Identification of novel sesquiterpene synthase genes that mediate the biosynthesis of valerianol, which was an unknown ingredient of tea

Chemistry, biosynthesis and biology of floral volatiles: roles in pollination and other functions

Covering: 2010 to 2023Floral volatiles are a chemically diverse group of plant metabolites that serve multiple functions. Their composition is shaped by environmental, ecological and evolutionary factors. This review will summarize recent advances in floral scent research from chemical, molecular and ecological perspectives. It will focus on the major chemical classes of floral volatiles, on notable new structures, and on recent discoveries regarding the biosynthesis and the regulation of volatile emission. Special attention will be devoted to the various functions of floral volatiles, not only as attractants for different types of pollinators, but also as defenses of flowers against enemies. We will also summarize recent findings on how floral volatiles are affected by abiotic stressors, such as increased temperatures and drought, and by other organisms, such as herbivores and flower-dwelling microbes. Finally, this review will indicate current research gaps, such as the very limited knowledge of the isomeric pattern of chiral compounds and its importance in interspecific interactions.

Cloning, Expression and Functional Characterization of a Novel α-Humulene Synthase, Responsible for the Formation of Sesquiterpene in Agarwood Originating from Aquilaria malaccensis

This study describes the cloning, expression and functional characterization of α-humulene synthase, responsible for the formation of the key aromatic compound α-humulene in agarwood originating from Aquilaria malaccensis. The partial sesquiterpene synthase gene from the transcriptome data of A. malaccensis was utilized for full-length gene isolation via a 3' RACE PCR. The complete gene, denoted as AmDG2, has an open reading frame (ORF) of 1671 bp and encodes for a polypeptide of 556 amino acids. In silico analysis of the protein highlighted several conserved motifs typically found in terpene synthases such as Asp-rich substrate binding (DDxxD), metal-binding residues (NSE/DTE), and cytoplasmic ER retention (RxR) motifs at their respective sites. The AmDG2 was successfully expressed in the E. coli:pET-28a(+) expression vector whereby an expected band of about 64 kDa in size was detected in the SDS-PAGE gel. In vitro enzyme assay using substrate farnesyl pyrophosphate (FPP) revealed that AmDG2 gave rise to two sesquiterpenes: α-humulene (major) and β-caryophyllene (minor), affirming its identity as α-humulene synthase. On the other hand, protein modeling performed using AlphaFold2 suggested that AmDG2 consists entirely of α-helices with short connecting loops and turns. Meanwhile, molecular docking via AutoDock Vina (Version 1.5.7) predicted that Asp307 and Asp311 act as catalytic residues in the α-humulene synthase. To our knowledge, this is the first comprehensive report on the cloning, expression and functional characterization of α-humulene synthase from agarwood originating from A. malaccensis species. These findings reveal a deeper understanding of the structure and functional properties of the α-humulene synthase and could be utilized for metabolic engineering work in the future.

Changes of Crocin and Other Crocetin Glycosides in Saffron Through Cooking Models, and Discovery of Rare Crocetin Glycosides in the Yellow Flowers of Freesia Hybrida

Crocetin glycosides such as crocin are noted as functional food materials since the preventive effects of crocin have been reported against chronic disease and cancer. However, it is unclear how these apocarotenoids are structurally changed through cooking for our intake. We examined such changes in crocetin glycosides (crocin, tricrocin, and crocin-3) contained in saffron (stigmas of Crocus sativus) through cooking models. These glycosides were almost kept stable in boiling for 20 min (a boiled cooking model), while hydrolysis of the ester linkage between glucose and the crocetin aglycone occurred in a grilled cooking model (180°C, 5 min), along with a 13-cis isomerization reaction in a part of crocetin subsequently generated. We further here revealed that the yellow petals of freesia (Freesia x hybrida) with yellow flowers accumulate two unique crocetin glycosides, which were identified to be crocetin (mono)neapolitanosyl ester and crocetin dineapolitanosyl ester. A similar result as above was obtained on their changes through the cooking models. Utility applications of the freesia flowers as edible flowers are also suggested in this study. Additionally, we evaluated singlet oxygen (¹O₂)-quenching activities of the crocetin glycosides contained in saffron and freesia, and crocetin and 13-cis crocetin contained in the grilled saffron, indicating that they possessed moderate ¹O₂-quenching activities (IC₅₀ 24–64 μM).

Functional characterization and substrate promiscuity of sesquiterpene synthases from Tripterygium wilfordii

Acyclic terpenes, commonly found in plants, are of high physiological importance and commercial value, and their diversity was controlled by different terpene synthases. During the screen of sesquiterpene synthases from Tripterygium wilfordii, we observed that Ses-TwTPS1-1 and Ses-TwTPS2 promiscuously accepted GPP, FPP, and GGPP to produce corresponding terpene alcohols (linalool/nerolidol/geranyllinalool). The Ses-TwTPS1-2, Ses-TwTPS3, and Ses-TwTPS4 also showed unusual substrate promiscuity by catalyzing GGPP or GPP in addition to FPP as substrate. Furthermore, key residues for the generation of diterpene product, (E, E)-geranyllinalool, were screened depending on mutagenesis studies. The functional analysis of Ses-TwTPS1-1:V199I and Ses-TwTPS1-2:I199V showed that Val in 199 site assisted the produce of diterpene product geranyllinalool by enzyme mutation studies, which indicated that subtle differences away from the active site could alter the product outcome. Moreover, an engineered sesquiterpene high-yielding yeast that produced 162 mg/L nerolidol in shake flask conditions was constructed to quickly identify the function of sesquiterpene synthases in vivo and develop potential applications in microbial fermentation. Our functional characterization of acyclic sesquiterpene synthases will give some insights into the substrate promiscuity of diverse acyclic terpene synthases and provide key residues for expanding the product portfolio.

A modern purification method for volatile sesquiterpenes produced by recombinant Escherichia coli carrying terpene synthase genes

Most volatile sesquiterpenes had been purified from plants using distillation and preparative gas chromatography, which is not applicable to many laboratories that do not possess a needed facility. Thus, this review focuses on a modern purification method for volatile sesquiterpenes using Escherichia coli cells that functionally express terpene synthase (Tps) genes. It was recently developed that recombinant E. coli cells carrying Tps genes were cultured in two-layer media (n-octane/TB medium) without harming the cells, and the volatile hydrophobic compounds trapped in the n-octane were purified by two-phase partition (alkane/alkaline 50% MeOH), silica gel column chromatography, and reversed-phase preparative high-performance liquid chromatography (if necessary). Consequently, it was found that the volatile sesquiterpenes are easily purified, the structures of which can then be determined by nuclear magnetic resonance, [α]_D and gas chromatography-mass spectrometry analyses. The antioxidant activities of several volatile sesquiterpenes are also presented in this review.