| Oriental sweet melon (Cucumis melo var. makuwa Makino) is favored by consumersmainly due to its special flavor, favorable taste and unique visual appearance. The planthormone ethylene plays an important part in controlling most of the metabolic pathwaysresponsible for ripening and senescence of melon. Aroma as one of mature physiologicalindexes closely related with ethylene and depends upon ethylene. However, until now, littlehas been known about the mechanism for the biosynthesis of aroma volatiles regulated byethylene. The inductive effect of key enzymes in the metabolic pathway on aroma needed tobe proved. Therefore, the objective of the present study is to explore the role of ethylene onaroma volatiles and biosynthetic pathway, which regulates the synthesis of aroma. Moreover,there is theoretical and practical significance for improving fruit quality of oriental sweetmelon.The oriental sweet melons used in the study consisted of three varieties,‘Caihong7’(ahighly aromatic cv.),‘Tianbao’(a less aromatic cv.) and ‘DX3-5’(a light aromatic cv.).Head-space solid phase microextraction (HS-SPME) was used to extract aromatic compoundsin three cultivars of melon during fruits development. Aromatic compounds were determinedand analyzed by gas chromatograph-mass spectrophotometer (GC-MS). The difference ofethylene production and the key aroma-related enzymes was investigated to determine therelation of ethylene and synthesis for aromatic compounds during the development of melon.On the basis, melons ‘Caihong7’ and ‘Tianbao’ were treated with10ppm of exogenousethylene and1mg·L-1of1-MCP, to investigate the effects on endogenous ethylene andaroma-related metabolites, such as species and content of aromatic compounds, mainprecursors in metabolic pathyway, activities of key enzymes and expression patterns of relatedgenes, in order to make clear the roles and regulation of ethylene on aroma volatiles and themetabolic pathway. Meanwhile, the result could provide theory to control aroma volatile inmelon, and this made important sense to improve fruit flavor and fruit quality. The key resultswere listed as follows:1. Comparison on ripening characteristics by three cultivars of oriental sweet melonsduring development was conducted. It was comfirmed that climacterics of ethylene andrespiration in ‘Caihong7’ made an advancement of3days relative to ‘Japanese Tianbao’ and‘DX3-5’. The soluble solids content were significantly higher from ripening ‘JapaneseTianbao’ and ‘DX3-5’ than ‘Caihong7’, and there is a similar maturity characteristic for‘Tianbao’ and ‘DX’.2. It was revealed that higher level of esters, especially for acetates from peel and fleshof mature ‘Caihong7’ and relatively lower level of alcohols and aldehydes contributing for thehighly aromatic of fruit. Higher content of alcohols and aldehydes, particularly for C6and C9aldehydes and alcohols in mature ‘Tianbao’ considered being important contributors to itscucumber-like flavor. Relatively lower levels of esters, C6and C9aldehydes and alcohols in ‘DX’ were crucial reasons to its light aromatic and cucumber-like flavor weaken than‘Tianbao’. The different components proportion of aroma compounds might be the reason tocause different types of fragrance among cultivars. Lipoxygenase (LOX), alcoholdehydrogenase (ADH) and alcohol acetyltransferase (AAT) as crucial aroma-related enzymes,had interaction effects on fragrance types.3. The results observed from ‘Caihong7’ and ‘Tianbao’ treated with exogenous ethyleneand1-methylcyclopropene (1-MCP), showing that the level of straight-chain esters derivedfrom fatty acids reduced in fruit treated with1-MCP. The production of ethylene andstraight-chain esters increased rapidly after the application of ethylene. These results revealedthat the synthesis of straight-chain esters in oriental sweet melon depends on ethylene. Themajor straight-chain esters including the majority of acetate, hexanoate and hexyl esters, suchas ethyl acetate, butyl acetate, hexyl acetate, ethyl hexanoate, butyl hexanoate,2-hexenylacetate, and (E)-2-hexenyl butanoate, etc. However, most levels of butanoate estersmaintained growth trend in ethylene/1-MCP or untreated ‘Caihong7’ and ‘Tianbao’ fruitthroughout the storage period. It seemed that the levels of butanoate esters were not under theregulation by ethylene completely.4. Free fatty acids (FA) component in oriental sweet melon fruit consisted of linolenicacid (LeA), linoleic acid (LA) and oleic acid (OA) was conducted by gas chromatograph tosuggest that not only the formation of esters was under ethylene control, but also FA as stepsupstream in the biosynthetic pathway of ester biosynthesis were under ethylene regulation.5. The activities of four key enzymes for aromatic compounds derived from FA pathwayinvestigated to clarify that, activities of LOX from upstream and AAT as the crucial enzymefrom the last step for synthesis of aroma volatiles, suggested that these enzymes weredependent on ethylene modulation. The differences of HPL activities in flesh between1-MCP-treated and control ‘Caihong7’ and ‘Tianbao’ were not significant after the applicationof1-MCP, demonstrating this enzyme was not under the regulation of ethylene completely.Indeed, significant differences of ADH activity among treatments were found at the beginningof storage, but not in peel and flesh tissues of ethylene-/1-MCP-treated and untreated fruitafter day9of storage, also indicating ADH activity was not under the regulation of ethyleneentirely.6. Real-time quantitative PCR was performed to exam the expression of two genes fromADH, comprising Cm-ADH1and Cm-ADH2, and four genes from AAT, including Cm-AAT1,Cm-AAT2, Cm-AAT3and Cm-AAT4. Results comfirmed that the expression of Cm-ADH1,Cm-ADH2and four genes from Cm-AAT family showing a positive regulation by ethylene.Whereas, compared with Cm-AAT1, Cm-AAT2and Cm-AAT4, the inhibition of Cm-AAT3expression in1-MCP-treated fruit was relatively weak; implying the regulation from ethyleneon Cm-AAT3was relatively weak.7. The levels of branched-chain, aromatic and sulfur-containing esters were investigatedin both cultivars. In addition, the effect of ethylene was greater on branched-chain ester and aromatic esters compared with sulfur-containing esters. The major branched-chain andaromatic esters consisted of butyl3-methyl acetate, isobutyl acetate, ethyl2-methyl butanoate,butyl2-methyl acetate, benzyl acetate and phenethyl acetate.8. The species and content of free amino acids was conducted by automatic amino acidanalyzer and seventeen kinds of free amino acids were detected in ‘Caihong7’ and ‘Tianbao’melon. Moreover, the variation of most amino acids content was under the regulation ofethylene. It was comfirmed that the metabolism of branched-chain amino acids, includingvaline, leucine and isoleucine besides phenylalanine as aromatic amino acids and cysteine assulfur-containing amino acids was catalyzed by ethylene, and then affected the synthesis ofaromatic compounds.9. Activities of three key enzymes for aroma volatiles derived from amino acids pathwaywere investigated and desmonstrated that ethylene had certain influences on activities ofaminotransferase (AT) and pyruvate dehydrogenase (PDH) activities in both cultivars.Activities of pyruvate decarboxylase (PDC) showed a clear pattern consistent with ethylene inpeel and flesh of ‘Tianbao’. While, the difference of PDC activities in peel betweenethylene-treated and control ‘Caihong7’ was not significant, implying that PDC activitieswere not completely under the regulation of ethylene.10. Branched-chain amino acid transaminase (BCAT) plays a crucial role in theconversion of branched-chain amino acids to aroma metabolites. Aromatic aminotransferase(ArAT) seems to have a major role in the metabolic pathway of aromatic amino acids. Theexpression patterns of both CmBCAT1and CmArAT1indicated up-regulation by ethyleneduring storage, in both peel and flesh tissues by RT-PCR. The reduction in CmArAT1andCmBCAT1transcript levels in the1-MCP-treated fruit was also concomitant with a reductionin aminotransferase enzyme activity, which indicated a role of CmArAT1and CmBCAT1inthe regulation of aminotransferase enzyme activity and in determining aroma volatilesformation under these conditions. |
