Overexpression of the 3-hydroxy-3-methylglutaryl-CoA synthase gene LcHMGS effectively increases the yield of monoterpenes and sesquiterpenes

LcMYB43 enhances monoterpene biosynthesis by activating 1-deoxy-D-xylulose-5-phosphate synthase gene expression in Litsea cubeba

MYB transcription factors are crucial regulators involved in various metabolic processes in plants, including terpene biosynthesis. Litsea cubeba, a member of the Lauraceae family, is rich in monoterpenes and regulates their biosynthesis via the key enzyme DXS in the MEP pathway. Seven DXS genes have been identified in this species, but the role of the MYB family in terpene biosynthesis remains unclear. This study conducted a genome-wide characterization of the R2R3-MYB gene family in L. cubeba, analyzing its phylogenetics, expression, and regulatory functions. A total of 129 R2R3-MYB members were identified, with expansion mechanisms involving tandem and segmental duplications. Expression analysis revealed that LcMYB43 activates LcDXS5, a key enzyme in monoterpene biosynthesis. Overexpression of LcMYB43 significantly increased monoterpene accumulation. Y1H, EMSA, and dual-luciferase assays showed that LcMYB43 directly binds to the CAACAG motif in the LcDXS5 promoter, activating its expression. These findings suggest that LcMYB43 enhances monoterpene biosynthesis by promoting LcDXS5 expression, providing new insights into the regulatory mechanisms of monoterpene biosynthesis.

Alcohol dehydrogenases regulated by a MYB44 transcription factor underlie Lauraceae citral biosynthesis

Lineage-specific terpenoids have arisen throughout the evolution of land plants and are believed to play a role in interactions between plants and the environment. Species-specific gene clusters in plants have provided insight on the evolution of secondary metabolism. Lauraceae is an ecologically important plant family whose members are also of considerable economic value given their monoterpene contents. However, the gene cluster responsible for the biosynthesis of monoterpenes remains yet to be elucidated. Here, a Lauraceae-specific citral biosynthetic gene cluster (CGC) was identified and investigated using a multifaceted approach that combined phylogenetic, collinearity, and biochemical analyses. The CGC comprises MYB44 as a regulator and 2 alcohol dehydrogenases (ADHs) as modifying enzymes, which derived from species-specific tandem and proximal duplication events. Activity and substrate divergence of the ADHs has resulted in the fruit of mountain pepper (Litsea cubeba), a core Lauraceae species, consisting of more than 80% citral. In addition, MYB44 negatively regulates citral biosynthesis by directly binding to the promoters of the ADH-encoding genes. The aggregation of citral biosynthetic pathways suggests that they may form the basis of important characteristics that enhance adaptability. The findings of this study provide insights into the evolution of and the regulatory mechanisms involved in plant terpene biosynthesis.

Overexpression of 1-deoxy-D-xylulose-5-phosphate reductoisomerase enhances the monoterpene content in Litsea cubeba

Monoterpenes are important components of plant essential oils and have long been used as raw materials for spices and food flavorings. Litsea cubeba is an economically aromatic plant species, the fruits of which produce essential oil with monoterpenes as the dominant components. As a branch point of carbon flow in the methyl erythritol phosphate (MEP) biosynthesis pathway, 1-deoxy-D-xylo-5-phosphate reductoisomerase (DXR) is a key rate-limiting enzyme that catalyzes the MEP pathway's second committed step. Therefore, DXR has become an effective regulation target to improve the biosynthesis of plant monoterpenes. In this study, we identified a DXR from L. cubeba, which was highly expressed in fruits, induced by MeJA and repressed by darkness. An enzyme assay showed that recombination LcDXR protein catalyzed DXP with NADPH as the cofactor. Transient overexpression of LcDXR significantly increased the content of monoterpenes in L. cubeba. Furthermore, LcDXR-overexpressing tobaccos were conducted and showed almost 5.9-fold increase in monoterpenes production, including limonene, α-pinene, eucalyptol, linalool, terpineol and camphor. Overexpression of LcDXR activated the metabolic flux of monoterpene biosynthesis through crosstalk and feedback mechanism. In addition, LcDXR-overexpressing tobaccos had no effect on phenotype of transgenic tobaccos. Our results demonstrate that LcDXR is a critical regulator of the monoterpene production in L. cubeba and other plants.

LcERF134 increases the production of monoterpenes by activating the terpene biosynthesis pathway in Litsea cubeba

Litsea cubeba, an aromatic species of the Lauraceae family, produces a diverse array of monoterpenes. The biosynthesis of monoterpenes is regulated by transcriptional factors (TFs), such as APETALA2/ethylene response factor (AP2/ERF). However, the regulatory mechanisms that control the AP2/ERF gene responsible for the biosynthesis of monoterpenes in L. cubeba have yet to be elucidated. Here, we identified an AP2/ERF gene, LcERF134, as an activator for the accumulation of citral and other monoterpenes. The expression level of LcERF134 was consistent with terpene synthase LcTPS42 in the pericarp. The transient overexpression of LcERF134 significantly increased monoterpene production in L. cubeba as well as the expression of rate-limiting genes involved in the monoterpene biosynthesis pathway. Furthermore, yeast one-hybrid, dual-luciferase and electrophoretic mobility shift assays demonstrated that LcERF134 activated the monoterpene biosynthesis pathway by directly binding to the GCC-box elements of the LcTPS42 and LcGPPS.SSU1 promoters. However, the overexpression of LcERF134 in tomatoes had no impact on the synthesis of monoterpenes, thus indicating that LcERF134 is a species-specific TF. Our research demonstrated that LcERF134 significantly increased the biosynthesis of monoterpenes by inducing the expression of LcTPS42 and LcGPPS.SSU1, thus offering insight into how to enhance the flavor of L. cubeba essential oil.

The chromosome-scale genome of Phoebe bournei reveals contrasting fates of terpene synthase (TPS)-a and TPS-b subfamilies

Terpenoids, including aromatic volatile monoterpenoids and sesquiterpenoids, function in defense against pathogens and herbivores. Phoebe trees are remarkable for their scented wood and decay resistance. Unlike other Lauraceae species investigated to date, Phoebe species predominantly accumulate sesquiterpenoids instead of monoterpenoids. Limited genomic data restrict the elucidation of terpenoid variation and functions. Here, we present a chromosome-scale genome assembly of a Lauraceae tree, Phoebe bournei, and identify 72 full-length terpene synthase (TPS) genes. Genome-level comparison shows pervasive lineage-specific duplication and contraction of TPS subfamilies, which have contributed to the extreme terpenoid variation within Lauraceae species. Although the TPS-a and TPS-b subfamilies were both expanded via tandem duplication in P. bournei, more TPS-a copies were retained and constitutively expressed, whereas more TPS-b copies were lost. The TPS-a genes on chromosome 8 functionally diverged to synthesize eight highly accumulated sesquiterpenes in P. bournei. The essential oil of P. bournei and its main component, β-caryophyllene, exhibited antifungal activities against the three most widespread canker pathogens of trees. The TPS-a and TPS-b subfamilies have experienced contrasting fates over the evolution of P. bournei. The abundant sesquiterpenoids produced by TPS-a proteins contribute to the excellent pathogen resistance of P. bournei trees. Overall, this study sheds light on the evolution and adaptation of terpenoids in Lauraceae and provides valuable resources for boosting plant immunity against pathogens in various trees and crops.

Advances in developing metabolically engineered microbial platforms to produce fourth-generation biofuels and high-value biochemicals

Producing bio-based chemicals is imperative to establish an eco-friendly circular bioeconomy. However, the compromised titer of these biochemicals hampers their commercial implementation. Advances in genetic engineering tools have enabled researchers to develop robust strains producing desired titers of the next-generation biofuels and biochemicals. The native and non-native pathways have been extensively engineered in various host strains via pathway reconstruction and metabolic flux redirection of lipid metabolism and central carbon metabolism to produce myriad biomolecules including alcohols, isoprenoids, hydrocarbons, fatty-acids, and their derivatives. This review has briefly covered the research efforts made during the previous decade to produce advanced biofuels and biochemicals through engineered microbial platforms along with the engineering approaches employed. The efficiency of the various techniques along with their shortcomings is also covered to provide a comprehensive overview of the progress and future directions to achieve higher titer of fourth-generation biofuels and biochemicals while keeping environmental sustainability intact.