A lavender ABC transporter confers resistance to monoterpene toxicity in yeast

Recent Advances and Multiple Strategies of Monoterpenoid Overproduction in Saccharomyces cerevisiae and Yarrowia lipolytica

Monoterpenoids are an important subclass of terpenoids that play important roles in the energy, cosmetics, pharmaceuticals, and fragrances fields. With the development of biotechnology, microbial synthesis of monoterpenoids has received great attention. Yeasts such Saccharomyces cerevisiae and Yarrowia lipolytica are emerging as potential hosts for monoterpenoids production because of unique advantages including rapid growth cycles, mature gene editing tools, and clear genetic background. Recently, advancements in metabolic engineering and fermentation engineering have significantly enhanced the accumulation of monoterpenoids in cell factories. First, this review introduces the biosynthetic pathway of monoterpenoids and comprehensively summarizes the latest production strategies, which encompass enhancing precursor flux, modulating the expression of rate-limited enzymes, suppressing competitive pathway flux, mitigating cytotoxicity, optimizing substrate utilization, and refining the fermentation process. Subsequently, this review introduces four representative monoterpenoids. Finally, we outline the future prospects for efficient construction cell factories tailored for the production of monoterpenoids and other terpenoids.

Biosynthesis Progress of High-Energy-Density Liquid Fuels Derived from Terpenes

High-energy-density liquid fuels (HED fuels) are essential for volume-limited aerospace vehicles and could serve as energetic additives for conventional fuels. Terpene-derived HED biofuel is an important research field for green fuel synthesis. The direct extraction of terpenes from natural plants is environmentally unfriendly and costly. Designing efficient synthetic pathways in microorganisms to achieve high yields of terpenes shows great potential for the application of terpene-derived fuels. This review provides an overview of the current research progress of terpene-derived HED fuels, surveying terpene fuel properties and the current status of biosynthesis. Additionally, we systematically summarize the engineering strategies for biosynthesizing terpenes, including mining and engineering terpene synthases, optimizing metabolic pathways and cell-level optimization, such as the subcellular localization of terpene synthesis and adaptive evolution. This article will be helpful in providing insight into better developing terpene-derived HED fuels.

Exploring the co-operativity of secretory structures for defense and pollination in flowering plants

MAIN CONCLUSION

In flowers multiple secretory systems cooperate to deliver specialized metabolites to support specific roles in defence and pollination. The collective roles of cell types, enzymes, and transporters are discussed. The interplay between reproductive strategies and defense mechanisms in flowering plants has long been recognized, with trade-offs between investment in defense and reproduction predicted. Glandular trichomes and secretory cavities or ducts, which are epidermal and internal structures, play a pivotal role in the secretion, accumulation, and transport of specialized secondary metabolites, and contribute significantly to defense and pollination. Recent investigations have revealed an intricate connection between these two structures, whereby specialized volatile and non-volatile metabolites are exchanged, collectively shaping their respective ecological functions. However, a comprehensive understanding of this profound integration remains largely elusive. In this review, we explore the secretory systems and associated secondary metabolism primarily in Asteraceous species to propose potential shared mechanisms facilitating the directional translocation of these metabolites to diverse destinations. We summarize recent advances in our understanding of the cooperativity between epidermal and internal secretory structures in the biosynthesis, secretion, accumulation, and emission of terpenes, providing specific well-documented examples from pyrethrum (Tanacetum cinerariifolium). Pyrethrum is renowned for its natural pyrethrin insecticides, which accumulate in the flower head, and more recently, for emitting an aphid alarm pheromone. These examples highlight the diverse specializations of secondary metabolism in pyrethrum and raise intriguing questions regarding the regulation of production and translocation of these compounds within and between its various epidermal and internal secretory systems, spanning multiple tissues, to serve distinct ecological purposes. By discussing the cooperative nature of secretory structures in flowering plants, this review sheds light on the intricate mechanisms underlying the ecological roles of terpenes in defense and pollination.

Engineering Saccharomyces cerevisiae for synthesis of β-myrcene and (E)-β-ocimene

Monoterpenes are among the important natural plant terpenes. Monoterpenes usually have the characteristics of volatility and strong aroma. β-Myrcene and its isomer (E)-β-ocimene are typical acyclic monoterpenes. They are high-value monoterpenes that have been widely applied in foods, cosmetics, and medicines. However, large-scale commercial production of β-myrcene and (E)-β-ocimene is restricted by their production method that mainly involves extraction from plant essential oils. Currently, an alternative synthetic route utilizing an engineered microbial platform was proposed for effective production. This study used a Saccharomyces cerevisiae strain previously constructed for squalene production as the starting strain. Farnesyl diphosphate synthase (Erg20) expression was weakened by promoter replacement and screened for optimal myrcene synthase (MS) and ocimene synthase (OS) activities. In the resulting S. cerevisiae engineered for β-myrcene and (E)-β-ocimene synthesis, titers of β-myrcene and (E)-β-ocimene were enhanced by a fusion expressing a mutant Erg20* with the obtained monoterpene synthase and optimizing the added solvent in a two-phase fermentation system. Finally, by scaling up in a 5-L fermenter, 8.12 mg/L of β-myrcene was obtained, which was first reported in yeast, and 34.56 mg/L of (E)-β-ocimene was obtained, which is the highest reported to date. This study provides a new synthesis route for β-myrcene and (E)-β-ocimene.

Supplementary Information

The online version contains supplementary material available at 10.1007/s13205-023-03818-2.

Performance of high solids enzymatic hydrolysis and bioethanol fermentation of food waste under the regulation of saponin

Bioethanol recovery from food waste through high solids enzymatic hydrolysis (HSEH) and high solids bioethanol fermentation (HSBF) alleviate the energy crisis. However, this cause decreased glucose and bioethanol yields due to the high solids content. In this study, saponin was introduced into food waste HSEH and HSBF systems to enhance the product yields. Under the regulation of saponin, the substrate released >90% of the theoretical reducing sugar. The glucose concentration increased by 137.41 g/L after 24 h of HSEH with 2.0% saponin. The bioethanol titer reached 73.2 g/L (1.0%-saponin). Untargeted metabolomics illustrating that saponin had higher antifungal properties at lower concentrations (0.5%-saponin) that caused a decrease in bioethanol yield. The addition of saponin concentrations of 1.0%∼3.0% promoted HSEH, HSBF, and the metabolism of Saccharomyces cerevisiae; thus, 1.0% was suggested for practical use. This study deepened the understanding of saponin in enhancing HSBF and provides theoretical support for further application.

Role of ABC Proteins in Secondary Metabolism and Immune (=Defensive) Response in Seaweeds

Laurencia seaweed species synthesize a broad range of secondary metabolites, mainly terpenes (e.g., elatol), exhibiting diverse ecological roles, such as defense against fouling and herbivores. Recently, an intricate cellular machinery was described concerning terpenes biosynthetic pathways, storage inside corps en cerise (CC), and regulated exocytosis in these species. But for seaweeds in general, the proteins involved in transmembrane transport of secondary metabolites remain unknown. Assays with Rhodamine-123 and cyclosporine A (CSA) revealed the presence of ABC transporters in CC membrane of Laurencia dendroidea. In vivo incubation assays with CSA resulted in CC morphological changes, reduced intracellular elatol concentrations, and increased biofouling cover on the seaweed surface. Cultivation assays in the presence of a marine pathogenic bacteria induced the expression of ABC proteins belonging to the subfamilies ABCB, ABCD, ABCF, and ABCG. The latter subfamily is known to be associated with the transport of plant terpenes. Our results shed new light on the role of ABC proteins in key mechanisms of the defensive system in seaweeds against fouling and herbivory.

Biosynthesis of monoterpenoid and sesquiterpenoid as natural flavors and fragrances

Terpenoids are a large class of plant-derived compounds, that constitute the main components of essential oils and are widely used as natural flavors and fragrances. The biosynthesis approach presents a promising alternative route in terpenoid production compared to plant extraction or chemical synthesis. In the past decade, the production of terpenoids using biotechnology has attracted broad attention from both academia and the industry. With the growing market of flavor and fragrance, the production of terpenoids directed by synthetic biology shows great potential in promoting future market prospects. Here, we reviewed the latest advances in terpenoid biosynthesis. The engineering strategies for biosynthetic terpenoids were systematically summarized from the enzyme, metabolic, and cellular dimensions. Additionally, we analyzed the key challenges from laboratory production to scalable production, such as key enzyme improvement, terpenoid toxicity, and volatility loss. To provide comprehensive technical guidance, we collected milestone examples of biosynthetic mono- and sesquiterpenoids, compared the current application status of chemical synthesis and biosynthesis in terpenoid production, and discussed the cost drivers based on the data of techno-economic assessment. It is expected to provide critical insights into developing translational research of terpenoid biomanufacturing.

PbABCG1 and PbABCG2 transporters are required for the emission of floral monoterpenes in Phalaenopsis bellina

Terrestrial plants emit volatiles into the atmosphere to attract both pollinators and the enemies of herbivores, for defense. Phalaenopsis bellina is a scented orchid species in which the main scent components are monoterpenes, including linalool and geraniol, and their derivatives. Here, we investigated whether ABC transporters are involved in floral scent emission. We carried out whole-genome identification of ABC transporter-related genes using four floral transcriptomics libraries of P. bellina. We identified 86 ABC subfamily G genes related to terpenoid transport. After comparing the gene expression patterns of P. bellina with that of Phalaenopsis aphrodite subsp. formosana, a scentless species, followed by gene-to-gene correlation analysis, PbABCG1 and PbABCG2 were selected. The temporal expression of both PbABCG1 and PbABCG2 was highly correlated with that of the key enzyme PbGDPS and the major transcription factor PbbHLH4 in monoterpene biosynthesis, with optimal expression on day 5 post-anthesis. Spatial gene expression analysis showed that PbABCG1 was highly expressed in sepals, whereas PbABCG2 was expressed in the lip. Subcellular localization with a GFP fusion protein revealed that both PbABCG1 and PbABCG2 are cytoplasmic membrane proteins. Co-downregulation of PbABCG1 and PbABCG2 using both double-strand RNA interference and tobacco rattle virus-based gene silencing led to a significant decrease in monoterpene emission, accompanied by an increase in the internal monoterpene pools. Furthermore, ectopic expression of PbABCG1 and PbABCG2 in an ABC16^- mutant yeast strain rescued its tolerance to geraniol. Altogether, our results indicate that PbABCG1 and PbABCG2 play substantial roles in monoterpene transport/emission in P. bellina floral scent.

The Genetic Complexity of Type-IV Trichome Development Reveals the Steps towards an Insect-Resistant Tomato

The leaves of the wild tomato Solanum galapagense harbor type-IV glandular trichomes (GT) that produce high levels of acylsugars (AS), conferring insect resistance. Conversely, domesticated tomatoes (S. lycopersicum) lack type-IV trichomes on the leaves of mature plants, preventing high AS production, thus rendering the plants more vulnerable to insect predation. We hypothesized that cultivated tomatoes engineered to harbor type-IV trichomes on the leaves of adult plants could be insect-resistant. We introgressed the genetic determinants controlling type-IV trichome development from S. galapagense into cv. Micro-Tom (MT) and created a line named “Galapagos-enhanced trichomes” (MT-Get). Mapping-by-sequencing revealed that five chromosomal regions of S. galapagense were present in MT-Get. Further genetic mapping showed that S. galapagense alleles in chromosomes 1, 2, and 3 were sufficient for the presence of type-IV trichomes on adult organs but at lower densities. Metabolic and gene expression analyses demonstrated that type-IV trichome density was not accompanied by the AS production and exudation in MT-Get. Although the plants produce a significant amount of acylsugars, those are still not enough to make them resistant to whiteflies. We demonstrate that type-IV glandular trichome development is insufficient for high AS accumulation. The results from our study provided additional insights into the steps necessary for breeding an insect-resistant tomato.

Identification of (-)-bornyl diphosphate synthase from Blumea balsamifera and its application for (-)-borneol biosynthesis in Saccharomyces cerevisiae

Borneol is a precious monoterpenoid with two chiral structures, (-)-borneol and (+)-borneol. Bornyl diphosphate synthase is the key enzyme in the borneol biosynthesis pathway. Many (+)-bornyl diphosphate synthases have been reported, but no (-)-bornyl diphosphate synthases have been identified. Blumea balsamifera leaves are rich in borneol, almost all of which is (-)-borneol. In this study, we identified a high-efficiency (-)-bornyl diphosphate synthase (BbTPS3) from B. balsamifera that converts geranyl diphosphate (GPP) to (-)-bornyl diphosphate, which is then converted to (-)-borneol after dephosphorylation in vitro. BbTPS3 exhibited a K_m value of 4.93 ± 1.38 μM for GPP, and the corresponding k_cat value was 1.49 s⁻¹. Multiple strategies were applied to obtain a high-yielding (-)-borneol producing yeast strain. A codon-optimized BbTPS3 protein was introduced into the GPP high-yield strain MD, and the resulting MD-B1 strain produced 1.24 mg·L^-1 (-)-borneol. After truncating the N-terminus of BbTPS3 and adding a Kozak sequence, the (-)-borneol yield was further improved by 4-fold to 4.87 mg·L^-1. Moreover, the (-)-borneol yield was improved by expressing the fusion protein module of ERG20^F96W-N127W-YRSQI-t14-BbTPS3K2, resulting in a final yield of 12.68 mg·L^-1 in shake flasks and 148.59 mg·L^-1 in a 5-L bioreactor. This work is the first reported attempt to produce (-)-borneol by microbial fermentation.

Metabolic engineering of microbes for monoterpenoid production

Monoterpenoids are an important class of natural products that are derived from the condensation of two five‑carbon isoprene subunits. They are widely used for flavouring, fragrances, colourants, cosmetics, fuels, chemicals, and pharmaceuticals in various industries. They can also serve as precursors for the production of many industrially important products. Currently, monoterpenoids are produced predominantly through extraction from plant sources. However, the small quantity of monoterpenoids in nature renders this method of isolation non-economically viable. Similarly impractical is the chemical synthesis of these compounds as they suffer from high energy consumption and pollutant discharge. Microbial biosynthesis, however, exists as a potential solution to these hindrances, but the transformation of cells into efficient factories remains a major impediment. Here, we critically review the recent advances in engineering microbes for monoterpenoid production, with an emphasis on categorized strategies, and discuss the challenges and perspectives to offer guidance for future engineering.

Plasticity engineering of plant monoterpene synthases and application for microbial production of monoterpenoids

Plant monoterpenoids with structural diversities have extensive applications in food, cosmetics, pharmaceuticals, and biofuels. Due to the strong dependence on the geographical locations and seasonal annual growth of plants, agricultural production for monoterpenoids is less effective. Chemical synthesis is also uneconomic because of its high cost and pollution. Recently, emerging synthetic biology enables engineered microbes to possess great potential for the production of plant monoterpenoids. Both acyclic and cyclic monoterpenoids have been synthesized from fermentative sugars through heterologously reconstructing monoterpenoid biosynthetic pathways in microbes. Acting as catalytic templates, plant monoterpene synthases (MTPSs) take elaborate control of the monoterpenoids production. Most plant MTPSs have broad substrate or product properties, and show functional plasticity. Thus, the substrate selectivity, product outcomes, or enzymatic activities can be achieved by the active site mutations and domain swapping of plant MTPSs. This makes plasticity engineering a promising way to engineer MTPSs for efficient production of natural and non-natural monoterpenoids in microbial cell factories. Here, this review summarizes the key advances in plasticity engineering of plant MTPSs, including the fundamental aspects of functional plasticity, the utilization of natural and non-natural substrates, and the outcomes from product isomers to complexity-divergent monoterpenoids. Furthermore, the applications of plasticity engineering for improving monoterpenoids production in microbes are addressed.

Bornyl Diphosphate Synthase From Cinnamomum burmanni and Its Application for (+)-Borneol Biosynthesis in Yeast

(+)-Borneol is a desirable monoterpenoid with effective anti-inflammatory and analgesic effects that is known as soft gold. (+)-bornyl diphosphate synthase is the key enzyme in the (+)-borneol biosynthesis pathway. Despite several reported (+)-bornyl diphosphate synthase genes, relatively low (+)-borneol production hinders the attempts to synthesize it using microbial fermentation. Here, we identified the highly specific (+)-bornyl diphosphate synthase CbTPS1 from Cinnamomum burmanni. An in vitro assay showed that (+)-borneol was the main product of CbTPS1 (88.70% of the total products), and the K _m value was 5.11 ± 1.70 μM with a k _cat value of 0.01 s^-1. Further, we reconstituted the (+)-borneol biosynthetic pathway in Saccharomyces cerevisiae. After tailored truncation and adding Kozak sequences, the (+)-borneol yield was improved by 96.33-fold to 2.89 mg⋅L^-1 compared with the initial strain in shake flasks. This work is the first reported attempt to produce (+)-borneol by microbial fermentation. It lays a foundation for further pathway reconstruction and metabolic engineering production of this valuable natural monoterpenoid.

Genomic and Transcriptomic Study for Screening Genes Involved in the Limonene Biotransformation of Penicillium digitatum DSM 62840

α-Terpineol has been widely used in daily chemical, pharmaceutical, food, and flavor industries due to its pleasant odor with high economic value and pharmacological action. Our previous study showed that Penicillium digitatum DSM 62840 was an efficient biocatalyst for the transformation of limonene to α-terpineol. Thus, it was meaningful to explore the genome features and the gene expression differences of strain DSM 62840 during limonene biotransformation, and the detailed bioconversion pathways. In this study, the functional genes related to limonene bioconversion were investigated using genome and transcriptome sequences analysis. The results showed that the P. digitatum DSM 62840 genome was estimated to be 29.09 Mb and it encoded 9,086 protein-encoding genes. The most annotated genes were associated to some protein metabolism and energy metabolism functions. When the threshold for differentially expressed genes (DEGs) was set at twofold ratio, a total of 4,128, and 4,148 DEGs were identified in P_L_12h (limonene-treated condition) compared with P_0h (blank) and P_12h (limonene-untreated blank), respectively. Among them, the expression levels of genes involved in the biosynthesis of secondary metabolites, energy metabolism and ATP-binding cassette (ABC) transporters were significantly altered during the biotransformation. And the reliability of these results was further confirmed by quantitative real-time polymerase chain reaction (RT-qPCR). Moreover, we found that the enzyme participated in limonene biotransformation was inducible. This enzyme was located in the microsome, and it was inhibited by cytochrome P450 inhibitors. This indicated that the cytochrome P450 may be responsible for the limonene bioconversion. Several differentially expressed cytochrome P450 genes were further identified, such as PDIDSM_85260 and PDIDSM_67430, which were significantly up-regulated with limonene treatment. These genes may be responsible for converting limonene to α-terpineol. Totally, the genomic and transcriptomic data could provide valuable information in the discovery of related-genes which was involved in limonene biotransformation, pathogenicity of fungi, and investigation of metabolites and biological pathways of strain DSM 62840.

Glandular trichomes: micro-organs with model status?

Glandular trichomes are epidermal outgrowths that are the site of biosynthesis and storage of large quantities of specialized metabolites. Besides their role in the protection of plants against biotic and abiotic stresses, they have attracted interest owing to the importance of the compounds they produce for human use; for example, as pharmaceuticals, flavor and fragrance ingredients, or pesticides. Here, we review what novel concepts investigations on glandular trichomes have brought to the field of specialized metabolism, particularly with respect to chemical and enzymatic diversity. Furthermore, the next challenges in the field are understanding the metabolic network underlying the high productivity of glandular trichomes and the transport and storage of metabolites. Another emerging area is the development of glandular trichomes. Studies in some model species, essentially tomato, tobacco, and Artemisia, are now providing the first molecular clues, but many open questions remain: How is the distribution and density of different trichome types on the leaf surface controlled? When is the decision for an epidermal cell to differentiate into one type of trichome or another taken? Recent advances in gene editing make it now possible to address these questions and promise exciting discoveries in the near future.

Engineering transport systems for microbial production

The rapid development in the field of metabolic engineering has enabled complex modifications of metabolic pathways to generate a diverse product portfolio. Manipulating substrate uptake and product export is an important research area in metabolic engineering. Optimization of transport systems has the potential to enhance microbial production of renewable fuels and chemicals. This chapter comprehensively reviews the transport systems critical for microbial production as well as current genetic engineering strategies to improve transport functions and thus production metrics. In addition, this chapter highlights recent advancements in engineering microbial efflux systems to enhance cellular tolerance to industrially relevant chemical stress. Lastly, future directions to address current technological gaps are discussed.