Sensory-Directed Genetic and Biochemical Characterization of Volatile Terpene Production in Kiwifruit

Integration of volatile and non-volatile metabolites and the transcriptome reveals the formation mechanisms of differential aroma compounds between Pyrus communis and Pyrus pyrifolia cultivars

Introduction

Aroma compounds are important flavor components in pear fruit. Among cultivated pears, fruits from Pyrus communis (hereafter referred to as P. communis) cultivars are famous for their abundant aroma, while the fruits of most Pyrus pyrifolia (hereafter referred to as P. pyrifolia) cultivars lack aroma compounds. A comparative study on the formation of differential aroma compounds between the two species could provide a theoretical foundation for improving the aroma quality of P. pyrifolia cultivars. However, there is a lack of systematic research on this subject.

Methods

An analysis of volatile and non-volatile metabolites was combined with transcriptome analysis to explore the formation mechanism of differential aroma compounds between three P. communis and three P. pyrifolia cultivars.

Results

In this study, a total of 510 volatile compounds were identified in the six cultivars. Of these, sixteen ester and alcohol compounds, including butyl acetate, hexyl acetate, ethyl-2-methylbutyrate, ethanol, butanol, propanol, and 2-methylbutanol, with higher contents in the P. communis cultivars than in the P. pyrifolia cultivars were identified as the primary differential aroma compounds. Among the possible synthesis pathways for these 16 aroma compounds, certain amino acid degradation processes, including isoleucine, valine, and alanine oxidation and threonine dehydration, were found to provide important intermediate substances for synthesis. Within the key enzyme genes in the synthesis pathway, several critical enzyme genes, including monoacylglycerol lipase (PcMAGL, pycom08g09340), threonine dehydrase (PcTD, pycom12g10020), and acyl CoA dehydrogenase (PcACD, pycom16g13880), might be important factors contributing to the disparity in aromatic compounds between P. communis and P. pyrifolia cultivars.

Discussion

The aforementioned results provide valuable information into the formation mechanisms of differential aroma compounds and offer novel target sites for enhancing pear aroma quality through gene editing.

EIL (ethylene-insensitive 3-like) transcription factors in apple affect both ethylene- and cold response-dependent fruit ripening

EIN3/EIL (ethylene-insensitive 3/EIN3-like) transcription factors are positive downstream transcriptional regulators of ethylene signalling. In apple (Malus × domestica), a small family of MdEIL genes was identified, with four expressed in fruit. Transgenic lines were generated to manipulate MdEIL1 expression, and fruits were sampled at harvest maturity and after cold treatment. Their fruit ripening behaviour was compared with control lines and contrasted to a ACC OXIDASE 1 antisense line (ACO1as) which produced no ripening associated ethylene. Two transgenic lines showed strong co-suppression of MdEIL1-4 expression as well as reduced ethylene production, softening and aroma production, while one overexpressing line showed enhanced ripening. Key genes involved in ethylene biosynthesis and ethylene-dependent genes involved in cell wall modification (MdXTH1, MdβGAL) and aroma biosynthesis (MdAFS1, MdoOMT1) were downregulated in the co-suppressed lines. Co-suppressed lines showed reduced softening/volatile production after cold treatment and in contrast to the ACO1as line, expression of cold response-dependent genes (MdCBF2, dehydrins MdDHN2, -14, -16 and MdNAC29a) remained cold-repressed. The action of MdEILs was shown using dual-luciferase reporter assays to occur through direct activation of MdAFS1, MdXTH1 and MdβGAL promoters. Exogenous ethylene was unable to further stimulate ripening promoter activation, but cold treatment could. Promoter deletion analysis identified potential EIL binding sites in the MdAFS1 and MdβGAL promoters and electrophoretic mobility shift assays showed that MdEIL1-3 could all bind to a 32 bp fragment in the MdAFS1 promoter. Together these results indicate that MdEILs contribute to a suite of apple fruit ripening attributes via activation of genes in an ethylene-dependent manner, but also in response to cold.

Transcriptomic and metabolomic study of the biosynthetic pathways of bioactive components in Amomum tsaoko fruits

Amomum tsaoko is a significant medicinal and edible plant with documented efficacy in the treatment of various diseases. Additionally, it is a crucial food additive and spice. 1,8-cineole and curcumin are the main bioactive compounds of A. tsaoko, and research on these compounds has mainly focused on their chemical composition and pharmacological activity, with relatively less exploration of synthetic pathways and identification of key genes. This study employed transcriptome sequencing and metabolomic analysis of A. tsaoko at five different developmental stages (May fruit - September fruit) to assess the accumulation patterns of terpenoid and curcuminoid compounds and to explore the key genes and transcription factors (TFs) involved in their synthesis pathways. The results showed that three genes encoding 1-deoxy-D-xylulose-5-phosphate synthase (DXS), hydroxymethylglutaryl-CoA synthase (HMGCS) and phosphomevalonate kinase (mvaK2) and TFs such as AP2-ERF, bHLH, WRKY were screened for involvement in terpenoid biosynthesis. In addition, three genes encoding trans-cinnamate 4-monooxygenase (C4H), curcumin synthase (CURS) and TFs such as MYB, bHLH, bZIP were screened for involvement in curcuminoid biosynthesis. This study provides a theoretical foundation for further research into the biosynthesis of active components in A. tsaoko, establishing a basis for in-depth investigations into the mechanisms underlying its medicinal quality formation. Additionally, it offers guidance for the utilisation of its aromatic components and natural pigments.

Kiwifruit spatiotemporal multiomics networks uncover key tissue-specific regulatory processes throughout the life cycle

Kiwifruit (Actinidia chinensis), a recently commercialized horticultural crop, is rich in various nutrient compounds. However, the regulatory networks controlling the dynamic changes in key metabolites among different tissues remain largely unknown. Here, high-resolution spatiotemporal datasets obtained by ultraperformance liquid chromatography-tandem mass spectrometry methodology and RNA-seq were employed to investigate the dynamic changes in the metabolic and transcriptional landscape of major kiwifruit tissues across different developmental stages, including from fruit skin, outer pericarp, inner pericarp, and fruit core. Kiwifruit spatiotemporal regulatory networks (KSRN) were constructed by integrating the 1,243 identified metabolites and co-expressed genes into 10 different clusters and 11 modules based on their biological functions. These networks allowed the generation of a global map for the major metabolic and transcriptional changes occurring throughout the life cycle of different kiwifruit tissues and discovery of the underlying regulatory networks. KSRN predictions confirmed previously established regulatory networks, including the spatiotemporal accumulation of anthocyanin and ascorbic acid (AsA). More importantly, the networks led to the functional characterization of three transcription factors: an A. chinensis ethylene response factor 1, which negatively controls sugar accumulation and ethylene production by perceiving the ripening signal, a basic-leucine zipper 60 (AcbZIP60) transcription factor, which is involved in the biosynthesis of AsA as part of the L-galactose pathway, and a transcription factor related to apetala 2.4 (RAP2.4), which directly activates the expression of the kiwi fruit aroma terpene synthase gene AcTPS1b. Our findings provide insights into spatiotemporal changes in kiwifruit metabolism and generate a valuable resource for the study of metabolic regulatory processes in kiwifruit as well as other fruits.

Naringenin chalcone carbon double-bond reductases mediate dihydrochalcone biosynthesis in apple leaves

Dihydrochalcones (DHCs) are flavonoids produced as a side branch of the phenylpropanoid pathway. DHCs are found at high concentrations in apples (Malus spp.) but not in pears (Pyrus spp.) or other members of the Rosaceae. Biosynthesis of DHCs in apple has been hypothesized to occur via reduction of p-coumaroyl CoA by a Malus × domestica hydroxycinnamoyl CoA double-bond reductase (MdHCDBR) followed by the action chalcone synthase to produce phloretin or via direct reduction of naringenin chalcone to phloretin via an unknown enzyme. In this study, we report that genetic downregulation of MdHCDBR does not reduce DHC concentrations in apple leaves. We used comparative transcriptome analysis to identify candidate naringenin chalcone reductases (NCRs), designated MdNCR1a-c, expressed in apple leaves but not fruit. These MdNCR1 genes form an expanded gene cluster found exclusively in apple. Transient expression of MdNCR1 genes in Nicotiana benthamiana leaves indicated they produced DHCs at high concentrations in planta. Recombinant MdNCR1 utilized naringenin chalcone to produce phloretin at high efficiency. Downregulation of NCR genes in transgenic apple reduced foliar DHC levels by 85% to 95%. Reducing DHC production redirected flux to the production of flavonol glycosides. In situ localization indicated that NCR proteins were likely found in the vacuolar membrane. Active site analysis of AlphaFold models indicated that MdNCR1a-c share identical substrate binding pockets, but the pockets differ substantially in related weakly active/inactive NCR proteins. Identifying the missing enzyme required for DHC production provides opportunities to manipulate DHC content in apple and other fruits and has other applications, e.g. in biofermentation and biopharming.

Elucidating the influence of volatile compounds on aroma profiles across peach (Prunus persica L.) cultivars and offspring exhibiting diverse flesh colors

Yellow- and white-fleshed peach fruits are favored for their diverse flesh colors. While carotenoid accumulation primarily dictates flesh color differences, the influence of volatile compounds on their aromas remains largely unexplored. Here, multiple analytical methods including odor importance assessment, hierarchical clustering, and aroma characterization analysis were employed to investigate volatile compositions and aroma characteristics of the two types of peach, as well as the offspring with identical parentage. Dihydro-β-ionone was the sole volatile exhibiting content and odor importance disparities between the two types of peach, and in descendant cultivars such volatiles encompassed theaspirane additionally. Respectively 16 and 30 important volatiles were identified in the two peach types and in the offspring cultivars, and subsequently overview of their aroma characteristics was obtained from a graphical perspective. The two peach types and offspring cultivars all revealed prevalent floral, fruity and caramel notes, whereas the higher odor activity values and especially the woody odors in the white-fleshed cultivars, as well as the differential balance degrees of the main odor directions defined the distinct aromas. By delving into the pivotal differences in odor directions and aroma profiles between the two types of peach, this research elucidates the aroma distinctions rooted in flesh color variations and paves the way for uncovering aroma formation mechanisms in fruits with varied flesh colors.

Characterization of Free and Glycosidically Bound Volatile and Non-Volatile Components of Shiikuwasha (Citrus depressa Hayata) Fruit

Shiikuwasha, a citrus fruit native to Okinawa, Japan, has various cultivation lines with distinct free volatile and non-volatile components. However, the glycosylated volatiles, which are sources of hidden aromas, remain unknown. This study aimed to characterize the chemical profiles of free and glycosidically bound volatile as well as non-volatile components in the mature fruits of six Shiikuwasha cultivation lines: Ishikunibu, Izumi kugani-like, Kaachi, Kohama, Nakamoto seedless, and Ogimi kugani. Free volatiles were analyzed using solid-phase microextraction-gas chromatography-mass spectrometry. Glycosides were collected via solid-phase extraction and hydrolyzed with β-glucosidase, and the released volatiles were measured. Additionally, the non-volatile components were determined using non-targeted proton nuclear magnetic resonance analysis. Total free and bound volatiles ranged from 457 to 8401 µg/L and from 104 to 548 µg/L, respectively, and the predominant free volatiles found were limonene, γ-terpinene, and p-cymene. Twenty volatiles were released from glycosides, including predominant 1-hexanol and benzyl alcohol, with Kaachi and Ogimi kugani showing higher concentrations. Principal component analysis (PCA) revealed that taste-related compounds like sucrose, citrate, and malate influenced line differentiation. The PCA of the combined data of free and bound volatile and non-volatile components showed flavor component variances across all lines. These findings provide valuable insights into the chemical profiles of Shiikuwasha fruits for fresh consumption and food and beverage processing.

Identification and characterization of a key LcTPS in the biosynthesis of volatile monoterpenes and sesquiterpenes in Litchi fruit

Litchi (Litchi chinensis Sonn.) has a desirable sweet taste and exotic aroma, making it popular in the markets. However, the biosynthesis of aroma volatiles in litchi fruit has rarely been investigated. In this study, the content and composition of volatile compounds were determined during litchi fruit ripening. In the mature green and mature red stages of litchi, 49 and 45 volatile compounds were detected, respectively. Monoterpenes were found to be the most abundant volatile compounds in mature red fruit, and their contents significantly increased compared to green fruit, mainly including citronellol, geraniol, myrcene, and D-limonene, which contributed to the aroma in litchi fruit. By comparing the expression profiles of the genes involved in the terpene synthesis pathway during fruit development, a terpene synthesis gene (LcTPS1-2) was identified and characterized as a major player in the synthesis of monoterpenes and sesquiterpenes. A subcellular localization analysis found LcTPS1-2 to be present in the plastid and cytoplasm. The recombinant LcTPS1-2 enzyme was able to catalyze the formation of three monoterpenes, myrcene, geraniol and citral, from geranyl pyrophosphate (GPP) and to convert farnesyl diphosphate (FPP) to a sesquiterpene, caryophyllene in vitro. Transgenic Arabidopsis thaliana plants overexpressing LcTPS1-2 exclusively released one monoterpene D-limonene, and three sesquiterpenes cis-thujopsene, (E)-β-famesene and trans-β-ionone. These results indicate that LcTPS1-2 plays an important role in the production of major volatile terpenes in litchi fruit and provides a basis for future investigations of terpenoid biosynthesis in litchi and other horticultural crops.

Comparison of Aroma and Taste Profiles of Kiwi Wine Fermented with/without Peel by Combining Intelligent Sensory, Gas Chromatography-Mass Spectrometry, and Proton Nuclear Magnetic Resonance

Kiwi wine (KW) is tipically made by fermenting juice from peeled kiwifruit, resulting in the disposal of peel and pomace as by-products. However, the peel contains various beneficial compounds, like phenols and flavonoids. Since the peel is edible and rich in these compounds, incorporating it into the fermentation process of KW presents a potential solution to minimize by-product waste. This study compared the aroma and taste profiles of KW from peeled (PKW) and unpeeled (UKW) kiwifruits by combining intelligent sensory technology, GC-MS, and 1H-NMR. Focusing on aroma profiles, 75 volatile organic compounds (VOCs) were identified in KW fermented with peel, and 73 VOCs in KW without peel, with 62 VOCs common to both. Among these compounds, rose oxide, D-citronellol, and bornylene were more abundant in UKW, while hexyl acetate, isoamyl acetate, and 2,4,5-trichlorobenzene were significantly higher in PKW. For taste profiles, E-tongue analysis revealed differences in the taste profiles of KW from the two sources. A total of 74 molecules were characterized using 1H-NMR. UKW exhibited significantly higher levels of tartrate, galactarate, N-acetylserotonin, 4-hydroxy-3-methoxymandelate, fumarate, and N-acetylglycine, along with a significantly lower level of oxypurinol compared to PKW. This study seeks to develop the theoretical understanding of the fermentation of kiwifruit with peel in sight of the utilization of the whole fruit for KW production, to increase the economic value of kiwifruit production.

In Vitro Comparative Analysis of the Effect and Structure-Based Influencing Factors of Flavonols on Lipid Accumulation

Flavonols represented by quercetin have been widely reported to have biological activities of regulating lipid metabolism. However, the differences in flavonols with different structures in lipid-lowering activity and the influencing factors remain unclear. In this study, the stability, transmembrane uptake ratio, and lipid metabolism regulation activities of 12 flavonol compounds in the 3T3-L1 cell model were systematically compared. The results showed that kaempferide had the highest cellular uptake ratio and the most potent inhibitory effect on adipogenesis at a dosing concentration of 20 μM, followed by isorhamnetin and kaempferol. They inhibited TG accumulation by more than 65% and downregulated the expression of PPARγ and SREBP1c by more than 60%. The other four aglycones, including quercetin, did not exhibit significant activity due to the structural instability in the cell culture medium. Meanwhile, five quercetin glucosides were quite stable but showed a low uptake ratio that no obvious activity was observed. Correlation analysis also showed that for 11 compounds except galangin, the activity was positively correlated with the cellular uptake ratio (p < 0.05, r = 0.6349). These findings may provide a valuable idea and insight for exploring the structure-based activity of flavonoids at the cellular level.

The kiwifruit amyloplast proteome (kfALP): a resource to better understand the mechanisms underlying amyloplast biogenesis and differentiation

The biogenesis and differentiation (B&D) of amyloplasts contributes to fruit flavor and color. Here, remodeling of starch granules, thylakoids and plastoglobules was observed during development and ripening in two kiwifruit (Actinidia spp.) cultivars - yellow-fleshed 'Hort16A' and green-fleshed 'Hayward'. A protocol was developed to purify starch-containing plastids with a high degree of intactness, and amyloplast B&D was studied using label-free-based quantitative proteomic analyses in both cultivars. Over 3000 amyloplast-localized proteins were identified, of which >98% were quantified and defined as the kfALP (kiwifruit amyloplast proteome). The kfALP data were validated by Tandem-Mass-Tag (TMT) labeled proteomics in 'Hort16A'. Analysis of the proteomic data across development and ripening revealed: 1) a conserved increase in the abundance of proteins participating in starch synthesis/degradation during both amyloplast B&D; 2) up-regulation of proteins for chlorophyll degradation and of plastoglobule-localized proteins associated with chloroplast breakdown and plastoglobule formation during amyloplast differentiation; 3) constitutive expression of proteins involved in ATP supply and protein import during amyloplast B&D. Interestingly, two different pathways of amyloplast B&D were observed in the two cultivars. In 'Hayward', significant increases in abundance of photosynthetic- and tetrapyrrole metabolism-related proteins were observed, but the opposite trend was observed in 'Hort16A'. In conclusion, analysis of the kfALP provides new insights into the potential mechanisms underlying amyloplast B&D with relevance to key fruit quality traits in contrasting kiwifruit cultivars.

Integrated Transcriptomic and Proteomic Analysis Identifies Novel Regulatory Genes Associated with Plant Growth Regulator-Induced Astringency in Grape Berries

Astringency influences the sensory characteristics and flavor quality of table grapes. We tested the astringency sensory attributes of berries and investigated the concentration of flavan-3-ols/proanthocyanidins (PAs) in skins after the application of the plant growth regulators CPPU and GA3 to the flowers and young berries of the "Summer Black" grape. Our results showed that CPPU and GA3 applications increase sensory astringency perception scores and flavan-3-ol/proanthocyanidin concentrations. Using integrated transcriptomic and proteomic analysis, differentially expressed transcripts and proteins associated with growth regulator treatment were identified, including those for flavonoid biosynthesis that contribute to the changes in sensory astringency levels. Transient overexpression of candidate astringency-related regulatory genes in grape leaves revealed that VvWRKY71, in combination with VvMYBPA1 and VvMYC1, could promote the biosynthesis of proanthocyanidins, while overexpression of VvNAC83 reduced the accumulation of proanthocyanidins. However, in transient promoter studies in Nicotiana benthamiana, VvWRKY71 repressed the promoter of VvMYBPA2, while VvNAC83 had no significant effect on the promoter activity of four PA-related genes, and VvMYBPA1 was shown to activate its own promoter. This study provides new insights into the molecular mechanisms of sensory astringency formation induced by plant growth regulators in grape berries.

Terpene Synthase Gene Family in Chinese Chestnut (Castanea mollissima BL.) Harbors Two Sesquiterpene Synthase Genes Implicated in Defense against Gall Wasp Dryocosmus kuriphilus

Chinese chestnut (Castanea mollissima BL.) is a well-known fruit tree that has been cultivated in East Asia for millennia. Leaves and buds of the plant can become seriously infested by the gall wasp Dryocosmus kuriphilus (GWDK), which results in gall formation and associated significant losses in fruit production. Herbivore-induced terpenes have been reported to play an important role in plant-herbivory interactions, and in this study, we show that upon herbivory by GWDK, four terpene-related compounds were significantly induced, while the concentrations of these four compounds in intact buds were relatively low. Among these compounds, (E)-nerolidol and (E, E)-α-farnesene have frequently been reported to be involved in plant herbivory defenses, which suggests direct and/or indirect functions in chestnut GWDK defenses. Candidate terpene synthase (TPS) genes that may account for (E)-nerolidol and (E, E)-α-farnesene terpene biosynthesis were characterized by transcriptomics and phylogenetic approaches, which revealed altered transcript levels for two TPSs: CmAFS, a TPS-g subfamily member, and CmNES/AFS, a TPS-b clade member. Both genes were dramatically upregulated in gene expression upon GWDK infestation. Furthermore, Agrobacterium tumefaciens-mediated transient overexpression in Nicotiana benthamiana showed that CmAFS catalyzed the formation of (E, E)-α-farnesene, while CmNES/AFS showed dual (E)-nerolidol and (E, E)-α-farnesene synthase activity. Biochemical assays of the recombinant CmAFS and CmNES/AFS proteins confirmed their catalytic activity in vitro, and the enzymatic products were consistent with two of the major volatile compounds released upon GWDK-infested chestnut buds. Subcellular localization demonstrated that CmAFS and CmNES/AFS were both localized in the cytoplasm, the primary compartment for sesquiterpene synthesis. In summary, we show that two novel sesquiterpene synthase genes CmAFS and CmNES/AFS are inducible by herbivory and can account for the elevated accumulation of (E, E)-α-farnesene and (E)-nerolidol upon GWDK infestation and may be implicated in chestnut defense against GWDK herbivores.

Metabolic engineering of a 1,8-cineole synthase enhances aphid repellence and increases trichome density in transgenic tobacco (Nicotiana tabacum L.)

BACKGROUND

The green peach aphid (Myzus persicae Sulzer) is a harmful agricultural pest that causes severe crop damage by directly feeding or indirectly vectoring viruses. 1,8-cineole synthase (CINS) is a multiproduct enzyme that synthesizes monoterpenes, with 1,8-cineole dominating the volatile organic compound profile. However, the relationship between aphid preference and CINS remains elusive.

RESULTS

Here, we present evidence that SoCINS, a protein from garden sage (Salvia officinalis), enhanced aphid repellence and increased trichome density in transgenic tobacco. Our results demonstrated that overexpression of SoCINS (SoCINS-OE) led to the emission of 1,8-cineole at a level of up to 181.5 ng per g fresh leaf. Subcellular localization assay showed that SoCINS localized to chloroplasts. A Y-tube olfactometer assay and free-choice assays revealed that SoCINS-OE plants had a repellent effect on aphids, without incurring developmental or fecundity-related penalties. Intriguingly, the SoCINS-OE plants displayed an altered trichome morphology, showing increases in trichome density and in the relative proportion of glandular trichomes, as well as enlarged glandular cells. We also found that SoCINS-OE plants had significantly higher jasmonic acid (JA) levels than wild-type plants. Furthermore, application of 1,8-cineole elicited increased JA content and trichome density.

CONCLUSION

Our results demonstrate that SoCINS-OE plants have a repellent effect on aphids, and suggest an apparent link between 1,8-cineole, JA and trichome density. This study presents a viable and sustainable approach for aphid management by engineering the expression of 1,8-cineole synthase gene in plants, and underscores the potential usefulness of monoterpene synthase for pest control. © 2023 Society of Chemical Industry.

The terpene synthase (TPS) gene family in kiwifruit shows high functional redundancy and a subset of TPS likely fulfil overlapping functions in fruit flavour, floral bouquet and defence

Volatile terpenes are important compounds that influence fruit flavour and aroma of kiwifruit. Terpenes in plants also impact on the floral bouquet and defence against pests and pathogens in leaves and fruit. To better understand the overlapping roles that terpenes may fulfil in plants, a systematic gene, chemical and biochemical analysis of terpenes and terpene synthases (TPS) was undertaken in Red5 kiwifruit (Actinidia spp.). Analysis of the Red5 genome shows it contains only 22 TPS gene models, of which fifteen encode full-length TPS. Thirteen TPS can account for the major terpene volatiles produced in different tissues of Red5 kiwifruit and in response to different stimuli. The small Red5 TPS family displays surprisingly high functional redundancy with five TPS producing linalool/nerolidol. Treatment of leaves and fruit with methyl jasmonate enhanced expression of a subset of defence-related TPS genes and stimulated the release of terpenes. Six TPS genes were induced upon herbivory of leaves by the economically important insect pest Ctenopseustis obliquana (brown-headed leaf roller) and emission, but not accumulation, of (E)- and (Z)-nerolidol was strongly linked to herbivory. Our results provide a framework to understand the overlapping biological and ecological roles of terpenes in Actinidia and other horticultural crops.

Identification of QTL controlling volatile terpene contents in tea plant (Camellia sinensis) using a high-aroma 'Huangdan' x 'Jinxuan' F1 population

Aroma is an important factor affecting the character and quality of tea. The improvement of aroma trait is a crucial research direction of tea plant breeding. Volatile terpenes, as the major contributors to the floral odors of tea products, also play critical roles in the defense responses of plants to multiple stresses. However, previous studies have largely focused on the aroma formation during the manufacture of tea or the comparison of raw tea samples. The mechanisms causing different aroma profiles between tea cultivars have remained underexplored. In the current study, a high-density genetic linkage map of tea plant was constructed based on an F₁ population of 'Huangdan' × 'Jinxuan' using genotyping by sequencing. This linkage map covered 1754.57 cM and contained 15 linkage groups with a low inter-marker distance of 0.47 cM. A total of 42 QTLs associated with eight monoterpene contents and 12 QTLs associated with four sesquiterpenes contents were identified with the average PVE of 12.6% and 11.7% respectively. Furthermore, six candidate genes related to volatile terpene contents were found in QTL cluster on chromosome 5 by RNA-seq analysis. This work will enrich our understanding of the molecular mechanism of volatile terpene biosynthesis and provide a theoretical basis for tea plant breeding programs for aroma quality improvement.

Alcohol acyl transferase genes at a high-flavor intensity locus contribute to ester biosynthesis in kiwifruit

Volatile esters are key compounds contributing to flavor intensity in commonly consumed fruits including apple (Malus domestica), strawberry (Fragaria spp.), and banana (Musa sapientum). In kiwifruit (Actinidia spp.), ethyl butanoate and other esters have been proposed to contribute fruity, sweet notes to commercial cultivars. Here, we investigated the genetic basis for ester production in Actinidia in an A. chinensis mapping population (AcMPO). A major quantitative trait loci for the production of multiple esters was identified at the high-flavor intensity (HiFI) locus on chromosome 20. This locus co-located with eight tandemly arrayed alcohol acyl transferase genes in the Red5 genome that were expressed in a ripening-specific fashion that corresponded with ester production. Biochemical characterization suggested two genes at the HiFI locus, alcohol acyl transferase 16-b/c (AT16-MPb/c), probably contributed most to the production of ethyl butanoate. A third gene, AT16-MPa, probably contributed more to hexyl butanoate and butyl hexanoate production, two esters that segregated in AcMPO. Sensory analysis of AcMPO indicated that fruit from segregating lines with high ester concentrations were more commonly described as being "fruity" as opposed to "beany". The downregulation of AT16-MPa-c by RNAi reduced ester production in ripe "Hort16A" fruit by >90%. Gas chromatography-olfactometry indicated the loss of the major "fruity" notes contributed by ethyl butanoate. A comparison of unimproved Actinidia germplasm with those of commercial cultivars indicated that the selection of fruit with high concentrations of alkyl esters (but not green note aldehydes) was probably an important selection trait in kiwifruit cultivation. Understanding ester production at the HiFI locus is a critical step toward maintaining and improving flavor intensity in kiwifruit.

Preparation, Shelf, and Eating Quality of Ready-to-Eat “Guichang” Kiwifruit: Regulation by Ethylene and 1-MCP

The acceptance of kiwifruit by consumers is significantly affected by its slow ripening and susceptibility to deterioration. Ready-to-eat “Guichang” kiwifruit and its preparation technology were studied by the regulation of ethylene and 1-MCP. Harvested kiwifruits were treated with 100–2000 μl L⁻¹ethylene for 36 h (20°C) and then treatment with 0–0.5 μl L⁻¹ 1-MCP. The results showed that the preservation effect of 0.5 μl L⁻¹ 1-MCP is inefficient when the soluble solid content of kiwifruit exceeded 15%. The ethylene-treated fruits reached an “edible window” after 24 h, but a higher concentration of ethylene would not further improve ripening efficiency, while the optimal ethylene concentration was 250 μl L⁻¹. Moreover, after 250 μl L⁻¹ ethylene treatment, 0.5 μl L⁻¹ 1-MCP would effectively prolong the “edible window” of fruits by approximately 19 days. The volatile component variety and ester content of 0.5 μl L⁻¹ 1-MCP-treated fruits were not different from those of the CK group. Principal component analysis and hierarchical cluster analysis indicated that the eating quality of fruits treated with 0.5 μl L⁻¹ 1-MCP was similar to that of fruits treated with ethylene. Consequently, ready-to-eat “Guichang” kiwifruit preparation includes ripening with 250 μl L⁻¹ (20°C, 36 h) ethylene without exceeding the 1-MCP threshold and then treated with 0.5 μl L⁻¹ 1-MCP (20°C, 24 h). This study highlights the first development of a facile and low-cost preparation technology for ready-to-eat “Guichang” kiwifruit, which could reduce the time for harvested kiwifruit to reach the “edible window” and prolong the “edible window” of edible kiwifruit.

Quantitative trait locus mapping of fruit aroma compounds in cucumber (Cucumber sativus L.) based on a recombinant inbred line population

The fresh and unique flavor of cucumber fruits, mainly composed of aldehydes and alcohols, is one of its most important fruit qualities. However, little is known about the genetic basis of aroma compounds in cucumber fruit and the related quantitative trait loci (QTLs). In this study, genomic screening of QTLs underlying aroma compounds was performed based on the genetic linkage map constructed using 1301 single-nucleotide polymorphism (SNP) markers from genotyping-by-sequencing of a recombinant inbred line (RIL) population developed from Q16 × Q24. Significant genetic variations of aroma compounds in the RIL population were observed, and a total of 28 QTLs were screened. A major QTL (qol8-2.1) related to (E,Z)-2,6-nonadien-1-ol was detected with a markedly high LOD score (10.97 in 2020 and 3.56 in 2019) between mk190 and mk204 on chromosome 2. Genome scans identified a cluster of nine lipoxygenase genes in this region. A significant positive correlation was detected between CsaV3_2G005360 (CsLOX08) and (E,Z)-2,6-nonadien-1-ol, and five amino acid variations were detected between the CsLOX08 protein sequences of the two parental lines. Based on the genome variation of CsLOX08, we developed an InDel marker. Genotyping of InDel markers was consistent with the content of (E,Z)-2,6-nonadien-1-ol in RILs, which were also verified in nine cucumber inbred lines. The results will give breeders guidance for obtaining better flavor in cucumber.

TPS-b family genes involved in signature aroma terpenes emission in ripe kiwifruit

Aroma is a critical factor influencing consumer acceptability of ripe fruit. When fruit are eaten, the aroma travels retronasally from the mouth into the olfactory receptors located in the nose after exhaling. In kiwifruit (Actinidia spp.), terpene volatiles such as α-terpinolene and 1,8-cineole have been shown to contribute to the characteristic aroma of ripe fruit. Notably, 1,8-cineole contributes a key floral/eucalyptus note to the aroma of ripe A. chinensis 'Hort16A' kiwifruit, based on sensory descriptive and discriminant analysis. Emission of α-terpinolene and 1,8-cineole in kiwifruit is induced by ethylene, and production peaks when fruit are at eating ripeness. Two monoterpene synthase TPS-b family genes have been isolated from the fruit of A. arguta and A. chinensis that produce α-terpinolene and 1,8-cineole, respectively. Here we discuss terpene volatiles with respect to fruit aroma and consumer sensory evaluation, analyze the gene structure and conserved motifs of TPS-b genes in published kiwifruit genomes and then construct a transcriptional regulatory network based on Actinidia TPS-b. These data provide further insights into the potential molecular mechanisms underlying signature monoterpene synthesis to improve flavor in kiwifruit.

Comparative analysis of volatile and carotenoid metabolites and mineral elements in the flesh of 17 kiwifruit

Kiwifruit contains abundant nutritive compounds and is highly favored by the consumers worldwide. Therefore, detailed metabolic profiling is important to provide theoretic basis for the improvement of kiwifruit quality. In this study, the levels of volatiles, carotenoids, and mineral elements in the flesh of 17 kiwifruit accessions were evaluated. Acids and esters were the main volatiles in kiwifruit. During these 17 kiwifruit accessions, "Chenhong," three "Jinyan," and two "Guichang" germplasms were specifically rich in aromatic esters, which might be associated with their special taste. The main carotenoids were lutein, β-carotene, and zeaxanthin, and their levels were also genotype specific, with the green-fleshed "Guichang" having the highest level of carotenoids, and red-fleshed "Fuhong" and "Chenhong" being rich in zeaxanthin. Partial correlation analysis showed that the contents of some mineral elements were significantly correlated with those of specific volatiles and carotenoids, indicating the impacts of mineral elements on the accumulation of volatiles and carotenoids in the kiwifruit flesh. These results indicated that the contents of carotenoids and volatiles seemed to be affected by mineral elements and also provided a new potential method for improving fruit flavor quality in production.

Identification of the Volatile Compounds and Observation of the Glandular Trichomes in Opisthopappus taihangensis and Four Species of Chrysanthemum

Opisthopappus taihangensis (Ling) Shih, a wild relative germplasm of chrysanthemum, releases a completely different fragrance from chrysanthemum species. We aimed to identify the volatile compounds of the leaves of O. taihangensis and four other Chrysanthemum species using headspace solid-phase micro-extraction combined with gas chromatography-mass spectrometry (HS-SPME-GC/MS). In total, 70 compounds were detected, and terpenoids accounted for the largest percentage in these five species. Many specific compounds were only emitted from O. taihangensis and not from the other four species. In particular, 1,8-cineole could be responsible for the special leaf fragrance of O. taihangensis as it accounted for the largest proportion of the compounds in O. taihangensis but a small or no proportion at all in other species. The glandular trichomes (GTs) in the leaves are the main organs responsible for the emission of volatiles. To explore the relationship between the emissions and the density of the GTs on the leaf epidermis, the shape and density of the GTs were observed and calculated, respectively. The results showed that the trichomes have two shapes in these leaves: T-shaped non-glandular trichomes and capitate trichomes. Histochemical staining analyses indicated that terpenoids are mainly emitted from capitate glandular trichomes. Correlation analysis showed that the volatile amount of terpenoids is highly related to the density of capitate trichomes. In O. taihangensis, the terpenoids content and density of capitate trichomes are the highest. We identified the diversity of leaf volatiles from O. taihangensis and four other Chrysanthemum species and found a possible relationship between the content of volatile compounds and the density of capitate trichomes, which explained the cause of the fragrance of O. taihangensis leaves.