Plant Metabolism

Irradiated or not, plant metabolism materials are constantly changing, which can be accelerated by factors such as mechanical damage, depression temperature storage, or modified atmosphere.

From: Encyclopedia of Nutrient and Health , 2016

Establish metabolomics

Diane Chiliad. Beckles , Ute Roessner , in Found Biotechnology and Agriculture, 2012

Compartmentation of plant metabolism

Plant metabolism is highly compartmented. The presence of several organelles, each performing specific physiological and metabolic roles, shows developmental plasticity, which ways that their in situ metabolic pool sizes will vary inside a cell and exist dependent on the developmental stage. Knowing the amount of metabolites in the compartment helps to determine if its content is sufficient to participate in the metabolic procedure of that organelle or prison cell. This data is critical for accurate pathway engineering. It is non trivial to obtain quantities of relatively pure fractions of subcellular organelles adequate for analysis. Non-aqueous subcellular fractionation tin can be used to divide organelles and is technically easier with non-bulky tissues such as leaves (Winter et al., 1993; Benkeblia et al., 2009), but great progress has too been made with hitherto difficult organs such every bit tubers (Farre et al., 2001, 2008). Use of this technique in combination with GC-MS revealed that most organic and amino acids and sugars are sequestered in the tuber vacuoles, simply that their concentrations are highest in the cytosol (Farre et al., 2008). This information can be used to make up one's mind the potential activation land of enzymes in each compartment and help to develop transgenic approaches for modifying enzyme activity.

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Chemic Ecology in Insects

Nib Hansson , Dieter Wicher , in Chemosensory Transduction, 2016

Insect–Plant Interactions: Exploiting the System

Found metabolism produces volatile cues that are used to detect a nutrient source for herbivores 58 (Figure 6). In response to such challenges, plants take evolved chemical defense systems for protection confronting herbivory that are comprehensively reviewed elsewhere. 59 Hither, nosotros focus on mechanisms involving chemical signaling, which is either direct or indirect. A direct defense includes the release of toxins or substances that human action within the insect nervous system such equally nicotine, caffeine, cocaine or other neuroactive substances. Biogenic amines, for example, act as competitive Orco antagonists. 60 In example of an indirect defense, the emitted compounds attract parasitoids or predators of the feeding insects and tin considerably reduce the herbivore number. 61 In addition to a constitutive blazon of defense, herbivory set on leads to induced defense responses. This induction on demand helps to salvage energy required to synthesize compounds. Effigy 6 illustrates the insect–plant interaction following an herbivore attack. Volatiles emitted from plant parts on which an insect has fed tin can alarm other parts to prepare a response to a potential attack. Such signals can evolve to a communication with neighboring plants and may exist designed to attract predators or parasitoids of the attacking herbivore. Such functional shifts of chemical signals released past plants have been recently reviewed. 62

Figure 6. Reaction chain of plant–insect interaction. Volatile cues emitted past a plant (kairomones) concenter herbivorous insects. The attacked plant releases alarm hormones/pheromones to inform nonattacked parts of the institute and neighboring plants. Odors are besides released that concenter predators and parasitoids of the herbivores. The predators and parasitoids attack the herbivorous insects thereby counteracting the herbivore assail.

Chemosignals tin can alter their functional role. For case, an amanuensis that originally served a defensive function, such as a resin secreted by a wounded found, may turn into an attractant that rewards a pollinator. 63 To attract insects equally pollinators, plants often release odors with a flowery note. Although allure is predominantly mediated via olfaction, other sensory modalities are also involved. For example, colorful flowers tin can deed as visual landmarks indicating nectar advantage. In improver to reward-connected attraction, some institute groups have evolved to attract pollinators by charade. 64 They do so, by promising a reward without providing information technology. To attract drosophilid flies an arum species, Arum palestinum, releases fruity-yeasty odors, 27 thereby activating ORs specifically tuned to attractive food odors. Some arum species such as the expressionless horse arum (Helicodiceros muscivorus) or the titan arum (Amorphophallus titanum) (Figure 5(B)) emit the smell of rotting meat, thereby mimicking an oviposition site for flesh feeding flies. 65,66 Other odors attract insects such equally flies and dung beetles past producing fecal or urine odors. It is also possible that these odors evolved to repel herbivores and that the allure of pollinators reflects a functional shift of the chemosignal. 67 Taken together, the deceptive plants emit odors as allomones (Effigy ii) to induce a beliefs in the attracted insects that is benign for the plant. To be convincing, however, it is sometimes necessary to complement chemosignaling with other sensory cues. 64 This is particularly the instance for sexually deceptive orchids that attract male pollinators past emitting female-specific sex pheromones. The visual bespeak from flowers that mimic insects (Figure five(C)) enhances the reproductive success. 68–lxx

Normally, floral scents released to attract pollinators act as synomones (Effigy ii), where the pollinator is rewarded (e.g., by nectar). An case for such an interaction is the attraction of the hawkmoth Manduca sexta by the odor plume from flowers of the jimsonweed Datura wrightii. The claiming for the moths is to discriminate the D. wrightii odor plume from the mélange of other odors. 71 Datura wrightii often coexists with other plants that release volatiles such as benzaldehyde, which also occurs in the D. wrightii bouquet. In current of air tunnel experiments, it was shown that the ability of moths to correctly navigate to the source of the D. wrightii mixture was decreased in the presence of a benzaldehyde background. 71 Nevertheless, the moth could cope with this situation by changing the processing of the olfactory data within the antennal lobe. Disturbing this flexibility in neuronal representation by inhibiting local interneuron signaling in the antennal lobe disrupted successful odour navigation. 71 Whereas M. sexta has an innate preference for sure night-blooming plants including D. wrightii, 72 the moths can learn to use other flowers for nectar feeding. These learning processes, including octopaminergic signaling, alter the neuronal representation of the previously unattractive scent bouquets but they exercise not override the representation of the innately preferred odor plumes. 72

In addition to nectar feeding, D. wrightii serves M. sexta also for oviposition. 73 Interestingly, there is a divergence in the floral scents inducing both behaviors. The D. wrightii scent boutonniere contains linalool, which occurs as the (+) and/or (−) enantiomer. Although feeding is contained of the enantiomeric class of linalool, oviposition is preferred for plants emitting (+) linalool. 73

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Agronomical and Related Biotechnologies

R.R. Pathak , ... N. Raghuram , in Comprehensive Biotechnology (2d Edition), 2011

4.xvi.four.v Manipulating Genes of C Metabolism

Sugars play an important role in plant growth and metabolism by providing carbon skeletons and energy for cellular metabolism. All the same, saccharide metabolism and signaling influences a number of processes involved in plant growth and development, such as seed germination, embryogenesis, flowering, and senescence, and accept also been implicated in hormone signaling. The genes involved in N metabolism and nitrate signaling are also tightly regulated past carbohydrate signaling mechanisms. A coordination between N and C metabolism is required at the amino acid synthesis level due to the requirement of carbon skeletons for their synthesis. SnRK1, a principal regulator in carbon signaling, is known to be linked to N and amino acid metabolism. Similarly to CDPK, GCN2 directly act on NR in plants. A mutant lacking GCN2 showed decreased expression of nitrate reducatse (nia1) gene in Arabidopsis. However, whether this had any direct implication on improvement of NUE remains to be validated. Recently, transgenic Arabidopsis plants overexpressing STP13, a member of sugar transporter family, showed increased rates of glucose uptake, higher internal sugar levels, and more total C per plant. STP13OX seedlings as well displayed improved Northward utilize, with the induction of a nitrate transporter and college total N per establish ( Effigy 3 ).

Figure iii. Schematic representation of key processes and enzymes involved in nitrogen metabolism in plants. Nitrate and ammonium ions are taken by transporters across the cell membrane, assimilated and incorporated into C metabolites to generate amino acids. The amino acids from degraded proteins in senescing tissues are remobilized into the North pool of the jail cell. All these processes are controlled by signaling molecules and transcription factors. NR, nitrate reductase; NiR, nitrite reductase; GS, glutamine synthetase; GOGAT, glutamate synthase; NO iii − , nitrate ion; NO ii − , nitrite ion; NH iv + , ammonium ion; Gln, glutamine; Glu, glutamate; AA, amino acids; αOG, two-oxoglutarate (α-ketoglutarate dehydrogenase); ICDH, isocitrate dehydrogenase; CS, citrate synthase; PK: pyruvate kinase; AGPase, ADP glucose phosphorylase; PPC, phosphoenol pyruvate carboxylase; SPS, sucrose phosphate synthase; PEP, phosphoenolpyruvate; OA, oxaloacetate; Air-conditioning CoA, acetyl coenzyme A.

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Polyketides and Other Secondary Metabolites Including Fatty Acids and Their Derivatives

Richard A. Dixon , in Comprehensive Natural Products Chemistry, 1999

1.28.four.1 Metabolism by Plant Tissues

Isoflavonoids may not be cease products of found metabolism. In addition to demonstrating their mobilization from vacuolar stores and subsequent metabolism (often to more highly modified isoflavonoid derivatives, see above), some studies have documented metabolism of endogenously applied isoflavonoids past plant tissue. However, the presence of contaminating microorganisms can seriously compromise the interpretation of such experiments. For example, studies with chickpea and mungbean seedlings indicated one-half lives for exogenously added daidzein ( four), formononetin (5) or coumestrol (nine) of ∼50 h. All the same, repeating these experiments with sterile mung bean seedlings revealed little appreciable metabolism of (5) (95% recovery later 24 h), although [xivC]-(4) was rapidly metabolized (8.5% recovery) with label incorporated into virtually cellular/chemical fractions, including the prison cell wall. 257

The interconversions of medicarpin (6) and its corresponding isoflavan vestitol (8) in alfalfa and red clover 102,103 have been described above. Ring opening of a pterocarpan to yield the respective isoflavan (110) has also been reported when phaseollin (17) is fed to bean cell interruption cultures, 258 and this is accompanied past the opening of the band formed from the cyclized prenyl side chain. Compound (17) is as well converted to (110) by the fungal pathogen Septoria nodorum. 259

The role of isoflavonoid degradation every bit a factor in the elicitor- and pathogen-induced aggregating of isoflavonoid phytoalexins received considerable attention when information technology was proposed that elicitation by abiotic elicitors or incompatible races of pathogens was associated with strongly inhibited phytoalexin degradation (assessed using exogenously practical radiolabeled phytoalexin), whereas an increased biosynthetic charge per unit was the major factor determining phytoalexin levels in response to biotic elicitors. 260,261 These conclusions were challenged when information technology was demonstrated, using fourteenCO2 labeling in vivo, that the half-lives of glyceollin (18) and its trihydroxypterocarpan forerunner (87) were long, ∼100 h and ∼38 h, respectively. 262 Apparently, the metabolic fates of exogenously applied and endogenously synthesized glyceollin are different. Studies of isoflavonoid turnover take subsequently been eclipsed past the vast body of work on the induced biosynthesis of these compounds, and more studies are needed to determine the biological one-half-lives and metabolic fates in planta of biologically active isoflavonoids.

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Development & Modification of Bioactivity

Arno Hazekamp , ... Renee 50. Ruhaak , in Comprehensive Natural Products II, 2010

iii.24.2.2.2 Occurrence of short-chain cannabinoids and other homologues

Most commonly, the acidic cannabinoids produced by plant metabolism contain a pentyl side concatenation, derived from the OA moiety. Cannabinoids with propyl side chains result if GPP condenses with divarinic acrid instead of OA, into cannabigerovarinic acid (CBGVA). The three known cannabinoid synthase enzymes are not selective for the length of the alkyl side chain, and will catechumen CBGVA into the propyl homologues of CBDA, THCA, and CBCA. 61 All concatenation lengths from –methyl to –pentyl take been found in naturally occurring cannabinoids, probably all arising from the incorporation of shorter chain homologues of OA. The side concatenation is important for the analogousness, selectivity, and pharmacological dominance for the cannabinoids receptors.

Many other pocket-size acidic cannabinoids take been identified over the years, including monomethyl and other types of esters. 38,62 The biosynthetic pathways explaining this variation have been studied. 59

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Arabidopsis thaliana

D. Szymanski , in Brenner'south Encyclopedia of Genetics (Second Edition), 2013

The Classical Genetic Map

Early on genetic screens identified loci that were of import for plant growth or metabolism, simply did non reveal the identity of the gene that controls the phenotype. The development of a genetic map of Arabidopsis was the outset quantum in bridging the gap between mutant phenotype and cistron identity. Using a large collection of Arabidopsis mutants with visually scorable phenotypes, Marten Koorneef and William Feenstra published a comprehensive linkage map that divided the genome into 500 map units on five chromosomes. The classical map was used by geneticists to locate the chromosomal positions of new mutations. In some cases, these mapping experiments defined a pocket-sized interval inside a chromosome that contained the mutation of interest. Although the classical genetic map was useful, it did non allow researchers to place the individual genes that were affected in mutant plants. To identify the affected gene, it was necessary to construct a map based on the DNA sequence of the genome.

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Carbohydrates and Their Derivatives Including Tannins, Cellulose, and Related Lignings

Darla P. Henderson , Eric J. Toone , in Comprehensive Natural Products Chemical science, 1999

three.13.vi.2.4 SHMT from plants

As noted above, SHMT plays two important roles in plant metabolism—in one-carbon metabolism, and in photorespiration. A SHMT was isolated from maize seedlings by Masuda et al. 300 in 1980. The enzyme is trimeric with a subunit molecular weight of twoscore kDa. As with other enzymes, reduced thiol was required for total activity. The poly peptide cleaves serine in the presence of tetrahydrofolate and 50-allothreonine, merely not l-threonine.

Neuburger and co-workers 249 reported mitochondrial and chloroplastic isoforms of a SHMT from spinach leafage. The proteins be as homotetramers with subunit molecular weights near l kDa and show Michaelis constants for serine of roughly 1 mM; no additional specificity data were reported. A like protein has been isolated from pea. 301

The SHMT from Solanum tuberosum has been cloned, although information on the sequence were not reported. 302 The protein shows loftier sequence homology to other aldolases.

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Agricultural and Related Biotechnologies

D. Johnston-Monje , M.N. Raizada , in Comprehensive Biotechnology (Second Edition), 2011

4.58.five Other Endophytic Mechanisms Affecting Plant Nutrient Status

Some endophytes appear to possess the ability to manipulate host plant metabolism to increase nutrient uptake and change nutrient homeostasis. When the AM fungus G. intraradices colonizes maize roots, one host response is to downregulate its own nitrate reductase, allowing the more efficient fungus to reduce all of the Due north assimilate [105]. On the other paw, infection of tomato roots with G. intraradices Sy167 upregulates expression of the high-affinity nitrate transporter LeNRT2.three in roots, stimulating greater found uptake of nitrate [106]. Broomsedge (Andropogon virginicus L.) grass infected by two AM fungi and growing in depression-P sand has college phosphorus-use efficiency at low Pi concentrations and maintains constant levels of tissue P non only past enhancing P uptake, but as well by altering plant patterns of P allocation and utilise [107]. How these mycorrhizae induce these changes in constitute-nutrient homeostasis is not known. The fungal endophyte P. indica can colonize the interior of a number of different constitute species roots, where it promotes plant growth. In Arabidopsis and tobacco roots, P. indica stimulates N uptake/nitrate reduction/accumulation past activating a host transcription factor which upregulates P. indica-responsive nitrate reductase and the starch-degrading enzyme glucan-water dikinase (SEX1) [108]. Alpine fescue (Festuca arundinacea) grown with the fungal endophyte Northward. coenophialum is able to take up greater amounts of soil nitrate and accumulate more than amino acids in its shoot, peculiarly arginine in leaves [109]. Faced with contest for N between the plant and its endophyte, it appears that the found increases its own sink need for Due north by upregulating the shoot glutamine synthetase activity (by 32%). Similarly, tall fescue (F. arundinacea Schreb.) plants infected with the fungal endophyte North. coenophialum (Morgan-Jones and Gams) experience elevated plant growth and increased tissue P content, suggesting that N. coenophialum is an additional P sink and stimulates the plant to increment its P uptake [110]. In poplar infected with a nitrogen-fixing endophytic Paenibacillus strain, the found's metabolic signature was altered, increasing asparagine and urea levels while reducing TCA sugars and organic acids [111]. Opposite to the above strategies, reduced levels of nitrate and amino acids in plant tissues take been reported in Lolium perenne infected with N. lolii [112]; information technology is hypothesized that these nutrient reductions make the constitute less attractive for herbivores.

Some of the signals used by endophytes to affect their hosts are starting to be discovered: Epichloë festucae releases reactive oxygen species (ROS) to communicate with its grass host, Fifty. perenne; when ROS levels are altered, the relationship switches from mutualistic to antagonistic, resulting in sickness and death of the plant [113]. A different elicitor was found in culture filtrates of growth-promoting microbe B. thuringiensis NEB17, which contain a novel bacteriocin protein called thurigen that enhances both soybean and corn biomass [114]. P. fluorescens B16 is a growth-promoting rhizobacteria that produces pyrroloquinoline quinine nether low-nutrient conditions; bacterial mutant studies using a cucumber-seedling bioassay showed this compound to be responsible for the observed institute growth promotion [115]. Information technology has long been known that Nod factors secreted by rhizobia are important in nodule germination, but these compounds are also able to affect other changes in the institute such equally increased uptake of calcium in soybean roots through unknown mechanisms [116]. Treatment of seeds of Z. mays (corn), O. sativa (rice), Beta vulgaris (sugarbeet), Thousand. max (soybean), P. vulgaris (bean), and Gossypium hirsutum (cotton), with Nod cistron BjV of B. japonicum 532C, resulted in enhanced seed germination and early growth nether lab and field conditions that can allow the developing seedlings optimal access to nutrients in the rhizosphere [117]. Transgenic rice overexpressing an early on nodulin gene ortholog, OsENOD93-1, had college shoot dry biomass, seed yield, full amino acids, and total N in roots [118]. Although the function of this gene is unknown, given its homology to legume genes involved in nodule formation, it is interesting to speculate that nonlegume plants may have an aboriginal bacterial-dependent plant growth-promotion pathway.

Modification of the soil via exudates is an important way that roots may increase the availability of nutrients. Plants have been shown to secrete up to 40% of their stock-still carbon through their root systems as amino acids, organic acids, sugars, phenolics, mucilage, proteins, and an array of additional secondary metabolites that may assist in optimizing their rhizospheres chemically and microbially [119]. An example of altered exudates affecting constitute diet was seen by infecting tall fescue (F. arundinacea Shreb.) with North. coenophialum (Morgan-Jones and Gams) mucus, which was observed to stimulate uptake and ship of greater P, Ca, Zn, and Cu in roots grown in low-P nutrient solution [28]. This event was specific to the establish DN2 genotype and appeared to be related to root growth reprogramming and an contradistinct pattern of acid exudation by roots. Northward. coenophialum infection has been shown to increase fescue release of organic carbon from its roots, resulting in higher microbial activity and respiration stimulated past changes in the rhizodeposits [120]. P-deprived tall fescue infected by this endophyte can too increment root exudation of phenolics past seven%, which results in a 375% increase in the rate of soil Atomic number 263+ reduction, a necessary step in iron uptake (Fe2+) by plants [110]. Under specific conditions, soil C and North pools can as well be increased by endophyte infection of tall fescue, acquired by either a reduction in soil microbial respiration [121] and/or a reduction in specific species of carbon-consuming rhizobacteria [122]. This may exist acquired by altered patterns of root exudation, or information technology may be caused by a buildup of endophyte-derived alkaloids in the soil [123]. In add-on to Neotyphodium, AM fungi have also been shown to change constitute exudates into the soil, including reducing the levels of full sugars exuded from roots, altering the proportions of exuded amino acids, reducing K+ and P leakage, and increasing the release of nitrogen, phenolics, and GAs [124].

In dissimilarity to Neotyphodium endophytes, which modify soils from within their constitute hosts, AM-like fungi are able to grow out from roots and able to modify soil directly. Equally mentioned previously, at least one species (http://www.nature.com/nature/journal/v413/n6853/full/413297a0.html) of AM seems to exist able to enhance the degradation of organic N [16,125] but how it does this is not known. Some AMs are able to affect the behavior of other soil microbes: G. mosseae inoculation on diverse plant species resulted in metabolic stimulation of bacterial rhizosphere populations including various groups of uncultured bacteria and Paenibacillus species, probable through altered exudates patterns into soil [126]. AM fungi themselves directly release big amounts of glycoprotein called glomalin into the soil, which may serve to aggregate soil particles, increment water retentiveness, chelate iron, or serve as an energy source for soil microbes [124]. A different written report on the issue of mycelial exudates from the AM fungus Glomus spp. MUCL 43205 showed that it induced increases in soil populations of several Gammaproteobacteria, including a group of Enterobacteriaceae, although what functional changes resulted in the rhizosphere are not articulate [127]. Likewise, the nearly ubiquitous root-colonizing fungi known every bit DSEs accept been shown to produce hyphae that leave the found root and blot organically jump mineral nutrients. These fungi have been shown to secrete cellulases, laccases, amylases, lipases, pectinases, xylanases, proteolytic enzymes, tyrosinases, and polyphenol oxidases, but it is not notwithstanding known whether these enzymes are secreted into the soil to aid in nutrient absorption [22].

As a final mechanism, some soil rhizosphere leaner and fungi that tin exist equally endophytes inside roots are able to mineralize organic or insoluble forms of Due north and P. As it has rarely been shown that endophytes exit the root to direct touch on the soil, information technology is non articulate how such credible phosphate solubilization or organic compound deposition could occur. For example, in a study of soybean endophytes, it was found that 49% were able to solubilize mineral phosphate, as compared to 52% of the foliage epiphytic bacteria, although information technology was not shown whether the endophytes traveled to found surfaces or soil where phosphate solubilization would be important [128]. In lettuce and maize, seed inoculation with phosphate-solubilizing strains of R. leguminosarum bv. phaseoli was shown to increase their P content past 6% and 8%, respectively, under field conditions, although information technology was not demonstrated that these unremarkably nodule-inhabiting leaner establish an endophytic lifestyle within these plants [129]. At that place are a few examples where validated endophytes take been shown to go out the institute to better nutrient bioavailability in the soil. For example, orchid seeds, which are small and nutrient poor for embryo development, possess endophytic Rhizoctonia fungi that abound out of the seed and enzymatically degrade the surrounding substrate to provide the embryo with nutrients for growth [130]. Similarly, the cardon cactus (Pachycereus pringlei) can abound on bare rock in Northern United mexican states with help from its seed-transmitted endophytes (mostly Bacillus spp., Klebsiella spp., Staphylococcus spp.), which appear to exit the seed to colonize and weather rock, liberating vital minerals for the developing bulb [95]. The cardon cactus-associated leaner can either solubilize inorganic phosphates by releasing organic acids, such as gluconic acid and 2-ketogluconic acid, or mineralize organic phosphates by secreting extracellular phosphatases [131].

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Integrative Plant Biochemistry

Björn Hamberger , ... Carl J. Douglas , in Recent Advances in Phytochemistry, 2006

Summary and Futurity Directions

We reconstructed the whole shikimate biosynthetic pathway, the central pathway of plant metabolism controlling metabolic flux from carbohydrate metabolism into the phenylpropanoid pathway, on the genomic level. Given that we focused on fully sequenced genomes, all families of the three species rice, Arabidopsis, and poplar should be represented in their total extent. We also included expression information for Arabidopsis and poplar that support functional classifications, providing an unprecedented view of the structure and expression of the gene families that encode shikimate pathway enzymes. This comparative data too immune u.s.a. to build on previous observations to highlight promising candidate genes that encode yet unknown enzymes for missing links in shikimate pathway required for phenylalanine biosynthesis (PNT and ADT candidates). In the future, it volition be necessary to functionally test PNT and ADT candidate genes for their hypothesized biochemical functions, and to characterize the enzymes. This may be achieved by heterologous expression of the candidate recombinant enzymes in East. coli or yeast expression systems. Yeast is a particularly attractive system to study the enzymes specific to phenylalanine biosynthesis, since yeast mutants dumb in this ability are bachelor. Thus, information technology may be possible to reconstruct the late steps of phenylalanine biosynthesis in yeast, using candidate Arabidopsis or poplar genes, resulting in the rescue of phenylalanine auxotrophy in impaired strains. Once identified, these novel plant enzymes required for phenylalanine biosynthesis could be of value in ingather improvement, for instance as targets for new herbicide evolution analogous to glyphosate, which targets the shikimate enzyme EPSPS synthase.

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Environmental Biotechnology and Safety

B. Van Aken , in Comprehensive Biotechnology (Second Edition), 2011

half dozen.20.2.3 Enzymes Involved in Phytoremediation of Organic Pollutants

In comparison to mammals and bacteria, piddling information is available about the plant metabolism of organic compounds. Very few establish enzymes involved in xenobiotic transformation have been isolated, purified, or characterized. Found enzymes potentially involved in the metabolism of organic pollutants tin can be classified co-ordinate to the green liver model (see Section half-dozen.20.2.2).

Enzymes involved in phase I of the green liver model (activation phase) are oxidative enzymes, such as cytochrome P-450 monooxygenases, peroxidases, and laccases. Oxidized metabolites produced by the add-on of -OH groups have been detected in constitute tissues exposed to chlorinated solvents, PCBs, and pesticides, suggesting the intervention of plant cytochrome P-450 monooxygenases [2]. In mammals, cytochrome P-450 monooxygenases are known to play a key office in the detoxification of a range of toxic xenobiotic compounds through hydroxylation and oxidative dealkylation [2]. Other enzymes have been hypothesized to catalyze oxidative transformations, including peroxidases and laccases. For example, peroxidases, remazol brilliant blue R (RBBR) oxidases, and cytochrome P-450 monooxygenases take been suggested to be implicated in PCB metabolism in plants [eighteen]. On the other mitt, constitute metabolism of nitro-substituted explosives involves the reduction of nitro groups catalyzed by type I or blazon II nitroreductases. Even though no nitroreductase involved in explosive degradation has been identified in plants, many institute enzymes accept type I or II nitroreductase action, including xanthine dehydrogenase and cytochrome P-450 reductase. Plant-mediated reductive dehalogenation of chlorinated compounds has also been reported, suggesting the intervention of hypothetical plant dehalogenases [25].

Phase II of the green liver model involves transferase-mediated conjugation of nucleophilic pollutants or their stage I metabolites with carbohydrates, amino acids, or peptides [2]. Primal enzymes commonly involved in stage II metabolism are glutathione S-transferases and glycosyltransferases catalyzing conjugation of xenobiotics with reduced glutathione and sugar molecules, respectively. Although glutathionyl conjugates have seldom been identified in establish tissues, evidence has been obtained of glycosyltransferase-mediated conjugations of diverse xenobiotic compounds, including chlorinated solvents, explosives, and Dichloro-diphenyl-trichloroethane metabolites. Glycosylation of trichloroethanol (TCEOH), a metabolite of TCE oxidation, was observed in TCE-exposed tobacco and poplar tissues [22]. Similarly, conjugation of TNT metabolites with half-dozen-carbon molecules was reported in establish hairy root cultures, suggesting a glycosylation reaction [25].

Phase III of the green liver model involves compartmentalization or sequestration of conjugates within plant tissues or structures. Although evidence of sequestration of unlike xenobiotic compounds or metabolites, including chlorinated compounds and explosives, inside plant tissues was obtained from the detection of nonextractable 14C-labeled fractions, the nature of the enzymes potentially involved in the process was not determined. One can hypothesize that enzymes involved in cell wall synthesis, such as laccases, are implicated in stage Three xenobiotic sequestration. On the other hand, storage of polar glutathionyl or glycosyl conjugates in the vacuole likely requires intervention of ATP-dependent membrane transporter proteins to cross the vacuole membrane [5, 14, 25].

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