Branched-chain and aromatic amino acid catabolism into aroma volatiles in Cucumis melo L. fruit

Dynamic changes and the effects of key procedures on the characteristic aroma compounds of Lu'an Guapian green tea during the manufacturing process

As a kind of green tea with unique multiple baking processes, the flavor code of Lu'an Guapian (LAGP) has recently been revealed. To improve and stabilize the quality of LAGP, further insight into the dynamic changes in odorants during the whole processing is required. In this study, 50 odorants were identified in processing tea leaves, 14 of which were selected for absolute quantification to profile the effect of processes. The results showed that spreading is crucial for key aroma generation and accumulation, while these odorants undergo significant changes at the deep baking stage. By adjusting the conditions of the spreading and deep baking, it was found that low-temperature (4 °C) spreading for 6 h and low-temperature with long-time baking (final leaf temperature: 102 °C, 45 min) could improve the overall aroma quality. These results provide a new direction for enhancing the quality of LAGP green tea.

Combining multivariate statistical analysis to characterize changes in amino acids and volatiles during growth of Lou onion pseudostems

BACKGROUND

The main edible part of the Lou onion is the pseudostem, which is highly valued for its distinctive flavour. However, harvesting decisions for the pseudostem are often based on size and market price, with little consideration given to flavour. By clarifying the growth of flavour in pseudostems, farmers and consumers may benefit from evidence-based insights that help optimize harvesting time and maximize flavour quality.

RESULTS

This study employed amino acid analysis and gas chromatography-ion migration spectroscopy (GC-IMS) to elucidate the compounds of the pseudostem across different growth phases, and 17 amino acids and 61 volatile substances. Subsequently, analysis revealed that 18 compounds, including arginine (Arg), aspartic acid (Asp), glutamic acid (Glu), valine (Val), (E)-2-nonenal, decanal, 2,4-nonadienal, 2-octenal, (Z)-4-decenal, 2,4-decadienal benzeneacetaldehyde, linalool, eugenol, (Z)-6-nonen-1-ol, methyl anthranilate, 2-acetylpyridine, 3-sec-butyl-2-methoxypyrazine, and 2,6-dichlorophenol, were the key compounds in determining the flavour characteristics of the pseudostems, as assessed by taste activity value and relative odour activity value calculations. In addition, correlation analysis, focusing on five amino acids and 38 volatile compounds with variable importance for predictive components scores of >1, identified anisaldehyde, eugenol, (Z)-6-nonen-1-ol, 2,4-decadienal, 3-sec-butyl-2-methoxypyrazine, Arg, Asp, and Val as the key differentiators and contributors to the pseudostems flavour profile.

CONCLUSION

During the rapid growth of Lou onions just before the emergence of flower stems, the pseudostem exhibited the most prominent flavour, making this stage most suitable for harvesting compared to the regreening growth stage and the rapid growth period of the aerial bulbs. © 2024 Society of Chemical Industry.

Identification of key aroma compounds of faba beans (Vicia faba) and their development during germination - a SENSOMICS approach

Faba beans are a promising source of valuable plant protein. However, their aroma impression is often a hindrance for the use in a broad range of food products. To develop mitigation strategies, a deeper insight into the faba bean aroma is required. Therefore, for the first time, the SENSOMICS concept was applied. First, 52 aroma active compounds in raw and malted faba beans were identified and semi-quantitatively preselected by aroma extract dilution analysis. Afterwards, the aroma compounds were quantified, odor activity values were calculated, and the 17 prominent odors were selected and used in the reconstitution of the faba bean aroma. Seven statistically significant key aroma compounds 3-methylbutanoic acid, (E)-non-2-enal, hexanal, methional, 3-methylbutanal, sotolon, and 2-methylbutan-1-ol were identified in omission experiments. Finally, their development upon malting was studied. To conclude, by knowing the key aroma compounds, specific mitigation strategies can be developed, which facilitates the broader use of faba beans.

Genetic regulation of volatile production in two melon introgression line collections with contrasting ripening behavior

The importance of melon aroma in determining fruit quality has been highlighted in recent years. The fruit volatile profile is influenced by the type of fruit ripening. Non-climacteric fruits contain predominantly aldehydes, while climacteric fruits mainly produce esters. Several genes have been described to participate in volatile organic compounds (VOCs) biosynthesis pathways, but knowledge in this area is still incomplete. In this work we analysed the volatile profile of two reciprocal Introgression Line (IL) collections generated from a cross between 'Piel de Sapo' (PS) and 'Védrantais' (VED) melons, differing in their aroma profile and ripening behaviour. SPME GC-MS was performed to identify genes responsible for VOCs formation. More than 1000 QTLs for many volatiles were detected taken together both populations. Introgressions on chromosomes 3, 5, 6, 7 and 8 modified ester-aldehyde balance and were correlated to ripening changes in both genetic backgrounds. Some previously identified QTLs for fruit ripening might be involved in these phenotypes, such as ETHQV8.1 on chromosome 8 and ETHQV6.3 on chromosome 6. PS alleles on chromosomes 2, 6, 10 and 11 were found to increase ester content when introgressed in VED melons. Terpenes showed to be affected by several genomic regions not related to ripening. In addition, several candidate genes have been hypothesized to be responsible for some of the QTLs detected. The analysis of volatile compounds in two reciprocal IL collections has increased our understanding of the relationship between ripening and aroma and offers valuable plant material to improve food quality in melon breeding programs.

The effects of different rootstocks on aroma components, activities and genes expression of aroma-related enzymes in oriental melon fruit

Grafting is widely applied in the cultivation of melon. In this study, 'Qinmi No.1' (Cucumis melo L.(QG)) and 'Ribenxuesong' (Cucurbita maxima Duch. (RG)) were used as rootstocks for 'Qingxin Yangjiaocui' (Cucumis melo L.). The results showed that grafting with muskmelon rootstocks had no significant effect on fruit aroma, but grafting with pumpkin rootstocks significantly reduced the odor intensity and odor preference scores of melon fruits. Compared with the fruits from self-grafted plants (SG), four new aromatic volatiles with a sweet smell were detected, the alcohol dehydrogenase (ADH) activity was significantly decreased at 30 DAP, but unaffected at 42 DAP in QG fruits. There was no difference for alcohol acetyltransferase (AAT) activity between QG and SG fruits. The expression level of CmADH2 was significantly higher at 30 DAP and 42 DAP, but CmAAT2 was significantly lower at 42 DAP in QG fruits compared with SG fruits. In RG fruits, the main aroma compounds including butanoic acid ethyl ester, 2-methyl-2-butene-1-al, and 2-methylheptan-1-al were absent, while the volatile compounds with unpleasant odor characteristics including trans, cis-2,6-nonadien-1-ol, (E,E)-2,4-heptadienal, octanoic acid, and styrene were detected. Compared with SG fruits, 1-nonanol and 1-heptanol with green odor characteristics were significantly increased, but eucalyptol and farnesene with fruity aroma characteristics were significantly decreased in RG fruits. The ADH activity of RG fruits was significantly lower than that of SG fruits at 30 DAP and the AAT activity was significantly lower than that of SG fruits at 42 DAP. In addition, the expression levels of CmADH and CmAAT homologs in RG fruits were significantly lower than those in SG or QG fruits. These results show that grafting with pumpkin rootstocks affected the main aroma components, reduced ADH and AAT activities, and down-regulated the expression levels of CmADHs and CmAATs in the melon fruits. This study reveals the mechanism of different rootstocks on melon fruit aroma quality, and lays a theoretical foundation for the selection of rootstocks in melon production. Future studies using overexpression or CRISPR/CAS system to obtain stable transgenic lines of genes encoding key aromatic volatiles, would be promising to effectively improve the flavor quality of melon.

Dynamic nitrogen reallocation in rice plants upon insect herbivory by a generalist lepidopteran pest Spodoptera litura (Fabricius)

Nitrogen (N) is a limited nutrient for both plants and herbivores. How plants reallocate N upon herbivore attack is vital for plant tolerance to herbivores. Here we investigated N reallocation in rice during a 2-day herbivore attack by a generalist herbivore Spodoptera litura and 2 days after herbivore removal. Labeled 15 N was translocated during insect attack from feeding-damaged leaves to roots, particularly to young roots. The amounts of chlorophyll and Rubisco were significantly reduced in the attacked leaves. Both free amino acids and nitrate accumulated in the damaged leaves and young roots, while ammonium content was decreased. Activities of nitrate reductase and glutamine synthetase were enhanced in feeding-damaged leaves but inhibited in young roots. The expression of amino acid transporters OsAAP6, OsAAT15, and jasmonate-responsive genes OsAOS, OsMAPK3, OsMAPK6 was induced in the damaged leaves. However, 2 days after herbivore removal, N uptake was increased and herbivory-induced 15 N transfer to roots was partially reverted back to the damaged leaves, resulting in N levels in the previously damaged leaves were even higher than that in control leaves. Collectively, our results indicate a dynamic N reallocation in rice responses to insect herbivory.

Research progress on branched-chain amino acid aminotransferases

Branched-chain amino acid aminotransferases, widely present in natural organisms, catalyze bidirectional amino transfer between branched-chain amino acids and branched-chain α-ketoacids in cells. Branched-chain amino acid aminotransferases play an important role in the metabolism of branched-chain amino acids. In this paper, the interspecific evolution and biological characteristics of branched-chain amino acid aminotransferases are introduced, the related research of branched-chain amino acid aminotransferases in animals, plants, microorganisms and humans is summarized and the molecular mechanism of branched-chain amino acid aminotransferase is analyzed. It has been found that branched-chain amino acid metabolism disorders are closely related to various diseases in humans and animals and plants, such as diabetes, cardiovascular diseases, brain diseases, neurological diseases and cancer. In particular, branched-chain amino acid aminotransferases play an important role in the development of various tumors. Branched-chain amino acid aminotransferases have been used as potential targets for various cancers. This article reviews the research on branched-chain amino acid aminotransferases, aiming to provide a reference for clinical research on targeted therapy for various diseases and different cancers.

Regulation of climacteric fruit ripening in melon: recent advances and future challenges

Fruit ripening is a complex and highly regulated process where tomato and strawberry have been the model species classically used for studying climacteric and non-climacteric fleshy fruit ripening types, respectively. Melon has emerged as an alternative ripening model because climacteric and non-climacteric cultivars exist, which makes it possible to dissect the regulation of ripening using a genetic approach. Several quantitative trait loci that regulate climacteric fruit ripening have been identified to date, and their combination in both climacteric and non-climacteric genetic backgrounds resulted in lines with different ripening behaviors, demonstrating that the climacteric intensity can be genetically modulated. This review discusses our current knowledge of the physiological changes observed during melon climacteric fruit ripening such as ethylene production, fruit abscission, chlorophyll degradation, firmness, and aroma, as well as their complex genetic control. From pioneer experiments in which ethylene biosynthesis was silenced, to the recent genetic edition of ripening regulators, current data suggest that the climacteric response is determined by the interaction of several loci under quantitative inheritance. The exploitation of the rich genetic diversity of melon will enable the discovery of additional genes involved in the regulation of the climacteric response, ultimately leading to breeding aromatic melon fruits with extended shelf life.

Effect of Peony (Paeonia ostii) Seed Meal Supplement on Enzyme Activities and Flavor Compounds of Chinese Traditional Soybean Paste during Fermentation

Peony seed meal (PSM) is the by-product obtained from peony seeds after oil extraction. In this study, PSM was incorporated into traditional koji-making, and its impacts on koji enzyme activities and flavor compounds in final products were investigated. In the process of koji fermentation, the optimal addition ratio of PSM to soybean was determined as 7:3. Under this ratio, the maximum enzyme activities of neutral protease, amylase, and glucoamylase were 1177.85, 686.58, and 1564.36 U/g, respectively, and the koji obtained was subjected to maturation. During post-fermentation, changes in the fermentation characteristics of the paste samples were monitored, and it was found that compared to the soybean paste without PSM, the enzyme activities maintained at a relatively good level. The PSM soybean paste contained a total of 80 flavor compounds and 11 key flavor compounds (OAV ≥ 1), including ethyl isovalerate, isovaleric acid, hexanal, phenylacetaldehyde, 3-Methyl-1-butanol 4-heptanone, 2-pentylfuran, methanethiol ester caproate, isoamyl acetate, 3-methyl-4-heptanone, and isovaleraldehyde. These findings could be used to improve the quality of traditional fermented paste, enrich its flavor, and simultaneously promote PSM as a valuable resource for fermented foods.

Advances in detecting fruit aroma compounds by combining chromatography and spectrometry

Fruit aroma is produced by volatile compounds, which can significantly enhance fruit flavor. These compounds are highly complex and have remarkable pharmacological effects. The synthesis, concentration, type, and quantity of fruit aroma substances are affected by various factors, both abiotic and biotic. To fully understand the aroma substances of various fruits and their influencing factors, detection technology can be used. Many methods exist for detecting aroma compounds, and approaches combining multiple instruments are widely used. This review describes and compares each detection technology and discusses the potential use of combined technologies to provide a comprehensive understanding of fruit aroma compounds and the factors influencing their synthesis. These results can inform the development and utilization of fruit aroma substances. © 2023 Society of Chemical Industry.

Natural variations in the Sl-AKR9 aldo/keto reductase gene impact fruit flavor volatile and sugar contents

The unique flavors of different fruits depend upon complex blends of soluble sugars, organic acids, and volatile organic compounds. 2-Phenylethanol and phenylacetaldehyde are major contributors to flavor in many foods, including tomato. In the tomato fruit, glucose, and fructose are the chemicals that most positively contribute to human flavor preferences. We identified a gene encoding a tomato aldo/keto reductase, Sl-AKR9, that is associated with phenylacetaldehyde and 2-phenylethanol contents in fruits. Two distinct haplotypes were identified; one encodes a chloroplast-targeted protein while the other encodes a transit peptide-less protein that accumulates in the cytoplasm. Sl-AKR9 effectively catalyzes reduction of phenylacetaldehyde to 2-phenylethanol. The enzyme can also metabolize sugar-derived reactive carbonyls, including glyceraldehyde and methylglyoxal. CRISPR-Cas9-induced loss-of-function mutations in Sl-AKR9 significantly increased phenylacetaldehyde and lowered 2-phenylethanol content in ripe fruit. Reduced fruit weight and increased soluble solids, glucose, and fructose contents were observed in the loss-of-function fruits. These results reveal a previously unidentified mechanism affecting two flavor-associated phenylalanine-derived volatile organic compounds, sugar content, and fruit weight. Modern varieties of tomato almost universally contain the haplotype associated with larger fruit, lower sugar content, and lower phenylacetaldehyde and 2-phenylethanol, likely leading to flavor deterioration in modern varieties.

The Dynamic Change in Aromatic Compounds and Their Relationship with CsAAAT Genes during the Post-Harvest Process of Oolong Tea

Formed by L-phenylalanine (L-phe) ammonia under the action of aromatic amino acid aminotransferases (AAATs), volatile benzenoids (VBs) and volatile phenylpropanoids (VPs) are essential aromatic components in oolong tea (Camellia sinensis). However, the key VB/VP components responsible for the aromatic quality of oolong tea need to be revealed, and the formation mechanism of VBs/VPs based on AAAT branches during the post-harvest process of oolong tea remains unclear. Therefore, in this study, raw oolong tea and manufacturing samples were used as the test materials, and targeted metabolomics combined with transcriptome analysis was also conducted. The results showed that thirteen types of VBs/VPs were identified, including nine types of VPs and four types of VBs. Based on the OAV calculation, in raw oolong tea, 2-hydroxy benzoic acid methyl ester and phenylethyl alcohol were identified as key components of the aromatic quality of oolong tea. As for the results from the selection of related genes, firstly, a total of sixteen candidate CsAAAT genes were selected and divided into two sub-families (CsAAAT1 and CsAAAT2); then, six key CsAAAT genes closely related to VB/VP formation were screened. The upregulation of the expression level of CsAAAT2-type genes may respond to light stress during solar-withering as well as the mechanical force of turnover. This study can help to understand the formation mechanism of aromatic compounds during oolong tea processing and provide a theoretical reference for future research on the formation of naturally floral and fruity aromas in oolong tea.

Transcriptome and UPLC-MS/MS reveal mechanisms of amino acid biosynthesis in sweet orange 'Newhall' after different rootstocks grafting

Sweet orange 'Newhall' (C. sinensis) is a popular fruit in high demand all over the world. Its peel and pulp are rich in a variety of nutrients and are widely used in catering, medicine, food and other industries. Grafting is commonly practiced in citrus production. Different rootstock types directly affect the fruit quality and nutritional flavor of citrus. However, the studies on citrus metabolites by grafting with different rootstocks are very limited, especially for amino acids (AAs). The preliminary test showed that there were significant differences in total amino acid content of two rootstocks (Poncirus trifoliata (CT) and C. junos Siebold ex Tanaka (CJ)) after grafting, and total amino acid content in the peel was higher than flesh. However, the molecular mechanism affecting amino acid differential accumulation remains unclear. Therefore, this study selected peel as the experimental material to reveal the amino acid components and differential accumulation mechanism of sweet orange 'Newhall' grafted with different rootstocks through combined transcriptome and metabolome analysis. Metabolome analysis identified 110 amino acids (AAs) and their derivatives in sweet orange 'Newhall' peels, with L-valine being the most abundant. L-asparagine was observed to be affected by both developmental periods and rootstock grafting. Weighted gene co-expression network analysis (WGCNA) combined with Redundancy Analysis (RDA) revealed eight hub structural genes and 41 transcription factors (TFs) that significantly influenced amino acid biosynthesis in sweet orange 'Newhall' peels. Our findings further highlight the significance of rootstock selection in enhancing the nutritional value of citrus fruits and might contribute to the development of functional citrus foods and nutritional amino acid supplements.

Integrated transcriptomic and metabolomic analysis of chili pepper fruits provides new insight into the regulation of the branched chain esters and capsaicin biosynthesis

Chili pepper (Capsicum spp.) is one of the world's most popular vegetables and spices. Aroma is an important quality indicator of pepper, but the nature of the related volatiles is still not clear. In this study, we investigated the fruit of two pepper varieties, one with strong fruity aroma 'CC' Capsicum chinense and one without 'TJ' Capsicum annuum at four different developmental stages using transcriptomic and metabolomic analysis. The results showed that the content of green leaf volatiles (GLVs) was higher in TJ than in CC and was higher in the young fruit stage in both varieties. GLVs content was positively correlated with the expression of 13-LOX1, 2, 5 and HPL. But the levels of branched-chain (BC) esters and capsaicin were higher in CC, and were positively correlated with the expression of IMPS4 and DADH1. Our findings shed light on the molecular mechanism of aroma biosynthesis in pepper and provide a theoretical basis for the molecular breeding of high-quality pepper fruits.

Arbuscular mycorrhizal fungus changes alfalfa (Medicago sativa) metabolites in response to leaf spot (Phoma medicaginis) infection, with subsequent effects on pea aphid (Acyrthosiphon pisum) behavior

Plant disease occurs simultaneously with insect attack. Arbuscular mycorrhizal fungi (AMF) modify plant biotic stress response. Arbuscular mycorrhizal fungi and pathogens may modify plant volatile organic compound (VOC) production and insect behavior. Nevertheless, such effects are rarely studied, particularly for mesocosms where component organisms interact with each other. Plant-mediated effects of leaf pathogen (Phoma medicaginis) infection on aphid (Acyrthosiphon pisum) infestation, and role of AMF (Rhizophagus intraradices) in modifying these interactions were elucidated in a glasshouse experiment. We evaluated alfalfa disease occurrence, photosynthesis, phytohormones, trypsin inhibitor (TI) and total phenol response to pathogen and aphid attack, with or without AMF, and aphid behavior towards VOCs from AMF inoculated and non-mycorrhizal alfalfa, with or without pathogen infection. AM fungus enhanced alfalfa resistance to pathogen and aphid infestation. Plant biomass, root : shoot ratio, net photosynthetic rate, transpiration rate, stomatal conductance, salicylic acid, and TI were significantly increased in AM-inoculated alfalfa. Arbuscular mycorrhizal fungi and pathogen significantly changed alfalfa VOCs. Aphids preferred VOCs of AM-inoculated and nonpathogen-infected to nonmycorrhizal and pathogen-infected alfalfa. We propose that AMF alter plant response to multiple biotic stresses in ways both beneficial and harmful to the plant host, providing a basis for strategies to manage pathogens and herbivore pests.

The dissection of tomato flavor: biochemistry, genetics, and omics

Flavor and quality are the major drivers of fruit consumption in the US. However, the poor flavor of modern commercial tomato varieties is a major cause of consumer dissatisfaction. Studies in flavor research have informed the role of volatile organic compounds in improving overall liking and sweetness of tomatoes. These studies have utilized and applied the tools of molecular biology, genetics, biochemistry, omics, machine learning, and gene editing to elucidate the compounds and biochemical pathways essential for good tasting fruit. Here, we discuss the progress in identifying the biosynthetic pathways and chemical modifications of important tomato volatile compounds. We also summarize the advances in developing highly flavorful tomato varieties and future steps toward developing a "perfect tomato".

Elucidating the importance of the catabolic enzyme, methionine-gamma-lyase, in stresses during Arabidopsis seed development and germination

The sulfur-containing essential amino acid, methionine, is a key metabolite in plant cells since it is used as a precursor for the synthesis of vital metabolites. The transcript level of methionine's catabolic enzyme, methionine γ-lyase (MGL), accumulates in the seeds to a high level compared to other organs. The aim of this study was to reveal the role of MGL during seed development and germination. Using [¹³C]S-methylmethionine (SMM), the mobile form of methionine that is used to feed flower stalks of wild-type (WT) plants, revealed that the contents of [¹³C]methionine in seeds were significantly reduced when the plants underwent heat and osmotic stresses. Moreover, the levels of [¹³C]isoleucine, a product of MGL, significantly increased. Also, using the MGL promoter and gene fused to the GUS reporter gene, it was demonstrated that the heat stress significantly increased the protein level in the seeds. Therefore, we can conclude that MGL became active under stresses apparently to produce isoleucine, which is used as an osmoprotectant and an energy source. Transgenic Arabidopsis thaliana RNAi seeds with targeted repression of AtMGL during the late developmental stages of seeds show that the seeds did not accumulate methionine when they were grown under standard growth conditions, unlike the mgl-2, a knockout mutant, which showed a three-fold higher level of methionine. Also, when the RNAi plants developed under mid-heat stress, the level of methionine significantly increased while the content of isoleucine decreased compared to the control seeds, which strengthened the assumption that MGL is active under stress. The germination efficiency of the RNAi lines and mgl seeds were similar to their controls. However, the seeds that developed during heat or salt stress showed significantly lower germination efficiency compared to the control seeds. This implies that MGL is important to maintain the ability of the seeds to germinate. The RNAi lines and mgl seeds that developed under regular conditions, but germinated during salt or osmotic stress, exhibited a lower germination rate, suggesting an essential role of MGL also during this process. The results of this study show the important role of AtMGL in seeds under stresses.

Novel insight into the evolution of volatile compounds during dynamic freeze-drying of Ziziphus jujuba cv. Huizao based on GC-MS combined with multivariate data analysis

To understand the evolution of aroma in jujubes during dynamic freeze drying (FD), the relationship between aroma compounds, precursors, and related enzyme activities were analyzed. Fifty-three volatiles were identified during FD processing. After FD, the total aroma contents were increased from 11,004 to 14,603 μg/kg, ketones content was significantly decreased by 54.11 %, resulted in the loss of creamy note in freeze-dried jujube (FDJ). Through the network analysis, serine, glycine, proline, valine, cysteine, arginine, glutamic acid, lysine and leucine had the significant correlation with pyrazines, dominated the roasty note of FDJ. Linoleic acid, α-linolenic acid and oleic acid with lipoxygenase had important effects on the increase of esters (from 412 to 9,486 μg/kg), contributed fruity and sweet notes of FDJ. Besides, through the Mantel test, the influence degree of factors on the formation of FDJ aroma was ranked as temperature > enzyme activity > fatty acids > amino acids.

Analysis of Volatile Aroma Components and Regulatory Genes in Different Kinds and Development Stages of Pepper Fruits Based on Non-Targeted Metabolome Combined with Transcriptome

Aroma is a crucial attribute affecting the quality of pepper and its processed products, which has significant commercial value. However, little is known about the composition of volatile aroma compounds (VACs) in pepper fruits and their potential molecular regulatory mechanisms. In this study, HS-SPME-GC-MS combined with transcriptome sequencing is used to analyze the composition and formation mechanism of VACs in different kinds and development stages of pepper fruits. The results showed that 149 VACs, such as esters, alcohols, aldehydes, and terpenoids, were identified from 4 varieties and 3 development stages, and there were significant quantitative differences among different samples. Volatile esters were the most important aroma components in pepper fruits. PCA analysis showed that pepper fruits of different developmental stages had significantly different marker aroma compounds, which may be an important provider of pepper's characteristic aroma. Transcriptome analysis showed that many differential genes (DEGs) were enriched in the metabolic pathways related to the synthesis of VACs, such as fatty acids, amino acids, MVA, and MEP in pepper fruits. In addition, we identified a large number of differential transcription factors (TFs) that may regulate the synthesis of VACs. Combined analysis of differential aroma metabolites and DEGs identified two co-expression network modules highly correlated with the relative content of VACs in pepper fruit. This study confirmed the basic information on the changes of VACs in the fruits of several Chinese spicy peppers at different stages of development, screened out the characteristic aroma components of different varieties, and revealed the molecular mechanism of aroma formation, providing a valuable reference for the quality breeding of pepper.

Rootstock identity in melon-pumpkin graft combinations determines fruit metabolite profile

Grafting has the potential to improve melon fruit yield and quality, but it is currently held that a lack of compatibility between the rootstock and scion compromises such an effect. To throw light on this subject, we studied melon-pumpkin graft combinations with different levels of compatibility to assess to the effect of the rootstock identity on melon fruit yield and quality, including total fruit ortho-diphenols, total flavonoids, and primary fruit metabolites. Melon cv. 'Kiran' (Ki) was grafted onto three pumpkin rootstocks, 'TZ-148' (TZ), 'Shimshon' (Sh), and '53006' (r53), characterized by high, moderate, and low compatibility, respectively. The non-grafted melon cultivar Ki was used as the control. The incompatible combination Ki/r53 gave the lowest fruit yield and the lowest average fruit weight. In that combination, the content of total ortho-diphenols increased vs. Ki and Ki/TZ and that of total flavonoids decreased vs. Ki/Sh. In addition, concentrations of the amino acids, glutamate, methionine, valine, alanine, glycine, and serine, increased in the pulp of the two compatible combinations, i.e., Ki/TZ and Ki/Sh, suggesting that rootstock identity and compatibility with melon Ki scion modulated amino acid synthesis. Our results show an association between rootstock identity (and level of compatibility with the scion) and an enhancement of fruit nutritional values, i.e., high concentrations of organic acids (determined as citrate, malate, fumarate, and succinate) and soluble carbohydrates (sucrose) were recorded in the pulp of the two compatible combinations, i.e., Ki/TZ and Ki/Sh.

Thorough Characterization of ETHQB3.5, a QTL Involved in Melon Fruit Climacteric Behavior and Aroma Volatile Composition

The effect of the QTL involved in climacteric ripening ETHQB3.5 on the fruit VOC composition was studied using a set of Near-Isogenic Lines (NILs) containing overlapping introgressions from the Korean accession PI 16375 on the chromosome 3 in the climacteric 'Piel de Sapo' (PS) genetic background. ETHQB3.5 was mapped in an interval of 1.24 Mb that contained a NAC transcription factor. NIL fruits also showed differences in VOC composition belonging to acetate esters, non-acetate esters, and sulfur-derived families. Cosegregation of VOC composition (23 out of 48 total QTLs were mapped) and climacteric ripening was observed, suggesting a pleiotropic effect of ETHQB3.5. On the other hand, other VOCs (mainly alkanes, aldehydes, and ketones) showed a pattern of variation independent of ETHQB3.5 effects, indicating the presence of other genes controlling non-climacteric ripening VOCs. Network correlation analysis and hierarchical clustering found groups of highly correlated compounds and confirmed the involvement of the climacteric differences in compound classes and VOC differences. The modification of melon VOCs may be achieved with or without interfering with its physiological behavior, but it is likely that high relative concentrations of some type of ethylene-dependent esters could be achieved in climacteric cultivars.

Postharvest chilling diminishes melon flavor via effects on volatile acetate ester biosynthesis

In postharvest handling systems, refrigeration can extend fruit shelf life and delay decay via slowing ripening progress; however, it selectively alters the biosynthesis of flavor-associated volatile organic compounds (VOCs), which results in reduced flavor quality. Volatile esters are major contributors to melon fruit flavor. The more esters, the more consumers enjoy the melon fruit. However, the effects of chilling on melon flavor and volatiles associated with consumer liking are yet to be fully understood. In the present study, consumer sensory evaluation showed that chilling changed the perception of melon fruit. Total ester content was lower after chilling, particularly volatile acetate esters (VAEs). Transcriptomic analysis revealed that transcript abundance of multiple flavor-associated genes in fatty acid and amino acid pathways was reduced after chilling. Additionally, expression levels of the transcription factors (TFs), such as NOR, MYB, and AP2/ERF, also were substantially downregulated, which likely altered the transcript levels of ester-associated pathway genes during cold storage. VAE content and expression of some key genes recover after transfer to room temperature. Therefore, chilling-induced changes of VAE profiles were consistent with expression patterns of some pathway genes that encode specific fatty acid- and amino acid-mobilizing enzymes as well as TFs involved in fruit ripening, metabolic regulation, and hormone signaling.

Telomere-to-telomere genome assembly of bitter melon (Momordica charantia L. var. abbreviata Ser.) reveals fruit development, composition and ripening genetic characteristics

Momordica charantia L. var. abbreviata Ser. (Mca), known as bitter gourd or bitter melon, is a Momordica variety with medicinal value and belongs to the Cucurbitaceae family. In view of the lack of genomic information on bitter gourd and other Momordica species and to promote Mca genomic research, we assembled a 295.6-Mb telomere-to-telomere (T2T) high-quality Mca genome with six gap-free chromosomes after Hi-C correction. This genome is anchored to 11 chromosomes, which is consistent with the karyotype information, and comprises 98 contigs (N50 of 25.4 Mb) and 95 scaffolds (N50 of 25.4 Mb). The Mca genome harbors 19 895 protein-coding genes, of which 45.59% constitute predicted repeat sequences. Synteny analysis revealed variations involved in fruit quality during the divergence of bitter gourd. In addition, assay for transposase-accessible chromatin by high-throughput sequencing and metabolic analysis showed that momordicosides and other substances are characteristic of Mca fruit pulp. A combined transcriptomic and metabolomic analysis revealed the mechanisms of pigment accumulation and cucurbitacin biosynthesis in Mca fruit peels, providing fundamental molecular information for further research on Mca fruit ripening. This report provides a new genetic resource for Momordica genomic studies and contributes additional insights into Cucurbitaceae phylogeny.

Arbuscular Mycorrhizal Fungus Alters Alfalfa (Medicago sativa) Defense Enzyme Activities and Volatile Organic Compound Contents in Response to Pea Aphid (Acyrthosiphon pisum) Infestation

Pea aphid (Acyrthosiphon pisum) infestation leads to withering, reduced yield, and lower quality of the host plant. Arbuscular mycorrhizal (AM) fungi have been found to enhance their host plants’ nutrient uptake, growth, and resistance to biotic stresses, including pathogen infection and insect pest infestation. Therefore, we evaluated the effects of AM fungus Rhizophagus intraradices on alfalfa defense responses to pea aphid infestation. Aphid infestation did not affect the colonization of AM fungus. The inoculation of AM fungus, on average, enhanced alfalfa catalase and the contents of salicylic acid and trypsin inhibitor by 101, 9.05, and 7.89% compared with non-mycorrhizal alfalfa, respectively. In addition, polyphenol oxidase activities significantly increased by six-fold after aphid infestation in mycorrhizal alfalfa. Moreover, the fungus significantly (p < 0.05) improved alfalfa shoot N content, net photosynthetic and transpiration rates, and shoot dry weight in aphid infected treatment. The aphid infestation changed the total volatile organic compounds (VOCs) in alfalfa, while AM fungus enhanced the contents of methyl salicylate (MeSA). The co-expression network analysis of differentially expressed genes (DEGs) and differentially expressed VOCs analysis showed that three DEGs, namely MS.gene23894, MS.gene003889, and MS.gene012415, positively correlated with MeSA both in aphid and AM fungus groups. In conclusion, AM fungus increased alfalfa’s growth, defense enzyme activities, hormones, and VOCs content and up-regulated VOC-related genes to enhance the alfalfa’s resistance following aphid infestation.

Characterization of key aroma compounds and regulation mechanism of aroma formation in local Binzi (Malus pumila × Malus asiatica) fruit

BACKGROUND

Volatile components are important secondary metabolites essential to fruit aroma quality, thus, in the past decades many studies have been extensively performed in clarifying fruit aroma formation. However, aroma components and biosynthesis in the fruit of Binzi (Malus pumila × Malus asiatica), an old local species with attractive aroma remain unknown.

RESULTS

We investigated two Binzi cultivars, 'Xiangbinzi' (here named high-fragrant Binzi, 'HFBZ') and 'Hulabin' (here named low-fragrant Binzi, 'LFBZ') by monitoring the variation of volatiles and their precursors by Gas Chromatography-Mass Spectrometer (GC-MS), as well as their related genes by RNA-seq during post-harvest ripening. We firstly confirmed that 'HFBZ' and 'LFBZ' fruit showed respiratory climacteric by detecting respiratory rate and ethylene emission during post-harvest; found that esters were the major aroma components in 'HFBZ' fruit, and hexyl 2-methylbutyrate was responsible for the 'fruity' note and most potent aroma component, followed by ethyl acetate, ethyl butanoate, (E)-2-hexenal, and 1-hexanol. Regarding aroma synthesis, fatty acid metabolism seemed to be more important than amino acid metabolism for aroma synthesis in 'HFBZ' fruit. Based on RNA-seq and quantitative reverse transcription PCR (RT-qPCR), LOX2a, LOX5a, ADH1, and AAT1 genes are pointed to the LOX pathway, which may play a vital role in the aroma formation of 'HFBZ' fruit.

CONCLUSION

Our study firstly investigated the aroma components and related genes of Binzi fruit, and provided an insight into the fragrant nature of Malus species.

Targeted approaches to improve tomato fruit taste

Tomato (Solanum lycopersicum) is the most valuable fruit and horticultural crop species worldwide. Compared with the fruits of their progenitors, those of modern tomato cultivars are, however, often described as having unsatisfactory taste or lacking flavor. The flavor of a tomato fruit arises from a complex mix of tastes and volatile metabolites, including sugars, acids, amino acids, and various volatiles. However, considerable differences in fruit flavor occur among tomato varieties, resulting in mixed consumer experiences. While tomato breeding has traditionally been driven by the desire for continual increases in yield and the introduction of traits that provide a long shelf-life, consumers are prepared to pay a reasonable premium for taste. Therefore, it is necessary to characterize preferences of tomato flavor and to define its underlying genetic basis. Here, we review recent conceptual and technological advances that have rendered this more feasible, including multi-omics-based QTL and association analyses, along with the use of trained testing panels, and machine learning approaches. This review proposes how the comprehensive datasets compiled to date could allow a precise rational design of tomato germplasm resources with improved organoleptic quality for the future.

Integrated metabolome and transcriptome analysis of the regulatory network of volatile ester formation during fruit ripening in pear

'Nanguo' pear (Pyrus ussuriensis Maxim.) is a typical climacteric fruit with an attractive aroma after postharvest ripening. Esters are the key volatile compounds determining the typical aroma formation. However, the mechanism of aroma-related ester formation remains largely unknown. In this study, we performed transcriptome and metabolome analyses to reveal the changes of aroma-related compounds during pear ripening in the optimal taste period (OTP). During the pear ripening process, typical fatty acid-derived volatile organic compounds (VOCs) are transformed from aldehydes, alcohols, and ketones to esters, where ethyl hexanoate, hexyl acetate, and ethyl butanoate are the dominant esters in the OTP. Rich aroma-related esters in the OTP are associated with the accumulation of important precursors of aroma volatiles, including linoleic acid, α-linolenic acid, γ-linolenic acid, and oleic acid. Genes encoding key biosynthetic enzymes are associated with the altered levels of aroma-related esters. The candidate genes associated with the high levels of aroma-related esters in 'Nanguo' pears are PuFAD2, PuLOX2, PuLOX5, and PuAAT. Additionally, transcription factor (TF) genes such as PuWRKY24, PuIAA29, and PuTINY may play crucial roles in aroma formation during fruit ripening. Hence, we summarized the TFs that regulate VOC metabolism in different fruit species. The results provided a foundation for further research on aroma-related esters in 'Nanguo' pears and could help to elucidate the mechanisms regulating fruit quality improvement.

Investigation of Biosynthetic Precursors of 3-Isobutyl-2-Methoxypyrazine Using Stable Isotope Labeling Studies in Bell Pepper Fruits (Capsicum annuum L.)

The biosynthesis of 3-isobutyl-2-methoxypyrazine (IBMP) in bell pepper fruits (Capsicum annuum L.) was investigated by in vivo feeding experiments with stable isotope-labeled precursors. Volatiles were extracted using headspace solid-phase microextraction (HS-SPME) and analyzed by comprehensive two-dimensional gas chromatography (GC×GC) coupled to a time-of-flight mass spectrometer (ToF-MS). Feeding experiments revealed incorporation of l-leucine and α-ketoisocaproic acid (α-KIC) as well as glycine and glyoxylic acid into IBMP. Furthermore, it has been shown that de novo biosynthesis of IBMP occurs in pericarp tissues of unripe bell pepper fruits, whereas pericarp tissues of ripe bell pepper fruits showed no capability of IBMP biosynthesis.

Amino acids metabolism as a source for aroma volatiles biosynthesis

Aroma volatiles are essential for plant ecological fitness and reproduction. Plants produce and use volatiles to attract pollinators and seed dispersers, repel herbivores and recruit their natural enemies, and communicate with other plants. Amino acids and their biosynthetic intermediates play key roles as precursors for the biosynthesis of plant volatiles. Different plants utilize different strategies and biosynthetic pathways to meet their specific biological needs. This review focuses on the different biosynthetic pathways that plants utilize to form amino acid-derived aroma volatiles, emphasizing their common and unique aspects and stressing the importance of the limiting enzymes residing in the primary-specialized metabolism interface. We also briefly review how biotechnology has used this interface and point to promising future directions for improving the quality of agricultural produce and the production of key volatiles.

A comparative metabolomics study of anthocyanins and taste components in Chinese bayberry (Morella rubra) with different flesh colors

The Chinese bayberry (Morella rubra Sieb. et Zucc.) is grown commercially in China and other Asian countries for its flavorful and appealing fruit. Here, two bayberry varieties differing in both color and flavor, namely, BDK ('Baidongkui') and DK ('Dongkui'), in China were compared. A total of 18 anthocyanins, three proanthocyanidins, and 229 primary metabolites were identified in the pulp of the two varieties; these were analyzed and compared using ultra-performance liquid chromatography-tandem mass spectrometry. The DK pulp showed higher concentrations of all 18 anthocyanins compared with BDK, apart from peonidin-3,5-O-diglucoside which was not detected in BDK and which was responsible for the formation of pink pulp in BDK. Principal component analysis and cluster analysis of the primary metabolites indicated that the two bayberry varieties had distinct metabolite profiles with approximately 37% (85/229) of the primary metabolome being significantly different. Of these, 62 metabolites were down-regulated and 23 metabolites were up-regulated in BDK relative to DK. Our results suggested that the flavor of the BDK fruit was different from DK, which could be explained by the reduced saccharide, organic acid, amino acid, and proanthocyanidin contents. These findings enhance our understanding of the metabolites responsible for color and taste differences in the Chinese bayberry.

Floral Scents and Fruit Aromas: Functions, Compositions, Biosynthesis, and Regulation

Floral scents and fruit aromas are crucial volatile organic compounds (VOCs) in plants. They are used in defense mechanisms, along with mechanisms to attract pollinators and seed dispersers. In addition, they are economically important for the quality of crops, as well as quality in the perfume, cosmetics, food, drink, and pharmaceutical industries. Floral scents and fruit aromas share many volatile organic compounds in flowers and fruits. Volatile compounds are classified as terpenoids, phenylpropanoids/benzenoids, fatty acid derivatives, and amino acid derivatives. Many genes and transcription factors regulating the synthesis of volatiles have been discovered. In this review, we summarize recent progress in volatile function, composition, biosynthetic pathway, and metabolism regulation. We also discuss unresolved issues and research perspectives, providing insight into improvements and applications of plant VOCs.

Branched-Chain Volatiles in Fruit: A Molecular Perspective

Branched-chain volatiles (BCVs) constitute an important family of fruit volatile metabolites essential to the characteristic flavor and aroma profiles of many edible fruits. Yet in contrast to other groups of volatile organic compounds important to fruit flavor such as terpenoids, phenylpropanoids, and oxylipins, the molecular biology underlying BCV biosynthesis remains poorly understood. This lack of knowledge is a barrier to efforts aimed at obtaining a more comprehensive understanding of fruit flavor and aroma and the biology underlying these complex phenomena. In this review, we discuss the current state of knowledge regarding fruit BCV biosynthesis from the perspective of molecular biology. We survey the diversity of BCV compounds identified in edible fruits as well as explore various hypotheses concerning their biosynthesis. Insights from branched-chain precursor compound metabolism obtained from non-plant organisms and how they may apply to fruit BCV production are also considered, along with potential avenues for future research that might clarify unresolved questions regarding BCV metabolism in fruits.

Cucurbitaceae genome evolution, gene function and molecular breeding

The Cucurbitaceae is one of the most genetically diverse plant families in the world. Many of them are important vegetables or medicinal plants and are widely distributed worldwide. The rapid development of sequencing technologies and bioinformatic algorithms has enabled the generation of genome sequences of numerous important Cucurbitaceae species. This has greatly facilitated research on gene identification, genome evolution, genetic variation and molecular breeding of cucurbit crops. So far, genome sequences of 18 different cucurbit species belonging to tribes Benincaseae, Cucurbiteae, Sicyoeae, Momordiceae and Siraitieae have been deciphered. This review summarizes the genome sequence information, evolutionary relationship, and functional genes associated with important agronomic traits (e.g., fruit quality). The progress of molecular breeding in cucurbit crops and prospects for future applications of Cucurbitaceae genome information are also discussed.

Alcohol Acyltransferase Is Involved in the Biosynthesis of C6 Esters in Apricot (Prunus armeniaca L.) Fruit

Short-chain esters derived from fatty acid contribute to the characteristic flavor of apricot fruit, and the biosynthesis of these compounds in fruit is catalyzed by alcohol acyltransferase (AAT). In this work, we investigated the AAT gene family via genome-wide scanning, and three AAT loci were identified in different linkage groups (LGs), with PaAAT1 (PARG22907m01) in LG7, PaAAT2 (PARG15279m01) in LG4, and PaAAT3 (PARG22697m01) in LG6. Phylogenetic analysis showed that PaAAT1 belongs to clade 3, while PaAAT2 and PaAAT3 belong to clade 1 and clade 2, respectively. In contrast, the three AAT genes present different expression patterns. Only PaAAT1 exhibited distinct patterns of fruit-specific expression, and the expression of PaAAT1 sharply increased during fruit ripening, which is consistent with the abundance of C4-C6 esters such as (E)-2-hexenyl acetate and (Z)-3-hexenyl acetate. The transient overexpression of PaAAT1 in Katy (KT) apricot fruit resulted in a remarkable decrease in hexenol, (E)-2-hexenol, and (Z)-3-hexenol levels while significantly increasing the corresponding acetate production (p < 0.01). A substrate assay revealed that the PaAAT1 protein enzyme can produce hexenyl acetate, (E)-2-hexenyl acetate, and (Z)-3-hexenyl acetate when C6 alcohols are used as substrates for the reaction. Taken together, these results indicate that PaAAT1 plays a crucial role in the production of C6 esters in apricot fruit during ripening.

Metabolomics Reveals Heterogeneity in the Chemical Composition of Green and White Spears of Asparagus (A. officinalis)

Green and white asparagus are quite different crops but can be harvested from the same plant. They have distinct morphological differences due to their mode of cultivation and they are characterised by having contrasting appearance and flavour. Significant chemical differences are therefore expected. Spears from three varieties of both green and white forms, harvested in two consecutive seasons were analysed using headspace GC-MS and LC-MS with an untargeted metabolomic workflow. Mainly C5 and C8 alcohols and aldehydes, and phenolic compounds were more abundant in green spears, whereas benzenoids, monoterpenes, unsaturated aldehydes and steroidal saponins were more abundant in white ones. Previously reported key asparagus volatiles and non-volatiles were detected at similar or not significantly different levels in the two asparagus types. Spatial metabolomics revealed also that many volatiles with known positive aroma attributes were significantly more abundant in the upper parts of the spears and showed a decreasing trend towards the base. These findings provide valuable insights into the metabolome of raw asparagus, the contrasts between green and white spears as well as the different chemical distributions along the stem.

Free Amino Acids Profile and Expression Analysis of Core Genes Involved in Branched-Chain Amino Acids Metabolism during Fruit Development of Longan (Dimocarpus longan Lour.) Cultivars with Different Aroma Types

Simple Summary

In this study, three longan cultivars, including non-aroma types ‘Shixia’ (SX), ‘Lidongben’ (LDB), and strong aroma type ‘Xiangcui’ (XC), were selected to analyze free amino acids (FAAs) variations at six distinct growth stages. The genome-wide identification and expression analysis of genes related to the branched-chain amino acids (BCAA) synthesis pathway were carried out. Results showed that thirty-six FAAs were identified, which increased drastically with fruit development until ripening. During the period of rapid fruit expansion, the aroma of XC changed from light to strong, and the contents of L-alanine and L-leucine were significantly higher than those of SX and LDB. The content of Leu was negatively correlated with the expression of DilBCAT1, -6, and -9 in three varieties, but positively correlated with DilBCAT16, indicating that these four genes may be responsible for the different synthesis and degradation of Leu among cultivars.

Abstract

Amino acids are important component of fruit nutrition and quality. In this study, three longan cultivars, including non-aroma types ‘Shixia’ (SX), ‘Lidongben’ (LDB), and strong aroma type ‘Xiangcui’ (XC), were selected to analyze free amino acids (FAAs) variations at six distinct growth stages (S1–S6). The genome-wide identification and expression analysis of genes related to the branched-chain amino acids (BCAA) synthesis pathway were carried out. Results showed that 36 FAAs were identified, and the total FAAs content ranged from 2601.0 to 9073.5 mg/kg, which increased drastically with fruit development until ripening. L-glutamic acid (Glu), L-alanine (Ala), L-arginine (Arg), γ-Aminobutyric acid (GABA), L-aspartic acid (Asp), L-leucine (Leu), hydroxyl-proline (Hypro), and L-serine (Ser) were the predominant FAAs (1619.9–7213.9 mg/kg) in pulp, accounting for 62.28–92.05% of the total amino acids. During the period of rapid fruit expansion (S2–S4), the aroma of XC changed from light to strong, and the contents of L-alanine (Ala) and L-leucine (Leu) were significantly higher than those of SX and LDB. Furthermore, a total of two 2-isopropyl malate synthase (IPMS), two 3-isopropyl malate dehydrogenase (IPMD), and 16 BCAA transferase (BCAT) genes were identified. The expression levels of DilBCAT1, -6, and -9 genes in XC were significantly higher than those in SX and LDB, while DilBCAT16 in XC was lower. The content of Leu was negatively correlated with the expression of DilBCAT1, -6, and -9 in three varieties, but positively correlated with DilBCAT16, indicating that these four genes may be responsible for the different synthesis and degradation of Leu among cultivars.

Volatile and Sensory Characterization of La Mancha Trujillo Melons over Three Consecutive Harvests

In this work, Trujillo melons were harvested across three years (2011–2013) in La Mancha region. Instrumental and sensory analysis were used for studying Trujillo melons. Solid phase extraction (SPE) was used for isolating free aroma compounds, and then, they were analysed by gas chromatography coupled with mass spectrometry (GC/MS). Fifty-five (55) volatile compounds were identified and quantified in La Mancha Trujillo melons over this three-year period. Experienced tasters evaluated the sensory profile of Trujillo melons, and it was characterized by jam/marmalade, cucumber, fresh fruit, sweet, green, honey and ripe fruit aroma descriptors and sweet, honey, jam/marmalade, cucumber, fresh fruit ripe fruit, spice and green flavour by mouth descriptors. This study represents the first complete aromatic characterization of Trujillo melons from La Mancha region. The obtained data suggested that these melons presented a great aromatic profile and that they represent a viable alternative for expanding the traditional market.

Formation of volatiles in response to tea green leafhopper (Empoasca onukii Matsuda) herbivory in tea plants: a multi-omics study

Combined transcriptome and metabolome analysis of fresh leaf infestation by tea green leafhoppers (Empoasca (Matsumurasca) onukii Matsuda) suggests roles for alternative pre-mRNA splicing and mRNAs in the regulation of aroma formation in tea plants. Oriental Beauty is a high-grade, oolong tea with a pronounced honey-like aroma and rich ripe fruit flavor that develops primarily as a result of the infestation of the fresh leaves by tea green leafhoppers (Empoasca (Matsumurasca) onukii Matsuda). Here, we used PacBio Iso-Seq and RNA-seq analyses to determine the full-length transcripts and gene expression profiles of fresh tea leaves in response to E. (M.) onukii herbivory. We investigated the relationship between RNA-seq, tea metabolites, and aroma response mechanisms in leaves infested by leafhoppers. We found 3644 differentially expressed genes, of which 2552 were up- and 1092 were down-regulated. A total of 49,913 alternative splicing events were predicted, including 324 differential AS events. Moreover, 3105 differentially expressed transcripts were also identified, of which 2295 were up- and 810 were down-regulated. The characterization of expression patterns of the key gene transcript isoforms involved in the aroma formation pathways identified 130 differentially expressed metabolites, 97 of which were up- and 33 were down-regulated. Two key aroma compounds (phenylacetaldehyde and 4-hydroxybenzaldehyde) were highly correlated with genes of the aroma formation pathways. Our results revealed that pre-mRNA AS plays a crucial role in the metabolic regulation surrounding aroma formation under leafhopper herbivory in tea plants.

Comparative Study of Volatile Compounds and Expression of Related Genes in Fruit from Two Apple Cultivars during Different Developmental Stages

Aromatic volatile compounds are important contributors to fruit quality that vary among different cultivars. Herein, headspace solid-phase microextraction coupled with gas chromatography-mass spectrometry was used to determine changes in volatile compounds and related gene expression patterns in "Ruixue" and "Fuji" apples (Malus domestica Borkh.) during fruit development and maturation. Volatile compounds detected in the fruit of both cultivars exhibited similar trends across different developmental stages. In the early stages of "Ruixue" fruit development (60 days after full bloom), there were fewer volatile compounds, mainly aldehydes (87.0%). During fruit maturation (180 days after full bloom), the types and amounts of volatile compounds increased, mainly including esters (37.6%), and alkenes (23.2%). The total volatile concentration, the types of major volatile compounds, and their relative content in both cultivars varied across different stages. Gene expression analysis indicated that the upregulation of MdLOX, MdAAT2, and MdADH3 was associated with increased aroma compound content, especially esters, during fruit development in both cultivars. Changes in the expression of MdArAT, MdACPD, MdADH3, MdAAT2, and MdLOX may lead to differences in volatile compounds between apple cultivars.

Genetic dissection of aroma biosynthesis in melon and its relationship with climacteric ripening

Aroma is an essential trait in melon fruit quality, but its complexity and genetic basis are still poorly understood. The aim of this study was the identification of quantitative trait loci (QTLs) underlying volatile organic compounds (VOCs) biosynthesis in melon rind and flesh, using a Recombinant Inbred Line (RIL) population from the cross 'Piel de Sapo' (PS) × 'Védrantais' (VED), two commercial varieties segregating for ripening behavior. A total of 82 VOCs were detected by gas chromatography-mass spectrometry (GC-MS), and 166 QTLs were identified. The main QTL cluster was on chromosome 8, collocating with the previously described ripening-related QTL ETHQV8.1, with an important role in VOCs biosynthesis. QTL clusters involved in esters, lipid-derived volatiles and apocarotenoids were also identified, and candidate genes have been proposed for ethyl 3-(methylthio)propanoate and benzaldehyde biosynthesis. Our results provide genetic insights for deciphering fruit aroma in melon and offer new tools for flavor breeding.