Characterization of Terpene synthase variation in flowers of wild aquilegia species from Northeastern Asia

Characterization of a group of germacrene A synthases involved in the biosynthesis of β-elemene from Atractylodis macrocephala

β-Elemene, an important component of the volatile oil of Atractylodis macrocephala, has been widely utilized as an antitumor drug for over 20 years. However, the germacrene A synthase (GAS) genes responsible for the biosynthesis of β-elemene in A. macrocephala were previously unidentified. In this study, two new AmGASs were identified from the A. macrocephala transcriptome, demonstrating their capability to convert farnesyl pyrophosphate into germacrene A, which subsequently synthesizes β-elemene through Cope rearrangement. Additionally, two highly catalytic AmGAS1 mutations, I307A and E392A, resulted in a 2.23-fold and 1.57-fold increase in β-elemene synthesis, respectively. Furthermore, precursor supply and fed-batch strategies were employed to enhance the precursor supply, resulting in β-elemene yields of 7.3 mg/L and 33.3 mg/L, respectively. These findings identify a promising candidate GAS for β-elemene biosynthesis and lay the foundation for further functional studies on terpene synthases in A. macrocephala.

Allelic variation of terpene synthases drives terpene diversity in the wild species of the Freesia genus

Terpene synthases (TPSs) play pivotal roles in conferring the structural diversity of terpenoids, which are mainly emitted from flowers, whereas the genetic basis of the release of floral volatile terpenes remains largely elusive. Though quite similar in sequence, TPS allelic variants still function divergently, and how they drive floral terpene diversity in closely related species remains unknown. Here, TPSs responsible for the floral scent of wild Freesia species were characterized, and the functions of their natural allelic variants, as well as the causal amino acid residues, were investigated in depth. Besides the 8 TPSs previously reported in modern cultivars, 7 additional TPSs were functionally evaluated to contribute to the major volatiles emitted from wild Freesia species. Functional characterization of allelic natural variants demonstrated that allelic TPS2 and TPS10 variants changed the enzymatic capacity while allelic TPS6 variants drove the diversity of floral terpene products. Further residue substitution analysis revealed the minor residues determining the enzyme catalytic activity and product specificity. The clarification of TPSs in wild Freesia species reveals that allelic TPS variants evolved differently to determine the interspecific floral volatile terpenes in the genus and might be used for modern cultivar improvement.

Functional Characterization of a (E)-β-Ocimene Synthase Gene Contributing to the Defense against Spodoptera litura

Soybean is a worldwide crop that offers valuable proteins, fatty acids, and phytonutrients to humans but is always damaged by insect pests or pathogens. Plants have captured sophisticated defense mechanisms in resisting the attack of insects and pathogens. How to protect soybean in an environment- or human-friendly way or how to develop plant-based pest control is a hotpot. Herbivore-induced plant volatiles that are released by multiple plant species have been assessed in multi-systems against various insects, of which (E)-β-ocimene has been reported to show anti-insect function in a variety of plants, including soybean. However, the responsible gene in soybean is unknown, and its mechanism of synthesis and anti-insect properties lacks comprehensive assessment. In this study, (E)-β-ocimene was confirmed to be induced by Spodoptera litura treatment. A plastidic localized monoterpene synthase gene, designated as GmOCS, was identified to be responsible for the biosynthesis of (E)-β-ocimene through genome-wide gene family screening and in vitro and in vivo assays. Results from transgenic soybean and tobacco confirmed that (E)-β-ocimene catalyzed by GmOCS had pivotal roles in repelling a S. litura attack. This study advances the understanding of (E)-β-ocimene synthesis and its function in crops, as well as provides a good candidate for further anti-insect soybean improvement.

Biosynthesis of α-Bisabolol by Farnesyl Diphosphate Synthase and α-Bisabolol Synthase and Their Related Transcription Factors in Matricaria recutita L

The essential oil of German chamomile (Matricaria recutita L.) is widely used in food, cosmetics, and the pharmaceutical industry. α-Bisabolol is the main active substance in German chamomile. Farnesyl diphosphate synthase (FPS) and α-bisabolol synthase (BBS) are key enzymes related to the α-bisabolol biosynthesis pathway. However, little is known about the α-bisabolol biosynthesis pathway in German chamomile, especially the transcription factors (TFs) related to the regulation of α-bisabolol synthesis. In this study, we identified MrFPS and MrBBS and investigated their functions by prokaryotic expression and expression in hairy root cells of German chamomile. The results suggest that MrFPS is the key enzyme in the production of sesquiterpenoids, and MrBBS catalyzes the reaction that produces α-bisabolol. Subcellular localization analysis showed that both MrFPS and MrBBS proteins were located in the cytosol. The expression levels of both MrFPS and MrBBS were highest in the extension period of ray florets. Furthermore, we cloned and analyzed the promoters of MrFPS and MrBBS. A large number of cis-acting elements related to light responsiveness, hormone response elements, and cis-regulatory elements that serve as putative binding sites for specific TFs in response to various biotic and abiotic stresses were identified. We identified and studied TFs related to MrFPS and MrBBS, including WRKY, AP2, and MYB. Our findings reveal the biosynthesis and regulation of α-bisabolol in German chamomile and provide novel insights for the production of α-bisabolol using synthetic biology methods.

The biochemical and molecular investigation of flower color and scent sheds lights on further genetic modification of ornamental traits in Clivia miniata

Clivia miniata is renowned for its evergreen and strap-like leaves, whereas its floral color and scent are lacking diversity. Here, anthocyanin, volatile terpene, and carotenoid metabolisms were integrally investigated in C. miniata flowers. The results showed that pelargonidins and lutein might cooperate to confer orange or yellow color to C. miniata flowers, but only a trace amount of (+)-limonene was detected. The expression levels of CmF3'H and CmDFR appeared to be responsible for the ratio of cyanidin and pelargonidin derivatives in C. miniata, and the low expression of CmF3'H was responsible for the lack of cyanidins in flowers. Moreover, the CmF3'H promoter could not be activated by CmMYBAs, suggesting that it was controlled by novel regulators. Only two CmTPSs were functional, with CmTPS2 responsible for (+)-limonene synthesis, contributing to the monotonous flower volatile terpenes of C. miniata. CmCCD1a and CmCCD1b were able to cleave carotenoids at the 5,6 (5',6'), and 9,10 (9',10') positions to generate volatile apocarotenoids, whereas the substrates found in low-quantities or specific subcellular localizations of CmCCD1s might constrain volatile apocarotenoid release. Consequently, activating F3'H and introducing novel F3'5'H or versatile TPS may be effective ways to modify the floral color and scent, respectively. Alternatively, modifying the carotenoid flux or CCD1 localization might affect floral color and scent simultaneously. Taking these results together, the present study provides a preliminary deciphering of the genetic constraints underlying flower color and scent development, and proposes possible schemes for further genetic modification of ornamental traits in C. miniata and other plants.