Duplication and specialization of NUDX1 in Rosaceae led to geraniol production in rose petals

Current insights into plant volatile organic compound biosynthesis

Plant-derived volatile organic compounds (VOCs) are essential for various ecological interactions, including plant communication, pollinator attraction, and defense against herbivores. Some VOCs are active ingredients with significant economic and medicinal value. For example, monoterpenoids such as linalool, geraniol, menthol, camphor, borneol, citral, and thymol are well-known for their flavor and aroma. Most monoterpenoids have a strong scent and physiological activity; some compounds, like thymoquinone, have excellent anti-cancer activities, making them important for pharmaceuticals and also beneficial in food and cosmetics. VOCs encompass a diverse range of chemical classes, such as terpenoids, benzenoids/phenylpropanoids, amino acid derivatives, and fatty acid-derived compounds. With the development of genomic, transcriptomic, and metabolomic techniques, significant progress has been made in the discovery of genes for the biosynthesis of VOCs. Herein, recent advances in the biosynthesis of plant-derived VOCs, focusing on two main classes: benzenoids/phenylpropanoids and monoterpenes, are discussed. It highlights the identification of a peroxisomal enzyme, benzaldehyde synthase, in petunia that elucidates the biosynthetic pathway of benzaldehyde, and a bifunctional enzyme, geranyl/farnesyl diphosphate synthase (RcG/FPPS1), in roses (Rosa chinensis "Old Blush") that contributes to the production of cytosolic geranyl diphosphate. Current understanding about canonical and non-canonical pathways for monoterpene formation and some approaches that are useful for gene discovery have been discussed. Open questions and future perspectives in this field have also been presented.

The MYC Gene RrbHLH105 Contributes to Salt Stress-Induced Geraniol in Rose by Regulating Trehalose-6-Phosphate Signalling

Rose (Rosa rugosa) is an important perfume plant, but its cultivation is significantly constrained by salt stress. Terpenes represent the most abundant volatile aromatic compounds in roses, yet little is known about how terpene metabolism responds to salt stress. In this study, salt-treated rose petals presented significant accumulation of monoterpenes, including geraniol, due to the disruption of jasmonic acid (JA) biosynthesis and signalling. Overexpression and silencing analyses revealed a MYC transcription factor involved in JA signalling (RrbHLH105) as a repressor of geraniol biosynthesis. RrbHLH105 was shown to activate the trehalose-6-phosphate synthase genes RrTPS5 and RrTPS8 by binding to the E-box (5'-CANNTG-3'). The increased trehalose-6-phosphate content and decreased geraniol content in rose petals overexpressing TPS5 or RrTPS8, along with the high accumulation of geraniol in petals where both RrbHLH105 and TPSs were cosilenced, indicate that trehalose signalling plays a role in the negative regulation of geraniol accumulation via the RrbHLH105-TPS module. In summary, the suppression of RrbHLH105 by salt stress leads to excessive geraniol accumulation through the inhibition of both RrbHLH105-mediated JA signalling and RrTPS-mediated trehalose signalling in rose petals. Additionally, this study highlights the emerging role of RrbHLH105 as a critical integrator of JA and trehalose signalling crosstalk.

Why do we like so much the smell of roses: The recipe for the perfect flower

The rose is the most cultivated ornamental plant in the world, and one of the reasons is that its fragrance is highly pleasant to humans. This raises the question of which volatile organic compounds (VOCs) emitted by flowers are involved in a rose odor-induced positive emotional response. Here, we invited participants to smell and rate the perceptual characteristics of roses whose VOCs were quantified. We revealed that (1) the more rose-specific the flower perception, the more pleasant the smell and (2) the rosy perception is driven by ionones and to a lesser extent by oxylipins while pleasantness by balanced proportion in the mixture of ionones, oxylipins, and 2-phenylethanol and derivatives. In the mixture, the proportion of some compounds, such as aliphatics and phenolic methyl esters, impact negatively the rose scent. Thus, the pleasure that roses bring to humans could be explained by the non-conscious perception of this unique mixture of compounds.

Antioxidant activity analysis of new interspecific hybrid germplasm thyme and oregano essential oils with different chemotypes

Thyme and oregano essential oils (EOs) and their components have numerous applications in the pharmaceutical, food, and cosmetic industries owing to their antioxidant, antibacterial, antifungal, anti-inflammatory, antiviral, and immunological properties. We attempted to create new chemotypes through the hybridization of thyme and oregano for functional EO research and product development. Here, we used interspecific hybridization to create new thyme and oregano germplasms with new EO chemotypes. The antioxidant activities of these new chemotype EOs were verified by DPPH, ABTS, and FRAP analyses. We determined that there are five types of thyme hybrid EOs: geraniol-type, carvacrol-type, thymol-type, parent polymerization-type, and α-terpineol/α-terpinyl acetate-type. Moreover, there are five types of oregano hybrid EOs: carvacrol-type, thymol-type, sabinene hydrate-type, parent polymerization-type, and carvacrol/sabinene hydrate-type. The geraniol, thymol, and carvacrol contents ranged from 30.45%, 1.21% and 0.00%, respectively, in the parents to 81.66%, 52.65%, and 46.16%, respectively, in the thyme hybrids. The carvacrol and thymol contents ranged from 2.33% to 24.18%, respectively, in the parents to 94.16% and 76.77%, respectively, in the oregano hybrids, indicating obvious heterosis. We further used three antioxidant assays, DPPH, ABTS, and FRAP, to analyse the antioxidant activity of thyme and oregano hybrid EO samples. The antioxidant capacities of carvacrol- and thymol-type EOs were significantly superior to those of other chemotypes. Our data suggest that carvacrol- and thymol-type EOs with greater antioxidant potential can be applied in many industries. In addition, the function of high geraniol-type thyme EO should be further researched. The results will also be very useful for the selection of new varieties, functional research on carvacrol-, thymol-, and geraniol-type essential oils, and product development of feed additives, cosmetics, etc.

Antioxidant and Antibacterial Activities of Chinese Native Thyme Essential Oils with Different Chemotypes

Thyme essential oils (EOs) have antioxidant, antiviral, antifungal, antibacterial, anti-inflammatory, and immunological properties and are used in medicine, food, feed additives, and cosmetics. Here, we made use of a multidimensional analytical method to analyze the differences in the chemical components, chemotypes, and antioxidant and antibacterial activities of EOs from 24 Chinese native thymes. These Chinese native thymes comprised 10 species (Thymus quinquecostatus, T. mongolicus, T. inaequalis, T. mandschuricus, T. curtus, T. amurensis, T. roseus, T. proximu, T. marschallianus, and T. altaicus) and two varieties (T. quinquecostatus var. asiaticus and T. quinquecostatus var. przewalskii). Four primary chemotype groups were identified, namely carvacrol, thymol, geraniol, and α-terpineol. The maximum carvacrol, thymol, geraniol, and α-terpineol contents were 72.4, 58.6, 59.5, and 65.4%, respectively. The antioxidant capacities of the thymol and carvacrol chemotype EOs were found to be significantly superior to the other chemotypes using three antioxidant assays: DPPH, ABTS, and FRAP. Moreover, the thymol and carvacrol EO chemotypes could significantly inhibit the growths of the common food-borne pathogenic bacteria Staphylococcus aureus and Escherichia coli. A correlation analysis between the EO components and the bacteria showed that thymol significantly positively correlated with the bacteria. In summary, we analyzed the thyme EOs' antioxidant and antibacterial activities, which laid a foundation for their use in medicines, foods, feed additives, and cosmetics. The results will also be very useful for the selection of wild thymes for functional research on carvacrol-, thymol-, geraniol-, and α-terpineol-rich essential oil chemotypes and the product development of feed additives, cosmetics, etc.

Phylogenetically distant enzymes localized in cytosol and plastids drive citral biosynthesis in lemongrass

Citral, a naturally occurring acyclic monoterpene aldehyde, is present in the essential oils of various plants, but only a few produce it in abundance. Despite its importance as a key aroma molecule, knowledge regarding the in-planta biosynthesis of citral and its metabolic origin remains limited. Here, we have elucidated the functions of an alcohol dehydrogenase (CfADH1) and an aldoketo-reductase (CfAKR2b) in citral biosynthesis in lemongrass (Cymbopogon flexuosus), one of the most cultivated aromatic crops for its citral-rich essential oil. Expression of both CfADH1 and CfAKR2b showed correlation with citral accumulation in different developmental stages. Recombinant CfADH1 and CfAKR2b, despite their sequence unrelatedness, catalyzed citral formation from geraniol with NADP cofactor. Virus-induced gene silencing in lemongrass and transient expression in lemon balm (Melissa officinalis) demonstrated the in-planta involvement of CfADH1 and CfAKR2b in citral biosynthesis. While CfADH1 exhibited a dual cytosolic/plastidial localization, CfAKR2b was localized to the cytosol. This was supported by higher citral-forming activity in the cytosolic fraction than in the chloroplast fraction of lemongrass leaf extract. Moreover, feeding lemongrass seedlings with inhibitors specific to the cytosolic mevalonate pathway and the plastidial methylerythritol phosphate pathway, combined with volatile profiling, supported the involvement of both pathways in citral formation. Taken together, our results indicate that high citral production has evolved in lemongrass through the recruitment of phylogenetically distant enzymes localized in both the cytosol and plastids.

The complexity of volatile terpene biosynthesis in roses: Particular insights into β-citronellol production

The fascinating scent of rose (Rosa genus) flowers has captivated human senses for centuries, making them one of the most popular and widely used floral fragrances. Despite much progress over the last decade, many biochemical pathways responsible for rose scents remain unclear. We analyzed the floral scent compositions from various rose varieties and selected the modern cultivar Rosa hybrida "Double Delight" as a model system to unravel the formation of rose dominant volatile terpenes, which contribute substantially to the rose fragrance. Key genes involved in rose terpene biosynthesis were functionally characterized. Cytosolic geranyl diphosphate (GPP) generated by geranyl/farnesyl diphosphate synthase (G/FPPS1) catalysis played a pivotal role in rose scent production, and terpene synthases in roses play an important role in the formation of most volatile terpenes, but not for geraniol, citral, or β-citronellol. Subsequently, a series of enzymes, including geraniol dehydrogenase, geranial reductase, 12-oxophytodienoate reductase, and citronellal reductase, were characterized as involved in the transformation of geraniol to β-citronellol in roses through three successive steps. Interestingly, the β-citronellol biosynthesis pathway appears to be conserved in other horticultural plants like Lagerstroemia caudata and Paeonia lactiflora. Our findings provide valuable insights into the biosynthesis of rose volatile terpenoid compounds and offer essential gene resources for future breeding and molecular modification efforts.

Genetic Variation, Polyploidy, Hybridization Influencing the Aroma Profiles of Rosaceae Family

BACKGROUND

The fragrance and aroma of Rosaceae plants are complex traits influenced by a multitude of factors, with genetic variation standing out as a key determinant which is largely impacted by polyploidy. Polyploidy serves as a crucial evolutionary mechanism in plants, significantly boosting genetic diversity and fostering speciation.

OBJECTIVE

This review focuses on the Rosaceae family, emphasizing how polyploidy influences the production of volatile organic compounds (VOCs), which are essential for the aromatic characteristics of economically important fruits like strawberries, apples, and cherries. The review delves into the biochemical pathways responsible for VOC biosynthesis, particularly highlighting the roles of terpenoids, esters alcohols, aldehydes, ketones, phenolics, hydrocarbons, alongside the genetic mechanisms that regulate these pathways. Key enzymes, such as terpene synthases and alcohol acyltransferases, are central to this process. This review further explores how polyploidy and hybridization can lead to the development of novel metabolic pathways, contributing to greater phenotypic diversity and complexity in fruit aromas. It underscores the importance of gene dosage effects, isoenzyme diversity, and regulatory elements in determining VOC profiles.

CONCLUSIONS

These findings provide valuable insights for breeding strategies aimed at improving fruit quality and aligning with consumer preferences. Present review not only elucidates the complex interplay between genomic evolution and fruit aroma but also offers a framework for future investigations in plant biology and agricultural innovation.

Evolution of the biosynthetic pathways of terpene scent compounds in roses

It is unknown why roses are terpene-rich, what the terpene biosynthetic pathways in roses are, and why only a few rose species produce the major components of rose essential oil. Here, we assembled two high-quality chromosome-level genomes for Rosa rugosa and Rosa multiflora. We also re-sequenced 132 individuals from the F1 progeny of Rosa chinensis and Rosa wichuraiana and 36 of their related species. Comparative genomics revealed that expansions of the 3-hydroxy-3-methylglutaryl-coenzyme A reductase (HMGR) and terpene synthases (TPSs) gene families led to the enrichment of terpenes in rose scent components. We constructed a terpene biosynthesis network and discovered a TPS-independent citronellol biosynthetic pathway in roses through gene functional identification, genome-wide association studies (GWASs), and multi-omic analysis. Heterologous co-expression of rose citronellol biosynthetic genes in Nicotiana benthamiana led to citronellol production. Our genomic and metabolomic analyses suggested that the copy number of NUDX1-1a determines the citronellol content in different rose species. Our findings not only provide additional genome and gene resources and reveal the evolution of the terpene biosynthetic pathways but also present a nearly complete scenario for terpenoid metabolism that will facilitate the breeding of fragrant roses and the production of rose oil.

Integrated Metabolomics and Transcriptomics Analysis Reveals New Insights into Triterpene Biosynthesis in Rosa rugosa

Rosa rugosa is highly regarded for its aesthetic and therapeutic qualities. In particular, R. rugosa's flowers are known to produce essential oils containing a mixture of volatile terpenes, phenylpropanoids, and other compounds. Despite this, extensive research exists on volatile terpenes in flowers, while the knowledge of non-volatile terpenes in distinct tissues is still limited. Using UPLC-ESI-MS/MS, a comprehensive analysis of the terpene metabolites in five different tissues of R. rugosa was conducted. These metabolites accumulated in distinct tissues, and the majority of them were triterpenoids. Transcriptome data were collected from five tissues using RNA-seq. Transcriptomics and metabolomics were utilized to evaluate the triterpene biosynthesis pathway, resulting in new insights into its regulation and biosynthesis. The RrOSC10 was identified as a key enzyme in converting 2,3-oxidosqualene into α-amyrin, potentially contributing to the triterpene biosynthesis pathway. Furthermore, the expression of the RrOSC10 gene was upregulated by salinity for 0.5 h and 1 h, with subsequent downregulation at 2 h. This study lays a foundation for future research on the biosynthesis and accumulation of triterpenes in R. rugosa.

Haplotype-resolved genome assembly of the diploid Rosa chinensis provides insight into the mechanisms underlying key ornamental traits

Roses are consistently ranked at the forefront in cut flower production. Increasing demands of market and changing climate conditions have resulted in the need to further improve the diversity and quality of traits. However, frequent hybridization leads to highly heterozygous nature, including the allelic variants. Therefore, the absence of comprehensive genomic information leads to them making it challenging to molecular breeding. Here, two haplotype-resolved chromosome genomes for Rosa chinensis 'Chilong Hanzhu' (2n = 14) which is high heterozygous diploid old Chinese rose are generated. An amount of genetic variation (1,605,616 SNPs, 209,575 indels) is identified. 13,971 allelic genes show differential expression patterns between two haplotypes. Importantly, these differences hold valuable insights into regulatory mechanisms of traits. RcMYB114b can influence cyanidin-3-glucoside accumulation and the allelic variation in its promoter leads to differences in promoter activity, which as a factor control petal color. Moreover, gene family expansion may contribute to the abundance of terpenes in floral scents. Additionally, RcANT1, RcDA1, RcAG1 and RcSVP1 genes are involved in regulation of petal number and size under heat stress treatment. This study provides a foundation for molecular breeding to improve important characteristics of roses.

The scent of roses, a bouquet of fragrance diversity

Roses have been domesticated since antiquity for their therapeutic, cosmetic, and ornamental properties. Their floral fragrance has great economic value, which has influenced the production of rose varieties. The production of rose water and essential oil is one of the most lucrative activities, supplying bioactive molecules to the cosmetic, pharmaceutical, and therapeutic industries. In recent years, major advances in molecular genetics, genomic, and biochemical tools have paved the way for the identification of molecules that make up the specific fragrance of various rose cultivars. The aim of this review is to highlight current knowledge on metabolite profiles, and more specifically on fragrance compounds, as well as the specificities and differences between rose species and cultivars belonging to different rose sections and how they contribute to modern roses fragrance.

Comparison of nocturnal and diurnal metabolomes of rose flowers and leaves

INTRODUCTION

Roses are one of the most essential ornamental flowers and are commonly used in perfumery, cosmetics, and food. They are rich in bioactive compounds, which are of interest for therapeutic effects.

OBJECTIVES

The objective of this study was to understand the kinds of changes that occur between the nocturnal and diurnal metabolism of rose and to suggest hypotheses.

METHODS

Reversed-phase ultrahigh-performance liquid chromatography coupled with electrospray ionization quadrupole time-of-flight mass spectrometry or triple quadrupole mass spectrometry (TQ MS/MS) was used for nontargeted metabolomics and hormonal profiling respectively. For metabolite annotation, accurate mass spectra were compared with those in databases.

RESULTS

The hormonal profile of flowers showed an increase in jasmonate at night, while that of leaves indicated an increase in the salicylic acid pathway. Nontargeted analyses of the flower revealed a switch in the plant's defense mechanisms from glycosylated metabolites during the day to acid metabolites at night. In leaves, a significant decrease in flavonoids was observed at night in favor of acid metabolism to maintain a level of protection. Moreover, it might be possible to place back some of the annotated molecules on the shikimate pathway.

CONCLUSION

The influence of day and night on the metabolome of rose flowers and leaves has been clearly demonstrated. The hormonal modulations occurring during the night and at day are consistent with the plant circadian cycle. A proposed management of the sesquiterpenoid and triterpenoid biosynthetic pathway may explain these changes in the flower. In leaves, the metabolic differences may reflect night-time regulation in favor of the salicylic acid pathway.

The MYB transcription factor RcMYB1 plays a central role in rose anthocyanin biosynthesis

Rose (Rosa hybrida) is one of most famous ornamental plants in the world, and its commodity value largely depends on its flower color. However, the regulatory mechanism underlying rose flower color is still unclear. In this study, we found that a key R2R3-MYB transcription factor, RcMYB1, plays a central role in rose anthocyanin biosynthesis. Overexpression of RcMYB1 significantly promoted anthocyanin accumulation in both white rose petals and tobacco leaves. In 35S:RcMYB1 transgenic lines, a significant accumulation of anthocyanins occurred in leaves and petioles. We further identified two MBW complexes (RcMYB1-RcBHLH42-RcTTG1; RcMYB1-RcEGL1-RcTTG1) associated with anthocyanin accumulation. Yeast one-hybrid and luciferase assays showed that RcMYB1 could active its own gene promoter and those of other EBGs (early anthocyanin biosynthesis genes) and LBGs (late anthocyanin biosynthesis genes). In addition, both of the MBW complexes enhanced the transcriptional activity of RcMYB1 and LBGs. Interestingly, our results also indicate that RcMYB1 is involved in the metabolic regulation of carotenoids and volatile aroma. In summary, we found that RcMYB1 widely participates in the transcriptional regulation of ABGs (anthocyanin biosynthesis genes), indicative of its central role in the regulation of anthocyanin accumulation in rose. Our results provide a theoretical basis for the further improvement of the flower color trait in rose by breeding or genetic modification.

A cytosolic bifunctional geranyl/farnesyl diphosphate synthase provides MVA-derived GPP for geraniol biosynthesis in rose flowers

Geraniol derived from essential oils of various plant species is widely used in the cosmetic and perfume industries. It is also an essential trait of the pleasant smell of rose flowers. In contrast to other monoterpenes which are produced in plastids via the methyl erythritol phosphate pathway, geraniol biosynthesis in roses relies on cytosolic NUDX1 hydrolase which dephosphorylates geranyl diphosphate (GPP). However, the metabolic origin of cytosolic GPP remains unknown. By feeding Rosa chinensis "Old Blush" flowers with pathway-specific precursors and inhibitors, combined with metabolic profiling and functional characterization of enzymes in vitro and in planta, we show that geraniol is synthesized through the cytosolic mevalonate (MVA) pathway by a bifunctional geranyl/farnesyl diphosphate synthase, RcG/FPPS1, producing both GPP and farnesyl diphosphate (FPP). The downregulation and overexpression of RcG/FPPS1 in rose petals affected not only geraniol and germacrene D emissions but also dihydro-β-ionol, the latter due to metabolic cross talk of RcG/FPPS1-dependent isoprenoid intermediates trafficking from the cytosol to plastids. Phylogenetic analysis together with functional characterization of G/FPPS orthologs revealed that the G/FPPS activity is conserved among Rosaceae species. Site-directed mutagenesis and molecular dynamic simulations enabled to identify two conserved amino acids that evolved from ancestral FPPSs and contribute to GPP/FPP product specificity. Overall, this study elucidates the origin of the cytosolic GPP for NUDX1-dependent geraniol production, provides insights into the emergence of the RcG/FPPS1 GPPS activity from the ancestral FPPSs, and shows that RcG/FPPS1 plays a key role in the biosynthesis of volatile terpenoid compounds in rose flowers.

BOX38, a DNA Marker for Selection of Essential Oil Yield of Rosa × rugosa

Rosa rugosa L. was a famous aromatic plant whose cultivars (Rosa × rugosa) have been widely used in the perfume industry in Asia. The perfume market looks for rose cultivars bearing higher essential oil, while the oil yields of most R. × rugosa have not been evaluated due to limiting conditions, such as insufficient cultivation areas. Here, we tested the yield and the aroma components of essential oil of 19 R. × rugosa. The results indicated that the yields of nerol, citronellol, and geraniol could represent an alternative index of the total yield of essential oil. Sequence syntenic analysis indicated that the Rosa genus specific cis-element Box38 was highly polymorphic. The Box38 region isolation of Rosa × rugosa by flanked primers proved that Box38 repeat number was significantly positively correlated with the essential oil yield of the corresponding cultivar. In the breeding of Rosa × rugosa, six-Box38-repeat could be a robust threshold for selection of high-essential-oil roses. Together, we found that Box38 was a DNA marker for essential oil yield and that it would be helpful in the early selection and breeding of essential oil roses.

Emerging mechanistic insights into the regulation of specialized metabolism in plants

Plants biosynthesize a broad range of natural products through specialized and species-specific metabolic pathways that are fuelled by core metabolism, together forming a metabolic network. Specialized metabolites have important roles in development and adaptation to external cues, and they also have invaluable pharmacological properties. A growing body of evidence has highlighted the impact of translational, transcriptional, epigenetic and chromatin-based regulation and evolution of specialized metabolism genes and metabolic networks. Here we review the forefront of this research field and extrapolate to medicinal plants that synthetize rare molecules. We also discuss how this new knowledge could help in improving strategies to produce useful plant-derived pharmaceuticals.

Identification and Characterization of Transcription Factors Involved in Geraniol Biosynthesis in Rosa chinensis

Fragrance is an important characteristic of rose flowers and is largely determined by the terpenes. Rose has a unique NUDX1 (NUDIX HYDROLASES 1)-dependent monoterpene geraniol biosynthesis pathway, but little is known about its transcriptional regulation. In this study, we characterized two China rose (Rosa chinensis) materials from the 'Old Blush' variety with contrasting aromas. We profiled the volatile metabolome of both materials, and the results revealed that geraniol was the main component that distinguishes the aroma of these two materials. We performed a comparative transcriptome analysis of the two rose materials, from which we identified the hydrolase RcNUDX1 as a key factor affecting geraniol content, as well as 17 transcription factor genes co-expressed with RcNUDX1. We also determined that the transcription factor RcWRKY70 binds to four W-box motifs in the promoter of RcNUDX1, repressing RcNUDX1 expression, based on yeast one-hybrid and transient dual-luciferase assays. These results provide important information concerning the transcriptional regulatory framework underlying the control of geraniol production in rose.