Characterization, Function, and Transcriptional Profiling Analysis of 3-Hydroxy-3-methylglutaryl-CoA Synthase Gene (GbHMGS1) towards Stresses and Exogenous Hormone Treatments in Ginkgo biloba

Research progress on the types, functions, biosynthesis, and metabolic regulation of ginkgo terpenoids

Ginkgo biloba L. is a relict plant endemic to China that is commonly considered a "living fossil". It contains unique medicinal compounds that play important roles in its response to various stresses and help maintain human health. Ginkgo terpenoids are known to be important active ingredients but have received less attention than flavonoids. Hence, this review focuses on recent progress in research on the pharmacological effects of ginkgo terpenoid and the bioactivities of different terpenoid monomers. Many key structural genes, enzyme-encoding genes, transcription factors, and noncoding RNAs involved in the ginkgo terpenoid pathway were identified. Finally, many external factors (ecological factors, hormones, etc.) that regulate the biosynthesis and metabolism of terpenoids were proposed. All these findings improve the understanding of the biosynthesis, accumulation, and medicinal functions of terpenoids. Finally, this review includes an in-depth discussion regarding the limitations of terpenoid-related studies and potential future research directions.

The Ginkgo biloba microRNA160-ERF4 module participates in terpene trilactone biosynthesis

Terpene trilactones (TTLs) are important secondary metabolites in ginkgo (Ginkgo biloba); however, their biosynthesis gene regulatory network remains unclear. Here, we isolated a G. biloba ethylene response factor 4 (GbERF4) involved in TTL synthesis. Overexpression of GbERF4 in tobacco (Nicotiana tabacum) significantly increased terpenoid content and upregulated the expression of key enzyme genes (3-hydroxy-3-methylglutaryl-CoA reductase [HMGR], 3-hydroxy-3-methylglutaryl-CoA synthase [HMGS], 1-deoxy-D-xylulose-5-phosphate reductoisomerase [DXR], 1-deoxy-D-xylulose-5-phosphate synthase [DXS], acetyl-CoA C-acetyltransferase [AACT], and geranylgeranyl diphosphate synthase [GGPPS]) in the terpenoid pathway in tobacco, suggesting that GbERF4 functions in regulating the synthesis of terpenoids. The expression pattern analysis and previous microRNA (miRNA) sequencing showed that gb-miR160 negatively regulates the biosynthesis of TTLs. Transgenic experiments showed that overexpression of gb-miR160 could significantly inhibit the accumulation of terpenoids in tobacco. Targeted inhibition and dual-luciferase reporter assays confirmed that gb-miR160 targets and negatively regulates GbERF4. Transient overexpression of GbERF4 increased TTL content in G. biloba, and further transcriptome analysis revealed that DXS, HMGS, CYPs, and transcription factor genes were upregulated. In addition, yeast 1-hybrid and dual-luciferase reporter assays showed that GbERF4 could bind to the promoters of the HMGS1, AACT1, DXS1, levopimaradiene synthase (LPS2), and GGPPS2 genes in the TTL biosynthesis pathway and activate their expression. In summary, this study investigated the molecular mechanism of the gb-miR160-GbERF4 regulatory module in regulating the biosynthesis of TTLs. It provides information for enriching the understanding of the regulatory network of TTL biosynthesis and offers important gene resources for the genetic improvement of G. biloba with high contents of TTLs.

Cloning and functional analysis of Gb4CL1 and Gb4CL2 from Ginkgo biloba

4-Coumarate-CoA ligase (4CL) gene plays vital roles in plant growth and development, especially the regulation of lignin metabolism and flavonoid synthesis. To investigate the potential function of 4CL in the lignin biosynthesis of Ginkgo biloba, this study identified two 4CL genes, Gb4CL1 and Gb4CL2, from G. biloba genome. Based on the phylogenetic tree analysis, Gb4CL1 and Gb4CL2 protein were classified into Class I, which has been confirmed to be involved in lignin biosynthesis. Therefore, it can be inferred that these two genes may also participate in lignin metabolism. The tissue-specific expression patterns of these two genes revealed that Gb4CL1 was highly expressed in microstrobilus, whereas Gb4CL2 was abundant in immature leaves. The onion transient expression assay indicated that Gb4CL1 was predominantly localized in the nucleus, indicating its potential involvement in nuclear functions, while Gb4CL2 was observed in the cell wall, suggesting its role in cell wall-related processes. Phytohormone response analysis revealed that the expression of both genes was upregulated in response to indole acetic acid, while methyl jasmonate suppressed it, gibberellin exhibited opposite effects on these genes. Furthermore, Gb4CL1 and Gb4CL2 expressed in all tissues containing lignin that showed a positive correlation with lignin content. Thus, these findings suggest that Gb4CL1 and Gb4CL2 are likely involved in lignin biosynthesis. Gb4CL1 and Gb4CL2 target proteins were successfully induced in Escherichia coli BL21 with molecular weights of 85.5 and 89.2 kDa, proving the integrity of target proteins. Our findings provided a basis for revealing that Gb4CL participated in lignin synthesis in G. biloba.

Plant secondary metabolites involved in the stress tolerance of long-lived trees

Ancient trees are natural wonders because of their longevity, having lived for hundreds or thousands of years, and their ability to withstand changing environments and a variety of stresses. These long-lived trees have sophisticated defense mechanisms, such as the production of specialized plant metabolites (SPMs). In this review, we provide an overview of the major biotic and abiotic stresses that long-lived trees often face, as well as an analysis of renowned ancient tree species and their unique protective SPMs against environmental stressors. We also discuss the synthesis and accumulation of defensive SPMs induced by environmental factors and endophytes in these trees. Furthermore, we conducted a comparative genomic analysis of 17 long-lived tree species and discovered significant expansions of SPM biosynthesis gene families in these species. Our comprehensive review reveals the crucial role of SPMs in high resistance in long-lived trees, providing a novel natural resource for plant defense, crop improvement and even the pharmaceutical industry.

Advances in Genome Sequencing and Natural Rubber Biosynthesis in Rubber-Producing Plants

Natural rubber (cis-1,4-polyisoprene, NR) is an important raw material utilized widely in the manufacturing of medical, agricultural, and industrial products. Rubber tree (Hevea brasiliensis) and several alternative rubber-producing plants (Taraxacum kok-saghyz, Lactuca sativa, and Parthenium argentatum) have the capability to produce high-quality NR. With the progress of genome sequencing, similar rubber biosynthesis pathways have been discovered among different rubber-producing plant species. NR is synthesized and stored in rubber particles, which are specialized organelles comprising a hydrophobic NR core surrounded by a lipid monolayer and membrane-bound proteins. The rubber transferase complex is considered to be the pivotal enzyme involved in catalyzing NR biosynthesis. However, the exact compositions of the RT complex in rubber-producing plants remain elusive and poorly understood. Here, we review the progress of genome sequencing, natural rubber biosynthesis, and the components of the RT complex in rubber-producing plants. We emphasize that identifying the detailed components of the RT complex holds great significance for exploring the mechanism of NR biosynthesis and accelerating molecular breeding in rubber-producing plants.

Combined nitrogen and drought stress leads to overlapping and unique proteomic responses in potato

Nitrogen deficient and drought-tolerant or sensitive potatoes differ in proteomic responses under combined (NWD) and individual stresses. The sensitive genotype 'Kiebitz' exhibits a higher abundance of proteases under NWD. Abiotic stresses such as N deficiency and drought affect the yield of Solanum tuberosum L. tremendously. Therefore, it is of importance to improve potato genotypes in terms of stress tolerance. In this study, we identified differentially abundant proteins (DAPs) in four starch potato genotypes under N deficiency (ND), drought stress (WD), or combined stress (NWD) in two rain-out shelter experiments. The gel-free LC-MS analysis generated a set of 1177 identified and quantified proteins. The incidence of common DAPs in tolerant and sensitive genotypes under NWD indicates general responses to this stress combination. Most of these proteins were part of the amino acid metabolism (13.9%). Three isoforms of S-adenosyl methionine synthase (SAMS) were found to be lower abundant in all genotypes. As SAMS were found upon application of single stresses as well, these proteins appear to be part of the general stress response in potato. Interestingly, the sensitive genotype 'Kiebitz' showed a higher abundance of three proteases (subtilase, carboxypeptidase, subtilase family protein) and a lower abundance of a protease inhibitor (stigma expressed protein) under NWD stress compared to control plants. The comparably tolerant genotype 'Tomba', however, displayed lower abundances of proteases. This indicates a better coping strategy for the tolerant genotype and a quicker reaction to WD when previously stressed with ND.

Expression Analysis of CoHMGS in Cornus officinalis and Subcellular Localization of the Enzyme It Encodes

Cornus officinalis is a perennial deciduous tree or shrub. Its mature fruits are extracted and used in Traditional Chinese Medicine, called Shanzhuyu. The characteristic active components of C. officinalis include loganin and morroniside, which belong to iridoid glycosides. 3-Hydroxy-3-methylglutaryl-CoA synthase (HMGS) is a key enzyme in the cytoplasmic mevalonate pathway providing the precursor molecules isopentenyl pyrophosphate and dimethylallyl pyrophosphate for isoprenoid biosynthesis such as sterols, triterpenes, and their derivatives such as iridoid glycosides. Different concentrations of methyl jasmonate (MeJA) and ethephon (ETH) solutions were sprayed on C. officinalis seedlings, and the effect of hormones on CoHMGS gene expression was detected by real-time fluorescence quantitative PCR. The quantitative real-time PCR results showed that 750 mg/L ETH treatment had the most significant induction effect on CoHMGS gene expression. The HPLC analysis of extracts revealed that the treatment could also significantly increase the content of morroniside and loganin in the leaves of C. officinalis. By use of a CoHMGS-green fluorescent protein (GFP) fusion construct for heterologous expression in tobacco, laser scanning confocal microscopy revealed a cytoplasmic localization. This preliminary study of the CoHMGS gene could prepare the ground for more precisely elucidating the synthesis of secondary metabolite in C. officinalis.

Characterization of a novel levopimaradiene synthase gene responsible for the biosynthesis of terpene trilactones in Ginkgo biloba

Terpene trilactones (TTLs) are the main medicinal compounds of Ginkgo biloba. Levopimaradiene synthase (LPS) is the crucial enzyme that catalyzes TTLs biosynthesis in G. biloba. In this study, a novel LPS gene (designated as GbLPS2) was cloned from G. biloba leaves. The open reading frame of GbLPS2 gene was 2520 bp in length, encoding a predicted polypeptide of 840 amino acids. Phylogenetic analysis revealed that the GbLPS2 was highly homologous with reported LPS proteins in other plants. On the basis of the genomic DNA (gDNA) template, a 4308 bp gDNA sequence of GbLPS2 and a 913 bp promoter sequence were amplified. Cis-acting elements in promoter analysis indicated that GbLPS2 could be regulated by methyl jasmonate (MeJA) and abscisic acid (ABA). Tissue-specific expression analysis revealed that GbLPS2 was mainly expressed in roots and ovulate strobilus. MeJA treatment could significantly induce the expression level of GbLPS2 and increase the content of TTLs. This study illustrates the structure and the tissue-specific expression pattern of GbLPS2 and demonstrates that exogenous hormones regulated the expression of GbLPS2 and TTL content in G. biloba. Our results provide a target gene for the enhancement of TTL content in G. biloba via genetic engineering.

Genome-wide identification and characterization of bHLH family genes from Ginkgo biloba

Basic helix–loop–helix (bHLH) proteins, one of the most important and largest transcription factor family in plants, play important roles in regulating growth and development, stress response. In recent years, many bHLH family genes have been identified and characterized in woody plants. However, a systematic analysis of the bHLH gene family has not been reported in Ginkgo biloba, the oldest relic plant species. In this study, we identifed a total of 85 GbbHLH genes from the genomic and transcriptomic databases of G. biloba, which were classified into 17 subfamilies based on the phylogenetic analysis. Gene structures analysis indicated that the number of exon–intron range in GbbHLHs from 0 to 12. The MEME analysis showed that two conserved motifs, motif 1 and motif 2, distributed in most GbbHLH protein. Subcellular localization analysis exhibited that most GbbHLHs located in nucleus and a few GbbHLHs were distributed in chloroplast, plasma membrane and peroxisome. Promoter cis-element analysis revealed that most of the GbbHLH genes contained abundant cis-elements that involved in plant growth and development, secondary metabolism biosynthesis, various abiotic stresses response. In addition, correlation analysis between gene expression and flavonoid content screened seven candidate GbbHLH genes involved in flavonoid biosynthesis, providing the targeted gene encoding transcript factor for increase the flavonoid production through genetic engineering in G. biloba.

Differential expression of the TwHMGS gene and its effect on triptolide biosynthesis in Tripterygium wilfordii

3-Hydroxy-3-methylglutaryl-CoA synthase (HMGS) is the first committed enzyme in the MVA pathway and involved in the biosynthesis of terpenes in Tripterygium wilfordii. The full-length cDNA and a 515 bp RNAi target fragment of TwHMGS were ligated into the pH7WG2D and pK7GWIWG2D vectors to respectively overexpress and silence, TwHMGS was overexpressed and silenced in T. wilfordii suspension cells using biolistic-gun mediated transformation, which resulted in 2-fold increase and a drop to 70% in the expression level compared to cells with empty vector controls. During TwHMGS overexpression, the expression of TwHMGR, TwDXR and TwTPS7v2 was significantly upregulated to the control. In the RNAi group, the expression of TwHMGR, TwDXS, TwDXR and TwMCT visibly displayed downregulation to the control. The cells with TwHMGS overexpressed produced twice higher than the control value. These results proved that differential expression of TwHMGS determined the production of triptolide in T. wilfordii and laterally caused different trends of relative gene expression in the terpene biosynthetic pathway. Finally, the substrate acetyl-CoA was docked into the active site of TwHMGS, suggesting the key residues including His247, Lys256 and Arg296 undergo electrostatic or H-bond interactions with acetyl-CoA.

Strategies for the production of biochemicals in bioenergy crops

Industrial crops are grown to produce goods for manufacturing. Rather than food and feed, they supply raw materials for making biofuels, pharmaceuticals, and specialty chemicals, as well as feedstocks for fabricating fiber, biopolymer, and construction materials. Therefore, such crops offer the potential to reduce our dependency on petrochemicals that currently serve as building blocks for manufacturing the majority of our industrial and consumer products. In this review, we are providing examples of metabolites synthesized in plants that can be used as bio-based platform chemicals for partial replacement of their petroleum-derived counterparts. Plant metabolic engineering approaches aiming at increasing the content of these metabolites in biomass are presented. In particular, we emphasize on recent advances in the manipulation of the shikimate and isoprenoid biosynthetic pathways, both of which being the source of multiple valuable compounds. Implementing and optimizing engineered metabolic pathways for accumulation of coproducts in bioenergy crops may represent a valuable option for enhancing the commercial value of biomass and attaining sustainable lignocellulosic biorefineries.

Oleoresin defenses in conifers: chemical diversity, terpene synthases and limitations of oleoresin defense under climate change

Conifers have evolved complex oleoresin terpene defenses against herbivores and pathogens. In co-evolved bark beetles, conifer terpenes also serve chemo-ecological functions as pheromone precursors, chemical barcodes for host identification, or nutrients for insect-associated microbiomes. We highlight the genomic, molecular and biochemical underpinnings of the large chemical space of conifer oleoresin terpenes and volatiles. Conifer terpenes are predominantly the products of the conifer terpene synthase (TPS) gene family. Terpene diversity is increased by cytochromes P450 of the CYP720B class. Many conifer TPS are multiproduct enzymes. Multisubstrate CYP720B enzymes catalyse multistep oxidations. We summarise known terpenoid gene functions in various different conifer species with reference to the annotated terpenoid gene space in a spruce genome. Overall, biosynthesis of terpene diversity in conifers is achieved through a system of biochemical radiation and metabolic grids. Expression of TPS and CYP720B genes can be specific to individual cell types of constitutive or traumatic resin duct systems. Induced terpenoid transcriptomes in resin duct cells lead to dynamic changes of terpene composition and quantity to fend off herbivores and pathogens. While terpenoid defenses have contributed much to the evolutionary success of conifers, under new conditions of climate change, these defences may become inconsequential against range-expanding forest pests.

Natural rubber biosynthesis in plants, the rubber transferase complex, and metabolic engineering progress and prospects

Natural rubber (NR) is a nonfungible and valuable biopolymer, used to manufacture ~50 000 rubber products, including tires and medical gloves. Current production of NR is derived entirely from the para rubber tree (Hevea brasiliensis). The increasing demand for NR, coupled with limitations and vulnerability of H. brasiliensis production systems, has induced increasing interest among scientists and companies in potential alternative NR crops. Genetic/metabolic pathway engineering approaches, to generate NR-enriched genotypes of alternative NR plants, are of great importance. However, although our knowledge of rubber biochemistry has significantly advanced, our current understanding of NR biosynthesis, the biosynthetic machinery and the molecular mechanisms involved remains incomplete. Two spatially separated metabolic pathways provide precursors for NR biosynthesis in plants and their genes and enzymes/complexes are quite well understood. In contrast, understanding of the proteins and genes involved in the final step(s)-the synthesis of the high molecular weight rubber polymer itself-is only now beginning to emerge. In this review, we provide a critical evaluation of recent research developments in NR biosynthesis, in vitro reconstitution, and the genetic and metabolic pathway engineering advances intended to improve NR content in plants, including H. brasiliensis, two other prospective alternative rubber crops, namely the rubber dandelion and guayule, and model species, such as lettuce. We describe a new model of the rubber transferase complex, which integrates these developments. In addition, we highlight the current challenges in NR biosynthesis research and future perspectives on metabolic pathway engineering of NR to speed alternative rubber crop commercial development.

Characterization and functional analysis of two novel 3-hydroxy-3-methylglutaryl-coenzyme A reductase genes (GbHMGR2 and GbHMGR3) from Ginkgo biloba

Terpene trilactones (TTLs) are the main secondary metabolites of Ginkgo biloba. As one of the rate-limiting enzymes in the mevalonic acid (MVA) pathway of TTL biosynthesis, 3-hydroxy-3-methylglutaryl-coenzyme A reductase (HMGR) catalyzes the 3-hydroxy-3-methylglutaryl coenzyme A to form MVA. In this study, two cDNA sequences of HMGR genes, namely, GbHMGR2 and GbHMGR3, were cloned from G. biloba. The protein sequences of GbHMGR2 and GbHMGR3, which contain several functional domains, were analyzed. Regulatory elements related to light, hormone, and stress response were detected in the promoter regions of GbHMGR2 and GbHMGR3. The catalytic activity of these genes was verified by a functional complement experiment in yeast. Quantitative real-time PCR (qRT-PCR) showed the distinct expression patterns of the two genes in different organs. The TTL contents in the organs were detected by high-performance liquid chromatography- evaporative light scattering detector. GbHMGR2 and GbHMGR3 were responded to cold, dark, methyl jasmonate (MJ), abscisic acid (ABA), salicylic acid (SA), and ethephon (Eth) treatments. The TTL contents were also regulated by cold, dark, MJ, ABA, SA, and Eth treatment. In conclusion, GbHMGR2 and GbHMGR3 may participate in the MVA pathway of TTL biosynthesis.

Evolutionary Conservation and Divergence of Genes Encoding 3-Hydroxy-3-methylglutaryl Coenzyme A Synthase in the Allotetraploid Cotton Species Gossypium hirsutum

Polyploidization is important for the speciation and subsequent evolution of many plant species. Analyses of the duplicated genes produced via polyploidization events may clarify the origin and evolution of gene families. During terpene biosynthesis, 3-hydroxy-3-methylglutaryl coenzyme A synthase (HMGS) functions as a key enzyme in the mevalonate pathway. In this study, we first identified a total of 53 HMGS genes in 23 land plant species, while no HMGS genes were detected in three green algae species. The phylogenetic analysis suggested that plant HMGS genes may have originated from a common ancestral gene before clustering in different branches during the divergence of plant lineages. Then, we detected six HMGS genes in the allotetraploid cotton species (Gossypium hirsutum), which was twice that of the two diploid cotton species (Gossypium raimondii and Gossypium arboreum). The comparison of gene structures and phylogenetic analysis of HMGS genes revealed conserved evolution during polyploidization in Gossypium. Moreover, the expression patterns indicated that six GhHMGS genes were expressed in all tested tissues, with most genes considerably expressed in the roots, and they were responsive to various phytohormone treatments and abiotic stresses. The sequence and expression divergence of duplicated genes in G. hirsutum implied the sub-functionalization of GhHMGS1A and GhHMGS1D as well as GhHMGS3A and GhHMGS3D, whereas it implied the pseudogenization of GhHMGS2A and GhHMGS2D. Collectively, our study unraveled the evolutionary history of HMGS genes in green plants and from diploid to allotetraploid in cotton and illustrated the different evolutionary fates of duplicated HMGS genes resulting from polyploidization.