Biosynthesis of Plant Triterpenoid Saponins: Genes, Enzymes and their Regulation

Phytotoxicity of Quillaja lancifolia Leaf Saponins and Their Bioherbicide Potential

Weeds are major threats to the integrity of agricultural and natural environments due to their invasive and competing potential. Bioherbicides are substances based on natural compounds that are biodegradable and often have low residual effects. Plant species able to produce and release phytotoxic compounds may represent effective bioherbicide sources. Leaves of Quillaja lancifolia D.Don (formerly Q. brasiliensis (A.St.-Hil. & Tul.) Mart.) produce water-soluble specialized metabolites of the saponin class that could be evaluated for phytotoxic activity and potential as natural herbicides. This study was conducted to examine the impacts of Q. lancifolia total saponins aqueous extract (AE) at 4 and 10% (w/v) and of two combined reverse-phase chromatography purified saponin fractions (QB) at 1 and 2% (w/v) on morpho-physiological parameters of Lactuca sativa (lettuce) and Echinochloa crus-galli (barnyardgrass) in pre- and post-emergence bioassays. QB was only tested in pre-emergence assays. In pre-emergence bioassays, the germination rate and germination kinetics were determined. Post-emergence evaluations included effects on seedling morphology, root and shoot length, dry mass, and chlorophyll content. Osmotic potential and pH analyses ruled out roles for these factors in the observed responses. AE had a high inhibitory impact on the germination of both lettuce and barnyardgrass. QB at 1% and 2% (w/v) significantly decreased the growth of lettuce seedlings germinated in its presence by more than 10-fold. Phytotoxic effects on the post-emergence growth of lettuce, especially at the highest concentration tested of AE (10% w/v), was also observed. The presence of quillaic acid-based triterpene saponins in AE and QB was confirmed using different analytical methods. Therefore, both saponin-enriched fraction and aqueous extracts of Q. lancifolia inhibited tested plant growth and development. The water solubility of saponins and the availability of a sustainable source of these molecules from the leaves of cultivated young Q. lancifolia plants make them attractive candidates for use as bioherbicides.

Comparative transcriptomic analysis of genes in the triterpene saponin biosynthesis pathway in leaves and roots of Ardisia kteniophylla A. DC., a plant used in traditional Chinese medicine

Ardisia kteniophylla (Primulaceae) is highly valued in traditional medicine due to its production of the pharmacologically active secondary metabolites, especially triterpenoid saponins in its roots. Although A. kteniophylla is very important in traditional medicine, the genetic basis for its production of triterpenoid saponins remains largely unknown. Therefore, we sequenced transcriptomes of A. kteniophylla to identify putative genes involved in production of triterpenoid saponins in both leaves and roots, and we used the transcriptomes to compare expression levels of these genes between the two organ systems. The production of triterpenoid saponins in plants is usually induced through hormonal signaling on account of the presence of pests. Thus, we treated plants with the hormones salicylic acid (SA) and methyl jasmonate (MeJA) and used quantitative real‐time PCR (qRT‐PCR) to investigate expression levels of genes involved in triterpenoid saponin biosynthesis. In total, we obtained transcriptomes for leaf and root tissues representing 52,454 unigenes. Compared with the leaf transcriptome, we found that 6092 unigenes were upregulated in the root, especially enzymes involved in the direct synthesis of triterpenoid saponins, while 6001 genes appeared downregulated, including those involved in precursory steps in the triterpenoid saponin biosynthesis pathway. Our results from qRT‐PCR indicate that genes within the upstream parts of the triterpenoid saponin biosynthesis pathway may be upregulated under exposure to the applied hormones, but downstream genes are downregulated. This suggests possible conflicting effects of SA and MeJA in promoting the production of secondary metabolites on the one hand, and, on the other, limiting plant growth processes to devote energy to combating pests. We also performed an analysis of transcription factors (TFs) and found 997 unique transcripts belonging to 16 TF families. Our data may help to facilitate future work on triterpene saponins biosynthesis in A. kteniophylla with potential pharmacological and molecular breeding applications.

Evolution of Structural Diversity of Triterpenoids

Plants have evolved to produce a blend of specialized metabolites that serve functional roles in plant adaptation. Among them, triterpenoids are one of the largest subclasses of such specialized metabolites, with more than 14,000 known structures. They play a role in plant defense and development and have potential applications within food and pharma. Triterpenoids are cyclized from oxidized squalene precursors by oxidosqualene cyclases, creating more than 100 different cyclical triterpene scaffolds. This limited number of scaffolds is the first step towards creating the vast structural diversity of triterpenoids followed by extensive diversification, in particular, by oxygenation and glycosylation. Gene duplication, divergence, and selection are major forces that drive triterpenoid structural diversification. The triterpenoid biosynthetic genes can be organized in non-homologous gene clusters, such as in Avena spp., Cucurbitaceae and Solanum spp., or scattered along plant chromosomes as in Barbarea vulgaris. Paralogous genes organized as tandem repeats reflect the extended gene duplication activities in the evolutionary history of the triterpenoid saponin pathways, as seen in B. vulgaris. We review and discuss examples of convergent and divergent evolution in triterpenoid biosynthesis, and the apparent mechanisms occurring in plants that drive their increasing structural diversity within and across species. Using B. vulgaris' saponins as examples, we discuss the impact a single structural modification can have on the structure of a triterpenoid and how this affect its biological properties. These examples provide insight into how plants continuously evolve their specialized metabolome, opening the way to study uncharacterized triterpenoid biosynthetic pathways.

Jasmonate responsive transcription factor WsMYC2 regulates the biosynthesis of triterpenoid withanolides and phytosterol via key pathway genes in Withania somnifera (L.) Dunal

Functional characterization of WsMYC2 via artificial microRNA mediated silencing and transient over-expression displayed significant regulatory role vis-à-vis withanolides and stigmasterol biosyntheses in Withania somnifera. Further, metabolic intensification corroborated well with higher expression levels of putative pathway genes. Additionally, copious expression of WsMYC2 in response to exogenous elicitors resulted in enhanced withanolides production. Withania somnifera, a high value multipurpose medicinal plant, is a rich reservoir of structurally diverse and biologically active triterpenoids known as withanolides. W. somnifera has been extensively pursued vis-à-vis pharmacological and chemical studies. Nonetheless, there exists fragmentary knowledge regarding the metabolic pathway and the regulatory aspects of withanolides biosynthesis. Against this backdrop, a jasmonate-responsive MYC2 transcription factor was identified and functionally characterized from W. somnifera. In planta transient over-expression of WsMYC2 showed significant enhancement of mRNA transcript levels which corroborated well with the enhanced content of withanolides and stigmasterol. Further, a comparative analysis of expression levels of some of the genes of triterpenoid pathway viz. WsCAS, WsCYP85A, WsCYP90B and WsCYP710A in corroboration with the over-expression and silencing of WsMYC2 suggested its positive influence on their regulation. These corroboratory approaches suggest that WsMYC2 has cascading effect on over-expression of multiple pathway genes leading to the increased triterpenoid biosynthesis in infiltered plants. Further, the functional validation of WsMYC2 was carried out by artificial micro-RNA mediated silencing. It resulted in significant reduction of withanolides and stigmasterol levels, indicative of crucial role of WsMYC2 in the regulation of their biosyntheses. Taken together, these non-complementary approaches provided unambiguous understanding of the regulatory role of WsMYC2 in context to withanolides and stigmasterol biosyntheses. Furthermore, the upstream promoter of WsMYC2 presented several cis-regulatory elements primarily related to phytohormone responsiveness. WsMYC2 displayed inducible nature in response to MeJA. It had substantial influence on the higher expression of WsMYC2 which was in consonance with enhanced accumulation of withanolides.

Postharvest UV-B Irradiation Stimulated Ginsenoside Rg1 Biosynthesis through Nitric Oxide (NO) and Jasmonic Acid (JA) in Panax quinquefolius Roots

The study highlights the influence and signal transduction mechanism of postharvest UV-B on the production of Rg1 in Panax quinquefolius roots during the drying process. The results showed that postharvest UV-B irradiation induced generation of nitric oxide (NO), jasmonic acid (JA), and ginsenoside Rg1 of P. quinquefolius roots. The UV-B-induced increase of Rg1 was suppressed by NO-specific scavenger (cPTIO) and NOS inhibitors (PBITU), JA synthesis inhibitor (SHAM), and JA synthesis inhibitor (PrGall), indicating that NO and JA played essential parts in UV-B-induced Rg1. External NO inhibitors treatment inhibited UV-B-induced accumulation of NO and JA, which suggested that NO was located upstream of the JA signal pathway. NO-caused Rg1 was inhibited by SHAM and PrGall, implying JA participated in transmitting signal NO to Rg1 accumulation. In other words, NO mediated the postharvest UV-B-induced Rg1 accumulation by the JA-dependent pathway in P. quinquefolius roots during the drying process, which helps us understand the underlying mechanisms involved in UV-B-induced Rg1 production and provides information helpful for P. quinquefolius production.

Triterpenoids

Covering 2014. Previous review: Nat. Prod. Rep., 2017, 34, 90-122 This review covers the isolation and structure determination of triterpenoids reported during 2014 including squalene derivatives, lanostanes, holostanes, cycloartanes, cucurbitanes, dammaranes, euphanes, tirucallanes, tetranortriterpenoids, quassinoids, lupanes, oleananes, friedelanes, ursanes, hopanes, serratanes, isomalabaricanes and saponins; 374 references are cited.

UGT73F17, a new glycosyltransferase from Glycyrrhiza uralensis, catalyzes the regiospecific glycosylation of pentacyclic triterpenoids

The regiospecific glycosylation of pentacyclic triterpenoids by UGT73F17, a new glycosyltransferase from Glycyrrhiza uralensis, is highlighted. UGT73F17 exhibited strict substrate specificity toward the carboxyl group at C-30/C-29 of pentacyclic triterpenoids, and showed high promiscuity to sugar donors. UGT73F17 represents the first identified triterpenoid 30/29-O-glycosyltransferase, and could be used as an effective biocatalyst to synthesize glycosyl ester saponins.

Molecular cloning of an ester-forming triterpenoid: UDP-glucose 28-O-glucosyltransferase involved in saponin biosynthesis from the medicinal plant Centella asiatica

Triterpene saponins include bioactive compounds with structures consisting of triterpene aglycones (sapogenins) and one or more sugar moieties linked through acetal or ester glycosidic linkages at one or more sites. Centella asiatica (L.) Urban is a medicinal plant that contains bioactive ursane-type saponins, such as madecassoside and asiaticoside. In this work, glucosylation of triterpenoids in C. asiatica was investigated starting with plant extracts. An enzyme capable of glucosylating asiatic and madecassic acids was partially purified. Proteomics methods and cDNA sequence data were employed as tools to obtain a full-length cDNA clone encoding a glucosyltransferase. The recombinant gene product, UGT73AD1, was functionally expressed in Escherichia coli and purified by immobilized metal-affinity chromatography. Purified recombinant UGT73AD1 was found to have a narrow specificity, glucosylating asiatic and madecassic acids at the C28 carboxyl. mRNA accumulated in all tissues tested (leaves, stems, roots and flowers), with highest expression in leaves. Thus, UGT73AD1 was identified as a triterpenoid carboxylic acid: UDP-glucose 28-O-glucosyltransferase that appears to be involved in saponin biosynthesis in C. asiatica.