Prenyltransferases catalyzing geranyldiphosphate formation in tomato fruit

Loss of tomato geranylgeranyl diphosphate synthase 2 increases monoterpenoid levels and enhances immune responses to bacterial infection

Many plastidial isoprenoids, including diterpenes and photosynthesis-related isoprenoids such as carotenoids and chlorophylls, derive from C20 geranylgeranyl diphosphate (GGPP), produced by GGPP synthase (GGPPS) enzymes. Heterodimers of GGPPS and non-catalytic type I small subunit (SSU-I) proteins produce C10 geranyl diphosphate (GPP), the precursor of monoterpenes. Three plastidial GGPPS isoforms, referred to as SlG1-3, are present in tomato (Solanum lycopersicum). Here we explored their contribution to the production of volatile organic compounds (VOCs) of isoprenoid origin under normal conditions and in response to infection with Pseudomonas syringae pathovar tomato (Pst). Edited lines lacking SlG2 showed a distinctive VOC profile compared to unedited (WT) plants and mutants impaired in SlG1 or SlG3. In particular, only slg2 mutants showed constitutively increased levels of GPP-derived hydroxylated monoterpenes (HMTPs). Upon Pst infection, slg2 plants accumulated higher levels of salicylic acid (SA) and exhibited increase resistance compared to WT controls, resulting in reduced levels of VOCs associated to cell death. Our findings suggest that SlG2 regulates GPP synthesis, potentially by specifically competing with other GGPPS isoforms for heterodimerization with SSU-I. Increased GPP production in slg2 plants could lead to higher HMTPs levels, which may result in elevated SA content, and subsequently enhanced protection against bacterial infection.

Plant terpenoid biosynthetic network and its multiple layers of regulation

Terpenoids constitute one of the largest and most chemically diverse classes of primary and secondary metabolites in nature with an exceptional breadth of functional roles in plants. Biosynthesis of all terpenoids begins with the universal five‑carbon building blocks, isopentenyl diphosphate (IPP) and its allylic isomer dimethylallyl diphosphate (DMAPP), which in plants are derived from two compartmentally separated but metabolically crosstalking routes, the mevalonic acid (MVA) and methylerythritol phosphate (MEP) pathways. Here, we review the current knowledge on the terpenoid precursor pathways and highlight the critical hidden constraints as well as multiple regulatory mechanisms that coordinate and homeostatically govern carbon flux through the terpenoid biosynthetic network in plants.

Isoprenyl diphosphate synthases of terpenoid biosynthesis in rose-scented geranium (Pelargonium graveolens)

The essential oil of Pelargonium graveolens (rose-scented geranium), an important aromatic plant, comprising mainly mono- and sesqui-terpenes, has applications in food and cosmetic industries. This study reports the characterization of isoprenyl disphosphate synthases (IDSs) involved in P. graveolens terpene biosynthesis. The six identified PgIDSs belonged to different classes of IDSs, comprising homomeric geranyl diphosphate synthases (GPPSs; PgGPPS1 and PgGPPS2), the large subunit of heteromeric GPPS or geranylgeranyl diphosphate synthases (GGPPSs; PgGGPPS), the small subunit of heteromeric GPPS (PgGPPS.SSUI and PgGPPS.SSUII), and farnesyl diphosphate synthases (FPPS; PgFPPS).All IDSs exhibited maximal expression in glandular trichomes (GTs), the site of aroma formation, and their expression except PgGPPS.SSUII was induced upon treatment with MeJA. Functional characterization of recombinant proteins revealed that PgGPPS1, PgGGPPS and PgFPPS were active enzymes producing GPP, GGPP/GPP, and FPP respectively, whereas both PgGPPS.SSUs and PgGPPS2 were inactive. Co-expression of PgGGPPS (that exhibited bifunctional G(G)PPS activity) with PgGPPS.SSUs in bacterial expression system showed lack of interaction between the two proteins, however, PgGGPPS interacted with a phylogenetically distant Antirrhinum majus GPPS.SSU. Further, transient expression of AmGPPS.SSU in P. graveolens leaf led to a significant increase in monoterpene levels. These findings provide insight into the types of IDSs and their role in providing precursors for different terpenoid components of P. graveolens essential oil.

Functional characterization of a geranylgeranyl diphosphate synthase in the leaf beetle Monolepta hieroglyphica

Geranylgeranyl diphosphate synthase (GGPPS) as the short-chain prenyltransferases for catalyzing the formation of the acyclic precursor (E)-GGPP has been extensively investigated in mammals, plants, and microbes, but its functional plasticity is poorly understood in insect species. Here, a single GGPPS in leaf beetle Monolepta hieroglyphica, MhieGGPPS, was functionally investigated. Phylogenetic analysis showed that MhieGGPPS was clustered in one clade with homologs and had six conserved motifs. Molecular docking results indicated that binding sites of dimethylallyl diphosphate (DMAPP), (E)-geranyl pyrophosphate (GPP), and (E)-farnesyl pyrophosphate (FPP) were in the chain-length determination region of MhieGGPPS, respectively. In vitro, recombiant MhieGGPPS could catalyze the formation of (E)-geranylgeraniol against different combinations of substrates including isopentenyl pyrophosphate (IPP)/DMAPP, IPP/(E)-GPP, and IPP/(E)-FPP, suggesting that MhieGGPPS could not only use (E)-FPP but also (E)-GPP and DMAPP as the allylic cosubstrates. In kinetic analysis, the (E)-FPP was most tightly bound to MhieGGPPS than that of others. It was proposed that MhieGGPPS as a multifunctional enzyme is differentiated from the other GGPPSs in the animals and plants, which only accepted (E)-FPP as the allylic cosubstrate. These findings provide valuable insights into understanding the functional plasticity of GGPPS in M. hieroglyphica and the novel biosynthesis mechanism in the isoprenoid pathway.

Geranyl diphosphate synthase large subunits OfLSU1/2 interact with small subunit OfSSUII and are involved in aromatic monoterpenes production in Osmanthus fragrans

Osmanthus fragrans is a famous ornamental tree species for its pleasing floral fragrance. Monoterpenoids are the core floral volatiles of O. fragrans flowers, which have tremendous commercial value. Geranyl diphosphate synthase (GPPS) is a key enzyme that catalyzes the formation of GPP, the precursor of monoterpenoids. However, there are no reports of GPPSs in O. fragrans. Here, we performed RNA sequencing on the O. fragrans flowers and identified three GPPSs. Phylogenetic tree analysis showed that OfLSU1/2 belonged to the GPPS.LSU branch, while the OfSSUII belonged to the GPPS.SSU branch. OfLSU1, OfLSU2 and OfSSUII were all localized in chloroplasts. Y2H and pull-down assays showed that OfLSU1 or OfLSU2 interacted with OfSSUII to form heteromeric GPPSs. Site mutation experiments revealed that the conserved CXXXC motifs of OfLSU1/2 and OfSSUII were essential for the interaction between OfLSU1/2 and OfSSUII. Transient expression experiments showed that OfLSU1, OfLSU2 and OfSSUII co-expressed with monoterpene synthase genes OfTPS1 or OfTPS2 improved the biosynthesis of monoterpenoids (E)-β-ocimene and linalool. The heteromeric GPPSs formed by OfLSU1/2 interacting with OfSSUII further improves the biosynthesis of monoterpenoids. Overall, these preliminary results suggested that the GPPSs play a key role in regulating the production of aromatic monoterpenes in O. fragrans.

Metagenomic analyses reveal microbial communities and functional differences between Daqu from seven provinces

Microbial communities perform the brewing function in Daqu. Macrogenomics and PICRUST II analyses revealed the differences in microbes and metabolic functions among Daqu from the seven Baijiu-producing provinces. Jiang-flavored Daqu (Guizhou, Shandong, and Hubei provinces) generally forms an aroma-producing functional microbiota with Kroppenstedtia, Bacillus, Thermoascus, Virgibacillus, and Thermomyces as the core, which promotes the metabolism of various amino acids and aroma compounds. Light-flavored Daqu (Shanxi Province) enriched the Saccharomycopsis, Saccharomyces, and lactic acid bacteria (LAB) microbiota through low-temperature fermentation. These microbes can synthesize alcohol and lactic acid but inhibit amino acid metabolism within the Light-flavored Daqu. Bifidobacterium and Saccharomycopsis were dominant in the Tao-flavored Daqu (Henan province). This unique microbial structure is beneficial for pyruvate fermentation to lactate. Research also found that Strong-flavored Daqu from Jiangsu and Sichuan provinces differed significantly. The microbial communities and metabolic pathways within Jiangsu Daqu were similar to those within Jiang-flavored Daqu, but Sichuan Daqu was dominated by Thermoascus, LAB, and Thermoactinomyces. In addition, Spearman correlation analysis indicated that Kroppenstedtia, Bacillus, and Thermomyces were not only positively related to flavor metabolism but also negatively correlated with Saccharomycopsis. This research will help establish a systematic understanding of the microbial community and functional characteristics in Daqu.

Farnesyl/geranylgeranyl diphosphate synthases regulate the biosynthesis of alarm pheromone in a unique manner in the vetch aphid Megoura viciae

Farnesyl/geranylgeranyl diphosphate synthases (FPPS/GGPPS) as the short-chain prenyltransferases catalyse the formation of the acyclic precursors (E)-FPP and (E)-GGPP for isoprenoid biosynthesis. Here, we first cloned the cDNAs encoding FPPS and GGPPS in the vetch aphid Megoura viciae (designated as MvFPPS and MvGGPPS). They had an open reading frame of 1185 and 930 bp in length, encoding 395 and 309 amino acids, with a theoretical isoelectric point of 6.52 and 6.21, respectively. Sequence alignment and phylogenetic analysis showed that MvFPPS and MvGGPPS shared the conserved aspartate-rich motifs characterized by all prenyltransferases identified to date and were clustered with their homologues in two large clades. RNA interference (RNAi) combined with gas chromatography/mass spectrometry (GC-MS) analysis showed that both MvFPPS and MvGGPPS were involved in the biosynthesis of alarm pheromone. Spatiotemporal expression profiling showed that the expression of MvFPPS and MvGGPPS was significantly higher in embryos than in other tissues. RNAi and GC-MS performed specifically in embryos corroborated the function of MvFPPS and MvGGPPS. In vitro, enzymatic activity assay and product analysis demonstrated that MvFPPS could catalysed the formation of (E)-FPP using DMAPP or (E)-GPP as the allylic cosubstrates in the presence of IPP, while MvGGPPS could only use (E)-GPP or (E)-FPP as cosubstrates. Functional interaction analysis using RNAi revealed that MvGGPPS exerts unidirectional functional compensation for MvFPPS. Moreover, it can regulate the biosynthesis of alarm pheromone by imposing a negative feedback regulation on MvFPPS. Our study helps to understand the molecular regulatory mechanism of terpenoid biosynthesis in the aphid.

A cytosolic bifunctional geranyl/farnesyl diphosphate synthase provides MVA-derived GPP for geraniol biosynthesis in rose flowers

Geraniol derived from essential oils of various plant species is widely used in the cosmetic and perfume industries. It is also an essential trait of the pleasant smell of rose flowers. In contrast to other monoterpenes which are produced in plastids via the methyl erythritol phosphate pathway, geraniol biosynthesis in roses relies on cytosolic NUDX1 hydrolase which dephosphorylates geranyl diphosphate (GPP). However, the metabolic origin of cytosolic GPP remains unknown. By feeding Rosa chinensis "Old Blush" flowers with pathway-specific precursors and inhibitors, combined with metabolic profiling and functional characterization of enzymes in vitro and in planta, we show that geraniol is synthesized through the cytosolic mevalonate (MVA) pathway by a bifunctional geranyl/farnesyl diphosphate synthase, RcG/FPPS1, producing both GPP and farnesyl diphosphate (FPP). The downregulation and overexpression of RcG/FPPS1 in rose petals affected not only geraniol and germacrene D emissions but also dihydro-β-ionol, the latter due to metabolic cross talk of RcG/FPPS1-dependent isoprenoid intermediates trafficking from the cytosol to plastids. Phylogenetic analysis together with functional characterization of G/FPPS orthologs revealed that the G/FPPS activity is conserved among Rosaceae species. Site-directed mutagenesis and molecular dynamic simulations enabled to identify two conserved amino acids that evolved from ancestral FPPSs and contribute to GPP/FPP product specificity. Overall, this study elucidates the origin of the cytosolic GPP for NUDX1-dependent geraniol production, provides insights into the emergence of the RcG/FPPS1 GPPS activity from the ancestral FPPSs, and shows that RcG/FPPS1 plays a key role in the biosynthesis of volatile terpenoid compounds in rose flowers.

Strengthening microbial cell factories for efficient production of bioactive molecules

High demand of bioactive molecules (food additives, antibiotics, plant growth enhancers, cosmetics, pigments and other commercial products) is the prime need for the betterment of human life where the applicability of the synthetic chemical product is on the saturation due to associated toxicity and ornamentations. It has been noticed that the discovery and productivity of such molecules in natural scenarios are limited due to low cellular yields as well as less optimized conventional methods. In this respect, microbial cell factories timely fulfilling the requirement of synthesizing bioactive molecules by improving production yield and screening more promising structural homologues of the native molecule. Where the robustness of the microbial host can be potentially achieved by taking advantage of cell engineering approaches such as tuning functional and adjustable factors, metabolic balancing, adapting cellular transcription machinery, applying high throughput OMICs tools, stability of genotype/phenotype, organelle optimizations, genome editing (CRISPER/Cas mediated system) and also by developing accurate model systems via machine-learning tools. In this article, we provide an overview from traditional to recent trends and the application of newly developed technologies, for strengthening the systemic approaches and providing future directions for enhancing the robustness of microbial cell factories to speed up the production of biomolecules for commercial purposes.

Transcriptional up-regulation of host-specific terpene metabolism in aphid-induced galls of Pistacia palaestina

Galling insects gain food and shelter by inducing specialized anatomical structures in their plant hosts. Such galls often accumulate plant defensive metabolites protecting the inhabiting insects from predation. We previously found that, despite a marked natural chemopolymorphism in natural populations of Pistacia palaestina, the monoterpene content in Baizongia pistaciae-induced galls is substantially higher than in leaves of their hosts. Here we show a general up-regulation of key structural genes in both the plastidial and cytosolic terpene biosynthetic pathways in galls as compared with non-colonized leaves. Novel prenyltransferases and terpene synthases were functionally expressed in Escherichia coli to reveal their biochemical function. Individual Pistacia trees exhibiting chemopolymorphism in terpene compositions displayed differential up-regulation of selected terpene synthase genes, and the metabolites generated by their gene products in vitro corresponded to the monoterpenes accumulated by each tree. Our results delineate molecular mechanisms responsible for the formation of enhanced monoterpene in galls and the observed intraspecific monoterpene chemodiversity displayed in P. palaestina. We demonstrate that gall-inhabiting aphids transcriptionally reprogram their host terpene pathways by up-regulating tree-specific genes, boosting the accumulation of plant defensive compounds for the protection of colonizing insects.

Genome-Wide Identification and Functional Characterization of the Trans-Isopentenyl Diphosphate Synthases Gene Family in Cinnamomum camphora

Trans-isopentenyl diphosphate synthases (TIDSs) genes are known to be important determinants for terpene diversity and the accumulation of terpenoids. The essential oil of Cinnamomum camphora, which is rich in monoterpenes, sesquiterpenes, and other aromatic compounds, has a wide range of pharmacological activities and has therefore attracted considerable interest. However, the TIDS gene family, and its relationship to the camphor tree (C. camphora L. Presl.), has not yet been characterized. In this study, we identified 10 TIDS genes in the genome of the C. camphora borneol chemotype that were unevenly distributed on chromosomes. Synteny analysis revealed that the TIDS gene family in this species likely expanded through segmental duplication events. Furthermore, cis-element analyses demonstrated that C. camphora TIDS (CcTIDS) genes can respond to multiple abiotic stresses. Finally, functional characterization of eight putative short-chain TIDS proteins revealed that CcTIDS3 and CcTIDS9 exhibit farnesyl diphosphate synthase (FPPS) activity, while CcTIDS1 and CcTIDS2 encode geranylgeranyl diphosphate synthases (GGPPS). Although, CcTIDS8 and CcTIDS10 were found to be catalytically inactive alone, they were able to bind to each other to form a heterodimeric functional geranyl diphosphate synthase (GPPS) in vitro, and this interaction was confirmed using a yeast two-hybrid assay. Furthermore, transcriptome analysis revealed that the CcTIDS3, CcTIDS8, CcTIDS9, and CcTIDS10 genes were found to be more active in C. camphora roots as compared to stems and leaves, which were verified by quantitative real-time PCR (qRT-PCR). These novel results provide a foundation for further exploration of the role of the TIDS gene family in camphor trees, and also provide a potential mechanism by which the production of camphor tree essential oil could be increased for pharmacological purposes through metabolic engineering.