Molecular cloning and expression of a gene encoding alcohol acyltransferase (MdAAT2) from apple (cv. Golden Delicious)

Genome-Wide Identification and Expression Pattern Analysis of BAHD Acyltransferase Family in Taxus mairei

BAHD acyltransferases are involved in catalyzing and regulating the secondary metabolism in plants. Despite this, the members of BAHD family and their functions have not been reported in the Taxus species. In this study, a total of 123 TwBAHD acyltransferases from Taxus wallichiana var. mairei genome were identified and divided into six clades based on phylogenetic analysis, of which Clade VI contained a Taxus-specific branch of 52 members potentially involved in taxol biosynthesis. Most TwBAHDs from the same clade shared similar conserved motifs and gene structures. Besides the typical conserved motifs within the BAHD family, the YPLAGR motif was also conserved in multiple clades of T. mairei. Moreover, only one pair of tandem duplicate genes was found on chromosome 1, with a Ka/Ks ratio < 1, indicating that the function of duplicate genes did not differentiate significantly. RNA-seq analysis revealed different expression patterns of TwBAHDs in MeJA induction and tissue-specific expression experiments. Several TwBAHD genes in the Taxus-specific branch were highly expressed in different tissues of T. mairei, suggesting an important role in the taxol pathway. This study provides comprehensive information for the TwBAHD gene family and sets up a basis for its potential functions.

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.

Comparative Analysis of Metabolites of 'Hongro' Apple Greasiness in Response to Temperature

Greasiness in apple skin reduces its quality, and its level varies depending on the variety. In this study, low-temperature (1 ± 0.5 °C) stored 'Hongro' and 'Fuji', which had differences in the occurrence of greasiness, were moved to room temperature (20 °C) and untargeted metabolite and fatty acids for skin and flesh along with quality changes due to greasiness occurrence were compared. Ethylene production differed noticeably between the two varieties and increased rapidly in 'Hongro' until 9 d of room-temperature storage. The ethylene production did not differ significantly between the two varieties on day 20 when greasiness occurred. According to the PLS-DA score plot, while 'Hongro' had similar amounts of unsaturated and saturated fatty acids, 'Fuji' had approximately twice as much unsaturated-fatty-acid content. 'Hongro', after 50 d of low-temperature (1 ± 0.5 °C) storage, produced excessive ethylene during room-temperature storage, which was directly related to greasiness development. As a result, the primary wax components of greasy 'Hongro' were nonacosane and nonacosan-10-ol. As the room-temperature storage period elapsed, pentyl linoleate and α-farnesene contents increased significantly. Furthermore, these greasiness-triggering characteristics of 'Hongro' may have been genetically influenced by the paternal parent used during breeding.

Expression of alcohol acyltransferase is a potential determinant of fruit volatile ester variations in Capsicum

KEY MESSAGE

The transcript level of alcohol acyltransferase 1 (AAT1) may be the main factor influencing the variations in volatile esters that characterizing the fruity/exotic aroma of pepper fruit. Volatile esters are key components for characterizing the fruity/exotic aroma of pepper (Capsicum spp.) fruit. In general, the volatile ester content in the fruit is the consequence of a delicate balance between their synthesis by alcohol acyltransferases (AATs) and degradation by carboxylesterases (CXEs). However, the precise role of these families of enzymes with regard to volatile ester content remains unexplored in Capsicum. In this study, we found that the volatile ester content was relatively low in C. annuum and much higher in C. chinense, particularly in pungent varieties. Additionally, fruits collected from multiple non-pungent C. chinense varieties, which harbor loss-of-function mutations in capsaicinoid biosynthetic genes, acyltransferase (Pun1), putative aminotransferase (pAMT), or putative ketoacyl-ACP reductase (CaKR1) were analyzed. The volatile ester contents of non-pungent C. chinense varieties (pamt/pamt) were equivalent to those of pungent varieties, but their levels were significantly lower in non-pungent NMCA30036 (pun12/pun12) and C. chinense (Cakr1/Cakr1) varieties. Multiple AAT-like sequences were identified from the pepper genome sequences, whereas only one CXE-like sequence was identified. Among these, AAT1, AAT2, and CXE1 were isolated from fruits of C. chinense and C. annuum. Gene expression analysis revealed that the AAT1 transcript level is a potential determinant of fruit volatile ester variations in Capsicum. Furthermore, enzymatic assays demonstrated that AAT1 is responsible for the biosynthesis of volatile esters in pepper fruit. Identification of a key gene for aroma biosynthesis in pepper fruit will provide a theoretical basis for the development of molecular tools for flavor improvement.

Family characteristics, phylogenetic reconstruction, and potential applications of the plant BAHD acyltransferase family

The BAHD acyltransferase family is a class of proteins in plants that can acylate a variety of primary and specialized secondary metabolites. The typically acylated products have greatly improved stability, lipid solubility, and bioavailability and thus show significant differences in their physicochemical properties and pharmacological activities. Here, we review the protein structure, catalytic mechanism, and phylogenetic reconstruction of plant BAHD acyltransferases to describe their family characteristics, acylation reactions, and the processes of potential functional differentiation. Moreover, the potential applications of the BAHD family in human activities are discussed from the perspectives of improving the quality of economic plants, enhancing the efficacy of medicinal plants, improving plant biomass for use in biofuel, and promoting stress resistance of land plants. This review provides a reference for the research and production of plant BAHD acyltransferases.

Changes in the VOC of Fruits at Different Refrigeration Stages of 'Ruixue' and the Participation of Carboxylesterase MdCXE20 in the Catabolism of Volatile Esters

Aroma is a crucial quality attribute of apple fruit, which significantly impacts its commercial value and consumer choice. Despite its importance the volatile aroma substances produced by the new variety 'Ruixue' after harvest remain unclear. In this study, we utilized headspace solid phase microextraction-gas chromatography-mass spectrometry (HS-SPME-GC-MS) to investigate the changes in volatile substances, fruit hardness, crispness, and related aroma synthase activity of commercially mature 'Ruixue' apples during cold storage. Our findings revealed a gradual decline in fruit firmness and brittleness of 'Ruixue' apples during cold storage, with hexyl acetate, hexyl caproate, and hexyl thiocyanate being the main hexyl esters detected. To gain a better understanding of the metabolic pathway of esters, we identified 42 MdCXE gene members that are associated with ester degradation. Through RT-qPCR analysis, we discovered that carboxylesterase MdCXE20 exhibited higher expression levels compared to other MdCXE genes during cold storage. To confirm the role of MdCXE20, we conducted a transient injection of apple fruits and observed that overexpression of MdCXE20 led to the degradation of esters such as hexyl hexanoate, butyl hexanoate, butyl 2-methylbutyrate, hexyl butyrate, and hexyl 2-methylbutyrate. The results of the study showed that the virus-induced gene silencing of MdCXE20 found the opposite results. Additionally, the esters of OE-MdCXE20 callus showed a lower content of ester VOC than the control callus, according to the homologous stable transformation of 'Wanglin' callus. Overall, these findings suggest that the MdCXE20 gene plays a crucial role in the decrease of esters in 'Ruixue' apples, which ultimately affects their flavor.

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.

Genetic analysis and QTL mapping of aroma volatile compounds in the apple progeny 'Fuji' × 'Cripps Pink'

Aroma is an essential trait for apple fruit quality, but the understanding of biochemical mechanisms underlying aroma formation is still limited. To better characterize and assess the genetic potential for improving aroma quality for breeding, many efforts have been paid to map quantitative trait loci (QTLs) using a saturated molecular linkage map. In the present study, aroma profiles in ripe fruit of F₁ population between 'Fuji' and 'Cripps Pink' were evaluated by gas chromatography-mass spectrometry (GC-MS) over 2019 and 2020 years, and the genetics of volatile compounds were dissected. In total, 38 volatile compounds were identified in 'Fuji' × 'Cripps Pink' population, including 23 esters, 3 alcohols, 7 aldehydes and 5 others. With the combination of aroma phenotypic data and constructed genetic linkage map, 87 QTLs were detected for 15 volatile compounds on 14 linkage groups (LGs). Among them, a set of QTLs associated with ester production identified and confirmed on LG 6. A candidate gene MdAAT6 in the QTL mapping interval was detected. Over-expression of MdAAT6 in tomato and apple fruits showed significantly higher esters accumulation compared to the control, indicating it was critical for the ester production. Our results give light on the mode of inheritance of the apple volatilome and provide new insights for apple flavor improvement in 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.

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.

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.

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.

Probing specificities of alcohol acyltransferases for designer ester biosynthesis with a high-throughput microbial screening platform

Alcohol acyltransferases (AATs) enables microbial biosynthesis of a large space of esters by condensing an alcohol and an acyl-CoA. However, substrate promiscuity of AATs prevents microbial biosynthesis of designer esters with high selectivity. Here, we developed a high-throughput microbial screening platform that facilitates rapid identification of AATs for designer ester biosynthesis. First, we established a microplate-based culturing technique with in situ fermentation and extraction of esters. We validated its capability in rapid profiling of the alcohol substrate specificity of 20 chloramphenicol acetyltransferase variants derived from Staphylococcus aureus (CAT_Sa ) for microbial biosynthesis of acetate esters with various exogeneous alcohol supply. By coupling the microplate-based culturing technique with a previously established colorimetric assay, we developed a high-throughput microbial screening platform for AATs. We demonstrated that this platform could not only probe the alcohol substrate specificity of both native and engineered AATs but also identify the beneficial mutations in engineered AATs for enhanced ester synthesis. We anticipate the high-throughput microbial screening platform provides a useful tool to identify novel wildtype and engineered AATs that have important roles in nature and industrial biocatalysis for designer bioester production.

Transcriptome and metabolite profiling analyses provide insight into volatile compounds of the apple cultivar 'Ruixue' and its parents during fruit development

BACKGROUND

Aroma is one the most crucial inherent quality attributes of fruit. 'Ruixue' apples were selected from a cross between 'Pink Lady' and 'Fuji', a later ripening yellow new cultivar. However, there is little known about the content and composition of aroma compounds in 'Ruixue' apples or the genetic characters of 'Ruixue' and its parents. In addition, the metabolic pathways for biosynthesis of aroma volatiles and aroma-related genes remain poorly understood.

RESULTS

Volatile aroma compounds were putatively identified using gas chromatography-mass spectrometry (GC-MS). Our results show that the profile of volatile compounds changes with ripening. Aldehydes were the dominant volatile compounds in early fruit development, with alcohols and esters increasing dramatically during maturation. On the basis of a heatmap dendrogram, these aroma compounds clustered into seven groups. In ripe fruit, esters and terpenoids were the main aroma volatiles in ripening fruit of 'Pink Lady' and 'Fuji' apples, and they included butyl 2-methylbutanoate; propanoic acid, hexyl ester; propanoic acid, hexyl ester; hexanoic acid, hexyl ester; acetic acid, hexyl ester and (Z, E)-α-farnesene. Interestingly, aldehydes and terpenoids were the dominant volatile aroma compounds in ripening fruit of 'Ruixue', and they mainly included hexanal; 2-hexenal; octanal; (E)-2-octenal; nonanal and (Z, E)-α-farnesene. By comparing the transcriptome profiles of 'Ruixue' and its parents fruits during development, we identified a large number of aroma-related genes related to the fatty acid, isoleucine and sesquiterpenoid metabolism pathways and transcription factors that may volatile regulate biosynthesis.

CONCLUSIONS

Our initial study facilitates a better understanding of the volatile compounds that affect fruit flavour as well as the mechanisms underlying differences in flavour between 'Ruixue' and its parents.

Transcriptional and epigenetic analysis reveals that NAC transcription factors regulate fruit flavor ester biosynthesis

Flavor-associated volatile chemicals make major contributions to consumers' perception of fruits. Although great progress has been made in establishing the metabolic pathways associated with volatile synthesis, much less is known about the regulation of those pathways. Knowledge of how those pathways are regulated would greatly facilitate efforts to improve flavor. Volatile esters are major contributors to fruity flavor notes in many species, providing a good model to investigate the regulation of volatile synthesis pathways. Here we initiated a study of peach (Prunus persica L. Batsch) fruits, and identified that the alcohol acyltransferase PpAAT1 contributes to ester formation. We next identified the transcription factor (TF) PpNAC1 as an activator of PpAAT1 expression and ester production. These conclusions were based on in vivo and in vitro experiments and validated by correlation in a panel of 30 different peach cultivars. Based on homology between PpNAC1 and the tomato (Solanum lycopersicum) TF NONRIPENING (NOR), we identified a parallel regulatory pathway in tomato. Overexpression of PpNAC1 enhances ripening in a nor mutant and restores synthesis of volatile esters in tomato fruits. Furthermore, in the NOR-deficient mutant tomatoes generated by CRISPR/Cas9, lower transcript levels of SlAAT1 were detected. The apple (Malus domestica) homolog MdNAC5 also stimulates MdAAT1 expression via binding to this gene's promoter. In addition to transcriptional control, epigenetic analysis showed that increased expression of NACs and AATs is associated with removal of the repressive mark H3K27me3 during fruit ripening. Our results support a conserved molecular mechanism in which NAC TFs activate ripening-related AAT expression, which in turn catalyzes volatile ester formation in multiple fruit species.

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.

Analysis of the postharvest storage characteristics of the new red-fleshed apple cultivar 'meihong'

This study examined the effects of postharvest storage conditions on the fruit quality of a new red-fleshed apple cultivar ('Meihong'). Mature 'Meihong' and 'Golden delicious' apples were exposed to room temperature, low temperature, and low temperature and 1-MCP, after which several fruit characteristics were evaluated (i.e., firmness, ethylene release rate, relative content of aroma components, phenolic compounds and antioxidant capacity, fruit softening-related enzyme activities, and related gene expression). Both 'Meihong' and 'Golden delicious' were ACS1-1/-2 heterozygotes, but the ethylene release rate in 'Meihong' fruits was lower than that in 'Golden delicious' fruits during storage. Therefore, 'Meihong' fruits are more conducive to storage. The low temperature storage with and without 1-MCP delayed fruit softening, decreased the ethylene release rate and ester aroma component content, and maintained total flavonoid and anthocyanin contents. Therefore, storage at low temperatures with 1-MCP or other preservatives may be useful for maintaining the 'Meihong' fruit quality.

3-D microstructural changes in relation to the evolution of quality during ripening of mango (Mangifera indica L. cv. Carabao)

BACKGROUND

The ripening of mango involves changes in texture, flavor, and color, affecting the quality of the fruit. Previous studies have investigated the physiology on the evolution of quality during ripening but only a few have looked at microstructural changes during ripening. None of them has provided an insight into the relationhip between 3-D microstructure and the evolution of quality during ripening. As the 3-D microstructure of fruit tissue determines its mechanical and gas-transport properties, it is likely to affect fruit texture, respiratory metabolism, and other ripening processes.

RESULTS

The present study focuses on the role of 3-D microstructural changes in relation to quality changes during mango ripening. Microstructural imaging using X-ray micro-computed tomography suggested the incidence of cell leakage, which was confirmed by the measurement of electrolyte leakage from the fruit peel. Due to cell leakage, porosity, pore connectivity, and pore local diameter were decreased whereas the tissue local diameter and pore specific area were increased. The decline in respiration and respiratory quotient during ripening followed the microstructural changes observed. Meanwhile, changes in aroma were observed such as a decrease in monoterpenes and an increase in esters and other fermentative metabolites.

CONCLUSION

Overall, the results provide a complete, integrated picture of microstructural changes during ripening accompanying the evolution of fruit quality, suggesting functional relationships between the two. © 2020 Society of Chemical Industry.

Transcriptome co-expression network analysis identifies key genes and regulators of ripening kiwifruit ester biosynthesis

BACKGROUND

Aroma is an important organoleptic quality for fruit and has a large influence on consumer preference. Kiwifruit esters undergo rapid and substantial changes contributing to the flavor during fruit ripening. Part of enzymes and their coding genes have been indicated potential candidates for flavor-related esters synthesis. However, there still exist obvious gaps in the biosynthetic pathways of esters and the mechanisms regulating ester biosynthesis in kiwifruit remain unknown.

RESULTS

Using gas chromatography-mass spectrometry (GC-MS), volatile compounds of kiwifruit were quantified in response to ethylene (ETH, 100 μl/l, 24 h, 20 °C) and 1-methylcyclopropene (1-MCP, 1 μl/l, 24 h, 20 °C). The results indicated that esters showed the most substantial changes enhanced by ethylene and were inhibited by 1-MCP. Correlations between RNA-seq results and concentrations of esters, constructed using Weighted Gene Co-Expression Network Analysis (WGCNA) indicated that three structural genes (fatty acid desaturase, AdFAD1; aldehyde dehydrogenase, AdALDH2; alcohol acyltransferase, AdAT17) had similar expression patterns that paralled the changes in total ester content, and AdFAD1 transcripts exhibited the highest correlation. In order to search for potential regulators for ester biosynthesis, 14 previously reported ethylene-responsive transcription factors (TFs) were included in the correlation analysis with esters and their biosynthetic genes. Using dual-luciferase assay, the in vivo regulatory activities of TFs on ester biosynthetic gene promoters were investigated and the results indicated that AdNAC5 and AdDof4 (DNA binding with one finger) trans-activated and trans-suppressed the AdFAD1 promoter.

CONCLUSIONS

The present study advanced the molecular basis of ripening-related ester biosynthesis in kiwifruit by identifying three biosynthetic related genes AdFAD1, AdALDH2 and AdAT17 by transcriptome analysis, and highlighted the function of two TFs by transactivation studies.

Genome-wide identification, characterization, expression and enzyme activity analysis of coniferyl alcohol acetyltransferase genes involved in eugenol biosynthesis in Prunus mume

Prunus mume, a traditional Chinese flower, is the only species of Prunus known to produce a strong floral fragrance, of which eugenol is one of the principal components. To explore the molecular mechanism of eugenol biosynthesis in P. mume, patterns of dynamic, spatial and temporal variation in eugenol were analysed using GC-MS. Coniferyl alcohol acetyltransferase (CFAT), a member of the BAHD acyltransferase family, catalyses the substrate of coniferyl alcohol to coniferyl acetate, which is an important substrate for synthesizing eugenol. In a genome-wide analysis, we found 90 PmBAHD genes that were phylogenetically clustered into five major groups with motif compositions relatively conserved in each cluster. The phylogenetic tree showed that the PmBAHD67-70 proteins were close to the functional CFATs identified in other species, indicating that these four proteins might function as CFATs. In this work, 2 PmCFAT genes, named PmCFAT1 and PmCFAT2, were cloned from P. mume ‘Sanlunyudie’, which has a strong fragrance. Multiple sequences indicated that PmCFAT1 contained two conserved domains, HxxxD and DFGWG, whereas DFGWG in PmCFAT2 was changed to DFGFG. The expression levels of PmCFAT1 and PmCFAT2 were examined in different flower organs and during the flowering stages of P. mume ‘Sanlunyudie’. The results showed that PmCFAT1 was highly expressed in petals and stamens, and this expression increased from the budding stage to the full bloom stage and decreased in the withering stage, consistent with the patterns of eugenol synthesis and emission. However, the peak of gene expression appeared earlier than those of eugenol synthesis and emission. In addition, the expression level of PmCFAT2 was higher in pistils and sepals than in other organs and decreased from the budding stage to the blooming stage and then increased in the withering stage, which was not consistent with eugenol synthesis. Subcellular localization analysis indicated that PmCFAT1 and PmCFAT2 were located in the cytoplasm and nucleus, while enzyme activity assays showed that PmCFAT1 is involved in eugenol biosynthesis in vitro. Overall, the results suggested that PmCFAT1, but not PmCFAT2, contributed to eugenol synthesis in P. mume.

PDC1, a pyruvate/α-ketoacid decarboxylase, is involved in acetaldehyde, propanal and pentanal biosynthesis in melon (Cucumis melo L.) fruit

Plant pyruvate decarboxylases (PDC) catalyze the decarboxylation of pyruvate to form acetaldehyde and CO₂ and are well known to play a key role in energy supply via fermentative metabolism in oxygen-limiting conditions. In addition to their role in fermentation, plant PDCs have also been hypothesized to be involved in aroma formation although, to date, there is no direct biochemical evidence for this function. We investigated the role of PDCs in fruit volatile biosynthesis, and identified a melon pyruvate decarboxylase, PDC1, that is highly expressed in ripe fruits. In vitro biochemical characterization of the recombinant PDC1 enzyme showed that it could not only decarboxylate pyruvate, but that it also had significant activity toward other straight- and branched-chain α-ketoacids, greatly expanding the range of substrates previously known to be accepted by the plant enzyme. RNAi-mediated transient and stable silencing of PDC1 expression in melon showed that this gene is involved in acetaldehyde, propanal and pentanal production, while it does not contribute to branched-chain amino acid (BCAA)-derived aldehyde biosynthesis in melon fruit. Importantly, our results not only demonstrate additional functions for the PDC enzyme, but also challenge the long standing hypothesis that PDC is involved in BCAA-derived aldehyde formation in fruit.

Abscisic acid is involved in aromatic ester biosynthesis related with ethylene in green apples

The production of aromatic volatiles such as esters during the ripening process in climacteric fruits is known to be controlled by ethylene. However, we here show that abscisic acid (ABA) application accelerated the onset of short-chain ester production (hexyl propionate, ethyl-2-methyl butyrate) and the expression of biosynthesis genes (MdAAT2 and MdBCAT1) during ripening of 'Orin' apple. ABA application also promoted the production of ethylene, and caused ethylene peak shifts correlated with the expression of ethylene synthesis genes (MdACS1/3 and MdACO1), suggesting that ABA may act jointly with ethylene as a positive regulator at the ripening stage of 'Orin' apple. Additionally, endogenous levels and expression of biosynthesis (MdNCED1) and signal transduction genes (MdABF2-like) of ABA increased towards ripening. Finally, the localization of the putative MdABF2-like protein binding element, AREB/ABF, was observed in the 5'-upstream region of MdACS1/3 and MdACO1.

Delayed response to cold stress is characterized by successive metabolic shifts culminating in apple fruit peel necrosis

BACKGROUND

Superficial scald is a physiological disorder of apple fruit characterized by sunken, necrotic lesions appearing after prolonged cold storage, although initial injury occurs much earlier in the storage period. To determine the degree to which the transition to cell death is an active process and specific metabolism involved, untargeted metabolic and transcriptomic profiling was used to follow metabolism of peel tissue over 180 d of cold storage.

RESULTS

The metabolome and transcriptome of peel destined to develop scald began to diverge from peel where scald was controlled using antioxidant (diphenylamine; DPA) or rendered insensitive to ethylene using 1-methylcyclopropene (1-MCP) beginning between 30 and 60 days of storage. Overall metabolic and transcriptomic shifts, representing multiple pathways and processes, occurred alongside α-farnesene oxidation and, later, methanol production alongside symptom development.

CONCLUSIONS

Results indicate this form of peel necrosis is a product of an active metabolic transition involving multiple pathways triggered by chilling temperatures at cold storage inception rather than physical injury. Among multiple other pathways, enhanced methanol and methyl ester levels alongside upregulated pectin methylesterases are unique to peel that is developing scald symptoms similar to injury resulting from mechanical stress and herbivory in other plants.

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.

Expanding the modular ester fermentative pathways for combinatorial biosynthesis of esters from volatile organic acids

Volatile organic acids are byproducts of fermentative metabolism, for example, anaerobic digestion of lignocellulosic biomass or organic wastes, and are often times undesired inhibiting cell growth and reducing directed formation of the desired products. Here, we devised a general framework for upgrading these volatile organic acids to high-value esters that can be used as flavors, fragrances, solvents, and biofuels. This framework employs the acid-to-ester modules, consisting of an AAT (alcohol acyltransferase) plus ACT (acyl CoA transferase) submodule and an alcohol submodule, for co-fermentation of sugars and organic acids to acyl CoAs and alcohols to form a combinatorial library of esters. By assembling these modules with the engineered Escherichia coli modular chassis cell, we developed microbial manufacturing platforms to perform the following functions: (i) rapid in vivo screening of novel AATs for their catalytic activities; (ii) expanding combinatorial biosynthesis of unique fermentative esters; and (iii) upgrading volatile organic acids to esters using single or mixed cell cultures. To demonstrate this framework, we screened for a set of five unique and divergent AATs from multiple species, and were able to determine their novel activities as well as produce a library of 12 out of the 13 expected esters from co-fermentation of sugars and (C2-C6) volatile organic acids. We envision the developed framework to be valuable for in vivo characterization of a repertoire of not-well-characterized natural AATs, expanding the combinatorial biosynthesis of fermentative esters, and upgrading volatile organic acids to high-value esters. Biotechnol. Bioeng. 2016;113: 1764-1776. © 2016 Wiley Periodicals, Inc.

The O-methyltransferase gene MdoOMT1 is required for biosynthesis of methylated phenylpropenes in ripe apple fruit

Phenylpropenes, such as eugenol and trans-anethole, are important aromatic compounds that determine flavour and aroma in many herbs and spices. Some apple varieties produce fruit with a highly desirable spicy/aromatic flavour that has been attributed to the production of estragole, a methylated phenylpropene. To elucidate the molecular basis for estragole production and its contribution to ripe apple flavour and aroma we characterised a segregating population from a Royal Gala (RG, estragole producer) × Granny Smith (GS, non-producer) apple cross. Two quantitative trait loci (QTLs; accounting for 9.2 and 24.8% of the variation) on linkage group (LG) 1 and LG2 were identified that co-located with seven candidate genes for phenylpropene O-methyltransferases (MdoOMT1-7). Of these genes, only expression of MdoOMT1 on LG1 increased strongly with ethylene and could be correlated with increasing estragole production in ripening RG fruit. Transient over-expression in tobacco showed that MdoOMT1 utilised a range of phenylpropene substrates and catalysed the conversion of chavicol to estragole. Royal Gala carried two alleles (MdoOMT1a, MdoOMT1b) whilst GS appeared to be homozygous for MdoOMT1b. MdoOMT1a showed a higher affinity and catalytic efficiency towards chavicol than MdoOMT1b, which could account for the phenotypic variation at the LG1 QTL. Multiple transgenic RG lines with reduced MdoOMT1 expression produced lower levels of methylated phenylpropenes, including estragole and methyleugenol. Differences in fruit aroma could be perceived in these fruit, compared with controls, by sensory analysis. Together these results indicate that MdoOMT1 is required for the production of methylated phenylpropenes in apple and that phenylpropenes including estragole may contribute to ripe apple fruit aroma.

Divergence in the enzymatic activities of a tomato and Solanum pennellii alcohol acyltransferase impacts fruit volatile ester composition

Tomato fruits accumulate a diverse set of volatiles including multiple esters. The content of ester volatiles is relatively low in tomato fruits (Solanum lycopersicum) and far more abundant in the closely related species Solanum pennellii. There are also qualitative variations in ester content between the two species. We have previously shown that high expression of a non-specific esterase is critical for the low overall ester content of S. lycopersicum fruit relative to S. pennellii fruit. Here, we show that qualitative differences in ester composition are the consequence of divergence in enzymatic activity of a ripening-related alcohol acyltransferase (AAT1). The S. pennellii AAT1 is more efficient than the tomato AAT1 for all the alcohols tested. The two enzymes have differences in their substrate preferences that explain the variations observed in the volatiles. The results illustrate how two related species have evolved to precisely adjust their volatile content by modulating the balance of the synthesis and degradation of esters.

The transcriptional response of apple alcohol acyltransferase (MdAAT2) to salicylic acid and ethylene is mediated through two apple MYB TFs in transgenic tobacco

Volatile esters are major factors affecting the aroma of apple fruits, and alcohol acyltransferases (AATs) are key enzymes involved in the last steps of ester biosynthesis. The expression of apple AAT (MdAAT2) is known to be induced by salicylic acid (SA) or ethylene in apple fruits, although the mechanism of its transcriptional regulation remains elusive. In this study, we reveal that two apple transcription factors (TFs), MdMYB1 and MdMYB6, are involved in MdAAT2 promoter response to SA and ethylene in transgenic tobacco. According to electrophoretic mobility shift assays, MdMYB1 or MdMYB6 can directly bind in vitro to MYB binding sites in the MdAAT2 promoter. In vivo, overexpression of the two MYB TFs can greatly enhance MdAAT2 promoter activity, as demonstrated by dual luciferase reporter assays in transgenic tobacco. In contrast to the promoter of MdMYB1 or MdMYB6, the MdAAT2 promoter cannot be induced by SA or ethephon (ETH) in transgenic tobacco, even in stigmas in which the MdAAT2 promoter can be highly induced under normal conditions. However, the induced MYB TFs can dramatically enhance MdAAT2 promoter activity under SA or ETH treatment. We conclude that MdMYB1 and MdMYB6 function in MdAAT2 responses to SA and ethylene in transgenic tobacco, suggesting that a similar regulation mechanism may exist in apple.

The AAT1 locus is critical for the biosynthesis of esters contributing to 'ripe apple' flavour in 'Royal Gala' and 'Granny Smith' apples

The 'fruity' attributes of ripe apples (Malus × domestica) arise from our perception of a combination of volatile ester compounds. Phenotypic variability in ester production was investigated using a segregating population from a 'Royal Gala' (RG; high ester production) × 'Granny Smith' (GS; low ester production) cross, as well as in transgenic RG plants in which expression of the alcohol acyl transferase 1 (AAT1) gene was reduced. In the RG × GS population, 46 quantitative trait loci (QTLs) for the production of esters and alcohols were identified on 15 linkage groups (LGs). The major QTL for 35 individual compounds was positioned on LG2 and co-located with AAT1. Multiple AAT1 gene variants were identified in RG and GS, but only two (AAT1-RGa and AAT1-GSa) were functional. AAT1-RGa and AAT1-GSa were both highly expressed in the cortex and skin of ripe fruit, but AAT1 protein was observed mainly in the skin. Transgenic RG specifically reduced in AAT1 expression showed reduced levels of most key esters in ripe fruit. Differences in the ripe fruit aroma could be perceived by sensory analysis. The transgenic lines also showed altered ratios of biosynthetic precursor alcohols and aldehydes, and expression of a number of ester biosynthetic genes increased, presumably in response to the increased substrate pool. These results indicate that the AAT1 locus is critical for the biosynthesis of esters contributing to a 'ripe apple' flavour.

Biochemical characteristics of a novel vegetative tissue geraniol acetyltransferase from a monoterpene oil grass (Palmarosa, Cymbopogon martinii var. Motia) leaf

Plants synthesize volatile alcohol esters on environmental insult or as metabolic induction during flower/fruit development. However, essential oil plants constitutively produce them as the oil constituents. Their synthesis is catalyzed by BAHD family enzymes called alcohol acyltransferases (AATs). However, no AAT has been characterized from plant foliage synthesizing acyclic monoterpenoids containing essential oils. Therefore, we have purified and biochemically characterized a geraniol: acetyl coenzyme A acetyltransferase (GAAT) from Palmarosa aroma grass (Cymbopogon martinii) leaf. MALDI-assisted proteomic study of the 43kDa monomeric enzyme revealed its sequence motif novelties e.g. relaxed conservation at Phe and Trp in DFGWG'. This suggests permissiveness of variations in the conserved motif without loss of catalytic ability. Also, some new conserved/semi-conserved motifs of AATs were recognized. The GAAT k(cat)/K(m) values (300-700M(-1)s(-1)) were low (a generic characteristic for secondary metabolism enzyme) but higher than those of some floral AATs. Wide substrate acceptability for catalyzing acetylation of diverse primary alcohols (chain of ≥C(6)) implied its catalytic description as a 'primary aliphatic alcohol acetyltransferase'. It signifies metabolic ability to deliver diverse aroma esters, should the acceptor alcohols be available in planta. To our knowledge, this is the first report of detailed kinetics of a vegetal monoterpenol acyltransferase.

The fruit ripening-related gene FaAAT2 encodes an acyl transferase involved in strawberry aroma biogenesis

Short-chain esters contribute to the blend of volatiles that define the strawberry aroma. The last step in their biosynthesis involves an alcohol acyltransferase that catalyses the esterification of an acyl moiety of acyl-CoA with an alcohol. This study identified a novel strawberry alcohol acyltransferase gene (FaAAT2) whose expression pattern during fruit receptacle growth and ripening is in accordance with the production of esters throughout strawberry fruit ripening. The full-length FaAAT2 cDNA was cloned and expressed in Escherichia coli and its activity was analysed with acyl-CoA and alcohol substrates. The semi-purified FaAAT2 enzyme had activity with C1-C8 straight-chain alcohols and aromatic alcohols in the presence of acetyl-CoA. Cinnamyl alcohol was the most efficient acyl acceptor. When FaAAT2 expression was transiently downregulated in the fruit receptacle by agroinfiltration, the volatile ester production was significantly reduced in strawberry fruit. The results suggest that FaAAT2 plays a significant role in the production of esters that contribute to the final strawberry fruit flavour.

Differential phylogenetic expansions in BAHD acyltransferases across five angiosperm taxa and evidence of divergent expression among Populus paralogues

Background

BAHD acyltransferases are involved in the synthesis and elaboration of a wide variety of secondary metabolites. Previous research has shown that characterized proteins from this family fall broadly into five major clades and contain two conserved protein motifs. Here, we aimed to expand the understanding of BAHD acyltransferase diversity in plants through genome-wide analysis across five angiosperm taxa. We focus particularly on Populus, a woody perennial known to produce an abundance of secondary metabolites.

Results

Phylogenetic analysis of putative BAHD acyltransferase sequences from Arabidopsis, Medicago, Oryza, Populus, and Vitis, along with previously characterized proteins, supported a refined grouping of eight major clades for this family. Taxon-specific clustering of many BAHD family members appears pervasive in angiosperms. We identified two new multi-clade motifs and numerous clade-specific motifs, several of which have been implicated in BAHD function by previous structural and mutagenesis research. Gene duplication and expression data for Populus-dominated subclades revealed that several paralogous BAHD members in this genus might have already undergone functional divergence.

Conclusions

Differential, taxon-specific BAHD family expansion via gene duplication could be an evolutionary process contributing to metabolic diversity across plant taxa. Gene expression divergence among some Populus paralogues highlights possible distinctions between their biochemical and physiological functions. The newly discovered motifs, especially the clade-specific motifs, should facilitate future functional study of substrate and donor specificity among BAHD enzymes.

VpAAT1, a gene encoding an alcohol acyltransferase, is involved in ester biosynthesis during ripening of mountain papaya fruit

Mountain papaya ( Vasconcellea pubescens ) is a climacteric fruit that develops a strong and characteristic aroma during ripening. Esters are the main volatile compounds produced by the fruit, and most of them are dependent on ethylene. As esters are synthesized through alcohol acyltransferases (AAT), a full-length cDNA (VpAAT1) was isolated that displayed the characteristic motifs of most plant acyltransferases. The full-length cDNA sequence was cloned and expressed in yeasts, obtaining a functional enzyme with high AAT activity toward the formation of benzyl acetate. The transcript accumulation pattern provided by qPCR analysis showed that the VpAAT1 gene is expressed exclusively in fruit tissues and that a high level of transcripts is accumulated during ripening. The increase in VpAAT1 transcripts in fruit is coincident with the increase in AAT activity; transcript accumulation is induced by ethylene, and it is avoided by 1-methylcyclopropene (1-MCP) treatment. The data indicate that VpAAT1 is involved in aroma formation and that ethylene plays a major role in regulating its expression.

An introduction to liquid chromatography-mass spectrometry instrumentation applied in plant metabolomic analyses

Over the past decade the application of non-targeted high-throughput metabolomic analysis within the plant sciences has gained ever increasing interest and has truly established itself as a valuable tool for plant functional genomics and studies of plant biochemical composition. Whilst proton nuclear magnetic resonance ((1)H-NMR) spectroscopy is particularly appropriate for the analysis of bulk metabolites and gas chromatography mass spectrometry (GC-MS) to the analysis of volatile organic compounds (VOC's) and derivatised primary metabolites, liquid chromatography (LC)-MS is highly applicable to the analysis of a wide range of semi-polar compounds including many secondary metabolites of interest to plant researchers and nutritionists. In view of the recent developments in the separation sciences, leading to the advent of ultra high performance liquid chromatography (UHPLC) and MS based technology showing the ever improving resolution of metabolite species and precision of mass measurements (sub-ppm accuracy now being achievable), this review sets out to introduce the background and update the reader upon LC, high performance (HP)LC and UHPLC, as well as the large range of MS instruments that are being applied in current plant metabolomic studies. As well as covering the theory behind modern day LC-MS, the review also discusses the most relevant metabolomics applications for the wide range of MS instruments that are currently being applied to LC.

Differential expression levels of aroma-related genes during ripening of apricot (Prunus armeniaca L.)

Fruit aroma is a complex trait, particularly in terms of the number of different biosynthetic pathways involved, the complexity of the final metabolites, and their regulation. In order to understand the underlying biochemical processes involved in apricot aroma, four cDNAs (Pa-aat, EU784138; Pa-adhEU395433; Pa-pdcEU395434; and Pa-loxEU439430) encoding an alcohol acyl transferase (AAT), alcohol dehydrogenase (ADH), pyruvate decarboxylase (PDC), and lipoxygenase (LOX), respectively, were isolated and characterized at four stages of maturity in Prunus armeniaca L. cv. Modesto. We observed a reduction in aldehyde and alcohol production between early-harvested fruit and late-harvest fruit, concomitant with an increase in ester production. qPCR analyses showed that the expression levels of the adh gene and the lox gene stayed constant at all stages. Interestingly, aat levels showed a sharp increase in the late-harvest stages concurrent with the changes observed in ester levels. The significance of these changes in relation to aroma production in apricot is discussed.

Overexpression of the apple alcohol acyltransferase gene alters the profile of volatile blends in transgenic tobacco leaves

Alcohol acyltransferases (AATs) are key enzymes in ester biosynthesis. Previous studies have found that AAT may be a stress-related gene. To investigate further the function of the apple alcohol acyltransferase gene (MdAAT2), transgenic tobacco plants overexpressing MdAAT2 were generated. Gas chromatography-mass spectroscopy analysis showed that the volatile blends were altered in these transgenic tobacco leaves. Although no apple-fruity volatile esters were detected in transgenic tobacco leaves, methyl caprylate, methyl caprate, and methyl dodecanoate were newly generated, and the concentrations of methyl benzoate and methyl tetradecanoate were significantly increased, suggesting that MdAAT2 may use medium-chain fatty acyl CoA and benzoyl-CoA as acyl donors together with methanol acceptors as substrates. Surprisingly, the concentrations of linalool were significantly increased in transgenic tobacco leaves, which may mediate the repellent effect on Myzus persicae (Sulzer) aphids. Using methyl jasmonate (MeJA) and wounding treatments, we found that MdAAT2 may substitute for the partial ability of MeJA to induce the production of linalool in transgenic plants. These data suggest that MdAAT2 may be involved in the response to the MeJA signal and may play a role in the response to biotic and abiotic stress.

A genomics approach reveals that aroma production in apple is controlled by ethylene predominantly at the final step in each biosynthetic pathway

Ethylene is the major effector of ripening in many fleshy fruits. In apples (Malus x domestica) the addition of ethylene causes a climacteric burst of respiration, an increase in aroma, and softening of the flesh. We have generated a transgenic line of 'Royal Gala' apple that produces no detectable levels of ethylene using antisense ACC OXIDASE, resulting in apples with no ethylene-induced ripening attributes. In response to external ethylene these antisense fruits undergo a normal climacteric burst and produced increasing concentrations of ester, polypropanoid, and terpene volatile compounds over an 8-d period. A total of 186 candidate genes that might be involved in the production of these compounds were mined from expressed sequence tags databases and full sequence obtained. Expression patterns of 179 of these were assessed using a 15,720 oligonucleotide apple microarray. Based on sequence similarity and gene expression patterns we identified 17 candidate genes that are likely to be ethylene control points for aroma production in apple. While many of the biosynthetic steps in these pathways were represented by gene families containing two or more genes, expression patterns revealed that only a single member is typically regulated by ethylene. Only certain points within the aroma biosynthesis pathways were regulated by ethylene. Often the first step, and in all pathways the last steps, contained enzymes that were ethylene regulated. This analysis suggests that the initial and final enzymatic steps with the biosynthetic pathways are important transcriptional regulation points for aroma production in apple.

Prokaryotic expression, purification, and sub-cellular localization of a novel alcohol acyltransferase from apple

The coding region of the alcohol acyltransferase gene (MdAAT2) from apple was sub-cloned into expression vectors, pET30a and pET32a, and introduced into E. coli for expression. The purified pET30a/MdAAT2 fusion proteins were used to immunize rabbits following standard protocols. The partially soluble fusion proteins had alcohol acyltransferase activity and were detected only in the pET32a/Origami B(DE3) expression system. Immunolocalization analysis indicated that MdAAT2 is mainly in the cytoplasm, in agreement with the prediction of sub-cellular localization obtained by the LOCSVMpsi program. Western blot analysis indicated that ester biosynthesis in different apple cultivars was related positively to the accumulation of MdAAT2.