Integrated volatile metabolome and transcriptome analyses provide insights into the warm aroma formation elicited by methyl jasmonate in carrot root

Abstract

Carrot is a highly significant vegetable cultivated worldwide and possesses a unique aroma with abundant edible and medicinal values. However, it remains largely unknown whether jasmonic acid could regulate aroma formation in carrot. Here, an integrated analysis of the volatile metabolome and transcriptome of carrot roots exposed to different concentrations of methyl jasmonate (MeJA) was performed. The results revealed 1,227 volatile organic compounds and 972 differential accumulated metabolites, with terpenes representing the largest portion. MeJA treatment evidently increased the relative odor activity values as well as the accumulation of most volatile compounds. In addition, 4,787 differentially expressed genes were identified and subjected to function enrichment analysis, indicating a role of terpene biosynthesis and metabolism in response to MeJA application. A network consisting of 4,680 transcription factor-structural pairs that showed highly significant positive correlations was constructed, which may be utilized as genetic targets for examining terpene accumulation and aroma formation elicited by methyl jasmonate. The results from the present work substantially improved our understanding of MeJA-mediated aroma formation in carrot.

Keywords: MeJA; carrot; terpenes; transcription-structural genes; transcriptome; volatile metabolome.

Figure 1

Distribution of volatile organic compounds in carrot root samples exposed to MeJA.

Figure 2

PCA analysis of the MeJA-treated samples. PC1 and PC2 represented the first and second principal component, respectively, whereas percentages indicated the explanatory power of these principal components on the dataset. Each point in the figure stood for a biological sample, and samples from the same group were represented by the same color.

Figure 3

Scatter plot of relative odor activity values for MeJA-treated root samples. The horizontal axis represented different groups, whereas the vertical axis indicated the rOAVs of flavor compounds.

Figure 4

Comparison of differential accumulated metabolites between MeJA-treated groups.

Figure 5

Venn diagram of distribution of differential metabolites in five comparisons.

Figure 6

Heat map of the differential volatile metabolites in MeJA-treated carrot roots. The red and green represented high and low levels of metabolites, respectively.

Figure 7

Six K-means clusters showed differential accumulated trends of volatile metabolites.

Figure 8

GO enrichment analysis of the differentially expressed genes. The top 50 GO terms with lowest p-value were displayed.

Figure 9

Different expression patterns of differentially expressed genes identified by K-means analysis.

Figure 10

Heat map representing the expression of terpenoid synthase genes in different samples after MeJA application. The red and green indicated high and low expression, respectively.

Figure 11

Co-expression network of TFs and terpenoid synthase genes in class 2 and 3 was constructed by Cytoscape (v.3.9.1) software. The diamonds around the periphery represented the structural genes, whereas different TF families were expressed as circles of different colors.

Figure 12

Quantitative real-time PCR validation of selected candidate genes. Error bars represent the standard deviation of three independent replicates.