Yeast expression of animal and plant P450s in optimized redox environments

Catalytic divergence of O-methyltransferases shapes the chemo-diversity of polymethoxylated bibenzyls in Dendrobium catenatum

Erianin, crepidatin, and chrysotobibenzyl are typical medicinal polymethoxylated bibenzyls (PMBs) that are commercially produced in Dendrobium species. PMBs' chemo-diversity is mediated by the manifold combinations of O-methylation and hydroxylation in a definite order, which remains unsolved. To unequivocally elucidate the methylation mechanism of PMBs, 15 possible intermediates in the biosynthetic pathway of PMBs were chemically synthesized. DcOMT1-5 were highly expressed in tissues where PMBs were biosynthesized, and their expression patterns were well-correlated with the accumulation profiles of PMBs. Moreover, cell-free orthogonal tests based on the synthesized intermediates further confirmed that DcOMT1-5 exhibited distinct substrate preferences and displayed hydroxyl-group regiospecificity during the sequential methylation process. The stepwise methylation of PMBs was discovered from SAM to dihydro-piceatannol (P) in the following order: P → 3-MeP → 4-OH-3-MeP → 4-OH-3,5-diMeP → 3,3'(4'),5-triMeP → 3,4,4',5-tetraMeP (erianin) or 3,3',4,5-tetraMeP (crepidatin) → 3,3',4,4',5-pentaMeP (chrysotobibenzyl). Furthermore, the regioselectivities of DcOMTs were investigated by ligand docking analyses which corresponded precisely with the catalytic activities. In summary, the findings shed light on the sequential catalytic mechanisms of PMB biosynthesis and provide a comprehensive PMB biosynthetic network in D. catenatum. The knowledge gained from this study may also contribute to the development of plant-based medicinal applications and the production of high-value PMBs.

Establishing cell suitability for high-level production of licorice triterpenoids in yeast

Yeast has been an indispensable host for synthesizing complex plant-derived natural compounds, yet the yields remained largely constrained. This limitation mainly arises from overlooking the importance of cell and pathway suitability during the optimization of enzymes and pathways. Herein, beyond conventional enzyme engineering, we dissected metabolic suitability with a framework for simultaneously augmenting cofactors and carbon flux to enhance the biosynthesis of heterogenous triterpenoids. We further developed phospholipid microenvironment engineering strategies, dramatically improving yeast's suitability for the high performance of endoplasmic reticulum (ER)-localized, rate-limiting plant P450s. Combining metabolic and microenvironment suitability by manipulating only three genes, NHMGR (NADH-dependent HMG-CoA reductase), SIP4 (a DNA-binding transcription factor)and GPP1 (Glycerol-1-phosphate phosphohydrolase 1), we enabled the high-level production of 4.92 g/L rare licorice triterpenoids derived from consecutive oxidation of β-amyrin by two P450 enzymes after fermentation optimization. This production holds substantial commercial value, highlighting the critical role of establishing cell suitability in enhancing triterpenoid biosynthesis and offering a versatile framework applicable to various plant natural product biosynthetic pathways.

OsCYP706C2 diverts rice strigolactone biosynthesis to a noncanonical pathway branch

Strigolactones exhibit dual functionality as regulators of plant architecture and signaling molecules in the rhizosphere. The important model crop rice exudes a blend of different strigolactones from its roots. Here, we identify the inaugural noncanonical strigolactone, 4-oxo-methyl carlactonoate (4-oxo-MeCLA), in rice root exudate. Comprehensive, cross-species coexpression analysis allowed us to identify a cytochrome P450, OsCYP706C2, and two methyl transferases as candidate enzymes for this noncanonical rice strigolactone biosynthetic pathway. Heterologous expression in yeast and Nicotiana benthamiana indeed demonstrated the role of these enzymes in the biosynthesis of 4-oxo-MeCLA, which, expectedly, is derived from carlactone as substrate. The oscyp706c2 mutants do not exhibit a tillering phenotype but do have delayed mycorrhizal colonization and altered root phenotype. This work sheds light onto the intricate complexity of strigolactone biosynthesis in rice and delineates its role in symbiosis and development.

Identification of the cytochrome P450s responsible for the biosynthesis of two types of aporphine alkaloids and their de novo biosynthesis in yeast

Aporphine alkaloids have diverse pharmacological activities; however, our understanding of their biosynthesis is relatively limited. Previous studies have classified aporphine alkaloids into two categories based on the configuration and number of substituents of the D-ring and have proposed preliminary biosynthetic pathways for each category. In this study, we identified two specific cytochrome P450 enzymes (CYP80G6 and CYP80Q5) with distinct activities toward (S)-configured and (R)-configured substrates from the herbaceous perennial vine Stephania tetrandra, shedding light on the biosynthetic mechanisms and stereochemical features of these two aporphine alkaloid categories. Additionally, we characterized two CYP719C enzymes (CYP719C3 and CYP719C4) that catalyzed the formation of the methylenedioxy bridge, an essential pharmacophoric group, on the A- and D-rings, respectively, of aporphine alkaloids. Leveraging the functional characterization of these crucial cytochrome P450 enzymes, we reconstructed the biosynthetic pathways for the two types of aporphine alkaloids in budding yeast (Saccharomyces cerevisiae) for the de novo production of compounds such as (R)-glaziovine, (S)-glaziovine, and magnoflorine. This study provides key insight into the biosynthesis of aporphine alkaloids and lays a foundation for producing these valuable compounds through synthetic biology.

Unraveling the role of cytochrome P450 enzymes in oleanane triterpenoid biosynthesis in arjuna tree

Triterpenoids (C30-isoprenoids) represent a major group of natural products with various physiological functions in plants. Triterpenoids and their derivatives have medicinal uses owing to diverse bioactivities. Arjuna (Terminalia arjuna) tree bark accumulates highly oxygenated β-amyrin-derived oleanane triterpenoids (e.g., arjunic acid, arjungenin, and arjunolic acid) with cardioprotective roles. However, biosynthetic routes and enzymes remain poorly understood. We mined the arjuna transcriptome and conducted cytochrome P450 monooxygenase (P450) assays using Saccharomyces cerevisiae and Nicotiana benthamiana to identify six P450s and two P450 reductases for oxidative modifications of oleanane triterpenoids. P450 assays using oleananes revealed a greater substrate promiscuity of C-2α and C-23 hydroxylases/oxidases than C-28 oxidases. CYP716A233 and CYP716A432 catalyzed β-amyrin/erythrodiol C-28 oxidation to produce oleanolic acid. C-2α hydroxylases (CYP716C88 and CYP716C89) converted oleanolic acid and hederagenin to maslinic acid and arjunolic acid. CYP716C89 also hydroxylated erythrodiol and oleanolic aldehyde. However, CYP714E107a and CYP714E107b catalyzed oleanolic acid/maslinic acid/arjunic acid, C-23 hydroxylation to form hederagenin, arjunolic acid and arjungenin, and hederagenin C-23 oxidation to produce gypsogenic acid, but at a lower rate than oleanolic acid C-23 hydroxylation. Overall, P450 substrate selectivity suggested that C-28 oxidation is the first P450-catalyzed oxidative modification in the arjuna triterpenoid pathway. However, the pathway might branch thereafter through C-2α/C-23 hydroxylation of oleanolic acid. Taken together, these results provided new insights into substrate range of P450s and unraveled biosynthetic routes of triterpenoids in arjuna. Moreover, complete elucidation and reconstruction of arjunolic acid pathway in S. cerevisiae and N. benthamiana suggested the utility of arjuna P450s in heterologous production of cardioprotective compounds.

Characterization of two CYP80 enzymes provides insights into aporphine alkaloid skeleton formation in Aristolochia contorta

Aporphine alkaloids are a large group of natural compounds with extensive pharmaceutical application prospects. The biosynthesis of aporphine alkaloids has been paid attentions in the past decades. Here, we determined the contents of four 1-benzylisoquinoline alkaloids and five aporphine alkaloids in root, stem, leaf, and flower of Aristolochia contorta Bunge, which belongs to magnoliids. Two CYP80 enzymes were identified and characterized from A. contorta. Both of them catalyze the unusual C-C phenol coupling reactions and directly form the aporphine alkaloid skeleton. AcCYP80G7 catalyzed the formation of hexacyclic aporphine corytuberine. AcCYP80Q8 catalyzed the formation of pentacyclic proaporphine glaziovine. Kingdom-wide phylogenetic analysis of the CYP80 family suggested that CYP80 first appeared in Nymphaeales. The functional divergence of hydroxylation and C-C (or C-O) phenol coupling preceded the divergence of magnoliids and eudicots. Probable crucial residues of AcCYP80Q8 were selected through sequence alignment and molecular docking. Site-directed mutagenesis revealed two crucial residues E284 and Y106 for the catalytic reaction. Identification and characterization of two aporphine skeleton-forming enzymes provide insights into the biosynthesis of aporphine alkaloids.

Characterization of cinnamate 4-hydroxylase (CYP73A) and p-coumaroyl 3'-hydroxylase (CYP98A) from Leucojum aestivum, a source of Amaryllidaceae alkaloids

Biosynthesis of Amaryllidaceae alkaloids (AA) starts with the condensation of tyramine with 3,4-dihydroxybenzaldehyde. The latter derives from the phenylpropanoid pathway that involves modifications of trans-cinnamic acid, p-coumaric acid, caffeic acid, and possibly 4-hydroxybenzaldehyde, all potentially catalyzed by hydroxylase enzymes. Leveraging bioinformatics, molecular biology techniques, and cell biology tools, this research identifies and characterizes key enzymes from the phenylpropanoid pathway in Leucojum aestivum. Notably, we focused our work on trans-cinnamate 4-hydroxylase (LaeC4H) and p-coumaroyl shikimate/quinate 3'-hydroxylase (LaeC3'H), two key cytochrome P450 enzymes, and on the ascorbate peroxidase/4-coumarate 3-hydroxylase (LaeAPX/C3H). Although LaeAPX/C3H consumed p-coumaric acid, it did not result in the production of caffeic acid. Yeasts expressing LaeC4H converted trans-cinnamate to p-coumaric acid, whereas LaeC3'H catalyzed specifically the 3-hydroxylation of p-coumaroyl shikimate, rather than of free p-coumaric acid or 4-hydroxybenzaldehyde. In vivo assays conducted in planta in this study provided further evidence for the contribution of these enzymes to the phenylpropanoid pathway. Both enzymes demonstrated typical endoplasmic reticulum membrane localization in Nicotiana benthamiana adding spatial context to their functions. Tissue-specific gene expression analysis revealed roots as hotspots for phenylpropanoid-related transcripts and bulbs as hubs for AA biosynthetic genes, aligning with the highest AAs concentration. This investigation adds valuable insights into the phenylpropanoid pathway within Amaryllidaceae, laying the foundation for the development of sustainable production platforms for AAs and other bioactive compounds with diverse applications.

Three types of enzymes complete the furanocoumarins core skeleton biosynthesis in Angelica sinensis

Furanocoumarins (FCs) are widely distributed secondary metabolites found in higher plants, including Apiaceae, Rutaceae, Moraceae, and Fabaceae. They play a crucial role in the physiological functions of plants and are well-known for their diverse pharmacological activities. As a representative plant of the Apiaceae family, Angelica sinensis is highly valued for its medicinal properties and FCs are one of the main ingredients of A. sinensis. However, the biosynthetic mechanism of FCs in A. sinensis remains poorly understood. In this study, we successfully cloned and verified three types of enzymes using genome analysis and in vitro functional verification, which complete the biosynthesis of the FCs core skeleton in A. sinensis. It includes a p-coumaroyl CoA 2'-hydroxylase (AsC2'H) responsible for umbelliferone formation, two UbiA prenyltransferases (AsPT1 and AsPT2) that convert umbelliferone to demethylsuberosin (DMS) and osthenol, respectively, and two CYP736 subfamily cyclases (AsDC and AsOD) that catalyze the formation of FCs core skeleton. Interestingly, AsOD was demonstrated to be a bifunctional cyclase and could catalyze both DMS and osthenol, but had a higher affinity to osthenol. The characterization of these enzymes elucidates the molecular mechanism of FCs biosynthesis, providing new insights and technologies for understanding the diverse origins of FCs biosynthesis.

Cloning and functional characterization of a cinnamate 4-hydroxylase gene from the hornwort Anthoceros angustus

Hornworts, as the sister group to liverworts and mosses, comprise bryophytes, which are critical in understanding the evolution of key land plant traits. Cinnamate 4-hydroxylase (C4H) catalyzes the second step of the phenylpropanoid pathway to synthesize the precursor of numerous phenolic compounds, such as lignin and flavonoids. However, C4H in the hornwort Anthoceros angustus has not yet been cloned and functionally characterized. In this work, we screened the transcriptome database of A. angustus and identified one C4H gene, AnanC4H. AnanC4H maintained conserved cytochrome P450 domains with other typical plant C4Hs. Ultraviolet B irradiation and exogenous application of methyl jasmonate (MeJA) induced the expression of AnanC4H to varying degrees. The coding sequence of AnanC4H was expressed in yeast, and the recombinant proteins were isolated. The recombinant proteins of AnanC4H catalyzed the conversion of trans-cinnamic acid to p-coumaric acid and catalyzed the conversion of 3-hydroxycinnamic acid to caffeic acid. AnanC4H showed higher affinity for trans-cinnamic acid than for 3-hydroxycinnamic acid, but there was no significant difference in the catalytic efficiency of AnanC4H for the two substrates in vitro. Moreover, the expression of AnanC4H in Arabidopsis thaliana led to an increase in both the lignin content and the number of lignified cells in stems. However, there was no significant change in flavonoid content in transgenic Arabidopsis plants.

Functional analysis of CYP71AV1 reveals the evolutionary landscape of artemisinin biosynthesis

Artemisinin biosynthesis, unique to Artemisia annua, is suggested to have evolved from the ancestral costunolide biosynthetic pathway commonly found in the Asteraceae family. However, the evolutionary landscape of this process is not fully understood. The first oxidase in artemisinin biosynthesis, CYP71AV1, also known as amorpha-4,11-diene oxidase (AMO), has specialized from ancestral germacrene A oxidases (GAOs). Unlike GAO, which exhibits catalytic promiscuity toward amorpha-4,11-diene, the natural substrate of AMO, AMO has lost its ancestral activity on germacrene A. Previous studies have suggested that the loss of the GAO copy in A. annua is responsible for the abolishment of the costunolide pathway. In the genome of A. annua, there are two copies of AMO, each of which has been reported to be responsible for the different product profiles of high- and low-artemisinin production chemotypes. Through analysis of their tissue-specific expression and comparison of their sequences with those of other GAOs, it was discovered that one copy of AMO (AMOHAP) exhibits a different transcript compared to the reported artemisinin biosynthetic genes and shows more sequence similarity to other GAOs in the catalytic regions. Furthermore, in a subsequent in vitro enzymatic assay, the recombinant protein of AMOHAP unequivocally demonstrated GAO activity. This result clearly indicates that AMOHAP is a GAO rather than an AMO and that its promiscuous activity on amorpha-4,11-diene has led to its misidentification as an AMO in previous studies. In addition, the divergent expression pattern of AMOHAP compared to that of the upstream germacrene A synthase may have contributed to the abolishment of costunolide biosynthesis in A. annua. Our findings reveal a complex evolutionary landscape in which the emergence of a new metabolic pathway replaces an ancestral one.

Sustainable production of natural products using synthetic biology: Ginsenosides

Synthetic biology approaches offer potential for large-scale and sustainable production of natural products with bioactive potency, including ginsenosides, providing a means to produce novel compounds with enhanced therapeutic properties. Ginseng, known for its non-toxic and potent qualities in traditional medicine, has been used for various medical needs. Ginseng has shown promise for its antioxidant and neuroprotective properties, and it has been used as a potential agent to boost immunity against various infections when used together with other drugs and vaccines. Given the increasing demand for ginsenosides and the challenges associated with traditional extraction methods, synthetic biology holds promise in the development of therapeutics. In this review, we discuss recent developments in microorganism producer engineering and ginsenoside production in microorganisms using synthetic biology approaches.

Two types of coumarins-specific enzymes complete the last missing steps in pyran- and furanocoumarins biosynthesis

Pyran- and furanocoumarins are key representatives of tetrahydropyrans and tetrahydrofurans, respectively, exhibiting diverse physiological and medical bioactivities. However, the biosynthetic mechanisms for their core structures remain poorly understood. Here we combined multiomics analyses of biosynthetic enzymes in Peucedanum praeruptorum and in vitro functional verification and identified two types of key enzymes critical for pyran and furan ring biosynthesis in plants. These included three distinct P. praeruptorum prenyltransferases (PpPT1-3) responsible for the prenylation of the simple coumarin skeleton 7 into linear or angular precursors, and two novel CYP450 cyclases (PpDC and PpOC) crucial for the cyclization of the linear/angular precursors into either tetrahydropyran or tetrahydrofuran scaffolds. Biochemical analyses of cyclases indicated that acid/base-assisted epoxide ring opening contributed to the enzyme-catalyzed tetrahydropyran and tetrahydrofuran ring refactoring. The possible acid/base-assisted catalytic mechanisms of the identified cyclases were theoretically investigated and assessed using site-specific mutagenesis. We identified two possible acidic amino acids Glu303 in PpDC and Asp301 in PpOC as vital in the catalytic process. This study provides new enzymatic tools in the epoxide formation/epoxide-opening mediated cascade reaction and exemplifies how plants become chemically diverse in terms of enzyme function and catalytic process.

Biosynthesis, herbivore induction, and defensive role of phenylacetaldoxime glucoside

Aldoximes are well-known metabolic precursors for plant defense compounds such as cyanogenic glycosides, glucosinolates, and volatile nitriles. They are also defenses themselves produced in response to herbivory; however, it is unclear whether aldoximes can be stored over a longer term as defense compounds and how plants protect themselves against the potential autotoxic effects of aldoximes. Here, we show that the Neotropical myrmecophyte tococa (Tococa quadrialata, recently renamed Miconia microphysca) accumulates phenylacetaldoxime glucoside (PAOx-Glc) in response to leaf herbivory. Sequence comparison, transcriptomic analysis, and heterologous expression revealed that 2 cytochrome P450 enzymes, CYP79A206 and CYP79A207, and the UDP-glucosyltransferase UGT85A123 are involved in the formation of PAOx-Glc in tococa. Another P450, CYP71E76, was shown to convert PAOx to the volatile defense compound benzyl cyanide. The formation of PAOx-Glc and PAOx in leaves is a very local response to herbivory but does not appear to be regulated by jasmonic acid signaling. In contrast to PAOx, which was only detectable during herbivory, PAOx-Glc levels remained high for at least 3 d after insect feeding. This, together with the fact that gut protein extracts of 3 insect herbivore species exhibited hydrolytic activity toward PAOx-Glc, suggests that the glucoside is a stable storage form of a defense compound that may provide rapid protection against future herbivory. Moreover, the finding that herbivory or pathogen elicitor treatment also led to the accumulation of PAOx-Glc in 3 other phylogenetically distant plant species suggests that the formation and storage of aldoxime glucosides may represent a widespread plant defense response.

John N, Zaidi F, Doyle F, Fasullo M. High-throughput screening of the Saccharomyces cerevisiae genome for 2-amino-3-methylimidazo [4,5-f] quinoline resistance identifies colon cancer-associated genes

Heterocyclic aromatic amines (HAAs) are potent carcinogenic agents found in charred meats and cigarette smoke. However, few eukaryotic resistance genes have been identified. We used Saccharomyces cerevisiae (budding yeast) to identify genes that confer resistance to 2-amino-3-methylimidazo[4,5-f] quinoline (IQ). CYP1A2 and NAT2 activate IQ to become a mutagenic nitrenium compound. Deletion libraries expressing human CYP1A2 and NAT2 or no human genes were exposed to either 400 or 800 µM IQ for 5 or 10 generations. DNA barcodes were sequenced using the Illumina HiSeq 2500 platform and statistical significance was determined for exactly matched barcodes. We identified 424 ORFs, including 337 genes of known function, in duplicate screens of the "humanized" collection for IQ resistance; resistance was further validated for a select group of 51 genes by growth curves, competitive growth, or trypan blue assays. Screens of the library not expressing human genes identified 143 ORFs conferring resistance to IQ per se. Ribosomal protein and protein modification genes were identified as IQ resistance genes in both the original and "humanized" libraries, while nitrogen metabolism, DNA repair, and growth control genes were also prominent in the "humanized" library. Protein complexes identified included the casein kinase 2 (CK2) and histone chaperone (HIR) complex. Among DNA Repair and checkpoint genes, we identified those that function in postreplication repair (RAD18, UBC13, REV7), base excision repair (NTG1), and checkpoint signaling (CHK1, PSY2). These studies underscore the role of ribosomal protein genes in conferring IQ resistance, and illuminate DNA repair pathways for conferring resistance to activated IQ.

Two CYP93A enzymes play a dual role in isoflavonoid biosynthesis in Glycine max L

Glycine max L. is rich in isoflavonoids with diverse biological activities. However, isoflavonoid biosynthetic pathway is not fully elucidated in soybean. In the present study, we investigated characteristics of all the thirteen CYP93 subfamily members, and found GmCYP93A1, GmCYP93A2, and GmCYP93A3 are closely clustered, preferentially expressed in roots, and highly inducible by elicitor. When expressed in yeast, GmCYP93A1 was active towards liquiritigenin, naringenin, and 3,9-dihydroxyptercarpan, GmCYP93A2 towards 3,9-dihydroxyptercarpan with strict substrate specificity, whereas GmCYP93A3 did not show any activity towards all the tested substrates. Both GmCYP93A1 and GmCYP93A2 could catalyze 3,9-dihydroxyptercarpan into daidzein and glycinol, with both hydroxylation and aryl migration activity. Site-directed mutagenesis assays revealed that mutation in Thr446 to Ser446 in heme-binding domain increased the enzyme activity of GmCYP93A1 towards 3,9-dihydroxyptercarpan, which highlights its key amino acid residues as shown with its molecular docking with 3,9-dihydroxyptercarpan and HEM. Overexpression of GmCYP93A1 and GmCYP93A2 in the soybean hairy roots reduced the content of daidzein, whereas knockdown of these two genes increased genistein content, indicating changes in expression level of GmCYP93A1 and GmCYP93A2 altered isoflavonoid flux in soybean. Our studies on the activity of GmCYP93A1 and GmCYP93A2 enriched diverse functions of CYP93 subfamily in soybean isoflavonoid pathway, which is valuable for further understanding and bioengineering of isoflavonoid pathway in soybean.

Sustainable metabolic engineering requires a perfect trifecta

The versatility of cellular metabolism in converting various substrates to products inspires sustainable alternatives to conventional chemical processes. Metabolism can be engineered to maximize the yield, rate, and titer of product generation. However, the numerous combinations of substrate, product, and organism make metabolic engineering projects difficult to navigate. A perfect trifecta of substrate, product, and organism is prerequisite for an environmentally and economically sustainable metabolic engineering endeavor. As a step toward this endeavor, we propose a reverse engineering strategy that starts with product selection, followed by substrate and organism pairing. While a large bioproduct space has been explored, the top-ten compounds have been synthesized mainly using glucose and model organisms. Unconventional feedstocks (e.g. hemicellulosic sugars and CO2) and non-model organisms are increasingly gaining traction for advanced bioproduct synthesis due to their specialized metabolic modes. Judicious selection of the substrate-organism-product combination will illuminate the untapped territory of sustainable metabolic engineering.

Discovery of unique CYP716C oxidase involved in pentacyclic triterpene biosynthesis from Camptotheca acuminata

Dozens of triterpenes have been isolated from Camptotheca acuminata, however, triterpene metabolism in this plant remains poorly understood. The common C28 carboxy located in the oleanane-type and ursane-type triterpenes indicates the existence of a functionally active triterpene, C28 oxidase, in this plant. Thorough mining and screening of the CYP716 genes were initiated using the multi-omics database for C. acuminata. Two CYP716A (CYP716A394 and CYP716A395) and three CYP716C (CYP716C80-CYP716C82) were identified based on conserved domain analyses and hierarchical cluster analyses. CYP716 microsomal proteins were prepared and their enzymatic activities were evaluated in vitro. The CYP716 classified into the CYP716C subfamily displays β-amyrin oxidation activity, and CYP716A displays α-amyrin and lupeol oxidation activity, based on gas chromatography-mass spectrometry analyses. The oxidation products were determined based on their mass and nuclear magnetic resonance spectrums. The optimum reaction conditions and kinetic parameters for CYP716C were determined, and functions were verified in Nicotiana benthaminana. Relative quantitative analyses revealed that these CYP716C genes were enriched in the leaves of C. acuminata plantlets after 60 d. These results indicate that CYP716C plays a dominant role in oleanane-type triterpene metabolism in the leaves of C. acuminata via a substrate-specific manner, and CYP716A is responsible for ursane- and lupane-type triterpene metabolism in fruit. This study provides valuable insights into the unique CYP716C-mediated oxidation step of pentacyclic triterpene biosynthesis in C. acuminata.

Hydroxylases involved in terpenoid biosynthesis: a review

Terpenoids are pervasive in nature and display an immense structural diversity. As the largest category of plant secondary metabolites, terpenoids have important socioeconomic value in the fields of pharmaceuticals, spices, and food manufacturing. The biosynthesis of terpenoid skeletons has made great progress, but the subsequent modifications of the terpenoid framework are poorly understood, especially for the functionalization of inert carbon skeleton usually catalyzed by hydroxylases. Hydroxylase is a class of enzymes that plays an important role in the modification of terpenoid backbone. This review article outlines the research progress in the identification, molecular modification, and functional expression of this class of enzymes in the past decade, which are profitable for the discovery, engineering, and application of more hydroxylases involved in the plant secondary metabolism.

Identification and characterization of cytochrome P450 CYP77A59 of loquat (Rhaphiolepis bibas) responsible for biosynthesis of phenylacetonitrile, a floral nitrile compound

MAIN CONCLUSION

Cytochrome P450 CYP77A59 is responsible for the biosynthesis of phenylacetonitrile in loquat flowers. Flowers of some plants emit volatile nitrile compounds, but the biosynthesis of these compounds is unclear. Loquat (Rhaphiolepis bibas) flowers emit characteristic N-containing volatiles, such as phenylacetonitrile (PAN), (E/Z)-phenylacetaldoxime (PAOx), and (2-nitroethyl)benzene (NEB). These volatiles likely play a defense role against pathogens and insects. PAN and NEB are commonly biosynthesized from L-phenylalanine via (E/Z)-PAOx. Two cytochrome P450s-CYP79D80 and "promiscuous fatty acid ω-hydroxylase" CYP94A90, which catalyze the formation of (E/Z)-PAOx from L-phenylalanine and NEB from (E/Z)-PAOx, respectively-are involved in NEB biosynthesis. However, the enzymes catalyzing the formation of PAN from (E/Z)-PAOx in loquat have not been identified. In this study, we aimed to identify candidate cytochrome P450s catalyzing PAN formation in loquat flowers. Yeast whole-cell biocatalyst assays showed that among nine candidate cytochrome P450s, CYP77A58 and CYP77A59 produced PAN from (E/Z)-PAOx. CYP77As catalyzed the dehydration of aldoximes, which is atypical of cytochrome P450; the reaction was NADPH-dependent, with an optimum temperature and pH of 40 °C and 8.0, respectively. CYP77As acted on (E/Z)-PAOx, (E/Z)-4-hydroxyphenylacetaldoxime, and (E/Z)-indole-3-acetaldoxime. Previously characterized CYP77As are known to hydroxylate fatty acids; loquat CYP77As did not act on tested fatty acids. We observed higher expression of CYP77A59 in flowers than in buds; expression of CYP77A58 was remarkably reduced in the flowers. Because the flowers, but not buds, emit PAN, CYP77A59 is likely responsible for the biosynthesis of PAN in loquat flowers. This study will help us understand the biosynthesis of floral nitrile compounds.

Lactucin Synthase Inactivation Boosts the Accumulation of Anti-inflammatory 8-Deoxylactucin and Its Derivatives in Chicory (Cichorium intybus L.)

For several sesquiterpene lactones (STLs) found in Asteraceae plants, very interesting biomedical activities have been demonstrated. Chicory roots accumulate the guaianolide STLs 8-deoxylactucin, lactucin, and lactucopicrin predominantly in oxalated forms in the latex. In this work, a supercritical fluid extract fraction of chicory STLs containing 8-deoxylactucin and 11β,13-dihydro-8-deoxylactucin was shown to have anti-inflammatory activity in an inflamed intestinal mucosa model. To increase the accumulation of these two compounds in chicory taproots, the lactucin synthase that takes 8-deoxylactucin as the substrate for the regiospecific hydroxylation to generate lactucin needs to be inactivated. Three candidate cytochrome P450 enzymes of the CYP71 clan were identified in chicory. Their targeted inactivation using the CRISPR/Cas9 approach identified CYP71DD33 to have lactucin synthase activity. The analysis of the terpene profile of the taproots of plants with edits in CYP71DD33 revealed a nearly complete elimination of the endogenous chicory STLs lactucin and lactucopicrin and their corresponding oxalates. Indeed, in the same lines, the interruption of biosynthesis resulted in a strong increase of 8-deoxylactucin and its derivatives. The enzyme activity of CYP71DD33 to convert 8-deoxylactucin to lactucin was additionally demonstrated in vitro using yeast microsome assays. The identified chicory lactucin synthase gene is predominantly expressed in the chicory latex, indicating that the late steps in the STL biosynthesis take place in the latex. This study contributes to further elucidation of the STL pathway in chicory and shows that root chicory can be positioned as a crop from which different health products can be extracted.

Identification of a second 16-hydroxytabersonine-O-methyltransferase suggests an evolutionary relationship between alkaloid and flavonoid metabolisms in Catharanthus roseus

The medicinal plant Catharanthus roseus biosynthesizes many important drugs for human health, including the anticancer monoterpene indole alkaloids (MIAs) vinblastine and vincristine. Over the past decades, the continuous increase in pharmaceutical demand has prompted several research groups to characterize MIA biosynthetic pathways for considering future metabolic engineering processes of supply. In line with previous work suggesting that diversification can potentially occur at various steps along the vindoline branch, we were here interested in investigating the involvement of distinct isoforms of tabersonine-16-O-methyltransferase (16OMT) which plays a pivotal role in the MIA biosynthetic pathway. By combining homology searches based on the previously characterized 16OMT1, phylogenetic analyses, functional assays in yeast, and biochemical and in planta characterizations, we identified a second isoform of 16OMT, referred to as 16OMT2. 16OMT2 appears to be a multifunctional enzyme working on both MIA and flavonoid substrates, suggesting that a constrained evolution of the enzyme for accommodating the MIA substrate has probably occurred to favor the apparition of 16OMT2 from an ancestral specific flavonoid-O-methyltransferase. Since 16OMT1 and 16OMT2 displays a high sequence identity and similar kinetic parameters for 16-hydroxytabersonine, we postulate that 16OMT1 may result from a later 16OMT2 gene duplication accompanied by a continuous neofunctionalization leading to an almost complete loss of flavonoid O-methyltransferase activity. Overall, these results participate in increasing our knowledge on the evolutionary processes that have likely led to enzyme co-optation for MIA synthesis.

Characterization of Arnebia euchroma PGT homologs involved in the biosynthesis of shikonin

Shikonin is a red naphthoquinone natural product from plants with high economical and medical values. The para-hydroxybenzoic acid geranyltransferase (PGT) catalyzes the key regulatory step of shikonin biosynthesis. PGTs from Lithospermum erythrorhizon have been well-characterized and used in industrial shikonin production. However, its perennial medicinal plant Arnebia euchroma accumulates much more pigment and the underlying mechanism remains obscure. Here, we discovered and characterized the different isoforms of AePGTs. Phylogenetic study and structure modeling suggested that the N-terminal of AePGT6 contributed to its highest activity among 7 AePGTs. Indeed, AePGT2 and AePGT3 fused with 60 amino acids from the N-terminal of AePGT6 showed even higher activity than AePGT6, while native AePGT2 and AePGT3 don't have catalytic activity. Our result not only provided a mechanistic explanation of high shikonin contents in Arnebia euchroma but also engineered a best-performing PGT to achieve the highest-to-date production of 3-geranyl-4-hydroxybenzoate acid, an intermedium of shikonin.

Manipulation of sterol homeostasis for the production of 24-epi-ergosterol in industrial yeast

Brassinolide (BL) is the most biologically active compound among natural brassinosteroids. However, the agricultural applications are limited by the extremely low natural abundance and the scarcity of synthetic precursors. Here, we employ synthetic biology to construct a yeast cell factory for scalable production of 24-epi-ergosterol, an un-natural sterol, proposed as a precursor for BL semi-synthesis. First, we construct an artificial pathway by introducing a Δ24(28) sterol reductase from plants (DWF1), followed by enzyme directed evolution, to enable de novo biosynthesis of 24-epi-ergosterol in yeast. Subsequently, we manipulate the sterol homeostasis (overexpression of ARE2, YEH1, and YEH2 with intact ARE1), maintaining a balance between sterol acylation and sterol ester hydrolysis, for the production of 24-epi-ergosterol, whose titer reaches to 2.76 g L-1 using fed-batch fermentation. The sterol homeostasis engineering strategy can be applicable for bulk production of other economically important phytosterols.

Tissue-preferential recruitment of electron transfer chains for cytochrome P450-catalyzed phenolic biosynthesis

Cytochrome P450 system consists of P450 monooxygenase and redox pattern(s). While the importance of monooxygenases in plant metabolism is well documented, the metabolic roles of the related redox components have been largely overlooked. Here, we show that distinct electron transfer chains are recruited in phenylpropanoid-monolignol P450 systems to support the synthesis and distribution of different classes of phenolics in different plant tissues. While Arabidopsis cinnamate 4-hydroxylase adopts conventional NADPH-cytochrome P450 oxidoreductase (CPR) electron transfer chain for its para-hydroxylation reaction, ferulate 5-hydroxylase uses both NADPH-CPR-cytochrome b₅ (CB5) and NADH-cytochrome b₅ reductase-CB5 chains to support benzene ring 5-hydroxylation, in which the former route is primarily recruited in the stem for syringyl lignin synthesis, while the latter dominates in the syntheses of 5-hydroxylated phenolics in seeds and seed coat suberin. Our study unveils an additional layer of complexity and versatility of P450 system that the plants evolved for diversifying phenolic repertoires.

Maize resistance to witchweed through changes in strigolactone biosynthesis

Maize (Zea mays) is a major staple crop in Africa, where its yield and the livelihood of millions are compromised by the parasitic witchweed Striga. Germination of Striga is induced by strigolactones exuded from maize roots into the rhizosphere. In a maize germplasm collection, we identified two strigolactones, zealactol and zealactonoic acid, which stimulate less Striga germination than the major maize strigolactone, zealactone. We then showed that a single cytochrome P450, ZmCYP706C37, catalyzes a series of oxidative steps in the maize-strigolactone biosynthetic pathway. Reduction in activity of this enzyme and two others involved in the pathway, ZmMAX1b and ZmCLAMT1, can change strigolactone composition and reduce Striga germination and infection. These results offer prospects for breeding Striga-resistant maize.

New dual functional CYP450 gene involves in isoflavone biosynthesis in Glycine max L

Glycine max L. accumulates a large amount of isoflavonoid compounds, which is beneficial for plant defense, plant-microbe symbiotic interactions, and human health. Several CYP450 subfamily genes are involved in the flavonoid biosynthetic pathway in plants. In the present study, we found 24 CYP82 subfamily genes were differentially expressed in various tissues of soybean, in Phytophthora sojae-infected soybean varieties and in soybean hairy roots treated with cell wall glucan elicitor. Six of them (GmCYP82A2, GmCYP82A3, GmCYP82A4, GmCYP82A23, GmCYP82C20 and GmCYP82D26) were co-expressed with other known isoflavonoid pathway genes in soybean. Their enzymatic activity in yeast feeding assays showed that only GmCYP82D26 was able to convert naringenin to daidzein with both aryl migration and dehydration function. When GmCYP82D26 was over-expressed in soybean hairy roots, the contents of the two major isoflavonoid aglycones in soybean (daidzein and genistein) were reduced, but total flavonoids were not affected. When GmCYP82D26 was suppressed by RNAi in the hairy roots, daidzein content was decreased but genistein content was increased, with unchanged total flavonoid content. GmCYP82D26 was found to be localized in the endoplasmic reticulum at subcellular level when transiently expressed in tobacco leaf epidermis. GmCYP82D26 gene was preferentially expressed in roots, with low expression level in other tissues in soybean. Homology modeling and molecular docking showed that GmCYP82D26 could form hydrogen bond with both HEM and naringenin at C5-OH and C4 carbonyl. All these results indicated that GmCYP82D26 possesses new and dual enzymatic activity, which bridges the two branches (daidzein and genistein branch) of isoflavonoid pathway in soybean.

The protein conformational basis of isoflavone biosynthesis

Isoflavonoids play important roles in plant defense and also exhibit a range of mammalian health-promoting activities. Their biosynthesis is initiated by two enzymes with unusual catalytic activities; 2-hydroxyisoflavanone synthase (2-HIS), a membrane-bound cytochrome P450 catalyzing a coupled aryl-ring migration and hydroxylation, and 2-hydroxyisoflavanone dehydratase (2-HID), a member of a large carboxylesterase family that paradoxically catalyzes dehydration of 2-hydroxyisoflavanones to isoflavone. Here we report the crystal structures of 2-HIS from Medicago truncatula and 2-HID from Pueraria lobata. The 2-HIS structure reveals a unique cytochrome P450 conformation and heme and substrate binding mode that facilitate the coupled aryl-ring migration and hydroxylation reactions. The 2-HID structure reveals the active site architecture and putative catalytic residues for the dual dehydratase and carboxylesterase activities. Mutagenesis studies revealed key residues involved in substrate binding and specificity. Understanding the structural basis of isoflavone biosynthesis will facilitate the engineering of new bioactive isoflavonoids.

The structure and function of two isoflavone biosynthetic enzymes are reported revealing a novel cytochrome P450 conformation and identification of key residues for dual dehydratase and carboxylesterase activities

The discovery of a key prenyltransferase gene assisted by a chromosome-level Epimedium pubescens genome

Epimedium pubescens is a species of the family Berberidaceae in the basal eudicot lineage, and a main plant source for the traditional Chinese medicine "Herba Epimedii". The current study achieved a chromosome-level genome assembly of E. pubescens with the genome size of 3.34 Gb, and the genome guided discovery of a key prenyltransferase (PT) in E. pubescens. Our comparative genomic analyses confirmed the absence of Whole Genome Triplication (WGT-γ) event shared in core eudicots and further revealed the occurrence of an ancient Whole Genome Duplication (WGD) event approximately between 66 and 81 Million Years Ago (MYA). In addition, whole genome search approach was successfully applied to identify 19 potential flavonoid PT genes and an important flavonoid PT (EpPT8) was proven to be an enzyme for the biosynthesis of medicinal compounds, icaritin and its derivatives in E. pubescens. Therefore, our results not only provide a good reference genome to conduct further molecular biological studies in Epimedium genus, but also give important clues for synthetic biology and industrial production of related prenylated flavonoids in future.

Production and stably maintenance of strigolactone by transient expression of biosynthetic enzymes in Nicotiana benthamiana

Strigolactones (SLs) are phytohormones that play an essential role in plant-microbe interactions. The instability of SLs makes it challenging to use them for application to agriculture. In this study, we successfully produced a large amount of the 4-deoxyorobanchol (4DO), one of SLs, in the leaves of Nicotiana benthamiana, using a transient expression system to express SL biosynthetic enzymes. Using this system, the yield of 4DO was 2.1 ± 0.3 μg/gFM (fresh mass). Treatment of leaves at 80°C for 16 h killed Agrobacterium and approximately half amount of 4DO was left in the leaves (1.0 μg/gFM (calculated based on the original FM) ± 0.3). Interestingly, incubation of dried leaves at room temperature for 1 month maintained an almost equal amount of 4DO (0.9 ± 0.2 μg/gFM) in the leaves. These results suggest that high accumulation of 4DO with stability for long periods can be achieved in plant leaves.

Cytochrome P450 mono-oxygenase CYP703A2 plays a central role in sporopollenin formation and ms5ms6 fertility in cotton

The double-recessive genic male-sterile (ms) line ms5 ms6 has been used to develop cotton (Gossypium hirsutum) hybrids for many years, but its molecular-genetic basis has remained unclear. Here, we identified the Ms5 and Ms6 loci through map-based cloning and confirmed their function in male sterility through CRISPR/Cas9 gene editing. Ms5 and Ms6 are highly expressed in stages 7-9 anthers and encode the cytochrome P450 mono-oxygenases CYP703A2-A and CYP703A2-D. The ms5 mutant carries a single-nucleotide C-to-T nonsense mutation leading to premature chain termination at amino acid 312 (GhCYP703A2-A^312aa ), and ms6 carries three nonsynonymous substitutions (D98E, E168K, and G198R) and a synonymous mutation (L11L). Enzyme assays showed that GhCYP703A2 proteins hydroxylate fatty acids, and the ms5 (GhCYP703A2-A^312aa ) and ms6 (GhCYP703A2-D^{D98E,E168K,G198R} ) mutant proteins have decreased enzyme activities. Biochemical and lipidomic analyses showed that in ms5 ms6 plants, C12-C18 free fatty acid and phospholipid levels are significantly elevated in stages 7-9 anthers, while stages 8-10 anthers lack sporopollenin fluorescence around the pollen, causing microspore degradation and male sterility. Overall, our characterization uncovered functions of GhCYP703A2 in sporopollenin formation and fertility, providing guidance for creating male-sterile lines to facilitate hybrid cotton production and therefore exploit heterosis for improvement of cotton.

Integrated multi-omics analysis and microbial recombinant protein system reveal hydroxylation and glycosylation involving nevadensin biosynthesis in Lysionotus pauciflorus

Background

Karst-adapted plant, Lysionotus pauciflours accumulates special secondary metabolites with a wide range of pharmacological effects for surviving in drought and high salty areas, while researchers focused more on their environmental adaptations and evolutions. Nevadensin (5,7-dihydroxy-6,8,4'-trimethoxyflavone), the main active component in L. pauciflours, has unique bioactivity of such as anti-inflammatory, anti-tubercular, and anti-tumor or cancer. Complex decoration of nevadensin, such as hydroxylation and glycosylation of the flavone skeleton determines its diversity and biological activities. The lack of omics data limits the exploration of accumulation mode and biosynthetic pathway. Herein, we integrated transcriptomics, metabolomics, and microbial recombinant protein system to reveal hydroxylation and glycosylation involving nevadensin biosynthesis in L. pauciflours.

Results

Up to 275 flavonoids were found to exist in L. pauciflorus by UPLC-MS/MS based on widely targeted metabolome analysis. The special flavone nevadensin (5,7-dihydroxy-6,8,4'-trimethoxyflavone) is enriched in different tissues, as are its related glycosides. The flavonoid biosynthesis pathway was drawn based on differential transcripts analysis, including 9 PAL, 5 C4H, 8 4CL, 6 CHS, 3 CHI, 1 FNSII, and over 20 OMTs.

Total 310 LpCYP450s were classified into 9 clans, 36 families, and 35 subfamilies, with 56% being A-type CYP450s by phylogenetic evolutionary analysis. According to the phylogenetic tree with AtUGTs, 187 LpUGTs clustered into 14 evolutionary groups (A-N), with 74% being E, A, D, G, and K groups.

Two LpCYP82D members and LpUGT95 were functionally identified in Saccharomyces cerevisiae and Escherichia coli, respectively. CYP82D-8 and CYP82D-1 specially hydroxylate the 6- or 8-position of A ring in vivo and in vitro, dislike the function of F6H or F8H discovered in basil which functioned depending on A-ring substituted methoxy. These results refreshed the starting mode that apigenin can be firstly hydroxylated on A ring in nevadensin biosynthesis. Furthermore, LpUGT95 clustered into the 7-OGT family was verified to catalyze 7-O glucosylation of nevadensin accompanied with weak nevadensin 5-O glucosylation function, firstly revealed glycosylation modification of flavones with completely substituted A-ring.

Conclusions

Metabolomic and full-length transcriptomic association analysis unveiled the accumulation mode and biosynthetic pathway of the secondary metabolites in the karst-adapted plant L. pauciflorus. Moreover, functional identification of two LpCYP82D members and one LpUGT in microbe reconstructed the pathway of nevadensin biosynthesis.

Supplementary Information

The online version contains supplementary material available at 10.1186/s12934-022-01921-2.

The tomato cytochrome P450 CYP712G1 catalyses the double oxidation of orobanchol en route to the rhizosphere signalling strigolactone, solanacol

Strigolactones (SLs) are rhizosphere signalling molecules and phytohormones. The biosynthetic pathway of SLs in tomato has been partially elucidated, but the structural diversity in tomato SLs predicts that additional biosynthetic steps are required. Here, root RNA-seq data and co-expression analysis were used for SL biosynthetic gene discovery. This strategy resulted in a candidate gene list containing several cytochrome P450s. Heterologous expression in Nicotiana benthamiana and yeast showed that one of these, CYP712G1, can catalyse the double oxidation of orobanchol, resulting in the formation of three didehydro-orobanchol (DDH) isomers. Virus-induced gene silencing and heterologous expression in yeast showed that one of these DDH isomers is converted to solanacol, one of the most abundant SLs in tomato root exudate. Protein modelling and substrate docking analysis suggest that hydroxy-orbanchol is the likely intermediate in the conversion from orobanchol to the DDH isomers. Phylogenetic analysis demonstrated the occurrence of CYP712G1 homologues in the Eudicots only, which fits with the reports on DDH isomers in that clade. Protein modelling and orobanchol docking of the putative tobacco CYP712G1 homologue suggest that it can convert orobanchol to similar DDH isomers as tomato.

Construction of an Escherichia coli cell factory to synthesize taxadien-5α-ol, the key precursor of anti-cancer drug paclitaxel

Paclitaxel (Taxol™), an alkaloid of diterpenoid family, is one of the most widely used anti-cancer drugs due to its effectiveness against a variety of tumors. Rather than directly extraction and chemical synthesis of paclitaxel or its intermediates from yew plants, construction of a microbial cell factory for paclitaxel biosynthesis will be more efficient and sustainable. The challenge for biosynthesis of paclitaxel lies on the insufficient precursor, such as taxadien-5α-ol. In this study, we report a recombinant Escherichia coli strain constructed with a heterologous mevalonate pathway, a taxadiene synthase from yew, and a cytochrome P450-mediated oxygenation system for the de novo production of taxadien-5α-ol, the first product of the multi-step taxadiene oxygenation metabolism. The key enzymes including taxadiene synthases and cytochrome P450 reductases were screened, and the linker for fusing taxadiene-5α-hydroxylase with its reductase partner cytochrome P450 reductase was optimized. By reducing the metabolic burden and optimizing the fermentation conditions, the final production of total oxygenated taxanes was raised up to 27 mg L-1 in a 50-mL flask cultivation, of which the yield of taxadien-5α-ol was 7.0 mg L-1, representing approximately a 12-fold and 23-fold improvements, respectively, as compared with the initial titers. The engineered MVA pathway for the overproduction of terpenoid precursors can serve as an efficient platform for the production of other valuable terpenoids.

Characterization of Cytochrome P450s with Key Roles in Determining Herbicide Selectivity in Maize

Safeners such as metcamifen and benoxacor are widely used in maize to enhance the selectivity of herbicides through the induction of key detoxifying enzymes, notably cytochrome P450 monooxygenases (CYPs). Using a combination of transcriptomics, proteomics, and functional assays, the safener-inducible CYPs responsible for herbicide metabolism in this globally important crop have been identified. A total of 18 CYPs belonging to clans 71, 72, 74, and 86 were safener-induced, with the respective enzymes expressed in yeast and screened for activity toward thiadiazine (bentazon), sulfonylurea (nicosulfuron), and triketone (mesotrione and tembotrione) chemistries. Herbicide metabolism was largely restricted to family CYP81A members from clan 71, notably CYP81A9, CYP81A16, and CYP81A2. Quantitative transcriptomics and proteomics showed that CYP81A9/CYP81A16 were dominant enzymes in safener-treated field maize, whereas only CYP81A9 was determined in sweet corn. The relationship between CYP81A sequence and activities were investigated by splicing CYP81A2 and CP81A9 together as a series of recombinant chimeras. CYP81A9 showed wide ranging activities toward the three herbicide chemistries, while CYP81A2 uniquely hydroxylated bentazon in multiple positions. The plasticity in substrate specificity of CYP81A9 toward multiple herbicides resided in the second quartile of its N terminal half. Further phylogenetic analysis of CYP81A9 showed that the maize enzyme was related to other CYP81As linked to agrochemical metabolism in cereals and wild grasses, suggesting this clan 71 CYP has a unique function in determining herbicide selectivity in arable crops.

Identification and characterization of unique 5-hydroxyisoflavonoid biosynthetic key enzyme genes in Lupinus albus

5-Hydroxyisoflavonoids, no 5-deoxyisoflavonoids, in Lupinus species, are due to lack of CHRs and Type II CHIs, and the key enzymes of isoflavonoid biosynthetic pathway in white lupin were identified. White lupin (Lupinus albus) is used as food ingredients owing to rich protein, low starch, and rich bioactive compounds such as isoflavonoids. The isoflavonoids biosynthetic pathway in white lupin still remains unclear. In this study, only 5-hydroxyisoflavonoids, but no 5-deoxyisoflavonoids, were detected in white lupin and other Lupinus species. No 5-deoxyisoflavonoids in Lupinus species are due to lack of CHRs and Type II CHIs. We further found that the CHI gene cluster containing both Type I and Type II CHIs possibly arose after the divergence of Lupinus with other legume clade. LaCHI1 and LaCHI2 identified from white lupin metabolized naringenin chalcone to naringenin in yeast and tobacco (Nicotiana benthamiana), and were bona fide Type I CHIs. We further identified two isoflavone synthases (LaIFS1 and LaIFS2), catalyzing flavanone naringenin into isoflavone genistein and also catalyzing liquiritigenin into daidzein in yeast and tobacco. In addition, LaG6DT1 and LaG6DT2 prenylated genistein at the C-6 position into wighteone. Two glucosyltransferases LaUGT1 and LaUGT2 metabolized genistein and wighteone into its 7-O-glucosides. Taken together, our study not only revealed that exclusive 5-hydroxyisoflavonoids do exist in Lupinus species, but also identified key enzymes in the isoflavonoid biosynthetic pathway in white lupin.

22R- but not 22S-hydroxycholesterol is recruited for diosgenin biosynthesis

Diosgenin is an important compound in the pharmaceutical industry and it is biosynthesized in several eudicot and monocot species, herein represented by fenugreek (a eudicot), and Dioscorea zingiberensis (a monocot). Formation of diosgenin can be achieved by the early C22,16-oxidations of cholesterol followed by a late C26-oxidation. This study reveals that, in both fenugreek and D. zingiberensis, the early C22,16-oxygenase(s) shows strict 22R-stereospecificity for hydroxylation of the substrates. Evidence against the recently proposed intermediacy of 16S,22S-dihydroxycholesterol in diosgenin biosynthesis was also found. Moreover, in contrast to the eudicot fenugreek, which utilizes a single multifunctional cytochrome P450 (TfCYP90B50) to perform the early C22,16-oxidations, the monocot D. zingiberensis has evolved two separate cytochrome P450 enzymes, with DzCYP90B71 being specific for the 22R-oxidation and DzCYP90G6 for the C16-oxidation. We suggest that the DzCYP90B71/DzCYP90G6 pair represent more broadly conserved catalysts for diosgenin biosynthesis in monocots.

CYP1B1 converts procarcinogens into genotoxins in Saccharomyces cerevisiae

CYP1B1 activates many chemical carcinogens into potent genotoxins, and allelic variants are risk factors in lung, breast, and prostate cancer. However, few eukaryotic genetic instability endpoints have been directly measured for CYP1B1-activated metabolites. In this study, we expressed human CYP1B1 in yeast strains that measure DNA damage-associated toxicity and frequencies of chromosomal translocations. DNA damage-associated toxicity was measured in a rad4 rad51 strain, defective in both DNA excision and recombinational repair. Frequencies of chromosomal translocations were measured in diploid yeast strains containing two his3 fragments. These strains were exposed to benzo[a]pyrene-7,8-dihydrodiol (BaP-DHD), aflatoxin B₁ (AFB₁), and the heterocyclic aromatic amines, 2-amino-3,8-dimethylimidazo[4,5-f]quinoxaline (MeIQx) and 2-amino-3-methylimidazo[4,5-f]quinoline (IQ). We observed that AFB₁, BaP-DHD, IQ, and MeIQx conferred toxicity in the DNA repair mutant expressing CYP1B1. Translocation frequencies increased eight-fold and three-fold after exposure to 50 μM AFB₁ and 33 μM BaP-DHD respectively. A DNA damage response was observed after AFB₁ exposure, as measured by the induction of the small subunit of ribonucleotide reductase, Rnr3. While CYP1B1-mediated activation of BaP-DHD and heterocyclic aromatic amines was expected, activation of AFB₁ to become a potent recombinagen was not expected. These studies demonstrate that chromosomal rearrangement is a useful genotoxic endpoint for CYP1B1-mediated carcinogen activation.

Removal of lycopene substrate inhibition enables high carotenoid productivity in Yarrowia lipolytica

Substrate inhibition of enzymes can be a major obstacle to the production of valuable chemicals in engineered microorganisms. Here, we show substrate inhibition of lycopene cyclase as the main limitation in carotenoid biosynthesis in Yarrowia lipolytica. To overcome this bottleneck, we exploit two independent approaches. Structure-guided protein engineering yields a variant, Y27R, characterized by complete loss of substrate inhibition without reduction of enzymatic activity. Alternatively, establishing a geranylgeranyl pyrophosphate synthase-mediated flux flow restrictor also prevents the onset of substrate inhibition by diverting metabolic flux away from the inhibitory metabolite while maintaining sufficient flux towards product formation. Both approaches result in high levels of near-exclusive β-carotene production. Ultimately, we construct strains capable of producing 39.5 g/L β-carotene at a productivity of 0.165 g/L/h in bioreactor fermentations (a 1441-fold improvement over the initial strain). Our findings provide effective approaches for removing substrate inhibition in engineering pathways for efficient synthesis of natural products.

Substrate inhibition has not been widely studied in the context of synthetic biology and metabolic engineering. Here, the authors report removal of lycopene substrate inhibition by two different strategies and enable high carotenoid productivity in Yarrowia lipolytica.

CRISPR/Cas9 targeted inactivation of the kauniolide synthase in chicory results in accumulation of costunolide and its conjugates in taproots

Chicory taproots accumulate sesquiterpene lactones lactucin, lactucopicrin, and 8-deoxylactucin, predominantly in their oxalated forms. The biosynthetic pathway for chicory sesquiterpene lactones has only partly been elucidated; the enzymes that convert farnesyl pyrophosphate to costunolide have been described. The next biosynthetic step of the conversion of costunolide to the tricyclic structure, guaianolide kauniolide, has so far not been elucidated in chicory. In this work three putative kauniolide synthase genes were identified in chicory named CiKLS1, CiKLS2, and CiKLS3. Their activity to convert costunolide to kauniolide was demonstrated in vitro using yeast microsome assays. Next, introduction of CRISPR/Cas9 reagents into chicory protoplasts was used to inactivate multiple chicory KLS genes and several chicory lines were successfully regenerated. The inactivation of the kauniolide synthase genes in chicory by the CRISPR/Cas9 approach resulted in interruption of the sesquiterpene lactone biosynthesis in chicory leaves and taproots. In chicory taproots, but not in leaves, accumulation of costunolide and its conjugates was observed to high levels, namely 1.5 mg/g FW. These results confirmed that all three genes contribute to STL accumulation, albeit to different extent. These observations demonstrate that three genes oriented in tandem on the chicory genome encode kauniolide synthases that initiate the conversion of costunolide toward the sesquiterpene lactones in chicory.

Isolation and Functional Characterization of Multiple NADPH-Cytochrome P450 Reductase Genes from Camellia sinensis in View of Catechin Biosynthesis

Catechins are critical constituents for the sensory quality and health-promoting benefits of tea. Cytochrome P450 monooxygenases are required for catechin biosynthesis and are dependent on NADPH-cytochrome P450 reductases (CPRs) to provide reducing equivalents for their activities. However, CPRs have not been identified in tea, and their relationship to catechin accumulation also remains unknown. Thus, three CsCPR genes were identified in this study, all of which had five CPR-related conserved domains and were targeted to the endoplasmic reticulum. These three recombinant CsCPR proteins could reduce cytochrome c using NADPH as an electron donor. Heterologous co-expression in yeast demonstrated that all the three CsCPRs could support the enzyme activities of CsC4H and CsF3'H. Correlation analysis indicated that the expression level of CsCPR1 (or CsCPR2 or CsCPR3) was positively correlated with 3',4',5'-catechin (or total catechins) content. Our results indicate that the CsCPRs are involved in the biosynthesis of catechins in tea leaves.

Genome-Wide Analysis of the Cytochrome P450 Gene Family Involved in Salt Tolerance in Gossypium hirsutum

Plant cytochrome P450 (P450) participates in a wide range of biosynthetic reactions and targets a variety of biological molecules. These reactions lead to various fatty acid conjugates, plant hormones, secondary metabolites, lignin, and various defensive compounds. In our previous research, transcriptome analysis was performed on the salt-tolerant upland cotton "Tongyan No. 1." Many differentially expressed genes (DEGs) belong to the P450 family, and their domains occur widely in plants. In this current research, P450 genes were identified in Gossypium hirsutum with the aid of bioinformatics methods for investigating phylogenetic relations, gene structure, cis-elements, chromosomal localization, and collinearity within a genome. qRT-PCR was conducted to analyze P450 gene expression patterns under salt stress. The molecular weights of the 156 P450 genes were in the range of 5,949.6-245,576.3 Da, and the length of the encoded amino acids for all the identified P450 genes ranged from 51 to 2,144. P450 proteins are divided into four different subfamilies based on phylogenetic relationship, gene structure, and chromosomal localization of gene replication. The length of P450 genes in upland cotton differs greatly, ranging from 1,500 to 13,000 bp. The number of exons in the P450 family genes ranged from 1 to 9, while the number of introns ranged from 0 to 8, and there were similar trends within clusters. A total of 31 cis-acting elements were identified by analyzing 1,500 bp promoter sequences. Differences were found in cis-acting elements among genes. The consistency between qRT-PCR and previous transcriptome analysis of salt tolerance DEGs indicated that they were likely to be involved in the salt tolerance of cotton seedlings. Our results provide valuable information on the evolutionary relationships of genes and functional characteristics of the gene family, which is beneficial for further study of the cotton P450 gene family.

Strigolactone biosynthesis catalyzed by cytochrome P450 and sulfotransferase in sorghum

Root parasitic plants such as Striga, Orobanche, and Phelipanche spp. cause serious damage to crop production world-wide. Deletion of the Low Germination Stimulant 1 (LGS1) gene gives a Striga-resistance trait in sorghum (Sorghum bicolor). The LGS1 gene encodes a sulfotransferase-like protein, but its function has not been elucidated. Since the profile of strigolactones (SLs) that induce seed germination in root parasitic plants is altered in the lgs1 mutant, LGS1 is thought to be an SL biosynthetic enzyme. In order to clarify the enzymatic function of LGS1, we looked for candidate SL substrates that accumulate in the lgs1 mutants and performed in vivo and in vitro metabolism experiments. We found the SL precursor 18-hydroxycarlactonoic acid (18-OH-CLA) is a substrate for LGS1. CYP711A cytochrome P450 enzymes (SbMAX1 proteins) in sorghum produce 18-OH-CLA. When LGS1 and SbMAX1 coding sequences were co-expressed in Nicotiana benthamiana with the upstream SL biosynthesis genes from sorghum, the canonical SLs 5-deoxystrigol and 4-deoxyorobanchol were produced. This finding showed that LGS1 in sorghum uses a sulfo group to catalyze leaving of a hydroxyl group and cyclization of 18-OH-CLA. A similar SL biosynthetic pathway has not been found in other plant species.

Total Biosynthesis of the Tubulin-Binding Alkaloid Colchicine

Colchicine (1) is a bioactive plant alkaloid from Colchicum and Gloriosa species that is used as a pharmaceutical treatment for inflammatory diseases, including gouty arthritis and familial Mediterranean fever. The activity of this alkaloid is attributed to its ability to bind tubulin dimers and inhibit microtubule assembly, which not only promotes anti-inflammatory effects, but also makes colchicine a potent mitotic poison. The biochemical origins of colchicine biosynthesis have been investigated for over 50 years, but only recently has the underlying enzymatic machinery become clear. Here, we report the discovery of multiple pathway enzymes from Gloriosa superba that allows for the reconstitution of a complete metabolic route to 1. This includes three enzymes that process a previously established tropolone-containing intermediate into 1 via tailoring of the nitrogen atom. We further demonstrate the total biosynthesis of enantiopure (-)-1 from primary metabolites via heterologous production in a model plant, thus enabling future efforts for the metabolic engineering of this medicinal alkaloid. Additionally, our results provide insight into the timing and tissue specificity for the late stage modifications required in colchicine biosynthesis, which are likely connected to the biological functions for this class of medicinal alkaloids in native producing plants.

Altering Sterol Composition Implied That Cholesterol Is Not Physiologically Associated With Diosgenin Biosynthesis in Trigonella foenum-graecum

Diosgenin serves as an important precursor of most steroidal drugs in market. Cholesterol was previously deemed as a sterol origin leading to diosgenin biosynthesis. This study reports that cholesterol is not in parallel with diosgenin biosynthesis in Trigonella foenum-graecum. We first perturbed its sterol composition using inhibitors specific for the upstream isoprenoid pathway enzymes, HMGR (3-hydroxy-3-methylgutaryl-CoA reductase) on the mevalonate (MVA) and DXR (1-deoxy-D-xylulose-5-phosphate reductoisomerase) on the 2-C-methyl-D-erythritol-4-phophate (MEP) pathways, and have revealed that diosgenin and cholesterol reversely or differently accumulated in either the MVA or the MEP pathway-suppressed plants, challenging the previously proposed role of cholesterol in diosgenin biosynthesis. To further investigate this, we altered the sterol composition by suppressing and overexpressing the 24-sterol methyltransferase type 1 (SMT1) gene in T. foenum-graecum, as SMT1 acts in the first committed step of diverting the carbon flux of cholesterol toward biosynthesis of 24-alkyl sterols. Knockdown of TfSMT1 expression led to increased cholesterol level but caused a large reduction of diosgenin. Diosgenin was increased upon the TfSMT1-overexpressing, which, however, did not significantly affect cholesterol biosynthesis. These data consistently supported that diosgenin biosynthesis in T. foenum-graecum is not associated with cholesterol. Rather, campesterol, a 24-alkyl sterol, was indicative of being correlative to diosgenin biosynthesis in T. foenum-graecum.

Divergent camptothecin biosynthetic pathway in Ophiorrhiza pumila

BACKGROUND

The anticancer drug camptothecin (CPT), first isolated from Camptotheca acuminata, was subsequently discovered in unrelated plants, including Ophiorrhiza pumila. Unlike known monoterpene indole alkaloids, CPT in C. acuminata is biosynthesized via the key intermediate strictosidinic acid, but how O. pumila synthesizes CPT has not been determined.

RESULTS

In this study, we used nontargeted metabolite profiling to show that 3α-(S)-strictosidine and 3-(S), 21-(S)-strictosidinic acid coexist in O. pumila. After identifying the enzymes OpLAMT, OpSLS, and OpSTR as participants in CPT biosynthesis, we compared these enzymes to their homologues from two other representative CPT-producing plants, C. acuminata and Nothapodytes nimmoniana, to elucidate their phylogenetic relationship. Finally, using labelled intermediates to resolve the CPT biosynthesis pathway in O. pumila, we showed that 3α-(S)-strictosidine, not 3-(S), 21-(S)-strictosidinic acid, is the exclusive intermediate in CPT biosynthesis.

CONCLUSIONS

In our study, we found that O. pumila, another representative CPT-producing plant, exhibits metabolite diversity in its central intermediates consisting of both 3-(S), 21-(S)-strictosidinic acid and 3α-(S)-strictosidine and utilizes 3α-(S)-strictosidine as the exclusive intermediate in the CPT biosynthetic pathway, which differs from C. acuminata. Our results show that enzymes likely to be involved in CPT biosynthesis in O. pumila, C. acuminata, and N. nimmoniana have evolved divergently. Overall, our new data regarding CPT biosynthesis in O. pumila suggest evolutionary divergence in CPT-producing plants. These results shed new light on CPT biosynthesis and pave the way towards its industrial production through enzymatic or metabolic engineering approaches.