Volatile Composition and Classification of Paeonia lactiflora Flower Aroma Types and Identification of the Fragrance-Related Genes

Abstract

Flower scent is one of the main ornamental characteristics of herbaceous peony, and the improvement of flower fragrance is a vital objective of herbaceous peony breeding. In this study, 87 herbaceous peony cultivars were divided into three groups (no/light fragrance, medium fragrance, and strong fragrance) based on their sensory evaluation scores, and 16 strong fragrance cultivars and one no fragrance cultivar were selected for subsequent analysis. Sixty-eight volatile components were detected in these 17 cultivars based on solid-phase microextraction (SPME) and gas chromatography/mass spectrometry (GC/MS), and 26 types were identified as important scent components. They were composed of terpenoids, benzenoids/phenylpropanoids, and fatty acid derivatives. According to the content and odor threshold of these main aroma components, the characteristic aroma substances of herbaceous peony were identified, including linalool, geraniol, citronellol, and phenylethyl alcohol (2-PE). The cultivars of strong scented herbaceous peony were divided into three types: rose scent, lily scent, and mixed scent. We explored the possible key genes of characteristic aroma substances in herbaceous peony petals with different odors through the qRT-PCR. The key genes encoding monoterpene biosynthesis were found to be PlDXS2, PlDXR1, PlMDS1, PlHDR1, PlGPPS3, and PlGPPS4. In addition, the linalool synthase (LIS) gene and the geraniol synthase (GES) gene were also found. PlAADC1, PlPAR1, and PlMAO1, related to the biosynthesis of 2-PE were detected, and the synthetic pathway of 2-PE was speculated. In conclusion, these findings revealed that the difference in gene expression of monoterpene and 2-PE synthesis pathway was related to the difference in the fragrance of herbaceous peony. This study explored the releasing pathway of herbaceous peony characteristic aroma substances and provided key genetic resources for fragrance improvement.

Keywords: floral fragrance; herbaceous peony; monoterpenes; phenylethyl alcohol; volatiles.

Figure 1

Aroma levels of floral scents in the flowers of 87 herbaceous peony cultivars. The detailed data are shown in the Supplementary Table S1.

Figure 2

Seventeen herbaceous peony cultivars, which were used for identifying and analyzing the aromatic compounds.

Figure 3

The types of volatile compounds in 17 cultivars. The numbers 1−17 represent 17 cultivars, and numbers are assigned, as described in Figure 2. Three biological replicates of the experiments were carried out. Retain only substances detected in all three biological replicates. The detailed data were shown in Supplementary Table S2.

Figure 4

Main volatile compounds of floral fragrance released by 17 herbaceous peony cultivars. 1−17 represents 17 cultivars, and numbers assigned as described in Figure 2. (A) The relative contents of 26 main volatile components from 17 cultivars. (B) The proportion of 26 main volatile components from 17 cultivars. (C) Trend comparison of relative content and sensory evaluation score. Data represent means and standard errors of three biological replicates, and the relative content is calculated using the internal standard method. Detailed data were shown in the Supplementary Table S4.

Figure 5

The odor classification, content, and OAV of 26 main volatile compounds from 17 cultivars. The numbers 1−17 represent 17 cultivars, and numbers are assigned, as described in Figure 2. (A) The relative contents of main volatile compounds in seven groups. (B) The proportion of main volatile components in seven groups. (C) The relative contents of 26 main volatile compounds. (D) The OAVs of 26 main volatile compounds. Data represent means and standard errors of three biological replicates. The detailed data were shown in Supplementary Tables S4 and S5.

Figure 6

The radar chart of volatile characteristics of 17 herbaceous peony cultivars. The numbers 1−17 represent 17 cultivars, and numbers are assigned, as described in Figure 2. (A) A rose scent. (B) A lily scent. (C) A mixed scent. Data represent means and standard errors of three biological replicates. The detailed data are shown in the Supplementary Table S5.

Figure 7

MEP pathway in herbaceous peony. The numbers 1−17 represent 17 cultivars, and numbers assigned are described in Figure 2. (A) Expression pattern of genes involved in the MEP pathway of herbaceous peony. Gene expression levels in different cultivars are represented by color gradations. (B) The relative contents of monoterpenes, citronellol, linalool, and geraniol in 11 cultivars. The detailed data are sourced from Figure 4 and Figure 5 and Supplementary Table S4. (C) Correlation heat map between monoterpenes release and gene expression levels. Correlation levels are represented by color gradations. (D) Correlation heat map between citronellol, linalool, and geraniol releases and TPS gene expression levels. Correlation levels are represented by color gradations. Three biological replicates of the experiments were carried out.

Figure 8

2-PE synthesis pathway in herbaceous peony. The numbers 1−17 represent 17 cultivars, and numbers are assigned, as described in Figure 2. (A) Expression pattern of genes involved in the three possible 2-PE synthesis pathways of herbaceous peony. Gene expression levels in different cultivars are represented by color gradations. (B) The relative contents of 2-PE in 11 cultivars. The detailed data are sourced from Figure 4 and Figure 5 and Supplementary Table S4. (C) Correlation heat map between 2-PE release and gene expression levels. Correlation levels are represented by color gradations. Three biological replicates of the experiments were carried out.

Figure 9

Synthetic pathway of characteristic aroma of herbaceous peony.