Abstract
BackgroundAroma 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.ResultsUsing 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.ConclusionsThe 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.
Highlights
Aroma is an important organoleptic quality for fruit and has a large influence on consumer preference
AdDof3, AdDof4 (DNA binding with one finger) and AdNAC5 were identified as targeting genes related to kiwifruit ripening and softening [20], and it was assumed that the remaining transcription factors (TFs) may be involved in other regulating other ripening related traits such as aroma
The results indicated that AdNAC5 could significantly transactivate the AdFAD1 promoter, with over 2-fold induction (Fig. 6a), whereas for the AdFAD1 promoter, AdDof4 acted as a repressor, reducing the transcriptional activity to 0.67, compared to the basal activity set as 1 (Fig. 6a)
Summary
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. Kiwifruit are widely preferred by consumers due to their flavor and high value of nutrient compounds The production of aroma volatile compounds is strongly ethylene-dependent [3]. In ethylene-suppressed kiwifruit, fruit softening was significantly delayed and aroma volatile production was dramatically reduced, which could be re-initiated by application of exogenous ethylene [4]. The aroma of kiwifruit is produced by a mixture of various volatile compounds, including alcohols, aldehydes, ketones, and esters [5], which makes kiwifruit a good model for understanding the biosynthesis and regulation of different volatile compounds
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