Engineering storage capacity for volatile sesquiterpenes in Nicotiana benthamiana leaves

In vitro activities and mechanisms of action of anti-cancer molecules from African medicinal plants: a systematic review

Cancer is one of the leading causes of death worldwide. In recent years, African countries have been faced with a rapid increase in morbidity and mortality due to this pathology. Management is often complicated by the high treatment costs, side effects and the increasing occurrence of resistance to treatments. The identification of new active ingredients extracted from endemic medicinal plants is definitively an interesting approach for the implementation of new therapeutic strategies: their extraction is often lower cost; their identification is based on an ethnobotanical history and a tradipratic approach; their use by low-income populations is simpler; this can help in the development of new synthetic molecules that are more active, more effective and with fewer side effects. The objective of this review is to document the molecules derived from African medicinal plants whose in vitro anti-cancer activities and the mechanisms of molecular actions have been identified. From the scientific databases Science Direct, PubMed and Google Scholar, we searched for publications on compounds isolated from African medicinal plants and having activity on cancer cells in culture. The data were analyzed in particular with regard to the cytotoxicity of the compounds and their mode of action. A total of 90 compounds of these African medicinal plants were selected. They come from nine chemical groups: alkaloids, flavonoids, polyphenols, quinones, saponins, steroids, terpenoids, xanthones and organic sulfides. These compounds have been associated with several cellular effects: i) Cytotoxicity, including caspase activation, alteration of mitochondrial membrane potential, and/or induction of reactive oxygen species (ROS); ii) Anti-angiogenesis; iii) Anti-metastatic properties. This review points out that the cited African plants are rich in active ingredients with anticancer properties. It also stresses that screening of these anti-tumor active ingredients should be continued at the continental scale. Altogether, this work provides a rational basis for the selection of phytochemical compounds for use in clinical trials.

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.

Improved antioxidant activities of spice require enrichment of distinct yet closely-related metabolic pathways

Improved biosynthesis of commercially and pharmacologically relevant phytometabolites through genetic and metabolic engineering is a lucrative strategy for crop improvement. However, identifying appropriate biosynthetic pathways pertaining to specific bioactivities has been challenging since the major metabolic pathways remain closely interconnected. Here we propose a reverse association strategy in which, based on the phytochemical profile, putative target metabolic pathways could be identified for increased production of phytochemicals. Dried seed fruits of Coriandrum sativum, Trachyspermum ammi, Cuminum cyminum, and Foeniculum vulgare (family Apiaceae) were subjected to untargeted gas chromatography-mass spectrometry-based phytochemical profiling followed by evaluation of the overall antioxidant profile using multiple antioxidant assays. Using bioinformatics approaches, specific phytochemical classes and the enrichment of their respective biosynthetic pathways were identified. Collectively, the data suggest enrichment of isoprenoids and fatty acids biosynthetic pathways. The close association of metabolic pathways with antioxidant capacities indicated a need for enrichment of specific yet closely-related metabolic pathways to achieve an improved quality of spices for better antioxidant effects.

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.

Acyl-CoA-dependent and acyl-CoA-independent avocado acyltransferases positively influence oleic acid content in nonseed triacylglycerols

In higher plants, acyl-CoA:diacylglycerol acyltransferase (DGAT) and phospholipid:diacylglycerol acyltransferase (PDAT) catalyze the terminal step of triacylglycerol (TAG) synthesis in acyl-CoA-dependent and -independent pathways, respectively. Avocado (Persea americana) mesocarp, a nonseed tissue, accumulates significant amounts of TAG (~70% by dry weight) that is rich in heart-healthy oleic acid (18:1). The oil accumulation stages of avocado mesocarp development coincide with high expression levels for type-1 DGAT (DGAT1) and PDAT1, although type-2 DGAT (DGAT2) expression remains low. The strong preference for oleic acid demonstrated by the avocado mesocarp TAG biosynthetic machinery represents lucrative biotechnological opportunities, yet functional characterization of these three acyltransferases has not been explored to date. We expressed avocado PaDGAT1, PaDGAT2, and PaPDAT1 in bakers' yeast and leaves of Nicotiana benthamiana. PaDGAT1 complemented the TAG biosynthesis deficiency in the quadruple mutant yeast strain H1246, and substantially elevated total cellular lipid content. In vitro enzyme assays showed that PaDGAT1 prefers oleic acid compared to palmitic acid (16:0). Both PaDGAT1 and PaPDAT1 increased the lipid content and elevated oleic acid levels when expressed independently or together, transiently in N. benthamiana leaves. These results indicate that PaDGAT1 and PaPDAT1 prefer oleate-containing substrates, and their coordinated expression likely contributes to sustained TAG synthesis that is enriched in oleic acid. This study establishes a knowledge base for future metabolic engineering studies focused on exploitation of the biochemical properties of PaDGAT1 and PaPDAT1.

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.

Acyl-CoA:diacylglycerol acyltransferase: Properties, physiological roles, metabolic engineering and intentional control

Acyl-CoA:diacylglycerol acyltransferase (DGAT, EC 2.3.1.20) catalyzes the last reaction in the acyl-CoA-dependent biosynthesis of triacylglycerol (TAG). DGAT activity resides mainly in membrane-bound DGAT1 and DGAT2 in eukaryotes and bifunctional wax ester synthase-diacylglycerol acyltransferase (WSD) in bacteria, which are all membrane-bound proteins but exhibit no sequence homology to each other. Recent studies also identified other DGAT enzymes such as the soluble DGAT3 and diacylglycerol acetyltransferase (EaDAcT), as well as enzymes with DGAT activities including defective in cuticular ridges (DCR) and steryl and phytyl ester synthases (PESs). This review comprehensively discusses research advances on DGATs in prokaryotes and eukaryotes with a focus on their biochemical properties, physiological roles, and biotechnological and therapeutic applications. The review begins with a discussion of DGAT assay methods, followed by a systematic discussion of TAG biosynthesis and the properties and physiological role of DGATs. Thereafter, the review discusses the three-dimensional structure and insights into mechanism of action of human DGAT1, and the modeled DGAT1 from Brassica napus. The review then examines metabolic engineering strategies involving manipulation of DGAT, followed by a discussion of its therapeutic applications. DGAT in relation to improvement of livestock traits is also discussed along with DGATs in various other eukaryotic organisms.

The chemotype core collection of genus Nicotiana

Sustainable production of chemicals and improving these biosources by engineering metabolic pathways to create efficient plant-based biofactories relies on the knowledge of available chemical/biosynthetic diversity present in the plant. Nicotiana species are well known for their amenability towards transformation and other new plant breeding techniques. The genus Nicotiana is primarily known through Nicotiana tabacum L., the source of tobacco leaves and all respective tobacco products. Due to the prevalence of the latter, N. tabacum and related Nicotiana species are one of the most extensively studied plants. The majority of studies focused solely on N. tabacum or other individual species for chemotyping. The present study analysed a diversity panel including 17 Nicotiana species and six accessions of Nicotiana benthamiana and created a data set that effectively represents the chemotype core collection of the genus Nicotiana. The utilisation of several analytical platforms and previously published libraries/databases enabled the identification and measurement of over 360 metabolites of a wide range of chemical classes as well as thousands of unknowns with dedicated spectral and chromatographic properties.

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.

An engineered extraplastidial pathway for carotenoid biofortification of leaves

Carotenoids are lipophilic plastidial isoprenoids highly valued as nutrients and natural pigments. A correct balance of chlorophylls and carotenoids is required for photosynthesis and therefore highly regulated, making carotenoid enrichment of green tissues challenging. Here we show that leaf carotenoid levels can be boosted through engineering their biosynthesis outside the chloroplast. Transient expression experiments in Nicotiana benthamiana leaves indicated that high extraplastidial production of carotenoids requires an enhanced supply of their isoprenoid precursors in the cytosol, which was achieved using a deregulated form of the main rate-determining enzyme of the mevalonic acid (MVA) pathway. Constructs encoding bacterial enzymes were used to convert these MVA-derived precursors into carotenoid biosynthetic intermediates that do not normally accumulate in leaves, such as phytoene and lycopene. Cytosolic versions of these enzymes produced extraplastidial carotenoids at levels similar to those of total endogenous (i.e. chloroplast) carotenoids. Strategies to enhance the development of endomembrane structures and lipid bodies as potential extraplastidial carotenoid storage systems were not successful to further increase carotenoid contents. Phytoene was found to be more bioaccessible when accumulated outside plastids, whereas lycopene formed cytosolic crystalloids very similar to those found in the chromoplasts of ripe tomatoes. This extraplastidial production of phytoene and lycopene led to an increased antioxidant capacity of leaves. Finally, we demonstrate that our system can be adapted for the biofortification of leafy vegetables such as lettuce.

Nature's Chemists: The Discovery and Engineering of Phytochemical Biosynthesis

Plants produce a diverse array of natural products, many of which have high pharmaceutical value or therapeutic potential. However, these compounds often occur at low concentrations in uncultivated species. Producing phytochemicals in heterologous systems has the potential to address the bioavailability issues related to obtaining these molecules from their natural source. Plants are suitable heterologous systems for the production of valuable phytochemicals as they are autotrophic, derive energy and carbon from photosynthesis, and have similar cellular context to native producer plants. In this review we highlight the methods that are used to elucidate natural product biosynthetic pathways, including the approaches leading to proposing the sequence of enzymatic steps, selecting enzyme candidates and characterizing gene function. We will also discuss the advantages of using plant chasses as production platforms for high value phytochemicals. In addition, through this report we will assess the emerging metabolic engineering strategies that have been developed to enhance and optimize the production of natural and novel bioactive phytochemicals in heterologous plant systems.

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.

The role of volatiles in plant communication

Volatiles mediate the interaction of plants with pollinators, herbivores and their natural enemies, other plants and micro-organisms. With increasing knowledge about these interactions the underlying mechanisms turn out to be increasingly complex. The mechanisms of biosynthesis and perception of volatiles are slowly being uncovered. The increasing scientific knowledge can be used to design and apply volatile-based agricultural strategies.

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.

Identification of the Bisabolol Synthase in the Endangered Candeia Tree (Eremanthus erythropappus (DC) McLeisch)

Candeia (Eremanthus erythropappus (DC) McLeisch, Asteraceae) is a Brazilian tree, mainly occurring in the cerrado areas. From ethnobotanical information its essential oil is known to have wound healing and nociceptive properties. These properties are ascribed to result from a sesquiterpene alcohol, (-)-α-bisabolol, which is present at high concentrations in this oil. Bisabolol is highly valued by the cosmetic industry because of its antibacterial, anti-inflammatory, skin-smoothing and wound healing properties. Over the past decades, Candeia timber has been collected at large scale for bisabolol extraction from wild reserves and the species is thereby at risk of extinction. To support the development of breeding and nursing practices that would facilitate sustainable cultivation of Candeia, we identified a terpene synthase gene, EeBOS1, that appears to control biosynthesis (-)-α-bisabolol in the plant. Expression of this gene in E. coli showed that EeBOS1 protein is capable of producing (-)-α-bisabolol from farnesyl pyrophosphate in vitro. Analysis of gene expression in different tissues from Candeia plants in different life stages showed a high correlation of EeBOS1 expression and accumulation of (-)-α-bisabolol. This work is the first step to unravel the pathway toward (-)-α-bisabolol in Candeia, and in the further study of the control of (-)-α-bisabolol production.