A dual positional specific lipoxygenase functions in the generation of flavor compounds during climacteric ripening of apple

The Composition of Volatiles and the Role of Non-Traditional LOX on Target Metabolites in Virgin Olive Oil from Autochthonous Dalmatian Cultivars

The lipoxygenase pathway has a significant influence on the composition of the volatile components of virgin olive oil (VOO). In this work, the influence of the maturity index (MI) on the activity of the lipoxygenase enzyme (LOX) in the fruits of the autochthonous Dalmatian olive cultivars Oblica, Levantinka and Lastovka was studied. The analysis of the primary oxidation products of linoleic acid in the studied cultivars showed that LOX synthesises a mixture of 9- and 13-hydroperoxides of octadecenoic acid in a ratio of about 1:2, which makes it a non-traditional plant LOX. By processing the fruits of MI~3, we obtained VOOs with the highest concentration of desirable C6 volatile compounds among the cultivars studied. We confirmed a positive correlation between MI, the enzyme activity LOX and the concentration of hexyl acetate and hexanol in cultivars Oblica and Lastovka, while no positive correlation with hexanol was observed in the cultivar Levantinka. A significant negative correlation was found between total phenolic compounds in VOO and LOX enzyme activity, followed by an increase in the MI of fruits. This article contributes to the selection of the optimal harvest time for the production of VOOs with the desired aromatic properties and to the knowledge of the varietal characteristics of VOOs.

The apple lipoxygenase MdLOX3 positively regulates zinc tolerance

Various abiotic stresses, especially heavy metals near factories around the world, limit plant growth and productivity worldwide. Zinc is a light gray transition metal, and excessive zinc will inactivate enzymes in the soil, weaken the biological function of microorganisms, and enter the food chain through enrichment, thus affecting human health. Lipoxygenase (LOX) can catalyze the production of fatty acid derivatives from phenolic triglycerides in plants and is an important pathway of fatty acid oxidation in plants, which usually begins under unfavorable conditions, especially under biotic and abiotic stresses. Lipoxygenase can be divided into 9-LOX and 13-LOX. MdLOX3 is a homolog of AtLOX3 and has been identified in apples (housefly apples). MdLOX3 has a typical conserved lipoxygenase domain, and promoter analysis shows that it contains multiple stress response elements. In addition, different abiotic stresses and hormonal treatments induced the MdLOX3 response. In order to explore the inherent anti-heavy metal mechanism of MdLOX3, this study verified the properties of MdLOX3 based on genetic analysis and overexpression experiments, including plant taproots length, plant fresh weight, chlorophyll, anthocyanins, MDA, relative electrical conductivity, hydrogen peroxide and superoxide anion, NBT\DAB staining, etc. In the experiment, overexpression of MdLOX3 in apple callus and Arabidopsis effectively enhanced the tolerance to zinc stress by improving the ability to clear ROS. Meanwhile, tomato materials with overexpression of ectopia grew better under excessive zinc ion stress. These results indicated that MdLOX3 had a good tolerance to heavy metal zinc. Homologous mutants are more sensitive to zinc, which proves that MdLOX3 plays an important positive role in zinc stressed apples, which broadens the range of action of LOX3 in different plants.

Identification, characterization, and expression of lipoxygenase genes in sweet cherry (Prunus avium L.) cv. Regina and their relationship with the development of an herbaceous off-flavor during fruit ripening

Flavor is an essential characteristic of fruit quality and is significant for consumers. Off-flavors have been reported in several fruits, including sweet cherry. This fruit has been reported to show an herbaceous/grassy-like flavor. The herbaceous off-flavor in sweet cherries detected in cultivar Regina has been related to the differential development of aroma compounds such as short-chain aldehydes and esters. One of the main biosynthesis pathways for these compounds is the fatty acid oxidation mediated by lipoxygenases (LOX). In order to have a better understanding of the biological basis of the differences in the volatile profile, the LOX gene expression profile was characterized during fruit development with and without herbaceous off-flavor. A genome-wide analysis of LOX in sweet cherry was carried out and compared to other species such as Arabidopsis, tomato, apple, prunus and strawberry. The structural features of 9-LOX and 13-LOX genes, encoded protein domains and their synteny were examined. Moreover, we analyzed the LOX expression at four developmental stages along ripening by RT-qPCR. Thirteen LOX gene candidates (six 9-LOX and seven 13-LOX) were identified. The 13-LOXs, PaLOX10, PaLOX11, and PaLOX12 were differentially expressed in herbaceous sweet cherries. Furthermore, their expression profile positively correlated with key volatile compounds linked to the herbaceous off-flavor. Overall, this study involves the genome-wide characterization of the LOX family in Prunus avium cv. Regina and provides information that can aid in studying LOX-related fruit deterioration in sweet cherries and associated species.

Transcription factors MdMYC2 and MdMYB85 interact with ester aroma synthesis gene MdAAT1 in apple

Volatile esters in apple (Malus domestica) fruit are the critical aroma components determining apple flavor quality. While the exact molecular regulatory mechanism remains unknown, jasmonic acid (JA) plays a crucial role in stimulating the synthesis of ester aromas in apples. In our study, we investigated the effects of methyl jasmonate (MeJA) on the production of ester aroma in apples. MeJA treatment significantly increased ester aroma synthesis, accompanied by the upregulation of several genes involved in the jasmonate pathway transduction. Specifically, expression of the gene MdMYC2, which encodes a transcription factor associated with the jasmonate pathway, and the R2R3-MYB transcription factor gene MdMYB85 increased upon MeJA treatment. Furthermore, the essential gene ALCOHOL ACYLTRANSFERASE 1 (MdAAT1), encoding an enzyme responsible for ester aroma synthesis, showed increased expression levels as well. Our investigation revealed that MdMYC2 and MdMYB85 directly interacted with the promoter region of MdAAT1, thereby enhancing its transcriptional activity. In addition, MdMYC2 and MdMYB85 directly bind their promoters and activate transcription. Notably, the interaction between MdMYC2 and MdMYB85 proteins further amplified the regulatory effect of MdMYB85 on MdMYC2 and MdAAT1, as well as that of MdMYC2 on MdMYB85 and MdAAT1. Collectively, our findings elucidate the role of the gene module consisting of MdMYC2, MdMYB85, and MdAAT1 in mediating the effects of JA and promoting ester aroma synthesis in apples.

Chemometric Classification of Apple Cultivars Based on Physicochemical Properties: Raw Material Selection for Processing Applications

Apple cultivars exhibit significant diversity in fruit quality traits, creating distinct consumption scenarios. This study aimed to assess the physicochemical parameters and sensory attributes differences among fifteen apple cultivars and identify characteristic qualities suitable for various processed apple products using chemometric analysis. Relatively large differences were registered between cultivars for deflection, peel color, titratable acidity (TA), the ratio of total soluble solid to titratable acidity (TSS/TA), hardness, soluble sugar, and volatile organic compound contents. Sensory results showed significant differences existed among the preferences for different processed products. Based on the above results, all cultivars could be distinguished into three main clusters. Cluster I (i.e., Aziteke, Bakeai, Magic Flute, Royal Gala, Red General, Red Delicious, and Zhongqiuwang) demonstrated favorable appearance, high sensory scores, and rich aroma volatile compounds, making them suitable for direct consumption. Cluster II (i.e., Fuburuisi, Sinike, Honglu, and Huashuo) exhibited a higher sugar and acid content, making them suitable for apple juice production. Cluster III (i.e., Miqila, Honey Crisp, Shandong Fuji, and Yanfu 3) were more suitable for fresh-cut apples due to their good flavor and undesirable appearance. Several chemometric analyses effectively assessed differences among apple cultivars.

Transcriptome and Metabolomics Integrated Analysis Reveals MdMYB94 Associated with Esters Biosynthesis in Apple (Malus × domestica)

Volatile esters are major aromas contributing to the organoleptic quality of apple fruit. However, the molecular mechanisms underlying the regulation of volatile ester biosynthesis in apple remain elusive. This study investigated the volatile profiles and transcriptomes of 'Qinguan' (QG) apple fruit during development and/or postharvest storage. Although the constitution of volatiles varied widely between the peel and flesh, the volatile profiles of the peel and flesh of ripening QG fruit were dominated by volatile esters. WGCNA results suggested that 19 genes belonging to ester biosynthesis pathways and 11 hub transcription factor genes potentially participated in the biosynthesis and regulation of esters. To figure out key regulators of ester biosynthesis, correlation network analysis, dual-luciferase assays, and yeast one-hybrid assay were conducted and suggested that MdMYB94 trans-activated the MdAAT2 promoter and participated in the regulation of ester biosynthesis. This study provides a framework for understanding ester biosynthesis and regulation in apple.

Physicochemical Attributes, Aroma Profile, and Sensory Quality of Organic Crimson Crisp Apples after Storage

The objective of this study was to determine the effect of various storage conditions (i.e., storage under regular air with temperature control, controlled atmosphere storage and storage after the application of the ethylene blocker (1-MCP)) on the flavor characteristics of Crimson Crisp apples (Malus domestica Borkh.), a scab-resistant cultivar suitable for organic agriculture. Highly ripe organically-grown apples (starch degradation 9.7) were stored under different conditions and evaluated for physicochemical attributes such as fruit weight, firmness, juiciness, total soluble solids, titratable acidity, pH value and aroma profile. The analysis of primary and secondary aroma compounds was conducted utilizing HS-SPME-GCMS and the results were integrated with sensory evaluation. Crimson Crisp apples stored under controlled atmosphere with (MCP) or without (CA) application of 1-MCP, had a higher overall quality than those stored in air (RA) after a storage period of six months. The results from sensory analysis indicated that storage with temperature control alone was not suitable for preserving the distinct properties such as crispness or juiciness of Crimson Crisp apples. However, a significant increase particularly in secondary aroma compounds in RA-stored apples was found accompanied by structural disorders of the apple tissue. While a significant decline in the volatile fraction of CA and MCP-stored apples was observed, PCA showed close correlation between the CA stored and the fresh apple samples. Furthermore, these results demonstrate that the applied measures which are taken to prolong the storage time of Crimson Crisp apples, significantly impact the biochemical reactions in the fruits which are responsible for formation of flavor. These findings underscore the significance of comprehensive aroma analysis for new agricultural products and emphasize the potential for improved the quality of organic apples through carefully considered harvest and storage regimes.

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.

Physicochemical Attributes, Volatile Profile and Sensory Quality of Organic Crimson Crisp Apples during On-Tree Maturation

When new apple cultivars are planted, knowledge of their maturation and ripening behavior and, as a consequence, the best time for their harvest is of utmost importance for providing fruits of the highest quality to consumers. In this study, we followed the on-tree maturation and ripening behavior of organic Crimson Crisp apples over a period of almost 3 months. With the weekly analyses of basic fruit quality attributes (weight, fruit firmness, total soluble solids, titratable acids, starch degradation) in combination with the fruit volatilome (primary and secondary volatiles) and sensory analysis, we obtained a holistic picture of the maturation and ripening properties of this new variety. We could show that at the recommended harvest window, which is based on the degree of starch degradation alone, the fruit development is not finished. Synthesis of aroma volatiles—which is strongly related to the expression of pronounced fruity, apple-like flavor—requires two additional weeks of on-tree ripening. Results indicate an upregulation of 13-LOX at very early maturation stages, while upregulation of 9-LOX and enzyme systems involved in the β-oxidation pathway requires a prolonged on-tree maturation period. The results of this study demonstrate that the analysis of basic fruit quality attributes is not sufficient for understanding the properties of apples. However, we demonstrate that the analysis of the fruit volatilome is a valuable and necessary tool for optimizing the quality of new apple varieties.

Pseudophosphorylation of Arabidopsis jasmonate biosynthesis enzyme lipoxygenase 2 via mutation of Ser600 inhibits enzyme activity

Jasmonates are oxylipin phytohormones critical for plant resistance against necrotrophic pathogens and chewing herbivores. An early step in their biosynthesis is catalyzed by non-heme iron lipoxygenases (LOX; EC 1.13.11.12). In Arabidopsis thaliana, phosphorylation of Ser⁶⁰⁰ of AtLOX2 was previously reported, but whether phosphorylation regulates AtLOX2 activity is unclear. Here, we characterize the kinetic properties of recombinant WT AtLOX2 (AtLOX2^WT). AtLOX2^WT displays positive cooperativity with α-linolenic acid (α-LeA, jasmonate precursor), linoleic acid (LA), and arachidonic acid (AA) as substrates. Enzyme velocity with endogenous substrates α-LeA and LA increased with pH. For α-LeA, this increase was accompanied by a decrease in substrate affinity at alkaline pH; thus, the catalytic efficiency for α-LeA was not affected over the pH range tested. Analysis of Ser⁶⁰⁰ phosphovariants demonstrated that pseudophosphorylation inhibits enzyme activity. AtLOX2 activity was not detected in phosphomimics Atlox2^S600D and Atlox2^S600M when α-LeA or AA were used as substrates. In contrast, phosphonull mutant Atlox2^S600A exhibited strong activity with all three substrates, α-LeA, LA, and AA. Structural comparison between the AtLOX2 AlphaFold model and a complex between 8R-LOX and a 20C polyunsaturated fatty acid suggests a close proximity between AtLOX2 Ser⁶⁰⁰ and the carboxylic acid head group of the polyunsaturated fatty acid. This analysis indicates that Ser⁶⁰⁰ is located at a critical position within the AtLOX2 structure and highlights how Ser⁶⁰⁰ phosphorylation could affect AtLOX2 catalytic activity. Overall, we propose that AtLOX2 Ser⁶⁰⁰ phosphorylation represents a key mechanism for the regulation of AtLOX2 activity and, thus, the jasmonate biosynthesis pathway and plant resistance.

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.

Identification of the key genes contributing to the LOX-HPL volatile aldehyde biosynthesis pathway in jujube fruit

Jujube (Ziziphus jujuba Mill.) is a traditional popular fruit widely grown in China. The volatiles in jujube determine its unique flavor and the high fruit quality required by consumers. However, the biosynthesis of volatiles in jujube were remain unknown. By using gas chromatography-mass spectrometry, there were 46 volatile compounds were identified and determined from three representative jujube fruit types at six developmental stages, including the dry-used (Z. jujuba cv. 'Junzao'), the fresh-used (Z. jujuba cv. 'Dongzao'), and wild jujube (Z. jujuba var. spinosa Hu. cv. 'Qingjiansuanzao'). The aldehydes were identified as major volatile contributors to flavor, of which (E)-2-hexenal was the primary volatile in jujube fruit. Then LOX and HPL gene family were identified in jujube, which were involved in aldehyde biosynthesis through the lipoxygenase-hydroperoxide lyase (LOX-HPL) pathway. Gene expression analysis suggested that ZjLOX3, ZjLOX4, and ZjHPL1 were highly correlated with the accumulation of (E)-2-hexenal, and their proteins were localized to the nucleus and cytoplasm. Transient over-expression of ZjLOX3, ZjLOX4, and ZjHPL1 in jujube fruit significantly enhanced the accumulation of (E)-2-hexenal. Our study provides valuable information on the major volatiles and their biosynthesis in different types of jujube fruit. These results will help determine flavor improvements for future breeding.

Research Progress on Genetic Basis of Fruit Quality Traits in Apple (Malus × domestica)

Identifying the genetic variation characteristics of phenotypic traits is important for fruit tree breeding. During the long-term evolution of fruit trees, gene recombination and natural mutation have resulted in a high degree of heterozygosity. Apple (Malus × domestica Borkh.) shows strong ecological adaptability and is widely cultivated, and is among the most economically important fruit crops worldwide. However, the high level of heterozygosity and large genome of apple, in combination with its perennial life history and long juvenile phase, complicate investigation of the genetic basis of fruit quality traits. With continuing augmentation in the apple genomic resources available, in recent years important progress has been achieved in research on the genetic variation of fruit quality traits. This review focuses on summarizing recent genetic studies on apple fruit quality traits, including appearance, flavor, nutritional, ripening, and storage qualities. In addition, we discuss the mapping of quantitative trait loci, screening of molecular markers, and mining of major genes associated with fruit quality traits. The overall aim of this review is to provide valuable insights into the mechanisms of genetic variation and molecular breeding of important fruit quality traits in apple.

Promoter cloning of PuLOX2S gene from "Nanguo" pears and screening of transcription factors by Y1H technique

Our previous study on differential proteome and transcriptome of refrigerated "Nanguo" pears found that the PuLOX2S gene was very active in the LOX pathway of aroma synthesis, but the regulation of expression behavior of the gene and how to mediate the aroma synthesis were still unknown. Partial genome sequences of PuLOX2S were cloned, and its promoter was analyzed by Tail-PCR. The PuLOX2S promoter sequences of 610 bp were isolated and identified using Plant CARE, which were composed of cis-acting elements, such as ABRE, AE-box, ARE, CAAT-box, Box 4, TCCC-motif, CAT-box, CGTCA-motif, G-Box, TATA-box, TCA-element, TGA-element, and TGACG-motif. The Y1H technology was used to determine whether proteins interacted with PuLOX2S based on the pGADT7-Chinese white pear cDNA library. The Y1H results were shown that 52 proteins could interact with the PuLOX2S promoter, which was compared with sequences in the GenBank database. The three genes PuERF12, PuMYB44, and PuRF2a were the candidate transcription factors of PuLOX2S and PuCDPK10 played an important role in the gene expression in Nanguo pears. Therefore, the results of this study supply important information for revealing new function of PuLOX2S and the regulation mechanism of expression behavior of the gene. It provides new ideas for the regulation of aroma synthesis in Nanguo pears. PRACTICAL APPLICATIONS: The gene PuLOX2S was very active in the LOX pathway of aroma synthesis, but the regulation of expression behavior of the gene and how to mediate the aroma synthesis were still unknown. We have successfully cloned the partial sequence of the gene and the 610 bp promoter sequence upstream of PuLOX2S and analyzed the structure of cis-acting elements. There are 52 proteins that interact with the PuLOX2S promoter revealed by the Y1H technique. Three transcription factors among the proteins can regulate the level of PuLOX2S expression, which provides new ideas for the regulation of aroma synthesis in "Nanguo" pears. Moreover, the study results could supply scientific information for the quality improvement and genetic modification of Nanguo pears.

Storage Stability of Nutritional Qualities, Enzyme Activities, and Volatile Compounds of "Hangjiao No. 2" Chili Pepper Treated With Different Concentrations of 1-Methyl Cyclopropene

This study aimed to determine the effects of different concentrations of 1-methyl cyclopropene (1-MCP) on the nutritional quality, antioxidant enzyme activities, and volatile compounds of "Hangjiao No.2" chili pepper during 12 days of storage at ambient temperature. The chili fruit were randomly selected and divided into four groups corresponding to the four treatments, thus, 0.5, 1.0, and 1.5 μl L^-1 1-MCP and a control. The analysis of the nutritional value, enzyme activities, and volatile compounds were determined at 3 days interval. The results showed that the malondialdehyde (MDA) content was lower in the fruit treated with 1-MCP compared to the control. The treatment with 1.5 μl L^-1 and the control showed the lowest superoxide dismutase (SOD) activity compared to the other treatments. Peroxidase (POD) and Catalase (CAT) were highest in the fruit treated with 0.5 μl L^-1 compared to the control and treatment with 1.0 μl L^-1. The 1.5 μl L^-1 treatment delayed the decline in vitamin C and protein content compared to the control. Nitrate levels increased 1.34-fold at 0.5 μl L^-1 and 2.01-fold in the control. Chlorophyll content degradation was delayed at 1.0 μl L^-1 compared to the control. A total of 88 volatile compounds, including terpenes, aldehydes, alkanes, esters, alcohols, acids, phenolic derivatives, ketones, and other aromatic compounds, were detected in "Hangjiao No.2" pepper during the 12-day storage period and treatment concentrations. The production of volatile terpenes was higher in the control than in the 1-MCP treatments, while the 0.5 μl L^-1 1-MCP treatment generally suppressed the production of volatile compounds during storage. Overall, the production of volatile compounds after treatment was higher in the "Hangjiao No.2" chili fruit treated with 1.0 μl L^-1 1-MCP than in the other treatments throughout the storage period. The results indicate that 1-MCP treatment was more effective in maintaining fruit quality, enhancing the activities of SOD, POD, and CAT, retarding the accumulation of MDA and restoring volatile aromas, with 1.0 μl L^-1 having the best preservative effect on "Hangjiao No.2" chili fruit during storage, which could be useful for future marketing and processing.

Exogenous ABA promotes aroma biosynthesis of postharvest kiwifruit after low-temperature storage

Exogenous ABA played a positive role in the accumulation and biosynthesis of aroma components of postharvest kiwifruit after low-temperature storage, especially the esters production during ripening. Low-temperature storage (LTS) generally affects the aroma formation associated with the decrease in aroma quality in kiwifruit. In this work, abscisic acid (ABA) treatment after LTS increased the production of aroma components in postharvest kiwifruit and enhanced the related enzyme activity, especially alcohol acyltransferase (AAT), branched amino acid transaminase (BCAT) and hydroperoxide lyase (HPL). Corresponding to the enzyme activity, the gene expression of AchnAAT, AchnADH, AchnBCAT and AchnHPL was significantly up-regulated by ABA. The principal component analysis further illustrated the differences in aroma components between ABA and the control. The positive correlation of aroma accumulation with the expression levels of AchnPDC and AchnLOX and the enzyme activities of BCAT and pyruvate decarboxylase (PDC) was also revealed by correlation analysis. In addition, the promoter sequences of the key genes involved in aroma biosynthesis contained multiple cis-elements (ABRE and G-box) of ABA-responsive proteins. Combining the transcriptome sequencing data, the promoting role of ABA signaling in the regulation of aroma biosynthesis of postharvest kiwifruit after LTS was discussed. This study would provide a reference for improving aroma quality of postharvest kiwifruit after LTS, as well the molecular mechanism of kiwifruit aroma fading after LTS.

Transcriptome and Metabolome Analyses Provide Insights Into the Composition and Biosynthesis of Grassy Aroma Volatiles in White-Fleshed Pitaya

Aroma is one of the major inherent quality characteristics in fruits. Understanding the composition of aroma volatiles and their biosynthesis mechanism is crucial to improving fruit quality. However, the biosynthesis mechanism of aroma volatiles has not been characterized yet in white-fleshed pitaya (Hylocereus undatus). This study was performed to investigate aroma volatiles and related gene expression patterns in the pulp of "mild grassy" and "strong grassy" aroma cultivars. Analysis of volatile composition and concentration showed that aldehydes, alcohols, esters, and alkenes were predominant in both cultivars. However, comparative analysis revealed a significant difference in the concentration of several metabolites, particularly hexanal and 1-hexanol. The results of the comparative transcriptome identified a large number of aroma-related differentially expressed genes. The majority of these genes were enriched in fatty acid and isoleucine degradation pathways. According to integrative analyses, changes in the expression of lipoxygenase pathway genes, specifically FAD, LOXs, HPLs, and ADHs, probably lead to the difference in strength of "grassy" aroma between both cultivars. The qRT-PCR of 18 aroma-related genes was performed to validate the transcriptome analysis. Our results identified key genes and pathways connected with the biosynthesis of aroma volatiles in white-fleshed pitaya. These results will be useful to dissect the genetic mechanism of fruit aroma in white-fleshed pitaya.

Genome Wide MeDIP-Seq Profiling of Wild and Cultivated Olives Trees Suggests DNA Methylation Fingerprint on the Sensory Quality of Olive Oil

Secondary metabolites are particularly important to humans due to their pharmaceutical properties. Moreover, secondary metabolites are key compounds in climate change adaptation in long-living trees. Recently, it has been described that the domestication of Olea subspecies had no major selection signature on coding variants and was mainly related to changes in gene expression. In addition, the phenotypic plasticity in Olea subspecies was linked to the activation of transposable elements in the genes neighboring. Here, we investigated the imprint of DNA methylation in the unassigned fraction of the phenotypic plasticity of the Olea subspecies, using methylated DNA immuno-precipitation sequencing (MeDIP-seq) for a high-resolution genome-wide DNA methylation profiling of leaves and fruits during fruit development in wild and cultivated olives from Turkey. Notably, the methylation profiling showed a differential DNA methylation in secondary metabolism responsible for the sensory quality of olive oil. Here, we highlight for the first time the imprint of DNA methylation in modulating the activity of the Linoleate 9S lipoxygenase in the biosynthesis of volatile aromatic compounds. Unprecedently, the current study reveals the methylation status of the olive genome during fruit ripening.

Effects of different processing steps on the flavor and colloidal properties of cloudy apple juice

BACKGROUND

Flavor loss is a common problem when manufacturing apple juice and is closely related to the properties of the colloidal pectin particles in cloudy juice. The flavor changes and particle properties of three varieties of apple juice ('Ralls', 'Golden Delicious' and 'Fuji') during processing were investigated.

RESULTS

Compared with manually pressed juice, juice made by industrial pulping and filtration contained larger particles, resulting in the 'sweet and sour' taste of the juice being relatively weak and the diversity of aromas narrower, as determined by E-nose analysis. Pulping and filtration, however, released some important flavor esters, such as butyl butyrate, hexyl-2-methyl butyrate, and hexyl butyrate. The transformation of volatile compounds during apple juice processing was closely related to the apple cultivar but, in all three varieties, the content of 1-hexanal and (E)-2-hexenal in the juice gradually decreased during processing. Pectinase treatment reduced the colloid particle size and increased the ζ-potential of the juice, resulting in better uniformity and stability, as well as increasing the content of nonanal. After pasteurization, the colloidal particles tended to aggregate and the ζ-potential decreased. Many volatile compounds decreased in concentration or disappeared after heat treatment.

CONCLUSION

The flavor and colloidal properties of cloudy apple juice changed markedly during processing and the effect of each processing step was different. © 2020 Society of Chemical Industry.

Identification, Comparison and Classification of Volatile Compounds in Peels of 40 Apple Cultivars by HS-SPME with GC-MS

Aroma is an important quality indicator for apples and has a great influence on the overall flavour and consumer acceptance. However, the information of the aroma volatile compounds in apple peels is largely unknown. In this study, evaluation of volatile compounds in peels of 40 apple cultivars was carried out using headspace solid-phase microextraction (HS-SPME) coupled with gas chromatography-mass spectrometry (GC-MS). A total of 78 volatile compounds were identified, including 47 esters, 12 aldehydes, 5 alcohols, 3 ketones, 1 acid and 10 others. Eight volatile compounds were common in all apple cultivars. Cultivar Changfu No. 2 contained the highest number of volatile compounds (47), while Qinyue contained the least (20). Honey Crisps had the highest volatile content, at 27,813.56 ± 2310.07 μg/kg FW, while Huashuo had the lowest volatile content, at 2041.27 ± 120.36 μg/kg FW. Principal component analysis (PCA) clustered the 40 apple cultivars into five groups. Aroma is cultivar-specific, volatile compounds of hexyl butyrate, hexyl 2-methylbutyrate and hexyl hexanoate, together with hexanal, (E)-2-hexenal, 1-hexanol, estragole and α-farnesene could be proposed for apple cultivar classification in the future.

Candidate genes and signatures of directional selection on fruit quality traits during apple domestication

PREMISE

During plant domestication, traits can be subject to a variety of types of selection, ranging from strong directional selection for traits such as seed or fruit size to diversifying selection for traits like color or flavor. These types of selection interact with other evolutionary processes including genetic bottlenecks and interspecific gene flow to generate different levels of genetic diversity across the genome and at target genes in domesticated lineages, but little is known about the impacts of these processes in perennial fruit crops.

METHODS

We used sequence capture by hybridization to examine patterns of diversity at a suite of candidate domestication and anonymous background genes in domesticated apple (Malus ×domestica) in comparison to its wild relatives Malus sieversii and Malus orientalis.

RESULTS

We found no change in average diversity at these candidate domestication genes across the three species. However, a subset of the genes did exhibit patterns of very high or very low diversity in M. ×domestica compared to its progenitor, M. sieversii. Of the genes with characterized function, the low-diversity genes mainly contributed to fruit quality traits like color and flavor, predicted to be under conscious, directional selection relatively late in the domestication process, while the high-diversity genes included a variety of functions.

CONCLUSIONS

Overall, these results are consistent with predictions based on the likely timing and nature of selection during domestication and open new avenues for understanding genes with high diversity in a perennial crop compared to its wild relatives.

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.

Expression of Castanea crenata Allene Oxide Synthase in Arabidopsis Improves the Defense to Phytophthora cinnamomi

Allene oxide synthase (AOS) is a key enzyme of the jasmonic acid (JA) signaling pathway. The AOS gene was previously found to be upregulated in an Asian chestnut species resistant to infection by the oomycete Phytophthora cinnamomi (Castanea crenata), while lower expression values were detected in the susceptible European chestnut (Castanea sativa). Here, we report a genetic and functional characterization of the C. crenata AOS (CcAOS) upon its heterologous gene expression in a susceptible ecotype of Arabidopsis thaliana, which contains a single AOS gene. It was found that Arabidopsis plants expressing CcAOS delay pathogen progression and exhibit more vigorous growth in its presence. They also show upregulation of jasmonic acid and salicylic acid-related genes. As in its native species, heterologous CcAOS localized to plastids, as revealed by confocal imaging of the CcAOS-eGFP fusion protein in transgenic Arabidopsis roots. This observation was confirmed upon transient expression in Nicotiana benthamiana leaf epidermal cells. To further confirm a specific role of CcAOS in the defense mechanism against the pathogen, we performed crosses between transgenic CcAOS plants and an infertile Arabidopsis AOS knockout mutant line. It was found that plants expressing CcAOS exhibit normal growth, remain infertile but are significantly more tolerant to the pathogen than wild type plants. Together, our results indicate that CcAOS is an important player in plant defense responses against oomycete infection and that its expression in susceptible varieties may be a valuable tool to mitigate biotic stress responses.

Evaluation of Physiological Characteristics, Soluble Sugars, Organic Acids and Volatile Compounds in 'Orin' Apples (Malus domestica) at Different Ripening Stages

‘Orin’ is a popular apple cultivar, which has a yellow-green appearance, pleasant taste, and unique aroma. However, few studies on the fruit quality characteristics of ‘Orin’ apples have been reported before. In this study, changes of the physiological characteristics were measured at different ripening stages, and the soluble sugars and organic acids were determined by high-performance liquid chromatography (HPLC). Volatile compounds were identified using the headspace solid-phase microextraction (HS-SPME) coupled with gas chromatography-mass spectrometry (GC-MS). During the fruit ripening, the ‘Orin’ apple fruit weight, size, and total soluble solid were gradually increased by contrast with the titratable acidity, and the firmness decreased. The content of four soluble sugars reached the maximum at the 180 days after full bloom (DAFB) stage. Malic acid was measured as the most abundant organic acid in ‘Orin’ apples. Ethyl butyrate, hexyl propanoate, hexyl acetate and butyl acetate belonging to esters with high odor activity values (OAVs) could be responsible for the typical aroma of ‘Orin’ apples. The aim of this work was to provide information on the flavor characteristics of ‘Orin’ apples and promote this apple cultivar for marketing and processing in the future.

Optimizing the Production of Recombinant Hydroperoxide Lyase in Escherichia coli Using Statistical Design

Hydroperoxide lyase (HPL) catalyzes the synthesis of volatiles C6 or C9 aldehydes from fatty acid hydroperoxides. These short carbon chain aldehydes, known as green leaf volatiles (GLV), are widely used in cosmetic industries and as food additives because of their “fresh green” aroma. To meet the growing demand for natural GLVs, the use of recombinant HPL as a biocatalyst in enzyme-catalyzed processes appears to be an interesting application. Previously, we cloned and expressed a 13-HPL from olive fruit in Escherichia coli and showed high conversion rates (up to 94%) during the synthesis of C6 aldehydes. To consider a scale-up of this process, optimization of the recombinant enzyme production is necessary. In this study, four host-vector combinations were tested. Experimental design and response surface methodology (RSM) were used to optimize the expression conditions. Three factors were considered, i.e., temperature, inducer concentration and induction duration. The Box–Behnken design consisted of 45 assays for each expression system performed in deep-well microplates. The regression models were built and fitted well to the experimental data (R2 coefficient > 97%). The best response (production level of the soluble enzyme) was obtained with E. coli BL21 DE3 cells. Using the optimal conditions, 2277 U L−1of culture of the soluble enzyme was produced in microliter plates and 21,920 U L−1of culture in an Erlenmeyer flask, which represents a 79-fold increase compared to the production levels previously reported.

Methyl jasmonate pretreatment improves aroma quality of cold-stored 'Nanguo' pears by promoting ester biosynthesis

Cold storage is widely used for delaying ripening and senescence; however, fruit aroma diminishes noticeably after long-term cold storage. The esters synthesized by the lipoxygenase (LOX) pathway are responsible for 'Nanguo' pear aroma. As methyl jasmonate (MeJA) is known to act on various fruit qualities, we investigated whether it acts via the LOX pathway in cold-stored 'Nanguo' pears. MeJA treatment increased the content of volatile esters and unsaturated fatty acids and the activities of alcohol acyltransferase, alcohol dehydrogenase, and LOX. It also up-regulated the expression of key genes (PuAAT, PuADH3, PuADH5, PuADH9, PuLOX1, and PuLOX3) in the LOX pathway and that of transcription factors (PuMYB21-like, PuMYB108-like, PuWRKY61, PuWRKY72, and PuWRKY31), whose genes were differentially expressed in preliminary transcriptome analysis. Therefore, considering its effects on LOX pathway-related genes and transcription factors, MeJA may be useful in preventing cold-storage-induced decline in ester biosynthesis, aroma, and consequently the quality of cold-stored 'Nanguo' pears.

Decoding altitude-activated regulatory mechanisms occurring during apple peel ripening

Apple (Malus domestica Borkh) is an important fruit crop cultivated in a broad range of environmental conditions. Apple fruit ripening is a physiological process, whose molecular regulatory network response to different environments is still not sufficiently investigated and this is particularly true of the peel tissue. In this study, the influence of environmental conditions associated with low (20 m) and high (750 m) altitude on peel tissue ripening was assessed by physiological measurements combined with metabolomic and proteomic analyses during apple fruit development and ripening. Although apple fruit ripening was itself not affected by the different environmental conditions, several key color parameters, such as redness and color index, were notably induced by high altitude. Consistent with this observation, increased levels of anthocyanin and other phenolic compounds, including cyanidin-3-O-galactoside, quercetin-3-O-rhamnoside, quercetin-3-O-rutinoside, and chlorogenic acid were identified in the peel of apple grown at high altitude. Moreover, the high-altitude environment was characterized by elevated abundance of various carbohydrates (e.g., arabinose, xylose, and sucrose) but decreased levels of glutamic acid and several related proteins, such as glycine hydroxymethyltransferase and glutamate-glyoxylate aminotransferase. Other processes affected by high altitude were the TCA cycle, the synthesis of oxidative/defense enzymes, and the accumulation of photosynthetic proteins. From the obtained data we were able to construct a metabolite-protein network depicting the impact of altitude on peel ripening. The combined analyses presented here provide new insights into physiological processes linking apple peel ripening with the prevailing environmental conditions.

Recombinant Lipoxygenases and Hydroperoxide Lyases for the Synthesis of Green Leaf Volatiles

Green leaf volatiles (GLVs) are mainly C₆- and in rare cases also C₉-aldehydes, -alcohols, and -esters, which are released by plants in response to biotic or abiotic stresses. These compounds are named for their characteristic smell reminiscent of freshly mowed grass. This review focuses on GLVs and the two major pathway enzymes responsible for their formation: lipoxygenases (LOXs) and fatty acid hydroperoxide lyases (HPLs). LOXs catalyze the peroxidation of unsaturated fatty acids, such as linoleic and α-linolenic acids. Hydroperoxy fatty acids are further converted by HPLs into aldehydes and oxo-acids. In many industrial applications, plant extracts have been used as LOX and HPL sources. However, these processes are limited by low enzyme concentration, stability, and specificity. Alternatively, recombinant enzymes can be used as biocatalysts for GLV synthesis. The increasing number of well-characterized enzymes efficiently expressed by microbial hosts will foster the development of innovative biocatalytic processes for GLV production.

Purification and Product Characterization of Lipoxygenase from Opium Poppy Cultures (Papaver somniferum L.)

Opium poppy (Papaver somniferum L.) is an ancient medicinal plant producing pharmaceutically important benzylisoquinoline alkaloids. In the present work we focused on the study of enzyme lipoxygenase (LOX, EC 1.13.11.12) from opium poppy cultures. LOX is involved in lipid peroxidation and lipoxygenase oxidation products of polyunsaturated fatty acids have a significant role in regulation of growth, development and plant defense responses to biotic or abiotic stress. The purpose of this study was to isolate and characterize LOX enzyme from opium poppy callus cultures. LOX was purified by ammonium sulfate precipitation and then followed by hydrophobic chromatography using Phenyl-Sepharose CL-4B and hydroxyapatite chromatography using HA Ultrogel sorbent. Sodium dodecyl sulphate-polyacrylamide gel electrophoresis (SDS-PAGE) analysis and immunoblotting revealed that LOX from opium poppy cultures was a single monomeric protein showing the relative molecular weight of 83 kDa. To investigate the positional specificity of the LOX reaction, purified LOX was incubated with linoleic acid and the products were analyzed by high-performance liquid chromatography in two steps, firstly with reverse phase (120-5 Nucleosil C18 column) and secondly with normal phase (Zorbax Rx-SIL column). LOX converted linoleic acid primarily to 13-hydroperoxy-(9Z,11E)-octadecadienoic acids (78%) and to a lesser extent 9-hydroperoxy-(10E,12Z)-octadecadienoic acids (22%). Characterization of LOX from opium poppy cultures provided valuable information in understanding LOX involvement in regulation of signaling pathways leading to biosynthesis of secondary metabolites with significant biological activity.

Biocatalytic Synthesis of Natural Green Leaf Volatiles Using the Lipoxygenase Metabolic Pathway

In higher plants, the lipoxygenase enzymatic pathway combined actions of several enzymes to convert lipid substrates into signaling and defense molecules called phytooxylipins including short chain volatile aldehydes, alcohols, and esters, known as green leaf volatiles (GLVs). GLVs are synthesized from C18:2 and C18:3 fatty acids that are oxygenated by lipoxygenase (LOX) to form corresponding hydroperoxides, then the action of hydroperoxide lyase (HPL) produces C6 or C9 aldehydes that can undergo isomerization, dehydrogenation, and esterification. GLVs are commonly used as flavors to confer a fresh green odor of vegetable to perfumes, cosmetics, and food products. Given the increasing demand in these natural flavors, biocatalytic processes using the LOX pathway reactions constitute an interesting application. Vegetable oils, chosen for their lipid profile are converted in natural GLVs with high added value. This review describes the enzymatic reactions of GLVs biosynthesis in the plant, as well as the structural and functional properties of the enzymes involved. The various stages of the biocatalytic production processes are approached from the lipid substrate to the corresponding aldehyde or alcoholic aromas, as well as the biotechnological improvements to enhance the production potential of the enzymatic catalysts.

A role for 9-lipoxygenases in maize defense against insect herbivory

Feeding by Spodoptera exigua (beet armyworm) larvae on Zea mays (maize) induces expression of 9-lipoxygenases to a greater extent than 13-lipoxygenases. Whereas 13-lipoxygenases have an established role in the synthesis of jasmonates that serve as defense signaling molecules in many plant species, relatively little is known about the role of 9-lipoxygenases in herbivore defense. Phylogenetic analysis of lipoxygenases from maize inbred lines B73 and W22 shows that, although most Lox genes are present in both lines, Lox12, a 9-lipoxygenase that has been implicated in fungal defense, is truncated and unlikely to encode a functional protein in W22. Two independent Mutator transposon insertions in another 9-lipoxygenase, Lox4, caused improved S. exigua growth on the mutant lines relative to wildtype W22. This observation suggests a function in herbivore defense for metabolic products downstream of maize Lox4, either through direct toxicity or a perhaps an as yet unknown signaling function.

Volatile Compound and Gene Expression Analyses Reveal Temporal and Spatial Production of LOX-Derived Volatiles in Pepino (Solanum muricatum Aiton) Fruit and LOX Specificity

Lipoxygenase (LOX) is an important contributor to aroma compounds in most fresh produce; however, little is known about the LOX pathway in pepino (Solanum muricatum Aiton) fruit. We explored the LOX aroma compounds produced by the flesh and the peel and identified eight putative LOX genes expressed in both tissues during fruit growth and development during two consecutive seasons. This study shows that pepino produces C5, C6, and C9 LOX-derived compounds. Odorant C9 volatiles were produced during immature stages with a concomitant decrease when the fruit ripens, whereas C5 and C6 compounds were formed throughout ripening. trans-2-Hexenal and its alcohol were produced in the peel, but not detected in the flesh. The expression of three genes, SmLOXD (putative 13-LOX), SmLOXB, and SmLOX5-like1 (putative 9-LOXs), increased during fruit ripening. These genes may account for aroma volatiles in pepino. Here, we discuss the possible roles of individual LOX genes in pepino.

Biochemistry of Apple Aroma: A Review

Flavour is a key quality attribute of apples defined by volatile aroma compounds. Biosynthesis of aroma compounds involves metabolic pathways in which the main precursors are fatty and amino acids, and the main products are aldehydes, alcohols and esters. Some enzymes are crucial in the production of volatile compounds, such as lipoxygenase, alcohol dehydrogenase, and alcohol acyltransferase. Composition and concentration of volatiles in apples may be altered by pre- and postharvest factors that cause a decline in apple flavour. Addition of biosynthetic precursors of volatile compounds may be a strategy to promote aroma production in apples. The present manuscript compiles information regarding the biosynthesis of volatile aroma compounds, including metabolic pathways, enzymes and substrates involved, factors that may affect their production and also includes a wide number of studies focused on the addition of biosynthetic precursors in their production.

Synthesis and Functions of Jasmonates in Maize

Of the over 600 oxylipins present in all plants, the phytohormone jasmonic acid (JA) remains the best understood in terms of its biosynthesis, function and signaling. Much like their eicosanoid analogues in mammalian system, evidence is growing for the role of the other oxylipins in diverse physiological processes. JA serves as the model plant oxylipin species and regulates defense and development. For several decades, the biology of JA has been characterized in a few dicot species, yet the function of JA in monocots has only recently begun to be elucidated. In this work, the synthesis and function of JA in maize is presented from the perspective of oxylipin biology. The maize genes responsible for catalyzing the reactions in the JA biosynthesis are clarified and described. Recent studies into the function of JA in maize defense against insect herbivory, pathogens and its role in growth and development are highlighted. Additionally, a list of JA-responsive genes is presented for use as biological markers for improving future investigations into JA signaling in maize.

Proteome changes in banana fruit peel tissue in response to ethylene and high-temperature treatments

Banana (Musa AAA group) is one of the most consumed fruits in the world due to its flavor and nutritional value. As a typical climacteric fruit, banana responds to ethylene treatment, which induces rapid changes of color, flavor (aroma and taste), sweetness and nutritional composition. It has also been reported that ripening bananas at temperatures above 24 °C inhibits chlorophyll breakdown and color formation but increases the rate of senescence. To gain fundamental knowledge about the effects of high temperature and ethylene on banana ripening, a quantitative proteomic study employing multiplex peptide stable isotope dimethyl labeling was conducted. In this study, green (immature) untreated banana fruit were subjected to treatment with 10 μL L(-1) of ethylene for 24 h. After ethylene treatment, treated and untreated fruit were stored at 20 or 30 °C for 24 h. Fruit peel tissues were then sampled after 0 and 1 day of storage, and peel color and chlorophyll fluorescence were evaluated. Quantitative proteomic analysis was conducted on the fruit peels after 1 day of storage. In total, 413 common proteins were identified and quantified from two biological replicates. Among these proteins, 91 changed significantly in response to ethylene and high-temperature treatments. Cluster analysis on these 91 proteins identified 7 groups of changed proteins. Ethylene treatment and storage at 20 °C induced 40 proteins that are correlated with pathogen resistance, cell wall metabolism, ethylene biosynthesis, allergens and ribosomal proteins, and it repressed 36 proteins that are associated with fatty acid and lipid metabolism, redox-oxidative responses, and protein biosynthesis and modification. Ethylene treatment and storage at 30 °C induced 32 proteins, which were mainly similar to those in group 1 but also included 8 proteins in group 3 (identified as chitinase, cinnamyl alcohol dehydrogenase 1, cysteine synthase, villin-2, leucine-transfer RNA ligase, CP47 protein and calmodulin) and repressed 43 proteins in 4 groups (groups 4-7), of which 6 were associated with photosynthesis II oxygen-evolving protein, the photosynthesis I reaction center, sugar metabolism, the redox-oxidative system and fatty acid metabolism. Differences in the response to ethylene and holding temperature at 30 °C were also revealed and have been discussed. The identities and quantities of the proteins found were linked with quality changes. This study demonstrates that ethylene and high temperature influence banana fruit ripening and senescence at the proteomic level and reveals the mechanisms by which high temperature accelerates banana fruit ripening.

Ethylene and 1-MCP regulate major volatile biosynthetic pathways in apple fruit

The effects of ethylene and 1-methylcyclopropene (1-MCP) on apple fruit volatile biosynthesis and gene expression were investigated. Statistical analysis identified 17 genes that changed significantly in response to ethylene and 1-MCP treatments. Genes encoding branched-chain amino acid aminotransferase (BCAT), aromatic amino acid aminotransferase (ArAT) and amino acid decarboxylases (AADC) were up-regulated during ripening and further enhanced by ethylene treatment. Genes related to fatty acid synthesis and metabolism, including acyl-carrier-proteins (ACPs), malonyl-CoA:ACP transacylase (MCAT), acyl-ACP-desaturase (ACPD), lipoxygenase (LOX), hydroperoxide lyase (HPL), alcohol dehydrogenase (ADH), pyruvate decarboxylase (PDC2), β-oxidation, acyl-CoA synthetase (ACS), enoyl-CoA hydratase (ECHD), acyl-CoA dehydrogenase (ACAD), and alcohol acyltransferases (AATs) also increased during ripening and in response to ethylene treatment. Allene oxide synthase (AOS), alcohol dehydrogenase 1 (ADH1), 3-ketoacyl-CoA thiolase and branched-chain amino acid aminotransferase 2 (BCAT2) decreased in ethylene-treated fruit. Treatment with 1-MCP and ethylene generally produced opposite effects on related genes, which provides evidence that regulation of these genes is ethylene dependent.

Volatile profiles of members of the USDA Geneva Malus Core Collection: utility in evaluation of a hypothesized biosynthetic pathway for esters derived from 2-methylbutanoate and 2-methylbutan-1-ol

The volatile ester and alcohol profiles of ripening apple fruit from 184 germplasm lines in the USDA Malus Germplasm Repository at the New York Agricultural Experiment Station in Geneva, NY, USA, were evaluated. Cluster analysis suggested biochemical relationships exist between several ester classes. A strong linkage was revealed between 2-methylbutanoate, propanoate, and butanoate esters, suggesting the influence of the recently proposed "citramalic acid pathway" in apple fruit. Those lines with a high content of esters formed from 2-methylbutan-1-ol and 2-methylbutanoate (2MB) relative to straight-chain (SC) esters (high 2MB/SC ratio) exhibited a marked increase in isoleucine and citramalic acid during ripening, but those lines with a low content did not. Thus, the data were consistent with the existence of the hypothesized citramalic acid pathway and suggest that the Geneva Malus Germplasm Repository, appropriately used, could be helpful in expanding our understanding of mechanisms for fruit volatile synthesis and other aspects of secondary metabolism.