Co-Compartmentation of Terpene Biosynthesis and Storage via Synthetic Droplet

Navigating the challenges of engineering composite specialized metabolite pathways in plants

Plants are a valuable source of diverse specialized metabolites with numerous applications. However, these compounds are often produced in limited quantities, particularly under unfavorable ecological conditions. To achieve sufficient levels of target metabolites, alternative strategies such as pathway engineering in heterologous systems like microbes (e.g., bacteria and fungi) or cell-free systems can be employed. Another approach is plant engineering, which aims to either enhance the native production in the original plant or reconstruct the target pathway in a model plant system. Although increasing metabolite production in the native plant is a promising strategy, these source plants are often exotic and pose significant challenges for genetic manipulation. Effective pathway engineering requires comprehensive prior knowledge of the genes and enzymes involved, as well as the precursor, intermediate, branching, and final metabolites. Thus, a thorough elucidation of the biosynthetic pathway is closely linked to successful metabolic engineering in host or model systems. In this review, we highlight recent advances in strategies for biosynthetic pathway elucidation and metabolic engineering. We focus on efforts to engineer complex, multi-step pathways that require the expression of at least eight genes for transient and three genes for stable transformation. Reports on the engineering of complex pathways in stably transformed plants remain relatively scarce. We discuss the major hurdles in pathway elucidation and strategies for overcoming them, followed by an overview of achievements, challenges, and solutions in pathway reconstitution through metabolic engineering. Recent advances including computer-based predictions offer valuable platforms for the sustainable production of specialized metabolites in plants.

Prospects for synthetic biology in 21st Century agriculture

Plant synthetic biology has emerged as a transformative field in agriculture, offering innovative solutions to enhance food security, provide resilience to climate change, and transition to sustainable farming practices. By integrating advanced genetic tools, computational modeling, and systems biology, researchers can precisely modify plant genomes to enhance traits such as yield, stress tolerance, and nutrient use efficiency. The ability to design plants with specific characteristics tailored to diverse environmental conditions and agricultural needs holds great potential to address global food security challenges. Here, we highlight recent advancements and applications of plant synthetic biology in agriculture, focusing on key areas such as photosynthetic efficiency, nitrogen fixation, drought tolerance, pathogen resistance, nutrient use efficiency, biofortification, climate resilience, microbiology engineering, synthetic plant genomes, and the integration of artificial intelligence (AI) with synthetic biology. These innovations aim to maximize resource use efficiency, reduce reliance on external inputs, and mitigate environmental impacts associated with conventional agricultural practices. Despite challenges related to regulatory approval and public acceptance, the integration of synthetic biology in agriculture holds immense promise for creating more resilient and sustainable agricultural systems, contributing to global food security and environmental sustainability. Rigorous multi-field testing of these approaches will undoubtedly be required to ensure reproducibility.

Advances in multi-enzyme co-localization strategies for the construction of microbial cell factory

Biomanufacturing, driven by technologies such as synthetic biology, offers significant potential to advance the bioeconomy and promote sustainable development. It is anticipated to transform traditional manufacturing and become a key industry in future strategies. Cell factories are the core of biomanufacturing. The advancement of synthetic biology and growing market demand have led to the production of a greater variety of natural products and increasingly complex metabolic pathways. However, this progress also presents challenges, notably the conflict between natural product production and chassis cell growth. This conflict results in low productivity and yield, adverse side effects, metabolic imbalances, and growth retardation. Enzyme co-localization strategies have emerged as a promising solution. This article reviews recent progress and applications of these strategies in constructing cell factories for efficient natural product production. It comprehensively describes the applications of enzyme-based compartmentalization, metabolic pathway-based compartmentalization, and synthetic organelle-based compartmentalization in improving product titers. The article also explores future research directions and the prospects of combining multiple strategies with advanced technologies.

Engineered poplar for bioproduction of the triterpene squalene

Building sustainable platforms to produce biofuels and specialty chemicals has become an increasingly important strategy to supplement and replace fossil fuels and petrochemical-derived products. Terpenoids are the most diverse class of natural products that have many commercial roles as specialty chemicals. Poplar is a fast growing, biomassdense bioenergy crop with many species known to produce large amounts of the hemiterpene isoprene, suggesting an inherent capacity to produce significant quantities of other terpenes. Here we aimed to engineer poplar with optimized pathways to produce squalene, a triterpene commonly used in cosmetic oils, a potential biofuel candidate, and the precursor to the further diversified classes of triterpenoids and sterols. The squalene production pathways were either re-targeted from the cytosol to plastids or co-produced with lipid droplets in the cytosol. Squalene and lipid droplet co-production appeared to be toxic, which we hypothesize to be due to disruption of adventitious root formation, suggesting a need for tissue specific production. Plastidial squalene production enabled up to 0.63 mg/g fresh weight in leaf tissue, which also resulted in reductions in isoprene emission and photosynthesis. These results were also studied through a technoeconomic analysis, providing further insight into developing poplar as a production host.

Endogenous Production and Vibrational Analysis of Heavy-Isotope-Labeled Peptides from Cyanobacteria

Stable isotope labeling is an extremely useful tool for characterizing the structure, tracing the metabolism, and imaging the distribution of natural products in living organisms using mass-sensitive measurement techniques. In this study, a cyanobacterium was cultured in 15 N/13 C-enriched media to endogenously produce labeled, bioactive oligopeptides. The extent of heavy isotope incorporation in these peptides was determined with LC-MS, while the overall extent of heavy isotope incorporation in whole cells was studied with nanoSIMS and AFM-IR. Up to 98 % heavy isotope incorporation was observed in labeled cells. Three of the most abundant peptides, microcystin-LR (MCLR), cyanopeptolin-A (CYPA), and aerucyclamide-A (ACAA), were isolated and further studied with Raman and FTIR spectroscopies and DFT calculations. This revealed several IR and Raman active vibrations associated with functional groups not common in ribosomal peptides, like diene, ester, thiazole, thiazoline, and oxazoline groups, which could be suitable for future vibrational imaging studies. More broadly, this study outlines a simple and relatively inexpensive method for producing heavy-labeled natural products. Manipulating the bacterial culture conditions by the addition of specific types and amounts of heavy-labeled nutrients provides an efficient means of producing heavy-labeled natural products for mass-sensitive imaging studies.

Plant Engineering to Enable Platforms for Sustainable Bioproduction of Terpenoids

Terpenoids represent the most diverse class of natural products, with a broad spectrum of industrial relevance including applications in green solvents, flavors and fragrances, nutraceuticals, colorants, and therapeutics. They are typically challenging to extract from their natural sources, where they occur in small amounts and mixtures of related but unwanted byproducts. Formal chemical synthesis, where established, is reliant on petrochemistry. Hence, there is great interest in developing sustainable solutions to assemble biosynthetic pathways in engineered host organisms. Metabolic engineering for chemical production has largely focused on microbial hosts, yet plants offer a sustainable production platform. In addition to containing the precursor pathways that generate the terpenoid building blocks as well as the cell structures and compartments required, or tractable localization for the enzymes involved, plants may provide a low input system to produce these chemicals using carbon dioxide and sunlight only. There have been significant recent advancements in the discovery of pathways to terpenoids of interest as well as strategies to boost yields in host plants. While part of the phytochemical field is focusing on the discovery of biosynthetic pathways, this review will focus on advancements using the pathway toolbox and toward engineering plants for the production of terpenoids. We will highlight strategies currently used to produce target products, optimization of known pathways to improve yields, compartmentalization of pathways within cells, and genetic tools developed to facilitate complex engineering of biosynthetic pathways. These advancements in Synthetic Biology are bringing engineered plant systems closer to commercially relevant hosts for the bioproduction of terpenoids.

Subcellular compartmentalization in the biosynthesis and engineering of plant natural products

Plant natural products (PNPs) are specialized metabolites with diverse bioactivities. They are extensively used in the pharmaceutical, cosmeceutical and food industries. PNPs are synthesized in plant cells by enzymes that are distributed in different subcellular compartments with unique microenvironments, such as ions, co-factors and substrates. Plant metabolic engineering is an emerging and promising approach for the sustainable production of PNPs, for which the knowledge of the subcellular compartmentalization of their biosynthesis is instrumental. In this review we describe the state of the art on the role of subcellular compartments in the biosynthesis of major types of PNPs, including terpenoids, phenylpropanoids, alkaloids and glucosinolates, and highlight the efforts to target biosynthetic pathways to subcellular compartments in plants. In addition, we will discuss the challenges and strategies in the field of plant synthetic biology and subcellular engineering. We expect that newly developed methods and tools, together with the knowledge gained from the microbial chassis, will greatly advance plant metabolic engineering.

Quantitative Raman chemical imaging of intracellular drug-membrane aggregates and small molecule drug precipitates in cytoplasmic organelles

Raman confocal microscopes have been used to visualize the distribution of small molecule drugs within different subcellular compartments. This visualization allows the discovery, characterization, and detailed analysis of the molecular transport phenomena underpinning the Volume of Distribution - a key parameter governing the systemic pharmacokinetics of small molecule drugs. In the specific case of lipophilic small molecules with large Volumes of Distribution, chemical imaging studies using Raman confocal microscopes have revealed how weakly basic, poorly soluble drug molecules can accumulate inside cells by forming stable, supramolecular complexes in association with cytoplasmic membranes or by precipitating out within organelles. To study the self-assembly and function of the resulting intracellular drug inclusions, Raman chemical imaging methods have been developed to measure and map the mass, concentration, and ionization state of drug molecules at a microscopic, subcellular level. Beyond the field of drug delivery, Raman chemical imaging techniques relevant to the study of microscopic drug precipitates and drug-lipid complexes which form inside cells are also being developed by researchers with seemingly unrelated scientific interests. Highlighting advances in data acquisition, calibration methods, and computational data management and analysis tools, this review will cover a decade of technological developments that enable the conversion of spectral signals obtained from Raman confocal microscopes into new discoveries and information about previously unknown, concentrative drug transport pathways driven by soluble-to-insoluble phase transitions occurring within the cytoplasmic organelles of eukaryotic cells.

Engineering Nicotiana tabacum trichomes for triterpenic acid production

In this work, we aimed at implementing the biosynthesis of triterpenic acids in Nicotiana tabacum glandular trichomes. Although endogenous genes coding for enzymes involved in such biosynthetic pathway are found in the Nicotiana tabacum genome, implementing such pathway specifically in glandular trichomes required to boost endogenous enzymatic activities. Five transgenes coding for a farnesyl-diphosphate synthase, a squalene synthase, a squalene epoxidase, a beta-amyrin synthase and a beta-amyrin 28-monooxygenase were introduced in N.tabacum, their expression being driven by pMALD1, a trichome-specific transcriptional promoter. This study aimed at testing whether sinking isoprenoid precursors localized in plastids, by exploiting potential cross-talks allowing the exchange of terpenoid pools from the chloroplast to the cytosol, could be a way to improve overall yield. By analyzing metabolites extracted from entire leaves, a low amount of ursolic acid was detected in plants expressing the five transgenes. Our study shows that the terpene biosynthetic pathway could be, in part, redirected in N.tabacum glandular trichomes with no deleterious phenotype at the whole plant level (chlorosis, dwarfism,…). In light of our results, possible ways to improve the final yield are discussed.

Making small molecules in plants: A chassis for synthetic biology-based production of plant natural products

Plant natural products have been extensively exploited in food, medicine, flavor, cosmetic, renewable fuel, and other industrial sectors. Synthetic biology has recently emerged as a promising means for the cost-effective and sustainable production of natural products. Compared with engineering microbes for the production of plant natural products, the potential of plants as chassis for producing these compounds is underestimated, largely due to challenges encountered in engineering plants. Knowledge in plant engineering is instrumental for enabling the effective and efficient production of valuable phytochemicals in plants, and also paves the way for a more sustainable future agriculture. In this manuscript, we briefly recap the biosynthesis of plant natural products, focusing primarily on industrially important terpenoids, alkaloids, and phenylpropanoids. We further summarize the plant hosts and strategies that have been used to engineer the production of natural products. The challenges and opportunities of using plant synthetic biology to achieve rapid and scalable production of high-value plant natural products are also discussed.

Visualization of a Limonene Synthesis Metabolon Inside Living Bacteria by Hyperspectral SRS Microscopy

Monitoring biosynthesis activity at single-cell level is key to metabolic engineering but is still difficult to achieve in a label-free manner. Using hyperspectral stimulated Raman scattering imaging in the 670-900 cm-1 region, localized limonene synthesis are visualized inside engineered Escherichia coli. The colocalization of limonene and GFP-fused limonene synthase is confirmed by co-registered stimulated Raman scattering and two-photon fluorescence images. The finding suggests a limonene synthesis metabolon with a polar distribution inside the cells. This finding expands the knowledge of de novo limonene biosynthesis in engineered bacteria and highlights the potential of SRS chemical imaging in metabolic engineering research.

Plant-based engineering for production of high-valued natural products

Covering: up to March 2022Plants are a unique source of complex specialized metabolites, many of which play significant roles in human society. In many cases, however, the availability of these metabolites from naturally occurring sources fails to meet current demands. Thus, there is much interest in expanding the production capacity of target plant molecules. Traditionally, plant breeding, chemical synthesis, and microbial fermentation are considered the primary routes towards large scale production of natural products. Here, we explore the advances, challenges, and future of plant engineering as a complementary path. Although plants are an integral part of our food and agricultural systems and sustain an extensive array of chemical constituents, their complex genetics and physiology have prevented the optimal exploitation of plants as a production chassis. We highlight emerging engineering tools and scientific advances developed in recent years that have improved the prospects of using plants as a sustainable and scalable production platform. We also discuss technological limitations and overall economic outlook of plant-based production of natural products.

Compartmentalization and transporter engineering strategies for terpenoid synthesis

Microbial cell factories for terpenoid synthesis form a less expensive and more environment-friendly approach than chemical synthesis and extraction, and are thus being regarded as mainstream research recently. Organelle compartmentalization for terpenoid synthesis has received much attention from researchers owing to the diverse physiochemical characteristics of organelles. In this review, we first systematically summarized various compartmentalization strategies utilized in terpenoid production, mainly plant terpenoids, which can provide catalytic reactions with sufficient intermediates and a suitable environment, while bypassing competing metabolic pathways. In addition, because of the limited storage capacity of cells, strategies used for the expansion of specific organelle membranes were discussed. Next, transporter engineering strategies to overcome the cytotoxic effects of terpenoid accumulation were analyzed. Finally, we discussed the future perspectives of compartmentalization and transporter engineering strategies, with the hope of providing theoretical guidance for designing and constructing cell factories for the purpose of terpenoid production.

Pathway Engineering, Re-targeting, and Synthetic Scaffolding Improve the Production of Squalene in Plants

Plants are increasingly becoming an option for sustainable bioproduction of chemicals and complex molecules like terpenoids. The triterpene squalene has a variety of biotechnological uses and is the precursor to a diverse array of triterpenoids, but we currently lack a sustainable strategy to produce large quantities for industrial applications. Here, we further establish engineered plants as a platform for production of squalene through pathway re-targeting and membrane scaffolding. The squalene biosynthetic pathway, which natively resides in the cytosol and endoplasmic reticulum, was re-targeted to plastids, where screening of diverse variants of enzymes at key steps improved squalene yields. The highest yielding enzymes were used to create biosynthetic scaffolds on co-engineered, cytosolic lipid droplets, resulting in squalene yields up to 0.58 mg/gFW or 318% higher than a cytosolic pathway without scaffolding during transient expression. These scaffolds were also re-targeted to plastids where they associated with membranes throughout, including the formation of plastoglobules or plastidial lipid droplets. Plastid scaffolding ameliorated the negative effects of squalene biosynthesis and showed up to 345% higher rates of photosynthesis than without scaffolding. This study establishes a platform for engineering the production of squalene in plants, providing the opportunity to expand future work into production of higher-value triterpenoids.

Altered Carbon Partitioning Enhances CO2 to Terpene Conversion in Cyanobacteria

Photosynthetic terpene production represents one of the most carbon and energy-efficient routes for converting CO2 into hydrocarbon. In photosynthetic organisms, metabolic engineering has led to limited success in enhancing terpene productivity, partially due to the low carbon partitioning. In this study, we employed systems biology analysis to reveal the strong competition for carbon substrates between primary metabolism (e.g., sucrose, glycogen, and protein synthesis) and terpene biosynthesis in Synechococcus elongatus PCC 7942. We then engineered key "source" and "sink" enzymes. The "source" limitation was overcome by knocking out either sucrose or glycogen biosynthesis to significantly enhance limonene production via altered carbon partitioning. Moreover, a fusion enzyme complex with geranyl diphosphate synthase (GPPS) and limonene synthase (LS) was designed to further improve pathway kinetics and substrate channeling. The synergy between "source" and "sink" achieved a limonene titer of 21.0 mg/L. Overall, the study demonstrates that balancing carbon flux between primary and secondary metabolism can be an effective approach to enhance terpene bioproduction in cyanobacteria. The design of "source" and "sink" synergy has significant potential in improving natural product yield in photosynthetic species.

Machine learning-informed and synthetic biology-enabled semi-continuous algal cultivation to unleash renewable fuel productivity

Algal biofuel is regarded as one of the ultimate solutions for renewable energy, but its commercialization is hindered by growth limitations caused by mutual shading and high harvest costs. We overcome these challenges by advancing machine learning to inform the design of a semi-continuous algal cultivation (SAC) to sustain optimal cell growth and minimize mutual shading. An aggregation-based sedimentation (ABS) strategy is then designed to achieve low-cost biomass harvesting and economical SAC. The ABS is achieved by engineering a fast-growing strain, Synechococcus elongatus UTEX 2973, to produce limonene, which increases cyanobacterial cell surface hydrophobicity and enables efficient cell aggregation and sedimentation. SAC unleashes cyanobacterial growth potential with 0.1 g/L/hour biomass productivity and 0.2 mg/L/hour limonene productivity over a sustained period in photobioreactors. Scaling-up the SAC with an outdoor pond system achieves a biomass yield of 43.3 g/m²/day, bringing the minimum biomass selling price down to approximately $281 per ton.

Growth limitation caused by mutual shading and the high harvest cost hamper algal biofuel production. Here, the authors overcome these two problems by designing a semi-continuous algal cultivation system and an aggregation-based sedimentation strategy to achieve high levels production of biomass and limonene.

ATP Drives Efficient Terpene Biosynthesis in Marine Thraustochytrids

Understanding carbon flux controlling mechanisms in a tangled metabolic network is an essential question of cell metabolism. Secondary metabolism, such as terpene biosynthesis, has evolved with low carbon flux due to inherent pathway constraints. Thraustochytrids are a group of heterotrophic marine unicellular protists and can accumulate terpenoids under the high-salt conditions in their natural environment. However, the mechanism behind terpene accumulation is not well understood. Here, we show that terpene biosynthesis in Thraustochytrium sp. ATCC 26185 is constrained by local thermodynamics in the mevalonate pathway. Thermodynamic analysis reveals metabolite limitation in the nondecarboxylative Claisen condensation of acetyl-coenzyme A (CoA) to the acetoacetyl-CoA step, catalyzed by the acetyl-CoA acetyltransferase (ACAT). Through a sodium-elicited mechanism, higher respiration leads to increased ATP investment into the mevalonate pathway, providing a strong thermodynamic driving force for enhanced terpene biosynthesis. Proteomic and metabolomic analyses further show that the increased ATP demands are fulfilled by shifting energy generation from carbohydrate to lipid oxidation. This study demonstrates a unique strategy in nature that uses ATP to drive a low-flux metabolic pathway, providing an alternative solution for efficient terpene metabolic engineering. IMPORTANCE Terpenoids are a large class of lipid molecules with important biological functions and diverse industrial and medicinal applications. Metabolic engineering for terpene production has been hindered by the low-flux distribution to its biosynthesis pathway. In practice, a high substrate load is generally required to reach high product titers. Here, we show that mevalonate-derived terpene biosynthesis is constrained by local pathway thermodynamics, which can only be partially relieved by increasing substrate levels. Through comparative omics and biochemical analyses, we discovered a unique mechanism for high terpene accumulation in marine protist thraustochytrids. Through a sodium-induced mechanism, thraustochytrids shift their energy metabolism from carbohydrate to lipid oxidation for enhanced ATP production, providing a strong thermodynamic driving force for efficient terpene biosynthesis. This study reveals an important mechanism in eukaryotes to overcome the thermodynamic constraint in low-flux pathways by increased ATP consumption. Engineering energy metabolism thus provides an important alternative to relieve flux constraints in low-flux and energy-consuming pathways.

Metabolic engineering of Yarrowia lipolytica for improving squalene production

The aim of this present research is to enhance the squalene production in Yarrowia lipolytica using pathway engineering and bioprocess engineering. Firstly, to improve the production of squalene, the endogenous HMG-CoA reductase (HMG1) was overexpressed in Y. lipolytica to yield 208.88 mg/L squalene. Secondly, the HMG1 and diacylglycerol acyltranferase (DGA1) were co-overexpressed, the derived recombinant Y. lipolytica SQ-1 strain produced 439.14 mg/L of squalene. Thirdly, by optimizing the fermentation medium, the improved titer of squalene with 514.34 mg/L was obtained by the engineered strain SQ-1 grown on YPD-80 medium. Finally, by optimizing the addition concentrations of acetate, citrate and terbinafine, the 731.18 mg/L squalene was produced in the engineered strain SQ-1 with the addition of 0.5 mg/L terbinafine. This work describes the highest reported squalene titer in Y. lipolytica to date. This study will provide the foundation for further engineering Y. lipolytica capable of cost-efficiently producing squalene.

Neem secretory cells: developmental cytology and indications of cell autotoxicity

The neem tree (Azadirachta indica A.Juss.) contains a range of biologically active compounds-mainly triterpenoids produced in single secretory cells, which are distributed among all plant parts. Neem secretions are toxic to animal cells, triggering autolytic mechanisms that culminate in cell disruption. However, little is known about the self-toxicity of these secretions to the cells that produce them. We carried out an anatomical, histochemical, and ultrastructural investigation of neem's single secretory cells in the shoot apex and in young leaves. We evaluated the morphological changes as possible evidences of stress reactions to their own secretions. The subcellular apparatus involved in synthesis and compartmentation was consistent with hydrophilic and lipophilic secretions. Polymorphic plastids devoid of thylakoids and abundant smooth endoplasmic reticulum in the later stages of differentiation are comparable with previous reports on neem cotyledons with regard to terpenoid synthesis. However, secretions were compartmentalized within autophagic vacuoles and periplasmic spaces instead of in terpenoid vesicles. Cellular swelling, increased vesiculation, dilatation of endoplasmic reticulum cisternae, mitochondrial hypertrophy in the cristolysis process, autolytic vacuoles, and vacuolar degeneration culminating in protoplast autolysis are all consistent with early indications of autotoxicity. The signaling stress reaction mechanism was expressed as cytoplasmic deposits of calcium salt and by the expression of a 70-kDa heat-shock protein. The morphological and histochemical changes in the secreting cells are comparable with those described in animal cells exposed to neem oil. Our data provide evidence of cell damage and signaling reactions linked to these cells' own secretions before autolysis.

Production of β-carotene in Saccharomyces cerevisiae through altering yeast lipid metabolism

Saccharomyces cerevisiae is a widely used cell factory for the production of fuels and chemicals. However, as a non-oleaginous yeast, S. cerevisiae has a limited production capacity for lipophilic compounds, such as β-carotene. To increase its accumulation of β-carotene, we engineered different lipid metabolic pathways in a β-carotene producing strain and investigated the relationship between lipid components and the accumulation of β-carotene. We found that overexpression of sterol ester synthesis genes ARE1 and ARE2 increased β-carotene yield by 1.5-fold. Deletion of phosphatidate phosphatase (PAP) genes (PAH1, DPP1, and LPP1) also increased β-carotene yield by twofold. Combining these two strategies resulted in a 2.4-fold improvement in β-carotene production compared with the starting strain. These results demonstrated that regulating lipid metabolism pathways is important for β-carotene accumulation in S. cerevisiae, and may also shed insights to the accumulation of other lipophilic compounds in yeast.

Connecting moss lipid droplets to patchoulol biosynthesis

Plant-derived terpenoids are extensively used in perfume, food, cosmetic and pharmaceutical industries, and several attempts are being made to produce terpenes in heterologous hosts. Native hosts have evolved to accumulate large quantities of terpenes in specialized cells. However, heterologous cells lack the capacity needed to produce and store high amounts of non-native terpenes, leading to reduced growth and loss of volatile terpenes by evaporation. Here, we describe how to direct the sesquiterpene patchoulol production into cytoplasmic lipid droplets (LDs) in Physcomitrium patens (syn. Physcomitrella patens), by attaching patchoulol synthase (PTS) to proteins linked to plant LD biogenesis. Three different LD-proteins: Oleosin (PpOLE1), Lipid Droplet Associated Protein (AtLDAP1) and Seipin (PpSeipin325) were tested as anchors. Ectopic expression of PTS increased the number and size of LDs, implying an unknown mechanism between heterologous terpene production and LD biogenesis. The expression of PTS physically linked to Seipin increased the LD size and the retention of patchoulol in the cell. Overall, the expression of PTS was lower in the anchored mutants than in the control, but when normalized to the expression the production of patchoulol was higher in the seipin-linked mutants.

A Review of Diatom Lipid Droplets

The dynamic nutrient availability and photon flux density of diatom habitats necessitate buffering capabilities in order to maintain metabolic homeostasis. This is accomplished by the biosynthesis and turnover of storage lipids, which are sequestered in lipid droplets (LDs). LDs are an organelle conserved among eukaryotes, composed of a neutral lipid core surrounded by a polar lipid monolayer. LDs shield the intracellular environment from the accumulation of hydrophobic compounds and function as a carbon and electron sink. These functions are implemented by interconnections with other intracellular systems, including photosynthesis and autophagy. Since diatom lipid production may be a promising objective for biotechnological exploitation, a deeper understanding of LDs may offer targets for metabolic engineering. In this review, we provide an overview of diatom LD biology and biotechnological potential.

Dynamic metabolic solutions to the sessile life style of plants

Plants are sessile organisms. To compensate for not being able to escape when challenged by unfavorable growth conditions, pests or herbivores, plants have perfected their metabolic plasticity by having developed the capacity for on demand dynamic biosynthesis and storage of a plethora of phytochemicals.

Covering: up to 2018

Plants are sessile organisms. To compensate for not being able to escape when challenged by unfavorable growth conditions, pests or herbivores, plants have perfected their metabolic plasticity by having developed the capacity for on demand synthesis of a plethora of phytochemicals to specifically respond to the challenges arising during plant ontogeny. Key steps in the biosynthesis of phytochemicals are catalyzed by membrane-bound cytochrome P450 enzymes which in plants constitute a superfamily. In planta, the P450s may be organized in dynamic enzyme clusters (metabolons) and the genes encoding the P450s and other enzymes in a specific pathway may be clustered. Metabolon formation facilitates transfer of substrates between sequential enzymes and therefore enables the plant to channel the flux of general metabolites towards biosynthesis of specific phytochemicals. In the plant cell, compartmentalization of the operation of specific biosynthetic pathways in specialized plastids serves to avoid undesired metabolic cross-talk and offers distinct storage sites for molar concentrations of specific phytochemicals. Liquid–liquid phase separation may lead to formation of dense biomolecular condensates within the cytoplasm or vacuole allowing swift activation of the stored phytochemicals as required upon pest or herbivore attack. The molecular grid behind plant plasticity offers an endless reservoir of functional modules, which may be utilized as a synthetic biology tool-box for engineering of novel biological systems based on rational design principles. In this review, we highlight some of the concepts used by plants to coordinate biosynthesis and storage of phytochemicals.

Mouse lipogenic proteins promote the co-accumulation of triacylglycerols and sesquiterpenes in plant cells

MAIN CONCLUSION

Mouse FIT2 protein redirects the cytoplasmic terpene biosynthetic machinery to lipid-droplet-forming domains in the ER and this relocalization supports the efficient compartmentalization and accumulation of sesquiterpenes in plant cells. Mouse (Mus musculus) fat storage-inducing transmembrane protein 2 (MmFIT2), an endoplasmic reticulum (ER)-resident protein with an important role in lipid droplet (LD) biogenesis in mammals, can function in plant cells to promote neutral lipid compartmentalization. Surprisingly, in affinity capture experiments, the Nicotiana benthamiana 5-epi-aristolochene synthase (NbEAS), a soluble cytoplasm-localized sesquiterpene synthase, was one of the most abundant proteins that co-precipitated with GFP-tagged MmFIT2 in transient expression assays in N. benthamiana leaves. Consistent with results of pull-down experiments, the subcellular location of mCherry-tagged NbEAS was changed from the cytoplasm to the LD-forming domains in the ER, only when co-expressed with MmFIT2. Ectopic co-expression of NbEAS and MmFIT2 together with mouse diacylglycerol:acyl-CoA acyltransferase 2 (MmDGAT2) in N. benthamiana leaves substantially increased the numbers of cytoplasmic LDs and supported the accumulation of the sesquiterpenes, 5-epi-aristolochene and capsidiol, up to tenfold over levels elicited by Agrobacterium infection alone. Taken together, our results suggest that MmFIT2 recruits sesquiterpene synthetic machinery to ER subdomains involved in LD formation and that this process can enhance the efficiency of sesquiterpene biosynthesis and compartmentalization in plant cells. Further, MmFIT2 and MmDGAT2 represent cross-kingdom lipogenic protein factors that may be used to engineer terpene accumulation more broadly in the cytoplasm of plant vegetative tissues.

Metabolic engineering for enhanced oil in biomass

The world is hungry for energy. Plant oils in the form of triacylglycerol (TAG) are one of the most reduced storage forms of carbon found in nature and hence represent an excellent source of energy. The myriad of applications for plant oils range across foods, feeds, biofuels, and chemical feedstocks as a unique substitute for petroleum derivatives. Traditionally, plant oils are sourced either from oilseeds or tissues surrounding the seed (mesocarp). Most vegetative tissues, such as leaves and stems, however, accumulate relatively low levels of TAG. Since non-seed tissues constitute the majority of the plant biomass, metabolic engineering to improve their low-intrinsic TAG-biosynthetic capacity has recently attracted significant attention as a novel, sustainable and potentially high-yielding oil production platform. While initial attempts predominantly targeted single genes, recent combinatorial metabolic engineering strategies have focused on the simultaneous optimization of oil synthesis, packaging and degradation pathways (i.e., 'push, pull, package and protect'). This holistic approach has resulted in dramatic, seed-like TAG levels in vegetative tissues. With the first proof of concept hurdle addressed, new challenges and opportunities emerge, including engineering fatty acid profile, translation into agronomic crops, extraction, and downstream processing to deliver accessible and sustainable bioenergy.

Cytosolic lipid droplets as engineered organelles for production and accumulation of terpenoid biomaterials in leaves

Cytosolic lipid droplets are endoplasmic reticulum-derived organelles typically found in seeds as reservoirs for physiological energy and carbon to fuel germination. Here, we report synthetic biology approaches to co-produce high-value sesqui- or diterpenoids together with lipid droplets in plant leaves. The formation of cytosolic lipid droplets is enhanced in the transient Nicotiana benthamiana system through ectopic production of WRINKLED1, a key regulator of plastid fatty acid biosynthesis, and a microalgal lipid droplet surface protein. Engineering of the pathways providing the universal C5-building blocks for terpenoids and installation of terpenoid biosynthetic pathways through direction of the enzymes to native and non-native compartments boost the production of target terpenoids. We show that anchoring of distinct biosynthetic steps onto the surface of lipid droplets leads to efficient production of terpenoid scaffolds and functionalized terpenoids. The co-produced lipid droplets "trap" the terpenoids in the cells.