Identification of Iridoid Glucoside Transporters in Catharanthus roseus

Integrated transcriptomic and proteomic analyses uncover the early response mechanisms of Catharanthus roseus under ultraviolet-B radiation

Catharanthus roseus produces a large array of terpenoid indole alkaloids (TIAs) that are important natural source for many drugs. Ultraviolet B (UVB) radiation have been proved to have regulatory effect towards biosynthesis of TIAs, which were meaningful for boost of TIA production. To decipher more comprehensive molecular characteristics in C. roseus under UVB radiation, integrated analysis of the nuclear proteome together with the transcriptome data under UVB radiation were performed. Expression of genes related to transmembrane transporters gradually increased during the prolonged exposure to UVB radiation. Some of known TIA transporters were affected by UVB. Abundance of proteins associated with spliceosome and nucleocytoplasmic transport increased. Homologs belonging to ORCA and CrWRKY transcription factors family increased at both transcriptomic and proteomic levels. At the same time, the numbers of differential alternative splicing events between UVB-radiated and white-light-treated plants continuously increased. These results suggest that the nucleus participated in early response of C. roseus under UVB radiation, where alternative splicing events occurred and might regulate multiple pathways. Furthermore, integrative omics analysis indicates that expression of enzymes at the terminal stages of seco-iridoid pathway decreased with the prolonged radiation exposure, potentially inhibiting further rise of TIA synthesis under extended UVB exposure.

The leaf idioblastome of the medicinal plant Catharanthus roseus is associated with stress resistance and alkaloid metabolism

Catharanthus roseus leaves produce a range of monoterpenoid indole alkaloids (MIAs) that include low levels of the anticancer drugs vinblastine and vincristine. The MIA pathway displays a complex architecture spanning different subcellular and cell type localizations, and is under complex regulation. As a result, the development of strategies to increase the levels of the anticancer MIAs has remained elusive. The pathway involves mesophyll specialized idioblasts where the late unsolved biosynthetic steps are thought to occur. Here, protoplasts of C. roseus leaf idioblasts were isolated by fluorescence-activated cell sorting, and their differential alkaloid and transcriptomic profiles were characterized. This involved the assembly of an improved C. roseus transcriptome from short- and long-read data, IDIO+. It was observed that C. roseus mesophyll idioblasts possess a distinctive transcriptomic profile associated with protection against biotic and abiotic stresses, and indicative that this cell type is a carbon sink, in contrast to surrounding mesophyll cells. Moreover, it is shown that idioblasts are a hotspot of alkaloid accumulation, suggesting that their transcriptome may hold the key to the in-depth understanding of the MIA pathway and the success of strategies leading to higher levels of the anticancer drugs.

Development of an efficient Agrobacterium-mediated transformation method and its application in tryptophan pathway modification in Catharanthus roseus

The biosynthetic pathway of Catharanthus roseus vinca alkaloids has a long research history, including not only identification of metabolic intermediates but also the mechanisms of inter-cellular transport and accumulation of biosynthesized components. Vinca alkaloids pathway begins with strictosidine, which is biosynthesized by condensing tryptamine from the tryptophan pathway and secologanin from the isoprenoid pathway. Therefore, increasing the supply of precursor tryptophan may enhance vinca alkaloid content or their metabolic intermediates. Many reports on the genetic modification of C. roseus use cultured cells or hairy roots, but few reports cover the production of transgenic plants. In this study, we first investigated a method for stably producing transgenic plants of C. roseus, then, using this technique, we modified the tryptophan metabolism system to produce transgenic plants with increased tryptophan content. Transformed plants were obtained by infecting cotyledons two weeks after sowing with Agrobacterium strain A13 containing a plant expression vector, then selecting with 1/2 B5 medium supplemented with 50 mg l-1 kanamycin and 20 mg l-1 meropenem. Sixty-eight regenerated plants were obtained from 4,200 cotyledons infected with Agrobacterium, after which genomic PCR analysis using NPTII-specific primers confirmed gene presence in 24 plants with a transformation rate of 0.6%. Furthermore, we performed transformation into C. roseus using an expression vector to join trpE8 and aroG4 genes, which are feedback-resistant mutant genes derived from Escherichia coli. The resulting transformed plants showed exactly the same morphology as the wild-type, albeit with a marked increase in tryptophan and alkaloids content, especially catharanthine in leaves.

Export of defensive glucosinolates is key for their accumulation in seeds

Plant membrane transporters controlling metabolite distribution contribute key agronomic traits1-6. To eliminate anti-nutritional factors in edible parts of crops, the mutation of importers can block the accumulation of these factors in sink tissues7. However, this often results in a substantially altered distribution pattern within the plant8-12, whereas engineering of exporters may prevent such changes in distribution. In brassicaceous oilseed crops, anti-nutritional glucosinolate defence compounds are translocated to the seeds. However, the molecular targets for export engineering of glucosinolates remain unclear. Here we identify and characterize members of the USUALLY MULTIPLE AMINO ACIDS MOVE IN AND OUT TRANSPORTER (UMAMIT) family-UMAMIT29, UMAMIT30 and UMAMIT31-in Arabidopsis thaliana as glucosinolate exporters with a uniport mechanism. Loss-of-function umamit29 umamit30 umamit31 triple mutants have a very low level of seed glucosinolates, demonstrating a key role for these transporters in translocating glucosinolates into seeds. We propose a model in which the UMAMIT uniporters facilitate glucosinolate efflux from biosynthetic cells along the electrochemical gradient into the apoplast, where the high-affinity H+-coupled glucosinolate importers GLUCOSINOLATE TRANSPORTERS (GTRs) load them into the phloem for translocation to the seeds. Our findings validate the theory that two differently energized transporter types are required for cellular nutrient homeostasis13. The UMAMIT exporters are new molecular targets to improve nutritional value of seeds of brassicaceous oilseed crops without altering the distribution of the defence compounds in the whole plant.

ABC transporters mined through comparative transcriptomics associate with organ-specific accumulation of picrosides in a medicinal herb, Picrorhiza kurroa

Picrorhiza kurroa Royle ex Benth is a valuable medicinal herb of North-Western Himalayas due to presence of two major bioactive compounds, picroside-I and picroside-II used in the preparation of several hepatoprotective herbal drugs. These compounds accumulate in stolons/rhizomes; however, biosynthesized in different organs, viz., picroside-I in shoots and picroside-II in roots. As of today, no information exists on what transporters are transporting these metabolites from shoots and roots to the final storage organ, stolon, which ultimately transforms into rhizome. The ATP-binding cassette (ABC) transporters are reported to transport majority of secondary metabolites, including terpenoids in plants, therefore, we mined P. kurroa transcriptomes to identify and shortlist potential candidates. A total of 99 ABC transporter-encoding transcripts were identified in 3 differential transcriptomes, PKSS (shoots), PKSTS (stolons), and PKSR (roots) of P. kurroa, based on in silico comparative analysis and transcript abundance. 15 of these transcripts were further validated for their association using qRT-PCR in shoots, roots and stolon tissues in P. kurroa accessions varying for picroside-I and picroside-II contents. Organ-specific expression analysis revealed that PkABCA1, PkABCG1, and PkABCB5 had comparatively elevated expression in shoots; PkABCB2 and PkABCC2 in roots; PkABCB3 and PkABCC1 in stolon tissues of P. kurroa. Co-expression network analysis using ABC genes as hubs further unravelled important interactions with additional components of biosynthetic machinery. Our study has provided leads, first to our knowledge as of today, on putative ABC transporters possibly involved in long distance and local transport of picrosides in P. kurroa organs, thus opening avenues for designing a suitable genetic intervention strategy.

Establishment of recombinant Catharanthus roseus stem cells stably overexpressing ORCA4 for terpenoid indole alkaloids biosynthesis

Catharanthus roseus is a perennial herb of the Apocynaceae family, from which about 200 kinds of alkaloids have been characterized. Most alkaloids from C. roseus are terpenoid indole alkaloids (TIAs), such as vinblastine and vincristine, which are widely used in the clinic for their good antitumor activity. However, they were only biosynthesized in C. roseus, and their content in C. roseus is extremely low. The access to these valuable compounds is by plant extraction or chemical semisynthesis from their precursors catharanthine and vindoline. Since catharanthine and vindoline are also obtained from C. roseus, the supply of vinblastine and vincristine makes it difficult to meet market demands. Therefore, how to improve the yield of TIAs is an attractive issue. In this study, we compared the regulatory effect of two critical transcription factors, octadecanoid-derivative responsive Catharanthus AP2-domain protein 3 (ORCA3) and octadecanoid-derivative responsive Catharanthus AP2-domain protein 4 (ORCA4), on the biosynthesis of TIAs in C. roseus. The results showed that overexpressing both two transcription factors could increase the accumulation of TIAs. The effect was more significant when ORCA4 was overexpressed. To acquire C. roseus TIAs on a continuous and consistent basis, we then created and acquired C. roseus stem cells stably overexpressing ORCA4. This is the first time a recombinant C. roseus stem cell system with stable ORCA4 overexpression has been developed, which not only provides new ideas for future research in this area but also breaches new life into the industrial application of using plant cell culture to obtain natural products.

Single-cell RNA sequencing provides a high-resolution roadmap for understanding the multicellular compartmentation of specialized metabolism

Monoterpenoid indole alkaloids (MIAs) are among the most diverse specialized metabolites in plants and are of great pharmaceutical importance. We leveraged single-cell transcriptomics to explore the spatial organization of MIA metabolism in Catharanthus roseus leaves, and the transcripts of 20 MIA genes were first localized, updating the model of MIA biosynthesis. The MIA pathway was partitioned into three cell types, consistent with the results from RNA in situ hybridization experiments. Several candidate transporters were predicted to be essential players shuttling MIA intermediates between inter- and intracellular compartments, supplying potential targets to increase the overall yields of desirable MIAs in native plants or heterologous hosts through metabolic engineering and synthetic biology. This work provides not only a universal roadmap for elucidating the spatiotemporal distribution of biological processes at single-cell resolution, but also abundant cellular and genetic resources for further investigation of the higher-order organization of MIA biosynthesis, transport and storage.

Reconstitution of monoterpene indole alkaloid biosynthesis in genome engineered Nicotiana benthamiana

Monoterpene indole alkaloids (MIAs) are a diverse class of plant natural products that include a number of medicinally important compounds. We set out to reconstitute the pathway for strictosidine, a key intermediate of all MIAs, from central metabolism in Nicotiana benthamiana. A disadvantage of this host is that its rich background metabolism results in the derivatization of some heterologously produced molecules. Here we use transcriptomic analysis to identify glycosyltransferases that are upregulated in response to biosynthetic intermediates and produce plant lines with targeted mutations in the genes encoding them. Expression of the early MIA pathway in these lines produces a more favorable product profile. Strictosidine biosynthesis was successfully reconstituted, with the best yields obtained by the co-expression of 14 enzymes, of which a major latex protein-like enzyme (MLPL) from Nepeta (catmint) is critical for improving flux through the iridoid pathway. The removal of endogenous glycosyltransferases does not impact the yields of strictosidine, highlighting that the metabolic flux of the pathway enzymes to a stable biosynthetic intermediate minimizes the need to engineer the endogenous metabolism of the host. The production of strictosidine in planta expands the range of MIA products amenable to biological synthesis.

The biosynthesis of strictosidine, a key intermediate of monoterpene indole alkaloids, was successfully reconstructed in Nicotiana benthamiana, demonstrating the potential of Nicotiana benthamiana as a bioproduction chassis for small molecules.

The emerging role of the nitrate and peptide transporter family: NPF in plant specialized metabolism

The nitrate and peptide transporter family (NPF) is one of the largest transporter families in the plant kingdom. The name of the family reflects the substrates (nitrate and peptides) identified for the two founding members CHL1 and PTR2 from Arabidopsis thaliana almost 30 years ago. However, since then, the NPF has emerged as a hotspot for transporters with a wide range of crucial roles in plant specialized metabolism. Recent prominent examples include 1) controlling accumulation of antinutritional glucosinolates in Brassica seeds, 2) deposition of heat-stress tolerance flavonol diglucosides to pollen coats 3) production of anti-cancerous monoterpene indole alkaloid precursors in Catharanthus roseus and 4) detoxification of steroid glycoalkaloids in ripening tomatoes. In this review, we turn the spotlight on the emerging role of the NPF in plant specialized metabolism and its potential for improving crop traits through transport engineering.

More than a Catharanthus plant: A multicellular and pluri-organelle alkaloid-producing factory

Plants represent a huge reservoir of natural products. A broad series of these compounds now find application for human health. In this respect, the monoterpene indole alkaloids (MIAs), particularly from Madagascar periwinkle, are a prominent example of plant specialized metabolites with an important therapeutic potential. However, the supply of MIA drugs has always been a challenge since the low-yield accumulation in planta. This mainly results from the complex architecture of the MIA biosynthetic pathway that involves several organs, tissue types and subcellular organelles. Here, we describe the most recent advances towards the elucidation of this pathway route as well as its spatial organization in planta. Besides allowing a better understanding of the MIA biosynthetic flux in the whole plant, such knowledge will also probably pave the way for the development of metabolic engineering strategies to sustain the MIA supply.

The ins and outs of transporters at plasma membrane and tonoplast in plant specialized metabolism

Covering: up to 2022Plants are organic chemists par excellence and produce an amazing array of diverse chemical structures. Whereas primary metabolites are essential for all living organisms and highly conserved, the specialized metabolites constitute the taxonomy-specific chemical languages that are key for fitness and survival. Allocation of plants' wide array of specialized metabolites in patterns that are fine-tuned spatiotemporally is essential for adaptation to the ever-changing environment and requires transport processes. Thus advancing our knowledge about transporters is important as also evidenced by the increasing number of transporters that control key quality traits in agriculture. In this review, we will highlight recently identified transporters and new insights related to already known transporters of plant specialized metabolites. Focus will be on the transport mechanism revealed by the biochemical characterization and how that links to its function in planta.

Phloem-specific localization of benzylisoquinoline alkaloid metabolism in opium poppy

Opium poppy is the only commercial source of the narcotic analgesics morphine and codeine, and semi-synthetic derivatives of the natural opiate precursor thebaine, including oxycodone and the opioid antagonist naloxone. The plant also accumulates the vasodilator and antitussive agents papaverine and noscapine, respectively, which together with morphine, codeine and thebaine comprise the major benzylisoquinoline alkaloids (BIAs) in opium poppy. A majority of enzymes involved in the highly branched BIA metabolism in opium poppy have now been discovered, with many specifically localized to sieve elements of the phloem based on immunofluorescence labeling techniques. Transcripts corresponding to sieve element-localized biosynthetic enzymes were detected in companion cells, as expected. The more recent application of shotgun proteomics has shown that several enzymes operating late in the morphine and noscapine biosynthetic pathways occur primarily in laticifers that are adjacent or proximal to sieve elements. BIA biosynthesis and accumulation in opium poppy involves three phloem cell types and implicates the translocation of key pathway intermediates between sieve elements and laticifers. The recent isolation of uptake transporters associated with laticifers supports an apoplastic rather than a symplastic route for translocation. In spite of the extensive elucidation of BIA biosynthetic enzymes in opium poppy, additional transporters and other auxiliary proteins are clearly necessary to support the complex spatial organization and dynamics involved in product formation and sequestration. In this review, we provide an update of BIA metabolism in opium poppy with a focus on the role of phloem in the biosynthesis of the major alkaloids.

Compartmentalization at the interface of primary and alkaloid metabolism

Plants produce many compounds used by humans as medicines, including alkaloids of the benzylisoquinoline (BIA), monoterpene indole (MIA) and tropane classes. The biosynthetic pathways of these pharmaceutical alkaloids are complex and spatially segregated across several tissues, cell-types and organelles. This review discusses the origin of primary metabolic inputs required by these specialized biosynthetic pathways and considers aspects relevant to their spatial organization. These factors are important for alkaloid production both in the native plants and for synthetic biology pathway reconstruction in microorganisms.

Studying Iridoid Transport in Catharanthus roseus by Grafting

The plant Catharanthus roseus is well known for its spatial separation of iridoid and monoterpenoid indole alkaloid (MIA) biosynthesis at both intracellular and intercellular levels, collectively suggested by RNA in situ hybridization, enzymatic and transcriptomic studies using leaf epidermis, and fluorescent protein tagging studies. Although documented in other plant species, the long-distance transport of iridoid glycosides, such as secologanin, has not been known in C. roseus until a recent study suggested that secologanin is transported from root to shoot, by grafting low iridoid/MIA mutant scions onto wild-type stock plants. This chapter describes the in vitro cultivation of C. roseus plants and grafting techniques to enable studies concerning iridoid/MIA transport between organs. The iridoid and MIA analysis methods are also provided.

Terpenoid indole alkaloid biosynthesis in Catharanthus roseus: effects and prospects of environmental factors in metabolic engineering

Plants synthesize a vast array of specialized metabolites that primarily contribute to their defense and survival under adverse conditions. Many of the specialized metabolites have therapeutic values as drugs. Biosynthesis of specialized metabolites is affected by environmental factors including light, temperature, drought, salinity, and nutrients, as well as pathogens and insects. These environmental factors trigger a myriad of changes in gene expression at the transcriptional and posttranscriptional levels. The dynamic changes in gene expression are mediated by several regulatory proteins that perceive and transduce the signals, leading to up- or down-regulation of the metabolic pathways. Exploring the environmental effects and related signal cascades is a strategy in metabolic engineering to produce valuable specialized metabolites. However, mechanistic studies on environmental factors affecting specialized metabolism are limited. The medicinal plant Catharanthus roseus (Madagascar periwinkle) is an important source of bioactive terpenoid indole alkaloids (TIAs), including the anticancer therapeutics vinblastine and vincristine. The emerging picture shows that various environmental factors significantly alter TIA accumulation by affecting the expression of regulatory and enzyme-encoding genes in the pathway. Compared to our understanding of the TIA pathway in response to the phytohormone jasmonate, the impacts of environmental factors on TIA biosynthesis are insufficiently studied and discussed. This review thus focuses on these aspects and discusses possible strategies for metabolic engineering of TIA biosynthesis. PURPOSE OF WORK: Catharanthus roseus is a rich source of bioactive terpenoid indole alkaloids (TIAs). The objective of this work is to present a comprehensive account of the influence of various biotic and abiotic factors on TIA biosynthesis and to discuss possible strategies to enhance TIA production through metabolic engineering.

Plant Secondary Metabolite Transporters: Diversity, Functionality, and Their Modulation

Secondary metabolites (SMs) play crucial roles in the vital functioning of plants such as growth, development, defense, and survival via their transportation and accumulation at the required site. However, unlike primary metabolites, the transport mechanisms of SMs are not yet well explored. There exists a huge gap between the abundant presence of SM transporters, their identification, and functional characterization. A better understanding of plant SM transporters will surely be a step forward to fulfill the steeply increasing demand for bioactive compounds for the formulation of herbal medicines. Thus, the engineering of transporters by modulating their expression is emerging as the most viable option to achieve the long-term goal of systemic metabolic engineering for enhanced metabolite production at minimum cost. In this review article, we are updating the understanding of recent advancements in the field of plant SM transporters, particularly those discovered in the past two decades. Herein, we provide notable insights about various types of fully or partially characterized transporters from the ABC, MATE, PUP, and NPF families including their diverse functionalities, structural information, potential approaches for their identification and characterization, several regulatory parameters, and their modulation. A novel perspective to the concept of "Transporter Engineering" has also been unveiled by highlighting its potential applications particularly in plant stress (biotic and abiotic) tolerance, SM accumulation, and removal of anti-nutritional compounds, which will be of great value for the crop improvement program. The present study creates a roadmap for easy identification and a better understanding of various transporters, which can be utilized as suitable targets for transporter engineering in future research.

Catch-22 in specialized metabolism: balancing defense and growth

Plants are unsurpassed biochemists that synthesize a plethora of molecules in response to an ever-changing environment. The majority of these molecules, considered as specialized metabolites, effectively protect the plant against pathogens and herbivores. However, this defense most probably comes at a great expense, leading to reduction of growth (known as the 'growth-defense trade-off'). Plants employ several strategies to reduce the high metabolic costs associated with chemical defense. Production of specialized metabolites is tightly regulated by a network of transcription factors facilitating its fine-tuning in time and space. Multifunctionality of specialized metabolites-their effective recycling system by re-using carbon, nitrogen, and sulfur, thus re-introducing them back to the primary metabolite pool-allows further cost reduction. Spatial separation of biosynthetic enzymes and their substrates, and sequestration of potentially toxic substances and conversion to less toxic metabolite forms are the plant's solutions to avoid the detrimental effects of metabolites they produce as well as to reduce production costs. Constant fitness pressure from herbivores, pathogens, and abiotic stressors leads to honing of specialized metabolite biosynthesis reactions to be timely, efficient, and metabolically cost-effective. In this review, we assess the costs of production of specialized metabolites for chemical defense and the different plant mechanisms to reduce the cost of such metabolic activity in terms of self-toxicity and growth.

Biosynthesis and synthetic biology of psychoactive natural products

Psychoactive natural products play an integral role in the modern world. The tremendous structural complexity displayed by such molecules confers diverse biological activities of significant medicinal value and sociocultural impact. Accordingly, in the last two centuries, immense effort has been devoted towards establishing how plants, animals, and fungi synthesize complex natural products from simple metabolic precursors. The recent explosion of genomics data and molecular biology tools has enabled the identification of genes encoding proteins that catalyze individual biosynthetic steps. Once fully elucidated, the "biosynthetic pathways" are often comparable to organic syntheses in elegance and yield. Additionally, the discovery of biosynthetic enzymes provides powerful catalysts which may be repurposed for synthetic biology applications, or implemented with chemoenzymatic synthetic approaches. In this review, we discuss the progress that has been made toward biosynthetic pathway elucidation amongst four classes of psychoactive natural products: hallucinogens, stimulants, cannabinoids, and opioids. Compounds of diverse biosynthetic origin - terpene, amino acid, polyketide - are identified, and notable mechanisms of key scaffold transforming steps are highlighted. We also provide a description of subsequent applications of the biosynthetic machinery, with an emphasis placed on the synthetic biology and metabolic engineering strategies enabling heterologous production.

Arabidopsis NPF4.6 and NPF5.1 Control Leaf Stomatal Aperture by Regulating Abscisic Acid Transport

The plant hormone abscisic acid (ABA) is actively synthesized in vascular tissues and transported to guard cells to promote stomatal closure. Although several transmembrane ABA transporters have been identified, how the movement of ABA within plants is regulated is not fully understood. In this study, we determined that Arabidopsis NPF4.6, previously identified as an ABA transporter expressed in vascular tissues, is also present in guard cells and positively regulates stomatal closure in leaves. We also found that mutants defective in NPF5.1 had a higher leaf surface temperature compared to the wild type. Additionally, NPF5.1 mediated cellular ABA uptake when expressed in a heterologous yeast system. Promoter activities of NPF5.1 were detected in several leaf cell types. Taken together, these observations indicate that NPF5.1 negatively regulates stomatal closure by regulating the amount of ABA that can be transported from vascular tissues to guard cells.

Subfunctionalization of Paralog Transcription Factors Contributes to Regulation of Alkaloid Pathway Branch Choice in Catharanthus roseus

Catharanthus roseus produces a diverse range of specialized metabolites of the monoterpenoid indole alkaloid (MIA) class in a heavily branched pathway. Recent great progress in identification of MIA biosynthesis genes revealed that the different pathway branch genes are expressed in a highly cell type- and organ-specific and stress-dependent manner. This implies a complex control by specific transcription factors (TFs), only partly revealed today. We generated and mined a comprehensive compendium of publicly available C. roseus transcriptome data for MIA pathway branch-specific TFs. Functional analysis was performed through extensive comparative gene expression analysis and profiling of over 40 MIA metabolites in the C. roseus flower petal expression system. We identified additional members of the known BIS and ORCA regulators. Further detailed study of the ORCA TFs suggests subfunctionalization of ORCA paralogs in terms of target gene-specific regulation and synergistic activity with the central jasmonate response regulator MYC2. Moreover, we identified specific amino acid residues within the ORCA DNA-binding domains that contribute to the differential regulation of some MIA pathway branches. Our results advance our understanding of TF paralog specificity for which, despite the common occurrence of closely related paralogs in many species, comparative studies are scarce.

Membrane transporters: the key drivers of transport of secondary metabolites in plants

This review summarizes the recent updates in the area of transporters of plant secondary metabolites, including their applied aspects in metabolic engineering of economically important secondary metabolites. Plants have evolved biosynthetic pathways to produce structurally diverse secondary metabolites, which serve distinct functions, including defense against pathogens and herbivory, thereby playing a pivotal role in plant ecological interactions. These compounds often display interesting bioactivities and, therefore, have been used as repositories of natural drugs and phytoceuticals for humans. At an elevated level, plant secondary metabolites could be cytotoxic to the plant cell itself; therefore, plants have developed sophisticated mechanisms to sequester these compounds to prevent cytotoxicity. Many of these valuable natural compounds and their precursors are biosynthesized and accumulated at diverse subcellular locations, and few are even transported to sink organs via long-distance transport, implying the involvement of compartmentalization via intra- and intercellular transport mechanisms. The transporter proteins belonging to different families of transporters, especially ATP binding cassette (ABC) and multidrug and toxic compound extrusion (MATE) have been implicated in membrane-mediated transport of certain plant secondary metabolites. Despite increasing reports on the characterization of transporter proteins and their genes, our knowledge about the transporters of several medicinally and economically important plant secondary metabolites is still enigmatic. A comprehensive understanding of the molecular mechanisms underlying the whole route of secondary metabolite transportome, in addition to the biosynthetic pathways, will aid in systematic and targeted metabolic engineering of high-value secondary metabolites. The present review embodies a comprehensive update on the progress made in the elucidation of transporters of secondary metabolites in view of basic and applied aspects of their transport mechanism.

Back to the plant: overcoming roadblocks to the microbial production of pharmaceutically important plant natural products

Microbial fermentation platforms offer a cost-effective and sustainable alternative to plant cultivation and chemical synthesis for the production of many plant-derived pharmaceuticals. Plant alkaloids, particularly benzylisoquinoline alkaloids and monoterpene indole alkaloids, and recently cannabinoids have become attractive targets for microbial biosynthesis owing to their medicinal importance. Recent advances in the discovery of pathway components, together with the application of synthetic biology tools, have facilitated the assembly of plant alkaloid and cannabinoid pathways in the microbial hosts Escherichia coli and Saccharomyces cerevisiae. This review highlights key aspects of these pathways in the framework of overcoming bottlenecks in microbial production to further improve end-product titers. We discuss the opportunities that emerge from a better understanding of the pathway components by further study of the plant, and strategies for generation of new and advanced medicinal compounds.

Exploring plant metabolic genomics: chemical diversity, metabolic complexity in the biosynthesis and transport of specialized metabolites with the tea plant as a model

The diversity and complexity of secondary metabolites in tea plants contribute substantially to the popularity of tea, by determining tea flavors and their numerous health benefits. The most significant characteristics of tea plants are that they concentrate the complex plant secondary metabolites into one leaf: flavonoids, alkaloids, theanine, volatiles, and saponins. Many fundamental questions regarding tea plant secondary metabolism remain unanswered. This includes how tea plants accumulate high levels of monomeric galloylated catechins, unlike the polymerized flavan-3-ols in most other plants, as well as how they are evolved to selectively synthesize theanine and caffeine, and how tea plants properly transport and store these cytotoxic products and then reuse them in defense. Tea plants coordinate many metabolic pathways that simultaneously take place in young tea leaves in response to both developmental and environmental cues. With the available genome sequences of tea plants and high-throughput metabolomic tools as great platforms, it is of particular interest to launch metabolic genomics studies using tea plants as a model system. Plant metabolic genomics are to investigate all aspects of plant secondary metabolism at the genetic, genome, and molecular levels. This includes plant domestication and adaptation, divergence and convergence of secondary metaboloic pathways. The biosynthesis, transport, storage, and transcriptional regulation mechanisms of all metabolites are of core interest in the plant as a whole. This review highlights relevant contexts of metabolic genomics, outstanding questions, and strategies for answering them, with aim to guide future research for genetic improvement of nutrition quality for healthier plant foods.

Cellular and Subcellular Compartmentation of the 2C-Methyl-D-Erythritol 4-Phosphate Pathway in the Madagascar Periwinkle

The Madagascar periwinkle (Catharanthus roseus) synthesizes the highly valuable monoterpene indole alkaloids (MIAs) through a long metabolic route initiated by the 2C-methyl-D-erythritol 4-phosphate (MEP) pathway. In leaves, a complex compartmentation of the MIA biosynthetic pathway occurs at both the cellular and subcellular levels, notably for some gene products of the MEP pathway. To get a complete overview of the pathway organization, we cloned four genes encoding missing enzymes involved in the MEP pathway before conducting a systematic analysis of transcript distribution and protein subcellular localization. RNA in situ hybridization revealed that all MEP pathway genes were coordinately and mainly expressed in internal phloem-associated parenchyma of young leaves, reinforcing the role of this tissue in MIA biosynthesis. At the subcellular level, transient cell transformation and expression of fluorescent protein fusions showed that all MEP pathway enzymes were targeted to plastids. Surprisingly, two isoforms of 1-deoxy-D-xylulose 5-phosphate synthase and 1-deoxy-D-xylulose 5-phosphate reductoisomerase initially exhibited an artifactual aggregated pattern of localization due to high protein accumulation. Immunogold combined with transmission electron microscopy, transient transformations performed with a low amount of transforming DNA and fusion/deletion experiments established that both enzymes were rather diffuse in stroma and stromules of plastids as also observed for the last six enzymes of the pathway. Taken together, these results provide new insights into a potential role of stromules in enhancing MIA precursor exchange with other cell compartments to favor metabolic fluxes towards the MIA biosynthesis.

The complexity of intercellular localisation of alkaloids revealed by single-cell metabolomics

Catharanthus roseus is a medicinal plant well known for producing bioactive compounds such as vinblastine and vincristine, which are classified as terpenoid indole alkaloids (TIAs). Although the leaves of this plant are the main source of these antitumour drugs, much remains unknown on how TIAs are biosynthesised from a central precursor, strictosidine, to various TIAs in planta. Here, we have succeeded in showing, for the first time in leaf tissue of C. roseus, cell-specific TIAs localisation and accumulation with 10 μm spatial resolution Imaging mass spectrometry (Imaging MS) and live single-cell mass spectrometry (single-cell MS). These metabolomic studies revealed that most TIA precursors (iridoids) are localised in the epidermal cells, but major TIAs including serpentine and vindoline are localised instead in idioblast cells. Interestingly, the central TIA intermediate strictosidine also accumulates in both epidermal and idioblast cells of C. roseus. Moreover, we also found that vindoline accumulation increases in laticifer cells as the leaf expands. These discoveries highlight the complexity of intercellular localisation in plant specialised metabolism.

A cytochrome P450 from the mustard leaf beetles hydroxylates geraniol, a key step in iridoid biosynthesis

Larvae of the leaf beetle Phaedon cochleariae synthesize the iridoid chysomelidial via the mevalonate pathway to repel predators. The normal terpenoid biosynthesis is integrated into the dedicated defensive pathway by the ω-hydroxylation of geraniol to (2E,6E)-2,6-dimethylocta-2,6-diene-1,8-diol (ω-OH-geraniol). Here we identify and characterize the P450 monooxygenase CYP6BH5 as the geraniol hydroxylase using integrated transcriptomics, proteomics and RNA interference (RNAi). In the fat body, 73 cytochrome P450s were identified, and CYP6BH5 was among those that were expressed specifically in fat body. Double stranded RNA mediated knockdown of CYP6BH5 led to a significant reduction of ω-hydroxygeraniol glucoside in the hemolymph and, later, of the chrysomelidial in the defensive secretion. Heterologously expressed CYP6BH5 converted geraniol to ω-OH-geraniol. In addition to geraniol, CYP6BH5 also catalyzes hydroxylation of other monoterpenols, such as nerol and citronellol to the corresponding α,ω-dihydroxy compounds.

GATA and Phytochrome Interacting Factor Transcription Factors Regulate Light-Induced Vindoline Biosynthesis in Catharanthus roseus

Catharanthus roseus is the exclusive source of an array of terpenoid indole alkaloids including the anticancer drugs vincristine and vinblastine, derived from the coupling of catharanthine and vindoline. Leaf-synthesized vindoline is regulated by light. A seven-step enzymatic process is involved in the sequential conversion of tabersonine to vindoline; however, the regulatory mechanism controlling the expression of genes encoding these enzymes has not been elucidated. Here, we identified CrGATA1, an Leu-Leu-Met domain GATA transcription factor that regulates light-induced vindoline biosynthesis in C. roseus seedlings. Expression of CrGATA1 and the vindoline pathway genes T16H2, T3O, T3R, D4H, and DAT was significantly induced by light. In addition, CrGATA1 activated the promoters of five light-responsive vindoline pathway genes in plant cells. Two GATC motifs in the D4H promoter were critical for CrGATA1-mediated transactivation. Transient overexpression of CrGATA1 in C. roseus seedlings resulted in up-regulation of vindoline pathway genes and increased vindoline accumulation. Conversely, virus-induced gene silencing of CrGATA1 in young C. roseus leaves significantly repressed key vindoline pathway genes and reduced vindoline accumulation. Furthermore, we showed that a C. roseus Phytochrome Interacting Factor, CrPIF1, is a repressor of CrGATA1 and vindoline biosynthesis. Transient overexpression or virus-induced gene silencing of CrPIF1 in C. roseus seedlings altered CrGATA1 and vindoline pathway gene expression in the dark. CrPIF1 repressed CrGATA1 and DAT promoter activity by binding to G/E-box/PBE elements. Our findings reveal a regulatory module involving Phytochrome Interacting Factor -GATA that governs light-mediated biosynthesis of specialized metabolites.

Organ-specific and genotype-dependent constitutive biosynthesis of secoiridoid glucosides in Centaurium erythraea Rafn, and its elicitation with methyl jasmonate

While bioactive properties of Centaurium erythraea Rafn secoiridoid glucosides (SG) are widely recognized, many aspects related to their biochemistry, metabolism and relationship to the overall plant physiology are not yet understood. Here we present for the first time an insight into the molecular background of organ-specific and genotype-dependent constitutive biosynthesis of secoiridoids in C. erythraea, by comparing chemical profiles and secoiridoid glucosides-related gene expression. Genes encoding enzymes for intermediate steps of secoiridoids biosynthesis up to secologanin have been identified by analysing transcriptomic data from C. erythraea leaves. Results suggest an organ-specific capacity for the production and accumulation of secoiridoid glucosides, and highlight leaves as the main biosynthesis site. They also point out that significant differences in SG content among various C. erythraea genotypes, are, at least partially, determined by different expression patterns of SG-related genes. The biosynthesis of SG in C. erythraea leaves is enhanced upon treatments with methyl jasmonate (MeJA), which causes reprogramming of SG-related gene expression, leading to an increased production of valuable bioactive compounds. The present study unveiled several rate-limiting genes (encoding GES, G8O, 8HGO, IS and 7DLGT) in SG biosynthesis. SLS and CPR are highlighted as important genes/enzymes that might regulate biosynthetic flux through SG pathway. Information gathered within this study will help us gain deeper insight into the SG metabolism and develop strategies for enhanced biosynthesis of specific secoiridoid glucosides in homologous or heterologous systems.

An engineered combinatorial module of transcription factors boosts production of monoterpenoid indole alkaloids in Catharanthus roseus

To fend off microbial pathogens and herbivores, plants have evolved a wide range of defense strategies such as physical barriers, or the production of anti-digestive proteins or bioactive specialized metabolites. Accumulation of the latter compounds is often regulated by transcriptional activation of the biosynthesis pathway genes by the phytohormone jasmonate-isoleucine. Here, we used our recently developed flower petal transformation method in the medicinal plant Catharanthus roseus to shed light on the complex regulatory mechanisms steering the jasmonate-modulated biosynthesis of monoterpenoid indole alkaloids (MIAs), to which the anti-cancer compounds vinblastine and vincristine belong. By combinatorial overexpression of the transcriptional activators BIS1, ORCA3 and MYC2a, we provide an unprecedented insight into the modular transcriptional control of MIA biosynthesis. Furthermore, we show that the expression of an engineered de-repressed MYC2a triggers a tremendous reprogramming of the MIA pathway, finally leading to massively increased accumulation of at least 23 MIAs. The current study unveils an innovative approach for future metabolic engineering efforts for the production of valuable bioactive plant compounds in non-model plants.