Overexpression of an isoprenyl diphosphate synthase in spruce leads to unexpected terpene diversion products that function in plant defense

Usefulness of the 1H NMR Multisuppression Approach for the Global Characterization of Monovarietal Extra-Virgin Olive Oils

Extra-virgin olive oil (EVOO) is one of the most appreciated vegetable oils worldwide, but its high price makes it prone to suffer adulteration with lower quality oils. Therefore, it is important to have methodologies able to study EVOO composition as a whole in a simple and fast way, in order to guarantee its quality and safety. For this purpose, in this study, commercial samples of five Spanish olive cultivars (Arbequina, Arroniz, Cornicabra, Hojiblanca, Picual) were studied by Proton Nuclear Magnetic Resonance (1H NMR) spectroscopy, using standard and multisuppression pulses. The aim was to explore the possibility of 1H NMR use to characterize in a single run and in a global way the composition of these monocultivar oils, regarding not only their main components (fatty acids supported on triglycerides) but also minor ones (squalene, sterols, diterpenic wax esters of phytol and geranylgeraniol, phenolic and secoiridoid derivatives, like tyrosol, hydroxytyrosol, oleacein, oleocanthal, and lignans, among others, and aldehydes). The use of univariate and multivariate statistical analyses confirmed the presence of compositional features that were specific to some olive varieties. The Arbequina and Arroniz oils showed the most characteristic features that allowed for clearly differentiating them from the others. In contrast, the discrimination between the Cornicabra, Hojiblanca and Picual oils was not so easily achieved.

Tocopherol and phylloquinone biosynthesis in chloroplasts requires the phytol kinase VITAMIN E PATHWAY GENE5 (VTE5) and the farnesol kinase (FOLK)

Chlorophyll degradation causes the release of phytol, which is converted into phytyl diphosphate (phytyl-PP) by phytol kinase (VITAMIN E PATHWAY GENE5 [VTE5]) and phytyl phosphate (phytyl-P) kinase (VTE6). The kinase pathway is important for tocopherol synthesis, as the Arabidopsis (Arabidopsis thaliana) vte5 mutant contains reduced levels of tocopherol. Arabidopsis harbors one paralog of VTE5, farnesol kinase (FOLK) involved in farnesol phosphorylation. Here, we demonstrate that VTE5 and FOLK harbor kinase activities for phytol, geranylgeraniol, and farnesol with different specificities. While the tocopherol content of the folk mutant is unchanged, vte5-2 folk plants completely lack tocopherol. Tocopherol deficiency in vte5-2 plants can be complemented by overexpression of FOLK, indicating that FOLK is an authentic gene of tocopherol synthesis. The vte5-2 folk plants contain only ∼40% of wild-type amounts of phylloquinone, demonstrating that VTE5 and FOLK both contribute in part to phylloquinone synthesis. Tocotrienol and menaquinone-4 were produced in vte5-2 folk plants after supplementation with homogentisate or 1,4-dihydroxy-2-naphthoic acid, respectively, indicating that their synthesis is independent of the VTE5/FOLK pathway. These results show that phytyl moieties for tocopherol synthesis are completely but, for phylloquinone production, only partially derived from geranylgeranyl-chlorophyll and phytol phosphorylation by VTE5 and FOLK.

Full-Length Transcriptome Sequencing and RNA-Seq Analysis Offer Insights into Terpenoid Biosynthesis in Blumea balsamifera (L.) DC

Blumea balsamifera (L.) DC., an important economic and medicinal herb, has a long history of being used as a traditional Chinese medicine. Its leaves have always been used as a raw material for the extraction of essential oils, comprising large amounts of terpenoids, which have good therapeutic effects on many diseases, such as eczema, bacterial infection, and hypertension. However, the genetic basis of terpenoid biosynthesis in this plant is virtually unknown on account of the lack of genomic data. Here, a combination of next-generation sequencing (NGS) and full-length transcriptome sequencing was applied to identify genes involved in terpenoid biosynthesis at five developmental stages. Then, the main components of essential oils in B. balsamifera were identified using GC-MS. Overall, 16 monoterpenoids and 20 sesquiterpenoids were identified and 333,860 CCS reads were generated, yielding 65,045 non-redundant transcripts. Among these highly accurate transcripts, 59,958 (92.18%) transcripts were successfully annotated using NR, eggNOG, Swissprot, KEGG, KOG, COG, Pfam, and GO databases. Finally, a total of 56 differently expressed genes (DEGs) involved in terpenoid biosynthesis were identified, including 38 terpenoid backbone genes and 18 TPSs, which provide a significant amount of genetic information for B. balsamifera. These results build a basis for resource protection, molecular breeding, and the metabolic engineering of this plant.

HDR, the last enzyme in the MEP pathway, differently regulates isoprenoid biosynthesis in two woody plants

Dimethylallyl diphosphate (DMADP) and isopentenyl diphosphate (IDP) serves as the universal C5 precursors of isoprenoid biosynthesis in plants. These compounds are formed by the last step of the 2-C-methyl-D-erythritol 4-phosphate (MEP) pathway, catalyzed by (E)-4-hydroxy-3-methylbut-2-en-1-yl diphosphate reductase (HDR). In this study, we investigated the major HDR isoforms of two woody plant species, Norway spruce (Picea abies) and gray poplar (Populus × canescens), to determine how they regulate isoprenoid formation. Since each of these species has a distinct profile of isoprenoid compounds, they may require different proportions of DMADP and IDP with proportionally more IDP being needed to make larger isoprenoids. Norway spruce contained two major HDR isoforms differing in their occurrence and biochemical characteristics. PaHDR1 produced relatively more IDP than PaHDR2 and it encoding gene was expressed constitutively in leaves, likely serving to form substrate for production of carotenoids, chlorophylls, and other primary isoprenoids derived from a C20 precursor. On the other hand, Norway spruce PaHDR2 produced relatively more DMADP than PaHDR1 and its encoding gene was expressed in leaves, stems, and roots, both constitutively and after induction with the defense hormone methyl jasmonate. This second HDR enzyme likely forms a substrate for the specialized monoterpene (C10), sesquiterpene (C15), and diterpene (C20) metabolites of spruce oleoresin. Gray poplar contained only one dominant isoform (named PcHDR2) that produced relatively more DMADP and the gene of which was expressed in all organs. In leaves, where the requirement for IDP is high to make the major carotenoid and chlorophyll isoprenoids derived from C20 precursors, excess DMADP may accumulate, which could explain the high rate of isoprene (C5) emission. Our results provide new insights into the biosynthesis of isoprenoids in woody plants under conditions of differentially regulated biosynthesis of the precursors IDP and DMADP.

Heteromerization of short-chain trans-prenyltransferase controls precursor allocation within a plastidial terpenoid network

Terpenes are the largest and most diverse class of plant specialized metabolites. Sesterterpenes (C25), which are derived from the plastid methylerythritol phosphate pathway, were recently characterized in plants. In Arabidopsis thaliana, four genes encoding geranylfarnesyl diphosphate synthase (GFPPS) (AtGFPPS1 to 4) are responsible for the production of GFPP, which is the common precursor for sesterterpene biosynthesis. However, the interplay between sesterterpenes and other known terpenes remain elusive. Here, we first provide genetic evidence to demonstrate that GFPPSs are responsible for sesterterpene production in Arabidopsis. Blockage of the sesterterpene pathway at the GFPPS step increased the production of geranylgeranyl diphosphate (GGPP)-derived terpenes. Interestingly, co-expression of sesterTPSs in GFPPS-OE (overexpression) plants rescued the phenotypic changes of GFPPS-OE plants by restoring the endogenous GGPP. We further demonstrated that, in addition to precursor (DMAPP/IPP) competition by GFPPS and GGPP synthase (GGPPS) in plastids, GFPPS directly decreased the activity of GGPPS through protein-protein interaction, ultimately leading to GGPP deficiency in planta. Our study provides a new regulatory mechanism of the plastidial terpenoid network in plant cells.

Artemisia annua L. plants lacking Bornyl diPhosphate Synthase reallocate carbon from monoterpenes to sesquiterpenes except artemisinin

The monoterpene camphor is produced in glandular secretory trichomes of the medicinal plant Artemisia annua, which also produces the antimalarial drug artemisinin. We have found that, depending on growth conditions, camphor can accumulate at levels ranging from 1- 10% leaf dry weight (LDW) in the Artemis F1 hybrid, which has been developed for commercial production of artemisinin at up to 1% LDW. We discovered that a camphor null (camphor-0) phenotype segregates in the progeny of self-pollinated Artemis material. Camphor-0 plants also show reduced levels of other less abundant monoterpenes and increased levels of the sesquiterpene precursor farnesyl pyrophosphate plus sesquiterpenes, including enzymatically derived artemisinin pathway intermediates but not artemisinin. One possible explanation for this is that high camphor concentrations in the glandular secretory trichomes play an important role in generating the hydrophobic conditions required for the non-enzymatic conversion of dihydroartemisinic acid tertiary hydroperoxide to artemisinin. We established that the camphor-0 phenotype associates with a genomic deletion that results in loss of a Bornyl diPhosphate Synthase (AaBPS) gene candidate. Functional characterization of the corresponding enzyme in vitro confirmed it can catalyze the first committed step in not only camphor biosynthesis but also in a number of other monoterpenes, accounting for over 60% of total volatiles in A. annua leaves. This in vitro analysis is consistent with loss of monoterpenes in camphor-0 plants. The AaBPS promoter drives high reporter gene expression in A. annua glandular secretory trichomes of juvenile leaves with expression shifting to non-glandular trichomes in mature leaves, which is consistent with AaBPS transcript abundance.

Bark Beetle Attack History Does Not Influence the Induction of Terpene and Phenolic Defenses in Mature Norway Spruce (Picea abies) Trees by the Bark Beetle-Associated Fungus Endoconidiophora polonica

Terpenes and phenolics are important constitutive and inducible conifer defenses against bark beetles and their associated fungi. In this study, the inducible defenses of mature Norway spruce (Picea abies) trees with different histories of attack by the spruce bark beetle, Ips typographus were tested by inoculation with the I. typographus-associated fungus Endoconidiophora polonica. We compared trees that had been under previous attack with those under current attack and those that had no record of attack. After fungal inoculation, the concentrations of mono-, sesqui-, and diterpenes in bark increased 3- to 9-fold. For the phenolics, the flavan-3-ols, catechin, and gallocatechin, increased significantly by 2- and 5-fold, respectively, while other flavonoids and stilbenes did not. The magnitudes of these inductions were not influenced by prior bark beetle attack history for all the major compounds and compound classes measured. Before fungal inoculation, the total amounts of monoterpenes, diterpenes, and phenolics (constitutive defenses) were greater in trees that had been previously attacked compared to those under current attack, possibly a result of previous induction. The transcript levels of many genes involved in terpene formation (isoprenyl diphosphate synthases and terpene synthases) and phenolic formation (chalcone synthases) were significantly enhanced by fungal inoculation suggesting de novo biosynthesis. Similar inductions were found for the enzymatic activity of isoprenyl diphosphate synthases and the concentration of their prenyl diphosphate products after fungal inoculation. Quantification of defense hormones revealed a significant induction of the jasmonate pathway, but not the salicylic acid pathway after fungal inoculation. Our data highlight the coordinated induction of terpenes and phenolics in spruce upon infection by E. polonica, a fungal associate of the bark beetle I. typographus, but provide no evidence for the priming of these defense responses by prior beetle attack.

Extensive sequence divergence between the reference genomes of Taraxacum kok-saghyz and Taraxacum mongolicum

Plants belonging to the genus Taraxacum are widespread all over the world, which contain rubber-producing and non-rubber-producing species. However, the genomic basis underlying natural rubber (NR) biosynthesis still needs more investigation. Here, we presented high-quality genome assemblies of rubber-producing T. kok-saghyz TK1151 and non-rubber-producing T. mongolicum TM5. Comparative analyses uncovered a large number of genetic variations, including inversions, translocations, presence/absence variations, as well as considerable protein divergences between the two species. Two gene duplication events were found in these two Taraxacum species, including one common ancestral whole-genome triplication and one subsequent round of gene amplification. In genomes of both TK1151 and TM5, we identified the genes encoding for each step in the NR biosynthesis pathway and found that the SRPP and CPT gene families have experienced a more obvious expansion in TK1151 compared to TM5. This study will have large-ranging implications for the mechanism of NR biosynthesis and genetic improvement of NR-producing crops.

Analysis of isoprenyl-phosphates by liquid chromatography-mass spectrometry

Isoprenoids in plants are synthesized following the plastidial methylerythritol-4-phosphate (MEP) pathway or the mevalonate pathway localized to the cytosol and peroxisomes. Isoprenyl-diphosphates (isoprenyl-PP) are important intermediates for the synthesis of chlorophyll, carotenoids, sterols, and other isoprenoids in plants. The quantification of isoprenyl-PP is challenging due to the amphipathic structure, the low abundance, and the susceptibility to hydrolysis during extraction and storage. Different methods for the measurement of isoprenyl-phosphates have been developed. Isoprenyl-phosphates can be measured after radioactive labeling or after derivatization. Liquid chromatography-mass spectrometry (LC-MS) methods provide enhanced sensitivity, but still require the extraction from large amounts of sample material. In the protocol presented here, the monophosphates and diphosphates of farnesol, geranylgeraniol and phytol are isolated from plant material with an isopropanol-containing buffer and quantified by LC-MS using citronellyl-P and citronellyl-PP as internal standards. With a low limit of detection for phytyl-P, geranylgeranyl-P, phytyl-PP, and geranylgeranyl-PP, isoprenyl-phosphates can be accurately measured in Arabidopsis leaves or seeds starting with only 20mg of fresh weight.

Engineering Production of a Novel Diterpene Synthase Precursor in Nicotiana benthamiana

Diterpene biosynthesis commonly originates with the methylerythritol phosphate (MEP) pathway in chloroplasts, leading to the C₂₀ substrate, geranylgeranyl pyrophosphate (GGPP). The previous work demonstrated that over-expression of genes responsible for the first and last steps in the MEP pathway in combination with GERANYLGERANYL PYROPHOSPHATE SYNTHASE (GGPPS) and CASBENE SYNTHASE (CAS) is optimal for increasing flux through to casbene in Nicotiana benthamiana. When the gene responsible for the last step in the MEP pathway, 4-HYDROXY-3-METHYLBUT-2-ENYL DIPHOSPHATE REDUCTASE (HDR), is removed from this combination, casbene is still produced but at lower amounts. Here, we report the unexpected finding that this reduced gene combination also results in the production of 16-hydroxy-casbene (16-OH-casbene), consistent with the presence of 16-hydroxy-geranylgeranyl phosphate (16-OH-GGPP) in the same material. Indirect evidence suggests the latter is formed as a result of elevated levels of 4-hydroxy-3-methyl-but-2-enyl pyrophosphate (HMBPP) caused by a bottleneck at the HDR step responsible for conversion of HMBPP to dimethylallyl pyrophosphate (DMAPP). Over-expression of a GERANYLLINALOOL SYNTHASE from Nicotiana attenuata (NaGLS) produces 16-hydroxy-geranyllinalool (16-OH-geranyllinalool) when transiently expressed with the same reduced combination of MEP pathway genes in N. benthamiana. This work highlights the importance of pathway flux control in metabolic pathway engineering and the possibility of increasing terpene diversity through synthetic biology.

Specific decorations of 17-hydroxygeranyllinalool diterpene glycosides solve the autotoxicity problem of chemical defense in Nicotiana attenuata

The native diploid tobacco Nicotiana attenuata produces abundant, potent anti-herbivore defense metabolites known as 17-hydroxygeranyllinalool diterpene glycosides (HGL-DTGs) whose glycosylation and malonylation biosynthetic steps are regulated by jasmonate signaling. To characterize the biosynthetic pathway of HGL-DTGs, we conducted a genome-wide analysis of uridine diphosphate glycosyltransferases (UGTs) and identified 107 family-1 UGT members. The transcript levels of three UGTs were highly correlated with the transcript levels two key HGL-DTG biosynthetic genes: geranylgeranyl diphosphate synthase (NaGGPPS) and geranyllinalool synthase (NaGLS). NaGLS's role in HGL-DTG biosynthesis was confirmed by virus-induced gene silencing. Silencing the Uridine diphosphate (UDP)-rhamnosyltransferase gene UGT91T1 demonstrated its role in the rhamnosylation of HGL-DTGs. In vitro enzyme assays revealed that UGT74P3 and UGT74P4 use UDP-glucose for the glucosylation of 17-hydroxygeranyllinalool (17-HGL) to lyciumoside I. Plants with stable silencing of UGT74P3 and UGT74P5 were severely developmentally deformed, pointing to a phytotoxic effect of the aglycone. The application of synthetic 17-HGL and silencing of the UGTs in HGL-DTG-free plants confirmed this phytotoxic effect. Feeding assays with tobacco hornworm (Manduca sexta) larvae revealed the defensive functions of the glucosylation and rhamnosylation steps in HGL-DTG biosynthesis. Glucosylation of 17-HGL is therefore a critical step that contributes to the resulting metabolites' defensive function and solves the autotoxicity problem of this potent chemical defense.

Fatty acid isoprenoid alcohol ester synthesis in fruits of the African Oil Palm (Elaeis guineensis)

The African Oil Palm (Elaeis guineensis; family Arecaceae) represents the most important oil crop for food and feed production and for biotechnological applications. Two types of oil can be extracted from palm fruits, the mesocarp oil which is rich in palmitic acid and in carotenoids (provitamin A) and tocochromanols (vitamin E), and the kernel oil with high amounts of lauric and myristic acid. We identified fatty acid phytyl esters (FAPEs) in the mesocarp and kernel tissues of mature fruits, mostly esterified with oleic acid and very long chain fatty acids. In addition, fatty acid geranylgeranyl esters (FAGGEs) accumulated in mesocarp and kernels to even larger amounts. In contrast, FAPEs and FAGGEs amounts and fatty acid composition in leaves were very similar. Analysis of wild accessions of African Oil Palm from Cameroon revealed a considerable variation in the amounts and composition of FAPEs and FAGGEs in mesocarp and kernel tissues. Exogenous supplementation of phytol or geranylgeraniol to mesocarp slices resulted in the incorporation of these alcohols into FAPEs and FAGGEs, respectively, indicating that they are synthesized via enzymatic reactions. Three candidate genes of the esterase/lipase/thioesterase (ELT) family were identified in the Oil Palm genome. The genes are differentially expressed in mesocarp tissue with EgELT1 showing the highest expression. Geranylgeraniol from FAGGE might be recycled and used as a substrate for the synthesis of carotenoids and tocotrienols during fruit development. Thus, FAPEs and FAGGEs in the mesocarp and kernel of Oil Palm provide an additional metabolic source for fatty acids and phytol or geranylgeraniol, respectively.

Analysis of the isoprenoid pathway intermediates, dimethylallyl diphosphate and isopentenyl diphosphate, from crude plant extracts by liquid chromatography tandem mass spectrometry

OBJECTIVE

We sought to develop a sensitive and accurate analytical method for the detection and quantification of IDP and DMADP as well as their monophosphate derivatives in crude plant extracts.

METHODS

A liquid chromatography method coupled to tandem mass spectrometry (LC-MS/MS) with multiple reaction monitoring (MRM) was established to measure the amounts of IDP and DMADP down to low picogram levels, which was linear over at least three orders of magnitude. Extracts were enriched using an anion exchanger, and chromatographic separation was achieved using a β-cyclodextrin column. A S-thiolodiphosphate analog of DMADP was employed as an internal standard.

RESULTS

Dilution series of authentic compounds were used to determine the limits of detection and quantification for IDP, DMADP and their corresponding monophosphates. A survey of plant species producing varying amounts of isoprenoids showed a corresponding variation in IDP and DMADP with the ratio of DMADP/IDP ranging from 4:1 to 2:1. Trace levels of isopentenyl monophosphate (IP) and dimethylallyl monophosphate (DMAP) were also detected.

CONCLUSION

The LC-MS/MS method described enables absolute quantification of in planta levels of IDP and DMADP for the first time. The method is also suitable for analysing bacterial and animal samples as well as enzyme assays.

Terpenoids are transported in the xylem sap of Norway spruce

Norway spruce is a conifer storing large amounts of terpenoids in resin ducts of various tissues. Parts of the terpenoids stored in needles can be emitted together with de novo synthesized terpenoids. Since previous studies provided hints on xylem transported terpenoids as a third emission source, we tested if terpenoids are transported in xylem sap of Norway spruce. We further aimed at understanding if they might contribute to terpenoid emission from needles. We determined terpenoid content and composition in xylem sap, needles, bark, wood and roots of field grown trees, as well as terpenoid emissions from needles. We found considerable amounts of terpenoids-mainly oxygenated compounds-in xylem sap. The terpenoid concentration in xylem sap was relatively low compared with the content in other tissues, where terpenoids are stored in resin ducts. Importantly, the terpenoid composition in the xylem sap greatly differed from the composition in wood, bark or roots, suggesting that an internal transport of terpenoids takes place at the sites of xylem loading. Four terpenoids were identified in xylem sap and emissions, but not within needle tissue, suggesting that these compounds are likely derived from xylem sap. Our work gives hints that plant internal transport of terpenoids exists within conifers; studies on their functions should be a focus of future research.

Optimization of extraction solvents, solid phase extraction and decoupling for quantitation of free isoprenoid diphosphates in Haematococcus pluvialis by liquid chromatography with tandem mass spectrometry

Isoprenoid diphosphates are important precursors actively participating in many downstream metabolisms; they are often in modified forms, e.g., protein-coupled or esterified form. Therefore, in vivo level of free isoprenoid diphosphates is quite low, ˜0.07 nmol/g fresh weight in plants. In order to directly measure the isoprenoid diphosphate pool during stress-induced accumulation of astaxanthin in Haematococcus pluvialis, the present study optimized several pretreatment procedures to enrich free isoprenoid diphosphates for high-pressure liquid chromatography with tandem mass spectrometry (HPLC-MS/MS) detection. Specifically, different extraction solvents, e.g., water, methanol, chloroform, and mixture of water, methanol, and chloroform (1:1:1, V/V/V), and solid phase extraction (SPE) columns (OASIS@ WAX and HLB Cartridges) were compared; and gentle decoupling by NaOH or trifluoroacetic acid (TFA) was introduced to release free isoprenoid diphosphates. Results found that solvent mixture of water, methanol and chloroform (1:1:1, V/V/V) showed the highest extraction efficiency (R_E) for five isoprenoid diphosphates, ranging from 76.83% to 92.43%; HLB column showed the balanced recoveries ranging from 75.29% to 87.54%; and incubation with low NaOH (˜4.7 mmol/L) at 4 °C significantly increased detectable isoprenoid diphosphates in algal cells, some of which were undetectable or in trace level before NaOH decoupling. The method was applied to H. pluvialis cells under various stresses. Low levels of isoprenoid diphosphates were determined in most of the stresses used, e.g., 0.19 ± 0.09 to 0.98 ± 0.06 mg/g fresh weight (FW) for IPP/DMAPP, 0.35 ± 0.07 mg/g FW for GGPP and undetectable for FPP and GPP; while isoprenoid diphosphates were significantly accumulated in the dark to 3.27 ± 0.05, 0.17 ± 0.09, 1.81 ± 0.16 and 0.58 ± 0.07 mg/g FW for IPP/DMAPP, GPP, FPP and GGPP, respectively. These results implied that isoprenoid diphosphates were exhausted by downstream carotenogenesis under stress. Our work emphasizes NaOH decoupling for exact quantitation of in vivo isoprenoid diphosphates.

Comparative proteomic analysis provides insights into the complex responses to Pseudoperonospora cubensis infection of cucumber (Cucumis sativus L.)

Cucumber (Cucumis sativus L.) is an important crop distributed in many countries. Downy mildew (DM) caused by the obligate oomycete Pseudoperonospora cubensis is especially destructive in cucumber production. So far, few studies on the changes in proteomes during the P. cubensis infection have been performed. In the present study, the proteomes of DM-resistant variety ‘ZJ’ and DM-susceptible variety ‘SDG’ under the P. cubensis infection were investigated. In total, 6400 peptides were identified, 5629 of which were quantified. KEGG analysis showed that a number of metabolic pathways were significantly altered under P. cubensis infection, such as terpenoid backbone biosynthesis, and selenocompound metabolism in ZJ, and starch and sucrose metabolism in SDG. For terpenoid backbone synthesis, 1-deoxy-D-xylulose-5-phosphate synthase, 1-deoxy-D-xylulose 5-phosphate reductoisomerase, 2-C-methyl-D-erythritol 2,4-cyclodiphosphate synthase, 4-hydroxy-3-methylbut-2-en-1-yl diphosphate synthase, and geranylgeranyl pyrophosphate synthase were significantly accumulated in ZJ rather than in SDG, suggesting that pathogen-induced terpenoids accumulation might play an important role in the resistance against P. cubensis infection. Furthermore, a number of pathogenesis-related proteins, such as endochitinases, peroxidases, PR proteins and heat shock proteins were identified as DAPs, suggesting that DM resistance was controlled by a complex network. Our data allowed us to identify and screen more potential proteins related to the DM resistance.

Functions of stone cells and oleoresin terpenes in the conifer defense syndrome

Conifers depend on complex defense systems against herbivores. Stone cells (SC) and oleoresin are physical and chemical defenses of Sitka spruce that have been separately studied in previous work. Weevil oviposit at the tip of the previous year's apical shoot (PYAS). We investigated interactions between weevil larvae and trees in controlled oviposition experiments with resistant (R) and susceptible (S) Sitka spruce. R trees have an abundance of SC in the PYAS cortex. SC are mostly absent in S trees. R trees and S trees also differ in the composition of oleoresin terpenes. Transcriptomes of R and S trees revealed differences in long-term weevil-induced responses. Performance of larvae was significantly reduced on R trees compared with S trees under experimental conditions that mimicked natural oviposition behavior at apical shoot tips and may be attributed to the effects of SC. In oviposition experiments designed for larvae to feed below the area of highest SC abundance, larvae showed an unusual feeding behavior and oleoresin appeared to function as the major defense. The results support a role for both SC and oleoresin terpenes and possible synergies between these traits in the defense syndrome of weevil-resistant Sitka spruce.

Phytol metabolism in plants

Phytol, the prenyl side chain of chlorophyll, is derived from geranylgeraniol by reduction of three double bonds. Recent results demonstrated that the conversion of geranylgeraniol to phytol is linked to chlorophyll synthesis, which is catalyzed by protein complexes associated with the thylakoid membranes. One of these complexes contains light harvesting chlorophyll binding like proteins (LIL3), enzymes of chlorophyll synthesis (protoporphyrinogen oxidoreductase, POR; chlorophyll synthase, CHLG) and geranylgeranyl reductase (GGR). Phytol is not only employed for the synthesis of chlorophyll, but also for tocopherol (vitamin E), phylloquinol (vitamin K) and fatty acid phytyl ester production. Previously, it was believed that phytol is derived from reduction of geranylgeranyl-diphosphate originating from the 4-methylerythritol-5-phosphate (MEP) pathway. The identification and characterization of two kinases, VTE5 and VTE6, involved in phytol and phytyl-phosphate phosphorylation, respectively, indicated that most phytol employed for tocopherol synthesis is derived from reduction of geranylgeranylated chlorophyll to (phytol-) chlorophyll. After hydrolysis from chlorophyll, free phytol is phosphorylated by the two kinases, and phytyl-diphosphate employed for the synthesis of tocopherol and phylloquinol. The reason why some chloroplast lipids, i.e. chlorophyll, tocopherol and phylloquinol, are derived from phytol, while others, i.e. carotenoids and tocotrienols (in some plant species) are synthesized from geranylgeraniol, remains unclear.

Isoprenyl diphosphate synthases: the chain length determining step in terpene biosynthesis

This review summarizes the recent developments in the study of isoprenyl diphosphate synthases with an emphasis on analytical techniques, product length determination, and the physiological consequences of manipulating expression in planta. The highly diverse structures of all terpenes are synthesized from the five carbon precursors dimethylallyl diphosphate and a varying number of isopentenyl diphosphate units through 1'-4 alkylation reactions. These elongation reactions are catalyzed by isoprenyl diphosphate synthases (IDS). IDS are classified depending on the configuration of the ensuing double bond as trans- and cis-IDS. In addition, IDS are further stratified by the length of their prenyl diphosphate product. This review discusses analytical techniques for the determination of product length and the factors that control product length, with an emphasis on alternative mechanisms. With recent advances in analytics, multiple IDS of Arabidopsis thaliana have been recently reinvestigated and demonstrated to yield products of different lengths than originally reported, which is summarized here. As IDS dictate prenyl diphosphate length and thereby which class of terpenes is ultimately produced, another focus of this review is the impact that altering IDS expression has on terpenoid natural product accumulation. Finally, recent findings regarding the ability of a few IDS to not catalyze 1'-4 alkylation reactions, but instead produce irregular products, with unusual connectivity, or act as terpene synthases, are also discussed.

A multiple near isogenic line (multi-NIL) RNA-seq approach to identify candidate genes underpinning QTL

This study demonstrates how identification of genes underpinning disease-resistance QTL based on differential expression and SNPs can be improved by performing transcriptomic analysis on multiple near isogenic lines. Transcriptomic analysis has been widely used to understand the genetic basis of a trait of interest by comparing genotypes with contrasting phenotypes. However, these approaches identify such large sets of differentially expressed genes that it proves difficult to isolate which genes underpin the phenotype of interest. This study tests whether using multiple near isogenic lines (NILs) can improve the resolution of RNA-seq-based approaches to identify genes underpinning disease-resistance QTL. A set of NILs for a major effect Fusarium crown rot-resistance QTL in barley on the 4HL chromosome arm were analysed under Fusarium crown rot using RNA-seq. Differential gene expression and single nucleotide polymorphism detection analyses reduced the number of putative candidates from thousands within individual NIL pairs to only one hundred and two genes, which were differentially expressed or contained SNPs in common across NIL pairs and occurred on 4HL. Our findings support the value of performing RNA-seq analysis using multiple NILs to remove genetic background effects. The enrichment analyses indicated conserved differences in the response to infection between resistant and sensitive isolines suggesting that sensitive isolines are impaired in systemic defence response to Fusarium pseudograminearum.

Terpene Moiety Enhancement by Overexpression of Geranyl(geranyl) Diphosphate Synthase and Geraniol Synthase Elevates Monomeric and Dimeric Monoterpene Indole Alkaloids in Transgenic Catharanthus roseus

Catharanthus roseus is the sole source of two of the most important anticancer monoterpene indole alkaloids (MIAs), vinblastine and vincristine and their precursors, vindoline and catharanthine. The MIAs are produced from the condensation of precursors derived from indole and terpene secoiridoid pathways. It has been previously reported that the terpene moiety limits MIA biosynthesis in C. roseus. Here, to overcome this limitation and enhance MIAs levels in C. roseus, bifunctional geranyl(geranyl) diphosphate synthase [G(G)PPS] and geraniol synthase (GES) that provide precursors for early steps of terpene moiety (secologanin) formation, were overexpressed transiently by agroinfiltration and stably by Agrobacterium-mediated transformation. Both transient and stable overexpression of G(G)PPS and co-expression of G(G)PPS+GES significantly enhanced the accumulation of secologanin, which in turn elevated the levels of monomeric MIAs. In addition, transgenic C. roseus plants exhibited increased levels of root alkaloid ajmalicine. The dimeric alkaloid vinblastine was enhanced only in G(G)PPS but not in G(G)PPS+GES transgenic lines that correlated with transcript levels of peroxidase-1 (PRX1) involved in coupling of vindoline and catharanthine into 3',4'-anhydrovinblastine, the immediate precursor of vinblastine. Moreover, first generation (T₁) lines exhibited comparable transcript and metabolite levels to that of T₀ lines. In addition, transgenic lines displayed normal growth similar to wild-type plants indicating that the bifunctional G(G)PPS enhanced flux toward both primary and secondary metabolism. These results revealed that improved availability of early precursors for terpene moiety biosynthesis enhanced production of MIAs in C. roseus at the whole plant level. This is the first report demonstrating enhanced accumulation of monomeric and dimeric MIAs including root MIA ajmalicine in C. roseus through transgenic approaches.

Silencing amorpha-4,11-diene synthase Genes in Artemisia annua Leads to FPP Accumulation

Artemisia annua is established as an efficient crop for the production of the anti-malarial compound artemisinin, a sesquiterpene lactone synthesized and stored in Glandular Secretory Trichomes (GSTs) located on the leaves and inflorescences. Amorpha-4,11-diene synthase (AMS) catalyzes the conversion of farnesyl pyrophosphate (FPP) to amorpha-4,11-diene and diphosphate, which is the first committed step in the synthesis of artemisinin. FPP is the precursor for sesquiterpene and sterol biosynthesis in the plant. This work aimed to investigate the effect of blocking the synthesis of artemisinin in the GSTs of a high artemisinin yielding line, Artemis, by down regulating AMS. We determined that there are up to 12 AMS gene copies in Artemis, all expressed in GSTs. We used sequence homology to design an RNAi construct under the control of a GST specific promoter that was predicted to be effective against all 12 of these genes. Stable transformation of Artemis with this construct resulted in over 95% reduction in the content of artemisinin and related products, and a significant increase in the FPP pool. The Artemis AMS silenced lines showed no morphological alterations, and metabolomic and gene expression analysis did not detect any changes in the levels of other major sesquiterpene compounds or sesquiterpene synthase genes in leaf material. FPP also acts as a precursor for squalene and sterol biosynthesis but levels of these compounds were also not altered in the AMS silenced lines. Four unknown oxygenated sesquiterpenes were produced in these lines, but at extremely low levels compared to Artemis non-transformed controls (NTC). This study finds that engineering A. annua GSTs in an Artemis background results in endogenous terpenes related to artemisinin being depleted with the precursor FPP actually accumulating rather than being utilized by other endogenous enzymes. The challenge now is to establish if this precursor pool can act as substrate for production of alternative sesquiterpenes in A. annua.

Metabolic alteration of Catharanthus roseus cell suspension cultures overexpressing geraniol synthase in the plastids or cytosol

Previous studies showed that geraniol could be an upstream limiting factor in the monoterpenoid pathway towards the production of terpenoid indole alkaloid (TIA) in Catharanthus roseus cells and hairy root cultures. This shortage in precursor availability could be due to (1) limited expression of the plastidial geraniol synthase resulted in a low activity of the enzyme to catalyze the conversion of geranyl diphosphate to geraniol; or (2) the limitation of geraniol transport from plastids to cytosol. Therefore, in this study, C. roseus's geraniol synthase (CrGES) gene was overexpressed in either plastids or cytosol of a non-TIA producing C. roseus cell line. The expression of CrGES in the plastids or cytosol was confirmed and the constitutive transformation lines were successfully established. A targeted metabolite analysis using HPLC shows that the transformed cell lines did not produce TIA or iridoid precursors unless elicited with jasmonic acid, as their parent cell line. This indicates a requirement for expression of additional, inducible pathway genes to reach production of TIA in this cell line. Interestingly, further analysis using NMR-based metabolomics reveals that the overexpression of CrGES impacts primary metabolism differently if expressed in the plastids or cytosol. The levels of valine, leucine, and some metabolites derived from the shikimate pathway, i.e. phenylalanine and tyrosine were significantly higher in the plastidial- but lower in the cytosolic-CrGES overexpressing cell lines. This result shows that overexpression of CrGES in the plastids or cytosol caused alteration of primary metabolism that associated to the plant cell growth and development. A comprehensive omics analysis is necessary to reveal the full effect of metabolic engineering.

Cell-type- and tissue-specific transcriptomes of the white spruce (Picea glauca) bark unmask fine-scale spatial patterns of constitutive and induced conifer defense

Plant defenses often involve specialized cells and tissues. In conifers, specialized cells of the bark are important for defense against insects and pathogens. Using laser microdissection, we characterized the transcriptomes of cortical resin duct cells, phenolic cells and phloem of white spruce (Picea glauca) bark under constitutive and methyl jasmonate (MeJa)-induced conditions, and we compared these transcriptomes with the transcriptome of the bark tissue complex. Overall, ~3700 bark transcripts were differentially expressed in response to MeJa. Approximately 25% of transcripts were expressed in only one cell type, revealing cell specialization at the transcriptome level. MeJa caused cell-type-specific transcriptome responses and changed the overall patterns of cell-type-specific transcript accumulation. Comparison of transcriptomes of the conifer bark tissue complex and specialized cells resolved a masking effect inherent to transcriptome analysis of complex tissues, and showed the actual cell-type-specific transcriptome signatures. Characterization of cell-type-specific transcriptomes is critical to reveal the dynamic patterns of spatial and temporal display of constitutive and induced defense systems in a complex plant tissue or organ. This was demonstrated with the improved resolution of spatially restricted expression of sets of genes of secondary metabolism in the specialized cell types.

A Conifer UDP-Sugar Dependent Glycosyltransferase Contributes to Acetophenone Metabolism and Defense against Insects

Acetophenones are phenolic compounds involved in the resistance of white spruce (Picea glauca) against spruce budworm (Choristoneura fumiferiana), a major forest pest in North America. The acetophenones pungenol and piceol commonly accumulate in spruce foliage in the form of the corresponding glycosides, pungenin and picein. These glycosides appear to be inactive against the insect but can be cleaved by a spruce β-glucosidase, PgβGLU-1, which releases the active aglycons. The reverse glycosylation reaction was hypothesized to involve a family 1 UDP-sugar dependent glycosyltransferase (UGT) to facilitate acetophenone accumulation in the plant. Metabolite and transcriptome profiling over a developmental time course of white spruce bud burst and shoot growth revealed two UGTs, PgUGT5 and PgUGT5b, that glycosylate pungenol. Recombinant PgUGT5b enzyme produced mostly pungenin, while PgUGT5 produced mostly isopungenin. Both UGTs also were active in vitro on select flavonoids. However, the context of transcript and metabolite accumulation did not support a biological role in flavonoid metabolism but correlated with the formation of pungenin in growing shoots. Transcript levels of PgUGT5b were higher than those of PgUGT5 in needles across different genotypes of white spruce. These results support a role of PgUGT5b in the biosynthesis of the glycosylated acetophenone pungenin in white spruce.

The jasmonate-responsive AaMYC2 transcription factor positively regulates artemisinin biosynthesis in Artemisia annua

The plant Artemisia annua is well known due to the production of artemisinin, a sesquiterpene lactone that is widely used in malaria treatment. Phytohormones play important roles in plant secondary metabolism, such as jasmonic acid (JA), which can induce artemisinin biosynthesis in A. annua. Nevertheless, the JA-inducing mechanism remains poorly understood. The expression of gene AaMYC2 was rapidly induced by JA and AaMYC2 binds the G-box-like motifs within the promoters of gene CYP71AV1 and DBR2, which are key structural genes in the artemisinin biosynthetic pathway. Overexpression of AaMYC2 in A. annua significantly activated the transcript levels of CYP71AV1 and DBR2, which resulted in an increased artemisinin content. By contrast, artemisinin content was reduced in the RNAi transgenic A. annua plants in which the expression of AaMYC2 was suppressed. Meanwhile, the RNAi transgenic A. annua plants showed lower sensitivity to methyl jasmonate treatment than the wild-type plants. These results demonstrate that AaMYC2 is a positive regulator of artemisinin biosynthesis and is of great value in genetic engineering of A. annua for increased artemisinin production.

A Geranylfarnesyl Diphosphate Synthase Provides the Precursor for Sesterterpenoid (C25) Formation in the Glandular Trichomes of the Mint Species Leucosceptrum canum

Plant sesterterpenoids, an important class of terpenoids, are widely distributed in various plants, including food crops. However, little is known about their biosynthesis. Here, we cloned and functionally characterized a plant geranylfarnesyl diphosphate synthase (Lc-GFDPS), the enzyme producing the C25 prenyl diphosphate precursor to all sesterterpenoids, from the glandular trichomes of the woody plant Leucosceptrum canum. GFDPS catalyzed the formation of GFDP after expression in Escherichia coli. Overexpressing GFDPS in Arabidopsis thaliana also gave an extract catalyzing GFDP formation. GFDPS was strongly expressed in glandular trichomes, and its transcript profile was completely in accordance with the sesterterpenoid accumulation pattern. GFDPS is localized to the plastids, and inhibitor studies indicated its use of isoprenyl diphosphate substrates supplied by the 2-C-methyl-D-erythritol 4-phosphate pathway. Application of a jasmonate defense hormone induced GFDPS transcript and sesterterpenoid accumulation, while reducing feeding and growth of the generalist insect Spodoptera exigua, suggesting that these C25 terpenoids play a defensive role. Phylogenetic analysis suggested that GFDPS probably evolved from plant geranylgeranyl diphosphate synthase under the influence of positive selection. The isolation of GFDPS provides a model for investigating sesterterpenoid formation in other species and a tool for manipulating the formation of this group in plants and other organisms.

Arabidopsis thaliana isoprenyl diphosphate synthases produce the C25 intermediate geranylfarnesyl diphosphate

Isoprenyl diphosphate synthases (IDSs) catalyze some of the most basic steps in terpene biosynthesis by producing the prenyl diphosphate precursors of each of the various terpenoid classes. Most plants investigated have distinct enzymes that produce the short-chain all-trans (E) prenyl diphosphates geranyl diphosphate (GDP, C10 ), farnesyl diphosphate (FDP, C15 ) or geranylgeranyl diphosphate (GGDP, C20 ). In the genome of Arabidopsis thaliana, 15 trans-product-forming IDSs are present. Ten of these have recently been shown to produce GGDP by genetic complementation of a carotenoid pathway engineered into Escherichia coli. When verifying the product pattern of IDSs producing GGDP by a new LC-MS/MS procedure, we found that five of these IDSs produce geranylfarnesyl diphosphate (GFDP, C25 ) instead of GGDP as their major product in enzyme assays performed in vitro. Over-expression of one of the GFDP synthases in A. thaliana confirmed the production of GFDP in vivo. Enzyme assays with A. thaliana protein extracts from roots but not other organs showed formation of GFDP. Furthermore, GFDP itself was detected in root extracts. Subcellular localization studies in leaves indicated that four of the GFDP synthases were targeted to the plastoglobules of the chloroplast and one was targeted to the mitochondria. Sequence comparison and mutational studies showed that the size of the R group of the 5th amino acid residue N-terminal to the first aspartate-rich motif is responsible for C25 versus C20 product formation, with smaller R groups (Ala and Ser) resulting in GGDP (C20 ) as a product and a larger R group (Met) resulting in GFDP (C25 ).

Improved white spruce (Picea glauca) genome assemblies and annotation of large gene families of conifer terpenoid and phenolic defense metabolism

White spruce (Picea glauca), a gymnosperm tree, has been established as one of the models for conifer genomics. We describe the draft genome assemblies of two white spruce genotypes, PG29 and WS77111, innovative tools for the assembly of very large genomes, and the conifer genomics resources developed in this process. The two white spruce genotypes originate from distant geographic regions of western (PG29) and eastern (WS77111) North America, and represent elite trees in two Canadian tree-breeding programs. We present an update (V3 and V4) for a previously reported PG29 V2 draft genome assembly and introduce a second white spruce genome assembly for genotype WS77111. Assemblies of the PG29 and WS77111 genomes confirm the reconstructed white spruce genome size in the 20 Gbp range, and show broad synteny. Using the PG29 V3 assembly and additional white spruce genomics and transcriptomics resources, we performed MAKER-P annotation and meticulous expert annotation of very large gene families of conifer defense metabolism, the terpene synthases and cytochrome P450s. We also comprehensively annotated the white spruce mevalonate, methylerythritol phosphate and phenylpropanoid pathways. These analyses highlighted the large extent of gene and pseudogene duplications in a conifer genome, in particular for genes of secondary (i.e. specialized) metabolism, and the potential for gain and loss of function for defense and adaptation.

Aphid-repellent pheromone E-β-farnesene is generated in transgenic Arabidopsis thaliana over-expressing farnesyl diphosphate synthase2

BACKGROUND AND AIMS

Plant-synthesized sesquiterpenes play a pivotal role in chemotactic interactions with insects. Biosynthesis of functionally diverse sesquiterpenes is dependent on the availability of a pool of the precursor farnesyldiphosphate (FDP). In Arabidopsis thaliana, FPS2, encoding cytosolic farnesyldiphosphate synthase, is implicated in the synthesis of cytosolic FDP, but it is not known whether enhanced levels of FDP have a commensurate effect on sesquiterpene-mediated defence responses. This study examined transgenic arabidopsis plants generated to over-express FPS2 in order to determine if any effects could be observed in the response of aphids, Myzus persicae.

METHODS

Transgenic arabidopsis plants were generated to over-express FPS2 to produce FPS2 in either the cytosol or the chloroplasts. Morphochemical analyses of the transgenic plants were carried out to detremine growth responses of roots and shoots, and for GC-MS profiling of sesquiterpenes. Aphid response to hyrdo-distillate extracts and head-space volatiles from transgenic plants was assessed using a bioassay.

KEY RESULTS

Either over-expression of FPS2 in the cytosol or targetting of its translated product to chlorplasts resulted in stimulatory growth responses of transgenic arabidopsis at early and late developmental stages. GC-MS analysis of hydro-distillate extracts from aerial parts of the plants revealed biosynthesis of several novel sesquiterpenes, including E-β-farnesene, an alarm pheromone of aphids. Both entrapped volatiles and hydro-distillate extracts of the transgenic leaves triggered agitation in aphids, which was related to both time and dose of exposure.

CONCLUSIONS

Over-expression of FPS2 in the cytosol and targeting of its translated product to chloroplasts in arabidopsis led to synthesis of several novel sesquiterpenes, including E-β-farnesene, and induced alarm responses in M. persicae. The results suggest a potential for engineering aphid-resistant strains of arabidopsis.

Metabolic engineering of higher plants and algae for isoprenoid production

Isoprenoids are a class of compounds derived from the five carbon precursors, dimethylallyl diphosphate, and isopentenyl diphosphate. These molecules present incredible natural chemical diversity, which can be valuable for humans in many aspects such as cosmetics, agriculture, and medicine. However, many terpenoids are only produced in small quantities by their natural hosts and can be difficult to generate synthetically. Therefore, much interest and effort has been directed toward capturing the genetic blueprint for their biochemistry and engineering it into alternative hosts such as plants and algae. These autotrophic organisms are attractive when compared to traditional microbial platforms because of their ability to utilize atmospheric CO2 as a carbon substrate instead of supplied carbon sources like glucose. This chapter will summarize important techniques and strategies for engineering the accumulation of isoprenoid metabolites into higher plants and algae by choosing the correct host, avoiding endogenous regulatory mechanisms, and optimizing potential flux into the target compound. Future endeavors will build on these efforts by fine-tuning product accumulation levels via the vast amount of available "-omic" data and devising metabolic engineering schemes that integrate this into a whole-organism approach. With the development of high-throughput transformation protocols and synthetic biology molecular tools, we have only begun to harness the power and utility of plant and algae metabolic engineering.