ATP-binding cassette transporter controls leaf surface secretion of anticancer drug components in Catharanthus roseus

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.

A twist in the ABC: regulation of ABC transporter trafficking and transport by FK506-binding proteins

Post-transcriptional regulation of ATP-binding cassette (ABC) proteins has been so far shown to encompass protein phosphorylation, maturation, and ubiquitination. Yet, recent accumulating evidence implicates FK506-binding proteins (FKBPs), a type of peptidylprolyl cis-trans isomerase (PPIase) proteins, in ABC transporter regulation. In this perspective article, we summarize current knowledge on ABC transporter regulation by FKBPs, which seems to be conserved over kingdoms and ABC subfamilies. We uncover striking functional similarities but also differences between regulatory FKBP-ABC modules in plants and mammals. We dissect a PPIase- and HSP90-dependent and independent impact of FKBPs on ABC biogenesis and transport activity. We propose and discuss a putative new mode of transient ABC transporter regulation by cis-trans isomerization of X-prolyl bonds.

Enhanced monoterpene emission in transgenic orange mint (Mentha × piperita f. citrata) overexpressing a tobacco lipid transfer protein (NtLTP1)

MAIN CONCLUSION

Overexpression of the tobacco lipid transfer protein (NtLTP1) gene in transgenic orange mint resulted in enhanced accumulation of monoterpenes in the cavity of head cells of glandular trichomes, which resulted in enhanced emission of monoterpenes from transgenic orange mints. Plants in the genus Mentha (Lamiaceae) produce volatile oils that accumulate in peltate glandular trichomes in the aerial parts of plants. A lipid transfer protein (NtLTP1) in tobacco showed glandular trichome-specific expression and supported the secretion of diterpenoid lipids from head cells of glandular trichomes (Choi et al., Plant J 70:480-491,2012). Here, we constructed transgenic orange mint (Mentha × piperita f. citrata) overexpressing the tobacco NtLTP1 gene via Agrobacterium-mediated transformation. Transgenic lines of orange mint overexpressing NtLTP1 were confirmed by genomic PCR and RT-PCR. Immunoblotting analysis using an NtLTP1 polyclonal antibody showed clear dark spots at the position of the lipid exudates from tobacco glandular trichomes and the squeezed out lipids from the glandular trichomes of transgenic orange mint. Heads of glandular trichomes in transgenic plants overexpressing the NtLTP1 gene showed a larger diameter than those of the wild-type control. The enhanced size of trichome heads in transgenic orange mint was confirmed by scanning electron microscopy. Volatile components were extracted from wild-type and transgenic orange mint by solid-phase microextraction (SPME) and analyzed by headspace-gas chromatography-mass spectrometry (HS/GC/MS). Linalyl acetate was the most abundant component among the eleven identified monoterpenes in the volatile compounds extracted from both the wild-type and transgenic lines of orange mint. Overexpression of NtLTP1 in transgenic orange mint plants resulted in enhanced emission of volatile monoterpenoids compared with that of volatile monoterpenoids in the wild-type control plants.

The ATP-binding cassette transporter OsPDR1 regulates plant growth and pathogen resistance by affecting jasmonates biosynthesis in rice

Jasmonates (JAs) are important regulators of plant growth, development, and defense. ATP-binding cassette (ABC) transporters participate in disease resistance by transporting JAs or antimicrobial secondary metabolites in dicotyledons. Here, we functionally characterized a JAs-inducible rice gene (OsPDR1) that encodes a member of the pleiotropic drug resistance (PDR) subfamily of ABC transporters. By affecting JAs biosynthesis, overexpression of OsPDR1 resulted in constitutive activation of defense-related genes and enhanced resistance to bacterial blight, whereas its mutation decreased pathogen resistance. In addition, overexpression and mutation of OsPDR1 resulted in decreased and increased plant growth at seedling stage, respectively, but eventually led to decreased grain yield. OsPDR1 encodes three splice isoforms, of which OsPDR1.2 and OsPDR1.3 contain a conserved glutamate residue in the "ENI-motif" of the first nucleotide-binding domain, while OsPDR1.1 does not. The three OsPDR1 transcripts are developmentally controlled and differentially regulated by JAs and pathogen infection. The OsPDR1.2- and OsPDR1.3-overexpressing plants exhibited higher JAs content and stronger growth inhibition and disease resistance than OsPDR1.1-overexpressing plants. These results indicated that alternative splicing affects the function of OsPDR1 gene in regulation of growth, development and disease resistance.

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.

AaABCG40 Enhances Artemisinin Content and Modulates Drought Tolerance in Artemisia annua

The phytohormone Abscisic acid (ABA) regulates plant growth, development, and responses to abiotic stresses, including senescence, seed germination, cold stress and drought. Several kinds of researches indicate that exogenous ABA can enhance artemisinin content in A. annua. Some transcription factors related to ABA signaling are identified to increase artemisinin accumulation through activating the artemisinin synthase genes. However, no prior study on ABA transporter has been performed in A. annua. Here, we identified a pleiotropic drug resistance (PDR) transporter gene AaPDR4/AaABCG40 from A. annua. AaABCG40 was expressed mainly in roots, leaves, buds, and trichomes. GUS activity is primarily observed in roots and the vascular tissues of young leaves in proAaABCG40: GUS transgenic A. annua plants. When AaABCG40 was transferred into yeast AD12345678, yeasts expressing AaABCG40 accumulated more ABA than the control. The AaABCG40 overexpressing plants showed higher artemisinin content and stronger drought tolerance. Besides, the expression of CYP71AV1 in OE-AaABCG40 plants showed more sensitivity to exogenous ABA than that in both wild-type and iAaABCG40 plants. According to these results, they strongly suggest that AaABCG40 is involved in ABA transport in A. annua.

The genuine localization of indole alkaloids in Vinca minor and Catharanthus roseus

Based on the occurrence of indole alkaloids in so-called "chloroform leaf surface extracts", it was previously deduced that these alkaloids are present in the cuticle at the leaf surface of Catharanthus roseus and Vinca minor. As no symplastic markers were found in these extracts this deduction seemed to be sound. However, since chloroform is known to destroy biomembranes very rapidly, these data have to be judged with scepticism. We reanalyzed the alleged apoplastic localization of indole alkaloids by employing slightly acidic aqueous surface extracts and comparing the corresponding alkaloid patterns with those of aqueous total leaf extracts. Whereas in the "chloroform leaf surface extracts" all alkaloids are present in the same manner as in the total leaf extracts, no alkaloids occur in the aqueous leaf surface extracts. These results clearly show that chloroform had rapidly destroyed cell integrity, and the related extracts also contain the alkaloids genuinely accumulated within the protoplasm. The related decompartmentation was verified by the massively enhanced concentration of amino acids in aqueous surface extracts of chloroform treated leaves. Furthermore, the chloroform-induced cell disintegration was vividly visualized by confocal laser scanning microscopical analyses, which clearly displayed a strong decrease in the chlorophyll fluorescence in chloroform treated leaves. These findings unequivocally display that the indole alkaloids are not located in the apoplastic space, but exclusively are present symplastically within the cells of V. minor and C. roseus leaves. Accordingly, we have to presume that also other leaf surface extracts employing organic solvents have to be re-investigated.

Lipid Transfer Proteins (AaLTP3 and AaLTP4) Are Involved in Sesquiterpene Lactone Secretion from Glandular Trichomes in Artemisia annua

In Artemisia annua plants, glandular trichomes (GTs) are responsible for the biosynthesis and secretion of sesquiterpene lactones including artemisinin/arteannuin B. Nonspecific lipid transfer proteins (LTPs) in plants bind and carry lipid molecules across the cell membrane and are also known as secretary proteins. Interestingly, the transcripts of LTP genes are exceptionally abundant in the GTs of A. annua. In the present study, we isolated two trichome-specific LTP genes (AaLTP3 and AaLTP4) from a Korean ecotype of A. annua. AaLTP3 was expressed abundantly in shoots, whereas AaLTP4 was expressed in flowers. The GUS signal driven by the AaLTP3 or AaLTP4 promoter in transgenic A. annua plants revealed that the AaLTP3 promoter was active on hair-like non-GTs and that the AaLTP4 promoter was active on GTs. Analysis of enhanced cyan fluorescent protein (ECFP) fluorescence fused with the AaLTP3 or AaLTP4 protein in transgenic tobacco revealed that ECFP florescence was very bright on secreted lipids of long GTs. Moreover, the florescence was also bright on the head cells of short trichomes and their secreted granules. Immunoblotting analysis of GT exudates in petioles of A. annua revealed a strong positive signal against the AaLTP4 antibody. Overexpression of AaLTP3 or AaLTP4 in transgenic A. annua plants resulted in enhanced production of sesquiterpene lactones (arteannuin B, artemisinin, dihydroartemisinic acid and artemisinic acid) compared with those of wild type. The present study shows that LTP genes (AaLTP3 or AaLTP4) play important roles in the sequestration and secretion of lipids in GTs of A. annua, which is useful for the enhanced production of sesquiterpene lactones by genetic engineering.

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.

EASI Transformation: An Efficient Transient Expression Method for Analyzing Gene Function in Catharanthus roseus Seedlings

The Catharanthus roseus plant is the exclusive source of the valuable anticancer terpenoid indole alkaloids, vinblastine (VB) and vincristine (VC). The recent availability of transcriptome and genome resources for C. roseus necessitates a fast and reliable method for studying gene function. In this study, we developed an Agrobacterium-mediated transient expression method to enable the functional study of genes rapidly in planta, conserving the compartmentalization observed in the VB and VC pathway. We focused on (1) improving the transformation method (syringe versus vacuum agroinfiltration) and cultivation conditions (seedling age, Agrobacterium density, and time point of maximum transgene expression), (2) improving transformation efficiency through the constitutive expression of the virulence genes and suppressing RNA silencing mechanisms, and (3) improving the vector design by incorporating introns, quantitative and qualitative reporter genes (luciferase and GUS genes), and accounting for transformation heterogeneity across the tissue using an internal control. Of all the parameters tested, vacuum infiltration of young seedlings (10-day-old, harvested 3 days post-infection) resulted in the strongest increase in transgene expression, at 18 - 57 fold higher than either vacuum or syringe infiltration of other seedling ages. Endowing the A. tumefaciens strain with the mutated VirGN54D or silencing suppressors within the same plasmid as the reporter gene further increased expression by 2 - 10 fold. For accurate measurement of promoter transactivation or activity, we included an internal control to normalize the differences in plant mass and transformation efficiency. Including the normalization gene (Renilla luciferase) on the same plasmid as the reporter gene (firefly luciferase) consistently yielded a high signal and a high correlation between RLUC and FLUC. As proof of principle, we applied this approach to investigate the regulation of the CroSTR1 promoter with the well-known activator ORCA3 and repressor ZCT1. Our method demonstrated the quantitative assessment of both the activation and repression of promoter activity in C. roseus. Our efficient Agrobacterium-mediated seedling infiltration (EASI) protocol allows highly efficient, reproducible, and homogenous transformation of C. roseus cotyledons and provides a timely tool for the community to rapidly assess the function of genes in planta, particularly for investigating how transcription factors regulate terpenoid indole alkaloid biosynthesis.

Cloning and characterization of a tyrosine decarboxylase involved in the biosynthesis of galanthamine in Lycoris aurea

Background

Galanthamine, one kind of Amaryllidaceae alkaloid extracted from the Lycoris species, is used in the treatment of Alzheimer’s disease. In regards to medical and economic importance, the biosynthesis and regulatory mechanism of the secondary metabolites in Lycoris remain uninvestigated.

Methods

BLAST was used to identify the sequence of tyrosine decarboxylase in the transcriptome of Lycoris aurea (L’Hér) Herb. The enzyme activity of this TYDC was determined by using heterologous expressed protein in the Escherichia coli cells. The related productive contents of tyramine were detected using High Performance Liquid Chromatography (HPLC). According to the available micro RNA sequencing profiles and degradome database of L. aurea, microRNA396 were isolated, which targets to LaTYDC1 and RNA Ligase-Mediated-Rapid Amplification of cDNA Ends (RLM-RACE) were used to confirm the cleavage. The expression levels of miR396 and LaTYDC1 were measured using a quantitative real-time polymerase chain reaction (qRT-PCR).

Results

LaTYDC1 was mainly expressed in root, bulb, leaf and flower fitting the models for galanthamine accumulation. This decarboxylase efficiently catalyzes tyrosine to tyramine conversion. Under methyl jasmonate (MeJA) treatment, the expression of LaTYDC1 and the content of tyramine sharply increase. The use of RLM-RACE confirms that miR396 promotes the degradation of LaTYDC1 mRNA. Under MeJA treatment, the expression of miR396 was suppressed while the expression level of LaTYDC1 sharply increased. Following the increase of the miR396 transcriptional level, LaTYDC1 was significantly repressed.

Conclusion

LaTYDC1 participates in the biosynthesis of galanthamine, and is regulated by miR396. This finding also provides genetic strategy for improving the yield of galanthamine in the future.

UPLC and ESI-MS analysis of metabolites of Rauvolfia tetraphylla L. and their spatial localization using desorption electrospray ionization (DESI) mass spectrometric imaging

Rauvolfia tetraphylla L. (family Apocynaceae), often referred to as the wild snakeroot plant, is an important medicinal plant and produces a number of indole alkaloids in its seeds and roots. The plant is often used as a substitute for Ravuolfia serpentine (L.) Benth. ex Kurz known commonly as the Indian snakeroot plant or sarphagandha in the preparation of Ayurvedic formulations for a range of diseases including hypertension. In this study, we examine the spatial localization of the various indole alkaloids in developing fruits and plants of R. tetraphylla using desorption electrospray ionization mass spectrometry imaging (DESI-MSI). A semi-quantitative analysis of the various indole alkaloids was performed using UPLC-ESI/MS. DESI-MS images showed that the distribution of ajmalcine, yohimbine, demethyl serpentine and mitoridine are largely localized in the fruit coat while that for ajmaline is restricted to mesocarp of the fruit. At a whole plant level, the ESI-MS intensities of many of the ions were highest in the roots and lesser in the shoot region. Within the root tissue, except sarpagine and ajmalcine, all other indole alkaloids occurred in the epidermal and cortex tissues. In leaves, only serpentine, ajmalcine, reserpiline and yohimbine were present. Serpentine was restricted to the petiolar region of leaves. Principal component analysis based on the presence of the indole alkaloids, clearly separated the four tissues (stem, leaves, root and fruits) into distinct clusters. In summary, the DESI-MSI results indicated a clear tissue localization of the various indole alkaloids, in fruits, leaves and roots of R. tetraphylla. While it is not clear of how such localization is attained, we discuss the possible pathways of indole alkaloid biosynthesis and translocation during fruit and seedling development in R. tetraphylla. We also briefly discuss the functional significance of the spatial patterns in distribution of metabolites.

Completion of the canonical pathway for assembly of anticancer drugs vincristine/vinblastine in Catharanthus roseus

The important anticancer drugs, vinblastine, vincristine and analogs, are composed of the monoterpenoid indole alkaloids (MIAs), catharanthine and vindoline, found uniquely in the medicinal plant, Catharanthus roseus. While 26 genes involved in the assembly of these two MIAs are known, two key reactions have eluded characterization to complete the documentation of the vinblastine pathway in this plant species. The assembly of these dimeric MIAs requires O-acetylstemmadenine oxidase (ASO) and a dual function geissoschizine synthase (GS) that reduces cathenamine to form geissoschizine, and that also reduces the ASO product to form a common intermediate for subsequent conversion by four separate hydrolases to catharanthine, tabersonine or vincadifformine, respectively. The in planta role of ASO is supported by identifying a single amino acid-substituted ASO mutant with very low enzyme activity and by virus-induced gene silencing of ASO to produce plants that accumulate O-acetylstemmadenine rather than catharanthine and vindoline found in wild-type (WT) plants. The in planta role of GS is supported by showing that a low GS-expressing mutant accumulating lower levels of catharanthine and vindoline also displays significantly lower tabersonine-forming activity in coupled enzyme assays than in the WT background. Gene expression analyses demonstrate that both ASO and GS are highly enriched in the leaf epidermis where the pathways for catharanthine and tabersonine biosynthesis are expressed. The full elucidation of this canonical pathway enables synthetic biology approaches for manufacturing a broad range of MIAs, including these dimers used in cancer treatment.

Towards Identification of the Substrates of ATP-Binding Cassette Transporters

Most ATP-binding cassette (ABC) proteins function in transmembrane transport, and plant genomes encode a large number of ABC transporters compared with animal or fungal genomes. These transporters have been classified into eight subfamilies according to their topology and phylogenetic relationships. Transgenic plants and mutants with altered ABC transporter expression or function have contributed to deciphering the physiological roles of these proteins, such as in plant development, responses to biotic and abiotic stress, or detoxification activities within the cell. In agreement with the diversity of these functions, a large range of substrates (e.g. hormones and primary and secondary metabolites) have been identified. We review in detail transporters for which substrates have been unambiguously identified. However, some cases are far from clear, because some ABC transporters have the ability to transport several structurally unrelated substrates or because the identification of their substrates was performed indirectly without any flux measurement. Various heterologous or homologous expression systems have been used to better characterize the transport activity and other biochemical properties of ABC transporters, opening the way to addressing new issues such as the particular structural features of plant ABC transporters, the bidirectionality of transport, or the role of posttranslational modifications.

Differential responses to Cd stress induced by exogenous application of Cu, Zn or Ca in the medicinal plant Catharanthus roseus

Cd(II) is one of the most widespread and toxic heavy metals and seriously threatens plant growth, furthermore negatively affecting human health. For survival from this metal stress, plants always fight with Cd(II) toxicity by themselves or using other external factors. The effects of second metals copper (Cu(II)), zinc (Zn(II)) and calcium (Ca(II)) on the Cd(II)-affected root morphology, Cd(II) translocation and metabolic responses in Catharanthus roseus were investigated under hydroponic conditions. We found that the Cd-stressed plants displayed the browning and rot root symptom, excess H₂O₂ content, lipid peroxidation and Cd(II) accumulation in plants. However, the supplement with second metals largely alleviated Cd-induced toxicity, including browning and rot roots, oxidative stress and internal Cd(II) accumulation. The amended effects at metabolic and transcriptional levels involved in different second metals share either common or divergent strategies. They commonly repressed Cd uptake and promoted Cd(II) translocation from roots to shoots with divergent mechanisms. High Zn(II) could activate MTs expression in roots, while Cu(II) or Ca(II) did not under Cd(II) stress condition. The presence of Ca(II) under Cd stress condition largely initiated occurrence of lateral roots. We then grouped a metabolic diagram integrating terpenoid indole alkaloid (TIA) accumulation and TIA pathway gene expression to elucidate the metabolic response of C. roseus to Cd(II) alone or combined with second metals. The treatment with 100 Cd(II) alone largely promoted accumulation of vinblastine, vindoline, catharanthine and loganin, whereas depressed or little changed the expression levels of genes detected here, compared to 0 Cd(II) control. In the presence of Cd(II), the supplement with second metals displayed specific effect on different alkaloid. Among them, the metal Ca(II) is especially beneficial for serpentine accumulation, Zn(II) mainly promoted tabersonine production. However, the addition of Cu(II) commonly depressed accumulation of most alkaloids detected here. Generally, we presented different mechanisms by which the second metals used to alleviate Cd (II) toxicity. This plant has potential application in phytoremediation of Cd(II), due to relatively substantial accumulation of biomass, as well as secondary metabolites TIAs used as pharmaceutical materials when facing Cd stress.

Specialized Plant Metabolism Characteristics and Impact on Target Molecule Biotechnological Production

Plant secondary metabolism evolved in the context of highly organized and differentiated cells and tissues, featuring massive chemical complexity operating under tight environmental, developmental and genetic control. Biotechnological demand for natural products has been continuously increasing because of their significant value and new applications, mainly as pharmaceuticals. Aseptic production systems of plant secondary metabolites have improved considerably, constituting an attractive tool for increased, stable and large-scale supply of valuable molecules. Surprisingly, to date, only a few examples including taxol, shikonin, berberine and artemisinin have emerged as success cases of commercial production using this strategy. The present review focuses on the main characteristics of plant specialized metabolism and their implications for current strategies used to produce secondary compounds in axenic cultivation systems. The search for consonance between plant secondary metabolism unique features and various in vitro culture systems, including cell, tissue, organ, and engineered cultures, as well as heterologous expression in microbial platforms, is discussed. Data to date strongly suggest that attaining full potential of these biotechnology production strategies requires being able to take advantage of plant specialized metabolism singularities for improved target molecule yields and for bypassing inherent difficulties in its rational manipulation.

Transcriptomics comparison reveals the diversity of ethylene and methyl-jasmonate in roles of TIA metabolism in Catharanthus roseus

Background

The medicinal plant, Catharanthus roseus (C. roseus), accumulates a wide range of terpenoid indole alkaloids (TIAs). Ethylene (ET) and methyl-jasmonate (MeJA) were previously reported as effective elicitors for the production of various valuable secondary metabolites of C. roseus, while a few ET or MeJA induced transcriptomic research is yet reported on this species. In this study, the de-novo transcriptome assembly of C. roseus is performed by using the next-generation sequencing technology.

Results

The result shows that phenolic biosynthesis genes respond specifically to ET in leaves, monoterpenoid biosynthesis genes respond specifically to MeJA in roots. By screening the database, 23 ATP-binding cassette (ABC) transporter partial sequences are identified in C. roseus. On this basis, more than 80 key genes that encode key enzymes (namely TIA pathway, transcriptional factor (TF) and candidate ABC transporter) of alkaloid synthesis in TIA biosynthetic pathways are chosen to explore the integrative responses to ET and MeJA at the transcriptional level. Our data indicated that TIA accumulation is strictly regulated by the TF ethylene responsive factor (ERF) and bHLH iridoid synthesis 1 (BIS1). The heatmap, combined with principal component analysis (PCA) of C. roseus, shows that ERF co-expression with ABC2 and ABC8 specific expression in roots affect the root-specific accumulation of vinblastine in C. roseus. On the contrast, BIS1 activities follow a similar pattern of ABC3 and CrTPT2 specific expression in leaves, which affects the leaf-specific accumulation of vindoline in C. roseus.

Conclusions

Results presented above illustrate that ethylene has a stronger effect than MeJA on TIA induction at both transcriptional and metabolite level. Furthermore, meta-analysis reveals that ERF and BIS1 form a positive feedback loop connecting two ABC transporters respectively and are actively involved in TIAs responding to ET and MeJA in C. roseus.

Electronic supplementary material

The online version of this article (10.1186/s12864-018-4879-3) contains supplementary material, which is available to authorized users.

Comparison of Strategies to Overcome Drug Resistance: Learning from Various Kingdoms

Drug resistance, especially antibiotic resistance, is a growing threat to human health. To overcome this problem, it is significant to know precisely the mechanisms of drug resistance and/or self-resistance in various kingdoms, from bacteria through plants to animals, once more. This review compares the molecular mechanisms of the resistance against phycotoxins, toxins from marine and terrestrial animals, plants and fungi, and antibiotics. The results reveal that each kingdom possesses the characteristic features. The main mechanisms in each kingdom are transporters/efflux pumps in phycotoxins, mutation and modification of targets and sequestration in marine and terrestrial animal toxins, ABC transporters and sequestration in plant toxins, transporters in fungal toxins, and various or mixed mechanisms in antibiotics. Antibiotic producers in particular make tremendous efforts for avoiding suicide, and are more flexible and adaptable to the changes of environments. With these features in mind, potential alternative strategies to overcome these resistance problems are discussed. This paper will provide clues for solving the issues of drug resistance.

Self-Resistance of Natural Product Producers: Past, Present, and Future Focusing on Self-Resistant Protein Variants

Nature is a prolific producers of bioactive natural products with an array of biological activities and impact on human and animal health. But with great power comes great responsibility, and the organisms that produce a bioactive compound must be resistant to its biological effects to survive during production/accumulation. Microorganisms, particularly bacteria, have developed different strategies to prevent self-toxicity. Here, we review a few of the major mechanisms including the mechanism of resistance with a focus on self-resistant protein variants, target proteins that contain amino acid substitutions to reduce the binding of the bioactive natural product, and therefore its inhibitory effects are highlighted in depth. We also try to identify some future avenues of research and challenges that need to be addressed.

Loganic Acid Methyltransferase: Insights into the Specificity of Methylation on an Iridoid Glycoside

Loganin is an iridoid glycoside of interest as both an intermediate in the biosynthesis of indole alkaloids in plants and as a bioactive compound itself. Loganic acid methyltransferase catalyzes the methylation of a monoterpenoid glycoside precursor to produce loganin and demonstrates stereospecificity for the (6S,7R) substrate. We have biochemically characterized this biocatalyst and elucidated the basis for its strict substrate specificity. These studies could help facilitate the design of new classes of monoterpenoid indole alkaloids of pharmaceutical interest.

Solution of the multistep pathway for assembly of corynanthean, strychnos, iboga, and aspidosperma monoterpenoid indole alkaloids from 19E-geissoschizine

The multistep assembly of catharanthine and tabersonine from strictosidine remains poorly characterized for understanding the biochemistry of anticancer monoterpenoid indole alkaloid (MIA) biosynthesis in the medicinal plant, Catharanthus roseus. The seven-step pathway from 19E-geissoschizine to four major MIA skeletons enables the assembly of catharanthine and tabersonine that complete the pathway for biosynthesis of the anticancer drugs, anhydrovinblastine and vincristine as well as for production of other biologically active MIAs.

Monoterpenoid indole alkaloids (MIAs) possess a diversity of alkaloid skeletons whose biosynthesis is poorly understood. A bioinformatic search of candidate genes, combined with their virus-induced gene silencing, targeted MIA profiling and in vitro/in vivo pathway reconstitution identified and functionally characterized six genes as well as a seventh enzyme reaction required for the conversion of 19E-geissoschizine to tabersonine and catharanthine. The involvement of pathway intermediates in the formation of four MIA skeletons is described, and the role of stemmadenine-O-acetylation in providing necessary reactive substrates for the formation of iboga and aspidosperma MIAs is described. The results enable the assembly of complex dimeric MIAs used in cancer chemotherapy and open the way to production of many other biologically active MIAs that are not easily available from nature.

Involvement of LeMRP, an ATP-binding cassette transporter, in shikonin transport and biosynthesis in Lithospermum erythrorhizon

Shikonin and its derivatives are important medicinal secondary metabolites accumulating in roots of Lithospermum erythrorhizon. Although some membrane proteins have been identified as transporters of secondary metabolites, the mechanisms underlying shikonin transport and accumulation in L. erythrorhizon cells still remain largely unknown. In this study, we isolated a cDNA encoding LeMRP, an ATP-binding cassette transporter from L. erythrorhizon, and further investigated its functions in the transport and biosynthesis of shikonin using the yeast transformation and transgenic hairy root methods, respectively. Real-time PCR was applied for expression analyses of LeMRP and shikonin biosynthetic enzyme genes. Functional analysis of LeMRP using the heterologous yeast cell expression system showed that LeMRP could be involved in shikonin transport. Transgenic hairy roots of L. erythrorhizon demonstrated that LeMRP overexpressing hairy roots produced more shikonin than the empty vector (EV) control. Real-time PCR results revealed that the enhanced shikonin biosynthesis in the overexpression lines was mainly caused by highly up-regulated expression of genes coding key enzymes (LePAL, HMGR, Le4CL and LePGT) involved in shikonin biosynthesis. Conversely, LeMRP RNAi decreased the accumulation of shikonin and effectively down-regulated expression level of the above genes. Typical inhibitors of ABC proteins, such as azide and buthionine sulphoximine, dramatically inhibited accumulation of shikonin in hairy roots. Our findings provide evidence for the important direct or indirect role of LeMRP in transmembrane transport and biosynthesis of shikonin.

Geissoschizine synthase controls flux in the formation of monoterpenoid indole alkaloids in a Catharanthus roseus mutant

A Catharanthus roseus mutant accumulates high levels of ajmalicine at the expense of catharanthine and vindoline. The altered chemistry depends on increased expression and biochemical activities of strictosidine β-glucosidase and ajmalicine synthase activities and reduced expression and biochemical activity of geissoschizine synthase. The Madagascar periwinkle [Catharanthus roseus (L.) G. Don] is a commercially important horticultural flower species and is a valuable source for several monoterpenoid indole alkaloids (MIAs), such as the powerful antihypertensive drug ajmalicine and the antineoplastic agents, vinblastine and vincristine. While biosynthesis of the common MIA precursor strictosidine and its reactive aglycones has been elucidated, the branch point steps leading to the formation of different classes of MIAs remain poorly characterized. Screening of 3600 ethyl methyl sulfonate mutagenized C. roseus plants using a simple thin-layer chromatography screen yielded a mutant (M2-0754) accumulating high levels of ajmalicine together with significantly lower levels of catharanthine and vindoline. Comparative bioinformatic analyses, virus-induced gene silencing, and biochemical characterization identified geissoschizine synthase, the gateway enzyme that controls flux for the formation of iboga and aspidosperma MIAs. The reduction of geissoschizine synthase transcripts in this high ajmalicine mutant, together with increased transcripts and enzyme activities of strictosidine β-glucosidase and of heteroyohimbine synthase, explains the preferential formation of ajmalicine in the mutant instead of catharanthine and vindoline that accumulates in the wild-type parent. Reciprocal crosses established that that the high ajmalicine phenotype is inherited as a Mendelian recessive trait.

A tabersonine 3-reductase Catharanthus roseus mutant accumulates vindoline pathway intermediates

Monoterpenoid indole alkaloids (MIAs) have remarkable biological properties that have led to their medical uses for a variety of human diseases. Mutagenesis has been used to generate plants with new alkaloid profiles and a useful screen for rapid comparison of MIA profiles is described. The MIA mutants identified are useful for investigating MIA biosynthesis and for targeted production of these specialised metabolites. The Madagascar periwinkle (Catharanthus roseus) is the sole source of the dimeric anticancer monoterpenoid indole alkaloids (MIAs), 3',4'-anhydrovinblastine and derivatives, which are formed via the coupling of the MIAs, catharanthine and vindoline. While intense efforts to identify parts of the complex pathways involved in the assembly of these dimers have been successful, our understanding of MIA biochemistry in C. roseus remains limited. A simple thin layer chromatography screen of 4000 ethyl methanesulfonate-metagenized M2 plants is described to identify mutant lines with altered MIA profiles. One mutant (M2-1865) accumulated reduced levels of vindoline inside the leaves in favour of high levels of tabersonine-2,3-epoxide and 16-methoxytabersonine-2,3-epoxide on the leaf surface. This MIA profile suggested that changes in tabersonine 3-reductase (T3R) activity might be responsible for the observed phenotype. Molecular cloning of mutant and wild type T3R revealed two nucleotide substitutions at cytosine residues 565 (CAT to TAT) and 903 (ACC to ACA) in the mutant corresponding to substitution (H189Y) and silent (T305T) amino acid mutations, respectively, in the protein. The single amino acid substitution in the mutant T3R protein diminished the biochemical activity of T3R by 95% that explained the reason for the low vindoline phenotype of the mutant. This phenotype was recessive and exhibited standard Mendelian single-gene inheritance. The stable formation and accumulation of epoxides in the M2-1865 mutant provides a dependable biological source of these two MIAs.

Activity of the purified plant ABC transporter NtPDR1 is stimulated by diterpenes and sesquiterpenes involved in constitutive and induced defenses

Within the plant ATP-binding cassette transporter family, pleiotropic drug resistance (PDR) transporters play essential functions, such as in hormone transport or defense against biotic and abiotic stresses. NtPDR1 from Nicotiana tabacum has been shown to be involved in the constitutive defense against pathogens through the secretion of toxic cyclic diterpenes, such as the antimicrobial substrates cembrene and sclareol from the leaf hairs (trichomes). However, direct evidence of an interaction between NtPDR1 and terpenes is lacking. Here, we stably expressed NtPDR1 in N. tabacum BY-2 suspension cells. NtPDR1 was purified as an active monomer glycosylated at a single site in the third external loop. NtPDR1 reconstitution in proteoliposomes stimulated its basal ATPase activity from 21 to 38 nmol of Pi·mg-1·min-1, and ATPase activity was further stimulated by the NtPDR1 substrates cembrene and sclareol, providing direct evidence of an interaction between NtPDR1 and its two substrates. Interestingly, NtPDR1 was also stimulated by capsidiol, a sesquiterpene produced by N. tabacum upon pathogen attack. We also monitored the transcriptional activity from the NtPDR1 promoter in situ with a reporter gene and found that, although NtPDR1 expression was limited to trichomes under normal conditions, addition of methyl jasmonate, a biotic stress hormone, induced expression in all leaf tissues. This finding indicated that NtPDR1 is involved not only in constitutive but also in induced plant defenses. In conclusion, we provide direct evidence of an interaction between the NtPDR1 transporter and its substrates and that NtPDR1 transports compounds involved in both constitutive (diterpenes) and induced (sesquiterpenes) plant defenses.

Involvement of LeMDR, an ATP-binding cassette protein gene, in shikonin transport and biosynthesis in Lithospermum erythrorhizon

BACKGROUND

Shikonin is a naphthoquinone secondary metabolite with important medicinal value and is found in Lithospermum erythrorhizon. Considering the limited knowledge on the membrane transport mechanism of shikonin, this study investigated such molecular mechanism.

RESULTS

We successfully isolated an ATP-binding cassette protein gene, LeMDR, from L. erythrorhizon. LeMDR is predominantly expressed in L. erythrorhizon roots, where shikonin accumulated. Functional analysis of LeMDR by using the yeast cell expression system revealed that LeMDR is possibly involved in the shikonin efflux transport. The accumulation of shikonin is lower in yeast cells transformed with LeMDR-overexpressing vector than that with empty vector. The transgenic hairy roots of L. erythrorhizon overexpressing LeMDR (MDRO) significantly enhanced shikonin production, whereas the RNA interference of LeMDR (MDRi) displayed a reverse trend. Moreover, the mRNA expression level of LeMDR was up-regulated by treatment with shikonin and shikonin-positive regulators, methyl jasmonate and indole-3-acetic acid. There might be a relationship of mutual regulation between the expression level of LeMDR and shikonin biosynthesis.

CONCLUSIONS

Our findings demonstrated the important role of LeMDR in transmembrane transport and biosynthesis of shikonin.

Identification of Iridoid Glucoside Transporters in Catharanthus roseus

Monoterpenoid indole alkaloids (MIAs) are plant defense compounds and high-value pharmaceuticals. Biosynthesis of the universal MIA precursor, secologanin, is organized between internal phloem-associated parenchyma (IPAP) and epidermis cells. Transporters for intercellular transport of proposed mobile pathway intermediates have remained elusive. Screening of an Arabidopsis thaliana transporter library expressed in Xenopus oocytes identified AtNPF2.9 as a putative iridoid glucoside importer. Eight orthologs were identified in Catharanthus roseus, of which three, CrNPF2.4, CrNPF2.5 and CrNPF2.6, were capable of transporting the iridoid glucosides 7-deoxyloganic acid, loganic acid, loganin and secologanin into oocytes. Based on enzyme expression data and transporter specificity, we propose that several enzymes of the biosynthetic pathway are present in both IPAP and epidermis cells, and that the three transporters are responsible for transporting not only loganic acid, as previously proposed, but multiple intermediates. Identification of the iridoid glucoside-transporting CrNPFs is an important step toward understanding the complex orchestration of the seco-iridioid pathway.

The ATP binding cassette transporter, VmTPT2/VmABCG1, is involved in export of the monoterpenoid indole alkaloid, vincamine in Vinca minor leaves

Vinca minor is a herbaceous plant from the Apocynaceae family known to produce over 50 monoterpene indole alkaloids (MIAs). These include several biologically active MIAs that have a range of pharmaceutical activities. The present study shows that the MIAs, vincamine, akuammicine, minovincinine, lochnericine and vincadifformine tend to be secreted on V. minor leaf surfaces. A secretion mechanism of MIAs, previously described for Catharanthus roseus, appears to be mediated by a member (CrTPT2) of the pleiotropic drug resistance ABC transporter subfamily. The molecular cloning of an MIA transporter (VmTPT2/VmABCG1) that is predominantly expressed in V. minor leaves was functionally characterized in yeast and established it as an MIA efflux transporter. The similar function of VmTPT2/VmABCG1 to CrTPT2 increases the likelihood that this MIA transporter family may have co-evolved within members of Apocynaceae family to secrete selected MIAs and to regulate leaf MIA surface chemistry.

Arabidopsis ABCG34 contributes to defense against necrotrophic pathogens by mediating the secretion of camalexin

Plant pathogens cause huge yield losses. Plant defense often depends on toxic secondary metabolites that inhibit pathogen growth. Because most secondary metabolites are also toxic to the plant, specific transporters are needed to deliver them to the pathogens. To identify the transporters that function in plant defense, we screened Arabidopsis thaliana mutants of full-size ABCG transporters for hypersensitivity to sclareol, an antifungal compound. We found that atabcg34 mutants were hypersensitive to sclareol and to the necrotrophic fungi Alternaria brassicicola and Botrytis cinereaAtABCG34 expression was induced by Abrassicicola inoculation as well as by methyl-jasmonate, a defense-related phytohormone, and AtABCG34 was polarly localized at the external face of the plasma membrane of epidermal cells of leaves and roots. atabcg34 mutants secreted less camalexin, a major phytoalexin in Athaliana, whereas plants overexpressing AtABCG34 secreted more camalexin to the leaf surface and were more resistant to the pathogen. When treated with exogenous camalexin, atabcg34 mutants exhibited hypersensitivity, whereas BY2 cells expressing AtABCG34 exhibited improved resistance. Analyses of natural Arabidopsis accessions revealed that AtABCG34 contributes to the disease resistance in naturally occurring genetic variants, albeit to a small extent. Together, our data suggest that AtABCG34 mediates camalexin secretion to the leaf surface and thereby prevents Abrassicicola infection.

Quinolizidine Alkaloid Biosynthesis in Lupins and Prospects for Grain Quality Improvement

Quinolizidine alkaloids (QAs) are toxic secondary metabolites found within the genus Lupinus, some species of which are commercially important grain legume crops including Lupinus angustifolius (narrow-leafed lupin, NLL), L. luteus (yellow lupin), L. albus (white lupin), and L. mutabilis (pearl lupin), with NLL grain being the most largely produced of the four species in Australia and worldwide. While QAs offer the plants protection against insect pests, the accumulation of QAs in lupin grain complicates its use for food purposes as QA levels must remain below the industry threshold (0.02%), which is often exceeded. It is not well understood what factors cause grain QA levels to exceed this threshold. Much of the early work on QA biosynthesis began in the 1970-1980s, with many QA chemical structures well-characterized and lupin cell cultures and enzyme assays employed to identify some biosynthetic enzymes and pathway intermediates. More recently, two genes associated with these enzymes have been characterized, however, the QA biosynthetic pathway remains only partially elucidated. Here, we review the research accomplished thus far concerning QAs in lupin and consider some possibilities for further elucidation and manipulation of the QA pathway in lupin crops, drawing on examples from model alkaloid species. One breeding strategy for lupin is to produce plants with high QAs in vegetative tissues while low in the grain in order to confer insect resistance to plants while keeping grain QA levels within industry regulations. With the knowledge achieved on alkaloid biosynthesis in other plant species in recent years, and the recent development of genomic and transcriptomic resources for NLL, there is considerable scope to facilitate advances in our knowledge of QAs, leading to the production of improved lupin crops.

An NPF transporter exports a central monoterpene indole alkaloid intermediate from the vacuole

Plants sequester intermediates of metabolic pathways into different cellular compartments, but the mechanisms by which these molecules are transported remain poorly understood. Monoterpene indole alkaloids, a class of specialized metabolites that includes the anticancer agent vincristine, antimalarial quinine and neurotoxin strychnine, are synthesized in several different cellular locations. However, the transporters that control the movement of these biosynthetic intermediates within cellular compartments have not been discovered. Here we present the discovery of a tonoplast localized nitrate/peptide family (NPF) transporter from Catharanthus roseus, CrNPF2.9, that exports strictosidine, the central intermediate of this pathway, into the cytosol from the vacuole. This discovery highlights the role that intracellular localization plays in specialized metabolism, and sets the stage for understanding and controlling the central branch point of this pharmacologically important group of compounds.

AaPDR3, a PDR Transporter 3, Is Involved in Sesquiterpene β-Caryophyllene Transport in Artemisia annua

Artemisinin, a sesquiterpenoid endoperoxide, isolated from the plant Artemisia annua L., is widely used in the treatment of malaria. Another sesquiterpenoid, β-caryophyllene having antibiotic, antioxidant, anticarcinogenic and local anesthetic activities, is also presented in A. annua. The role played by sesquiterpene transporters in trichomes and accumulation of these metabolites is poorly understood in A. annua and in trichomes of other plant species. We identified AaPDR3, encoding a pleiotropic drug resistance (PDR) transporter located to the plasma membrane from A. annua. Expression of AaPDR3 is tissue-specifically and developmentally regulated in A. annua. GUS activity is primarily restricted to T-shaped trichomes of old leaves and roots of transgenic A. annua plants expressing proAaPDR3: GUS. The level of β-caryophyllene was decreased in transgenic A. annua plants expressing AaPDR3-RNAi while transgenic A. annua plants expressing increased levels of AaPDR3 accumulated higher levels of β-caryophyllene. When AaPDR3 was expressed in transformed yeast, yeasts expressing AaPDR3 accumulated more β-caryophyllene, rather than germacrene D and β-farnesene, compared to the non-expressing control.

Transcriptional Regulation and Transport of Terpenoid Indole Alkaloid in Catharanthus roseus: Exploration of New Research Directions

As one of the model medicinal plants for exploration of biochemical pathways and molecular biological questions on complex metabolic pathways, Catharanthus roseus synthesizes more than 100 terpenoid indole alkaloids (TIAs) used for clinical treatment of various diseases and for new drug discovery. Given that extensive studies have revealed the major metabolic pathways and the spatial-temporal biosynthesis of TIA in C. roseus plant, little is known about subcellular and inter-cellular trafficking or long-distance transport of TIA end products or intermediates, as well as their regulation. While these transport processes are indispensable for multi-organelle, -tissue and -cell biosynthesis, storage and their functions, great efforts have been made to explore these dynamic cellular processes. Progress has been made in past decades on transcriptional regulation of TIA biosynthesis by transcription factors as either activators or repressors; recent studies also revealed several transporters involved in subcellular and inter-cellular TIA trafficking. However, many details and the regulatory network for controlling the tissue-or cell-specific biosynthesis, transport and storage of serpentine and ajmalicine in root, catharanthine in leaf and root, vindoline specifically in leaf and vinblastine and vincristine only in green leaf and their biosynthetic intermediates remain to be determined. This review is to summarize the progress made in biosynthesis, transcriptional regulation and transport of TIAs. Based on analysis of organelle, tissue and cell-type specific biosynthesis and progresses in transport and trafficking of similar natural products, the transporters that might be involved in transport of TIAs and their synthetic intermediates are discussed; according to transcriptome analysis and bioinformatic approaches, the transcription factors that might be involved in TIA biosynthesis are analyzed. Further discussion is made on a broad context of transcriptional and transport regulation in order to guide our future research.

The basic helix-loop-helix transcription factor BIS2 is essential for monoterpenoid indole alkaloid production in the medicinal plant Catharanthus roseus

Monoterpenoid indole alkaloids (MIAs) are produced as plant defence compounds. In the medicinal plant Catharanthus roseus, they comprise the anticancer compounds vinblastine and vincristine. The iridoid (monoterpenoid) pathway forms one of the two branches that feed MIA biosynthesis and its activation is regulated by the transcription factor (TF) basic helix-loop-helix (bHLH) iridoid synthesis 1 (BIS1). Here, we describe the identification and characterisation of BIS2, a jasmonate (JA)-responsive bHLH TF expressed preferentially in internal phloem-associated parenchyma cells, which transactivates promoters of iridoid biosynthesis genes and can homodimerise or form heterodimers with BIS1. Stable overexpression of BIS2 in C. roseus suspension cells and transient ectopic expression of BIS2 in C. roseus petal limbs resulted in increased transcript accumulation of methylerythritol-4-phosphate and iridoid pathway genes, but not of other MIA genes or triterpenoid genes. Transcript profiling also indicated that BIS2 expression is part of an amplification loop, as it is induced by overexpression of either BIS1 or BIS2. Accordingly, silencing of BIS2 in C. roseus suspension cells completely abolished the JA-induced upregulation of the iridoid pathway genes and subsequent MIA accumulation, despite the presence of induced BIS1, indicating that BIS2 is essential for MIA production in C. roseus.

Plant ABC transporters: time for biochemistry?

ATP-binding cassette (ABC) proteins form a large and ubiquitous family, most members of which are membrane-associated primary transporters. Plant genomes code for a particularly large number of these ABC proteins, with more than 120 genes present in both Arabidopsis thaliana and Oryza sativa (rice). Although plant ABC transporters were initially identified as detoxifiers, sequestering xenobitotics into the vacuole, they were later found to be involved in a wide range of essential physiological processes. Currently, the exact substrates transported by most of these transporters are still unknown and we therefore cannot exclude that a single substrate (e.g. a hormone) is responsible for the diversity of physiological roles. This gap in our knowledge is mainly due to the fact that only a few studies have used direct methods to identify the substrates of these membrane transporters. To address this issue, transport assays involving isolated cells, vesicular membranes or reconstituted liposomes are essential. In this review, we will highlight the importance of the direct biochemical characterization of plant ABC transporters and give some insights into the current status of the homologous and heterologous expression of such proteins.

Isolation of Cells Specialized in Anticancer Alkaloid Metabolism by Fluorescence-Activated Cell Sorting

Plant specialized metabolism often presents a complex cell-specific compartmentation essential to accomplish the biosynthesis of valuable plant natural products. Hence, the disclosure and potential manipulation of such pathways may depend on the capacity to isolate and characterize specific cell types. Catharanthus roseus is the source of several medicinal terpenoid indole alkaloids, including the low-level anticancer vinblastine and vincristine, for which the late biosynthetic steps occur in specialized mesophyll cells called idioblasts. Here, the optical, fluorescence, and alkaloid-accumulating properties of C. roseus leaf idioblasts are characterized, and a methodology for the isolation of idioblast protoplasts by fluorescence-activated cell sorting is established, taking advantage of the distinctive autofluorescence of these cells. This achievement represents a crucial step for the development of differential omic strategies leading to the identification of candidate genes putatively involved in the biosynthesis, pathway regulation, and transmembrane transport leading to the anticancer alkaloids from C. roseus.

Secondary metabolites in plants: transport and self-tolerance mechanisms

Plants produce a host of secondary metabolites with a wide range of biological activities, including potential toxicity to eukaryotic cells. Plants generally manage these compounds by transport to the apoplast or specific organelles such as the vacuole, or other self-tolerance mechanisms. For efficient production of such bioactive compounds in plants or microbes, transport and self-tolerance mechanisms should function cooperatively with the corresponding biosynthetic enzymes. Intensive studies have identified and characterized the proteins responsible for transport and self-tolerance. In particular, many transporters have been isolated and their physiological functions have been proposed. This review describes recent progress in studies of transport and self-tolerance and provides an updated inventory of transporters according to their substrates. Application of such knowledge to synthetic biology might enable efficient production of valuable secondary metabolites in the future.

Cell-specific localization of alkaloids in Catharanthus roseus stem tissue measured with Imaging MS and Single-cell MS

Catharanthus roseus (L.) G. Don is a medicinal plant well known for producing antitumor drugs such as vinblastine and vincristine, which are classified as terpenoid indole alkaloids (TIAs). The TIA metabolic pathway in C. roseus has been extensively studied. However, the localization of TIA intermediates at the cellular level has not been demonstrated directly. In the present study, the metabolic pathway of TIA in C. roseus was studied with two forefront metabolomic techniques, that is, Imaging mass spectrometry (MS) and live Single-cell MS, to elucidate cell-specific TIA localization in the stem tissue. Imaging MS indicated that most TIAs localize in the idioblast and laticifer cells, which emit blue fluorescence under UV excitation. Single-cell MS was applied to four different kinds of cells [idioblast (specialized parenchyma cell), laticifer, parenchyma, and epidermal cells] in the stem longitudinal section. Principal component analysis of Imaging MS and Single-cell MS spectra of these cells showed that similar alkaloids accumulate in both idioblast cell and laticifer cell. From MS/MS analysis of Single-cell MS spectra, catharanthine, ajmalicine, and strictosidine were found in both cell types in C. roseus stem tissue, where serpentine was also accumulated. Based on these data, we discuss the significance of TIA synthesis and accumulation in the idioblast and laticifer cells of C. roseus stem tissue.

Plant ABC Transporters Enable Many Unique Aspects of a Terrestrial Plant's Lifestyle

Terrestrial plants have two to four times more ATP-binding cassette (ABC) transporter genes than other organisms, including their ancestral microalgae. Recent studies found that plants harboring mutations in these transporters exhibit dramatic phenotypes, many of which are related to developmental processes and functions necessary for life on dry land. These results suggest that ABC transporters multiplied during evolution and assumed novel functions that allowed plants to adapt to terrestrial environmental conditions. Examining the literature on plant ABC transporters from this viewpoint led us to propose that diverse ABC transporters enabled many unique and essential aspects of a terrestrial plant's lifestyle, by transporting various compounds across specific membranes of the plant.

Transcriptome Analysis of Secondary Metabolism Pathway, Transcription Factors, and Transporters in Response to Methyl Jasmonate in Lycoris aurea

Lycoris aurea, a medicinal species of the Amaryllidaceae family, is used in the practice of traditional Chinese medicine (TCM) because of its broad pharmacological activities of Amaryllidaceae alkaloids. Despite the officinal and economic importance of Lycoris species, the secondary mechanism for this species is relatively deficient. In this study, we attempted to characterize the transcriptome profiling of L. aurea seedlings with the methyl jasmonate (MeJA) treatment to uncover the molecular mechanisms regulating plant secondary metabolite pathway. By using short reads sequencing technology (Illumina), two sequencing cDNA libraries prepared from control (Con) and 100 μM MeJA-treated (MJ100) samples were sequenced. A total of 26,809,842 and 25,874,478 clean reads in the Con and MJ100 libraries, respectively, were obtained and assembled into 59,643 unigenes. Among them, 41,585 (69.72%) unigenes were annotated by basic local alignment search tool similarity searches against public sequence databases. These included 55 Gene Ontology (GO) terms, 128 Kyoto Encyclopedia of Genes and Genomes (KEGG) pathways, and 25 Clusters of Orthologous Groups (COG) families. Additionally, 4,175 differentially expressed genes (DEGs; false discovery rate ≤ 0.001 and |log₂ Ratio| ≥ 1) with 2,291 up-regulated and 1,884 down-regulated, were found to be affected significantly under MeJA treatment. Subsequently, the DEGs encoding key enzymes involving in the secondary metabolite biosynthetic pathways, transcription factors, and transporter proteins were also analyzed and summarized. Meanwhile, we confirmed the altered expression levels of the unigenes that encode transporters and transcription factors using quantitative real-time PCR (qRT-PCR). With this transcriptome sequencing, future genetic and genomics studies related to the molecular mechanisms associated with the chemical composition of L. aurea may be improved. Additionally, the genes involved in the enrichment of secondary metabolite biosynthesis-related pathways could enhance the potential applications of L. aurea.

Characterization of a second secologanin synthase isoform producing both secologanin and secoxyloganin allows enhanced de novo assembly of a Catharanthus roseus transcriptome

Background

Transcriptome sequencing offers a great resource for the study of non-model plants such as Catharanthus roseus, which produces valuable monoterpenoid indole alkaloids (MIAs) via a complex biosynthetic pathway whose characterization is still undergoing. Transcriptome databases dedicated to this plant were recently developed by several consortia to uncover new biosynthetic genes. However, the identification of missing steps in MIA biosynthesis based on these large datasets may be limited by the erroneous assembly of close transcripts and isoforms, even with the multiple available transcriptomes.

Results

Secologanin synthases (SLS) are P450 enzymes that catalyze an unusual ring-opening reaction of loganin in the biosynthesis of the MIA precursor secologanin. We report here the identification and characterization in C. roseus of a new isoform of SLS, SLS2, sharing 97 % nucleotide sequence identity with the previously characterized SLS1. We also discovered that both isoforms further oxidize secologanin into secoxyloganin. SLS2 had however a different expression profile, being the major isoform in aerial organs that constitute the main site of MIA accumulation. Unfortunately, we were unable to find a current C. roseus transcriptome database containing simultaneously well reconstructed sequences of SLS isoforms and accurate expression levels. After a pair of close mRNA encoding tabersonine 16-hydroxylase (T16H1 and T16H2), this is the second example of improperly assembled transcripts from the MIA pathway in the public transcriptome databases. To construct a more complete transcriptome resource for C. roseus, we re-processed previously published transcriptome data by combining new single assemblies. Care was particularly taken during clustering and filtering steps to remove redundant contigs but not transcripts encoding potential isoforms by monitoring quality reconstruction of MIA genes and specific SLS and T16H isoforms. The new consensus transcriptome allowed a precise estimation of abundance of SLS and T16H isoforms, similar to qPCR measurements.

Conclusions

The C. roseus consensus transcriptome can now be used for characterization of new genes of the MIA pathway. Furthermore, additional isoforms of genes encoding distinct MIA biosynthetic enzymes isoforms could be predicted suggesting the existence of a higher level of complexity in the synthesis of MIA, raising the question of the evolutionary events behind what seems like redundancy.

Electronic supplementary material

The online version of this article (doi:10.1186/s12864-015-1678-y) contains supplementary material, which is available to authorized users.

Completion of the seven-step pathway from tabersonine to the anticancer drug precursor vindoline and its assembly in yeast

Antitumor substances related to vinblastine and vincristine are exclusively found in the Catharanthus roseus (Madagascar periwinkle), a member of the Apocynaceae plant family, and continue to be extensively used in cancer chemotherapy. Although in high demand, these valuable compounds only accumulate in trace amounts in C. roseus leaves. Vinblastine and vincristine are condensed from the monoterpenoid indole alkaloid (MIA) precursors catharanthine and vindoline. Although catharanthine biosynthesis remains poorly characterized, the biosynthesis of vindoline from the MIA precursor tabersonine is well understood at the molecular and biochemical levels. This study uses virus-induced gene silencing (VIGS) to identify a cytochrome P450 [CYP71D1V2; tabersonine 3-oxygenase (T3O)] and an alcohol dehydrogenase [ADHL1; tabersonine 3-reductase (T3R)] as candidate genes involved in the conversion of tabersonine or 16-methoxytabersonine to 3-hydroxy-2,3-dihydrotabersonine or 3-hydroxy-16-methoxy-2,3-dihydrotabersonine, which are intermediates in the vindorosine and vindoline pathways, respectively. Biochemical assays with recombinant enzymes confirm that product formation is only possible by the coupled action of T3O and T3R, as the reaction product of T3O is an epoxide that is not used as a substrate by T3R. The T3O and T3R transcripts were identified in a C. roseus database representing genes preferentially expressed in leaf epidermis and suggest that the subsequent reaction products are transported from the leaf epidermis to specialized leaf mesophyll idioblast and laticifer cells to complete the biosynthesis of these MIAs. With these two genes, the complete seven-gene pathway was engineered in yeast to produce vindoline from tabersonine.

Phytochemical genomics of the Madagascar periwinkle: Unravelling the last twists of the alkaloid engine

The Madagascar periwinkle produces a large palette of Monoterpenoid Indole Alkaloids (MIAs), a class of complex alkaloids including some of the most valuable plant natural products with precious therapeutical values. Evolutionary pressure on one of the hotspots of biodiversity has obviously turned this endemic Malagasy plant into an innovative alkaloid engine. Catharanthus is a unique taxon producing vinblastine and vincristine, heterodimeric MIAs with complex stereochemistry, and also manufactures more than 100 different MIAs, some shared with the Apocynaceae, Loganiaceae and Rubiaceae members. For over 60 years, the quest for these powerful anticancer drugs has inspired biologists, chemists, and pharmacists to unravel the chemistry, biochemistry, therapeutic activity, cell and molecular biology of Catharanthus roseus. Recently, the "omics" technologies have fuelled rapid progress in deciphering the last secret of strictosidine biosynthesis, the central precursor opening biosynthetic routes to several thousand MIA compounds. Dedicated C. roseus transcriptome, proteome and metabolome databases, comprising organ-, tissue- and cell-specific libraries, and other phytogenomic resources, were developed for instance by PhytoMetaSyn, Medicinal Plant Genomic Resources and SmartCell consortium. Tissue specific library screening, orthology comparison in species with or without MIA-biochemical engines, clustering of gene expression profiles together with various functional validation strategies, largely contributed to enrich the toolbox for plant synthetic biology and metabolic engineering of MIA biosynthesis.