanoate biosynthesis is a promising strategy for bioplastic production in high biomass crops

Polyhydroxyalkanoates (PHAs) are bacterial carbon storage polymers with diverse plastic-like properties. PHA biosynthesis in transgenic plants is being developed as a way to reduce the cost and increase the sustainability of industrial PHA production. The homopolymer polyhydroxybutyrate (PHB) is the simplest form of these biodegradable polyesters. Plant peroxisomes contain the substrate molecules and necessary reducing power for PHB biosynthesis, but peroxisomal PHB production has not been explored in whole soil-grown transgenic plants to date. We generated transgenic sugarcane (Saccharum sp.) with the three-enzyme Ralstonia eutropha PHA biosynthetic pathway targeted to peroxisomes. We also introduced the pathway into Arabidopsis thaliana, as a model system for studying and manipulating peroxisomal PHB production. PHB, at levels up to 1.6%-1.8% dry weight, accumulated in sugarcane leaves and A. thaliana seedlings, respectively. In sugarcane, PHB accumulated throughout most leaf cell types in both peroxisomes and vacuoles. A small percentage of total polymer was also identified as the copolymer poly (3-hydroxybutyrate-co-3-hydroxyvalerate) in both plant species. No obvious deleterious effect was observed on plant growth because of peroxisomal PHA biosynthesis at these levels. This study highlights how using peroxisomal metabolism for PHA biosynthesis could significantly contribute to reaching commercial production levels of PHAs in crop plants.

Rhodobacter sphaeroides uses a reductive route via propionyl coenzyme A to assimilate 3-hydroxypropionate

3-Hydroxypropionate is a product or intermediate of the carbon metabolism of organisms from all three domains of life. However, little is known about how carbon derived from 3-hydroxypropionate is assimilated by organisms that can utilize this C(3) compound as a carbon source. This work uses the model bacterium Rhodobacter sphaeroides to begin to elucidate how 3-hydroxypropionate can be incorporated into cell constituents. To this end, a quantitative assay for 3-hydroxypropionate was developed by using recombinant propionyl coenzyme A (propionyl-CoA) synthase from Chloroflexus aurantiacus. Using this assay, we demonstrate that R. sphaeroides can utilize 3-hydroxypropionate as the sole carbon source and energy source. We establish that acetyl-CoA is not the exclusive entry point for 3-hydroxypropionate into the central carbon metabolism and that the reductive conversion of 3-hydroxypropionate to propionyl-CoA is a necessary route for the assimilation of this molecule by R. sphaeroides. Our conclusion is based on the following findings: (i) crotonyl-CoA carboxylase/reductase, a key enzyme of the ethylmalonyl-CoA pathway for acetyl-CoA assimilation, was not essential for growth with 3-hydroxypropionate, as demonstrated by mutant analyses and enzyme activity measurements; (ii) the reductive conversion of 3-hydroxypropionate or acrylate to propionyl-CoA was detected in cell extracts of R. sphaeroides grown with 3-hydroxypropionate, and both activities were upregulated compared to the activities of succinate-grown cells; and (iii) the inactivation of acuI, encoding a candidate acrylyl-CoA reductase, resulted in a 3-hydroxypropionate-negative growth phenotype.

Profiling hydroxycinnamoyl-coenzyme A thioesters: unlocking the back door of phenylpropanoid metabolism

In plants, 20 to 30% of photosynthetically fixed carbon is directed toward lignin and other phenylpropanoid compounds for which hydroxycinnamoyl-coenzyme A (CoA) esters are key intermediates. CoA thioesters, ubiquitous metabolites found in all living cells (often at trace levels), have traditionally been challenging to measure. Here we report a hydrophilic interaction liquid chromatography (HILIC) method, coupled with tandem mass spectrometry (MS/MS), that allows simultaneous sensitive quantification of previously undetectable hydroxycinnamoyl-CoA esters and an extended range of acyl-CoAs from plant tissues. This method provides rapid liquid chromatography (LC) analysis (10 min/sample) and the ability for qualitative assessment of acyl-CoAs by MS/MS precursor ion scanning.

Cloning and transcription analysis of the Candida rugosa propionyl-CoA dehydrogenase gene and its expression in Pichia pastoris

The propionyl-CoA dehydrogenase (PACD) gene was cloned from Candida rugosa by the cDNA RACE technique. The full cDNA of the PACD gene has a length of 1408 bp, which contains a complete open reading frame (ORF) of 1329 bp, coding for 442 amino acids. The cDNA of PACD was cloned into the expression plasmid pPIC9K and transformed into Pichia pastoris GS115. The recombinant protein was purified by Ni-NTA affinity chromatography, and its size was observed to be approximately 49 kDa as estimated by SDS-PAGE. Anti-His antibodies were used to characterise the recombinant PACD by western-blot analysis. The recombinant protein retained the activity of catalysing propionyl-CoA to acryloyl-CoA. The results of dot-blotting hybridisation using a PACD cDNA probe indicated that the PACD mRNA level was modified at different stages: mRNA levels were low for the first 36 h, then increased through 48 h and eventually reached a stable level. These results indicate that propionate induction could significantly activate PACD mRNA expression. Information from this study will be helpful in elucidating the metabolic pathway for 3-hydroxypropionic acid production in C. rugosa.

pex5 Mutants that differentially disrupt PTS1 and PTS2 peroxisomal matrix protein import in Arabidopsis

PEX5 and PEX7 are receptors required for the import of peroxisome-bound proteins containing one of two peroxisomal targeting signals (PTS1 or PTS2). To better understand the role of PEX5 in plant peroxisomal import, we characterized the Arabidopsis (Arabidopsis thaliana) pex5-10 mutant, which has a T-DNA insertion in exon 5 of the PEX5 gene. Sequencing results revealed that exon 5, along with the T-DNA, is removed in this mutant, resulting in a truncated pex5 protein. The pex5-10 mutant has germination defects and is completely dependent on exogenous Suc for early seedling establishment, based on poor utilization of seed-storage fatty acids. This mutant also has delayed development and reduced fertility, although adult pex5-10 plants appear normal. Peroxisomal metabolism of indole-3-butyric acid, propionate, and isobutyrate also is disrupted. The pex5-10 mutant has reduced import of both PTS1 and PTS2 proteins, and enzymatic processes that occur in peroxisomes are disrupted. To specifically study the import and importance of PTS1 proteins, we made a truncated PEX5 construct lacking the PTS1-binding region (PEX5(454)). Transformation of this construct into pex5-10 resulted in the rescue of PTS2 import, thereby creating a line with PTS1-specific import defects. The pex5-10 (PEX5(454)) plants still had developmental defects, although restoring PTS2 import resulted in a less severe mutant phenotype. Comparison of pex5-10 and pex5-10 (PEX5(454)) phenotypes can separate the import mechanisms for enzymes acting in different peroxisomal processes, including indole-3-butyric acid/2,4-dichlorophenoxybutyric acid oxidation, isobutyrate and propionate metabolism, and photorespiration.

Peroxisome biogenesis and function

Peroxisomes are small and single membrane-delimited organelles that execute numerous metabolic reactions and have pivotal roles in plant growth and development. In recent years, forward and reverse genetic studies along with biochemical and cell biological analyses in Arabidopsis have enabled researchers to identify many peroxisome proteins and elucidate their functions. This review focuses on the advances in our understanding of peroxisome biogenesis and metabolism, and further explores the contribution of large-scale analysis, such as in sillco predictions and proteomics, in augmenting our knowledge of peroxisome function In Arabidopsis.

Two D-2-hydroxy-acid dehydrogenases in Arabidopsis thaliana with catalytic capacities to participate in the last reactions of the methylglyoxal and beta-oxidation pathways

The Arabidopsis thaliana locus At5g06580 encodes an ortholog to Saccharomyces cerevisiae d-lactate dehydrogenase (AtD-LDH). The recombinant protein is a homodimer of 59-kDa subunits with one FAD per monomer. A substrate screen indicated that AtD-LDH catalyzes the oxidation of d- and l-lactate, d-2-hydroxybutyrate, glycerate, and glycolate using cytochrome c as an electron acceptor. AtD-LDH shows a clear preference for d-lactate, with a catalytic efficiency 200- and 2000-fold higher than that for l-lactate and glycolate, respectively, and a K(m) value for d-lactate of approximately 160 microm. Knock-out mutants showed impaired growth in the presence of d-lactate or methylglyoxal. Collectively, the data indicated that the protein is a d-LDH that participates in planta in the methylglyoxal pathway. Web-based bioinformatic tools revealed the existence of a paralogous protein encoded by locus At4g36400. The recombinant protein is a homodimer of 61-kDa subunits with one FAD per monomer. A substrate screening revealed highly specific d-2-hydroxyglutarate (d-2HG) conversion in the presence of an organic cofactor with a K(m) value of approximately 580 microm. Thus, the enzyme was characterized as a d-2HG dehydrogenase (AtD-2HGDH). Analysis of knock-out mutants demonstrated that AtD-2HGDH is responsible for the total d-2HGDH activity present in A. thaliana. Gene coexpression analysis indicated that AtD-2HGDH is in the same network as several genes involved in beta-oxidation and degradation of branched-chain amino acids and chlorophyll. It is proposed that AtD-2HGDH participates in the catabolism of d-2HG most probably during the mobilization of alternative substrates from proteolysis and/or lipid degradation.

Arabidopsis Chy1 null mutants are deficient in benzoic acid-containing glucosinolates in the seeds

The specific set of reactions that lead to the synthesis of benzoic acid in plants is still unclear, and even the subcellular compartment in which these reactions occur is unknown. Biosynthesis of both vegetative tissues and seeds of Arabidopsis thaliana contain a class of defense compounds termed glucosinolates, but only the seeds synthesize and store high levels of two glucosinolate compounds that contain a benzoic acid moiety. To identify genes involved in the synthesis of benzoic acid (directly or via benzaldehyde) in Arabidopsis, we analysed the levels of benzoylated glucosinolates in several lines that carry mutations in genes with homology to Pseudomonas fluorescens feruloyl-CoA hydratase, an enzyme that converts feruloyl-CoA to vanillin and acetyl-CoA, a reaction analogous to the conversion of cinnamoyl-CoA to benzaldehyde. We show here that mutations in the gene At5g65940, previously shown to encode a peroxisomal protein with beta-hydroxyisobutyryl-CoA hydrolase activity and designated as Chy1, lead to a deficiency of benzoic acid-containing glucosinolates in the seeds. Furthermore, Chy1 exhibits cinnamoyl-CoA hydrolase activity with a K(m) of 2.9 mum. Our findings suggest that at least a part of benzoic acid biosynthesis occurs in the peroxisomes, although the specific pathway that leads to benzoic acid and the specific biochemical role of Chy1 remain unclear.

Disruption of Arabidopsis CHY1 reveals an important role of metabolic status in plant cold stress signaling

To study cold signaling, we screened for Arabidopsis mutants with altered cold-induced transcription of a firefly luciferase reporter gene driven by the CBF3 promoter (CBF3-LUC). One mutant, chy1-10, displayed reduced cold-induction of CBF3-LUC luminescence. RNA gel blot analysis revealed that expression of endogenous CBFs also was reduced in the chy1 mutant. chy1-10 mutant plants are more sensitive to freezing treatment than wild-type after cold acclimation. Both the wild-type and chy1 mutant plants are sensitive to darkness-induced starvation at warm temperatures, although chy1 plants are slightly more sensitive. This dark-sensitivity is suppressed by cold temperature in the wild-type but not in chy1. Constitutive CBF3 expression partially rescues the sensitivity of chy1-10 plants to dark treatment in the cold. The chy1 mutant accumulates higher levels of reactive oxygen species, and application of hydrogen peroxide can reduce cold-induction of CBF3-LUC in wild-type. Map-based cloning of the gene defective in the mutant revealed a nonsense mutation in CHY1, which encodes a peroxisomal beta-hydroxyisobutyryl (HIBYL)-CoA hydrolase needed for valine catabolism and fatty acid beta-oxidation. Our results suggest a role for peroxisomal metabolism in cold stress signaling, and plant tolerance to cold stress and darkness-induced starvation.

Molecular characterization of organelle-type Nudix hydrolases in Arabidopsis

Nudix (for nucleoside diphosphates linked to some moiety X) hydrolases act to hydrolyze ribonucleoside and deoxyribonucleoside triphosphates, nucleotide sugars, coenzymes, or dinucleoside polyphosphates. Arabidopsis (Arabidopsis thaliana) contains 27 genes encoding Nudix hydrolase homologues (AtNUDX1 to -27) with a predicted distribution in the cytosol, mitochondria, and chloroplasts. Previously, cytosolic Nudix hydrolases (AtNUDX1 to -11 and -25) were characterized. Here, we conducted a characterization of organelle-type AtNUDX proteins (AtNUDX12 to -24, -26, and -27). AtNUDX14 showed pyrophosphohydrolase activity toward both ADP-ribose and ADP-glucose, although its K(m) value was approximately 100-fold lower for ADP-ribose (13.0+/-0.7 microm) than for ADP-glucose (1,235+/-65 microm). AtNUDX15 hydrolyzed not only reduced coenzyme A (118.7+/-3.4 microm) but also a wide range of its derivatives. AtNUDX19 showed pyrophosphohydrolase activity toward both NADH (335.3+/-5.4 microm) and NADPH (36.9+/-3.5 microm). AtNUDX23 had flavin adenine dinucleotide pyrophosphohydrolase activity (9.1+/-0.9 microm). Both AtNUDX26 and AtNUDX27 hydrolyzed diadenosine polyphosphates (n=4-5). A confocal microscopic analysis using a green fluorescent protein fusion protein showed that AtNUDX15 is distributed in mitochondria and AtNUDX14 -19, -23, -26, and -27 are distributed in chloroplasts. These AtNUDX mRNAs were detected ubiquitously in various Arabidopsis tissues. The T-DNA insertion mutants of AtNUDX13, -14, -15, -19, -20, -21, -25, -26, and -27 did not exhibit any phenotypical differences under normal growth conditions. These results suggest that Nudix hydrolases in Arabidopsis control a variety of metabolites and are pertinent to a wide range of physiological processes.

Identification and characterization of Arabidopsis indole-3-butyric acid response mutants defective in novel peroxisomal enzymes

Genetic evidence suggests that indole-3-butyric acid (IBA) is converted to the active auxin indole-3-acetic acid (IAA) by removal of two side-chain methylene units in a process similar to fatty acid beta-oxidation. Previous studies implicate peroxisomes as the site of IBA metabolism, although the enzymes that act in this process are still being identified. Here, we describe two IBA-response mutants, ibr1 and ibr10. Like the previously described ibr3 mutant, which disrupts a putative peroxisomal acyl-CoA oxidase/dehydrogenase, ibr1 and ibr10 display normal IAA responses and defective IBA responses. These defects include reduced root elongation inhibition, decreased lateral root initiation, and reduced IBA-responsive gene expression. However, peroxisomal energy-generating pathways necessary during early seedling development are unaffected in the mutants. Positional cloning of the genes responsible for the mutant defects reveals that IBR1 encodes a member of the short-chain dehydrogenase/reductase family and that IBR10 resembles enoyl-CoA hydratases/isomerases. Both enzymes contain C-terminal peroxisomal-targeting signals, consistent with IBA metabolism occurring in peroxisomes. We present a model in which IBR3, IBR10, and IBR1 may act sequentially in peroxisomal IBA beta-oxidation to IAA.

Structure and function of plant acyl-CoA oxidases

Acyl-CoA oxidases (in peroxisomes) and acyl-CoA dehydrogenases (in mitochondria) catalyse the first step in fatty acid beta-oxidation, the pathway responsible for lipid catabolism and plant hormone biosynthesis. The interplay and differences between peroxisomal and mitochondrial beta-oxidation processes are highlighted by the variation in the enzymes involved. Structure and sequence comparisons are made with a focus on the enzyme's mechanistic means to control electron transfer paths, reactivity towards molecular oxygen, and spatial and architectural requirements for substrate discrimination.