Exogenous Application of a Plant Elicitor Induces Volatile Emission in Wheat and Enhances the Attraction of an Aphid Parasitoid Aphidius gifuensis

Abstract

It is well known that plant elicitors can induce plant defense against pests. The herbivore-induced plant volatile (HIPV) methyl salicylate (MeSA), as a signaling hormone involved in plant pathogen defense, is used to recruit natural enemies to protect wheat and other crops. However, the defense mechanism remains largely unknown. Here, the headspace volatiles of wheat plants were collected and analyzed by gas chromatography-mass spectrometry (GC-MS), gas chromatography with electroantennographic detection (GC-EAD) and principal component analysis (PCA). The results showed that exogenous application of MeSA induced qualitative and quantitative changes in the volatiles emitted from wheat plants, and these changes were mainly related to Carveol, Linalool, m-Diethyl-benzene, p-Cymene, Nonanal, D-limonene and 6-methyl-5-Hepten-2-one. Then, the electroantennogram (EAG) and Y-tube bioassay were performed to test the physiological and behavioral responses of Aphidius gifuensis Ashmesd to the active volatile compounds (p-Cymene, m-Diethyl-benzene, Carveol) that identified by using GC-EAD. The female A. gifuensis showed strong physiological responses to 1 μg/μL p-Cymene and 1 μg/μL m-Diethyl-benzene. Moreover, a mixture blend was more attractive to female A. gifuensis than a single compound. These findings suggested that MeSA could induce wheat plant indirect defense against wheat aphids through attracting parasitoid in the wheat agro-ecosystem.

Keywords: HIPVs; biological control; indirect defense; methyl salicylate; wheat aphid.

Figure 1

Responses of female Aphidius gifuensis (n = 30) in a Y-tube olfactometer between the arm with the untreated plants (control) and the arm treated with either 1 mmol/L MeSA or the 0.5 mmol/L MeSA plants.

Figure 2

Responses of female Aphidius gifuensis (n = 30) in a Y-tube olfactometer between the arm with the untreated plant extracts and the arm treated with either 1 mmol/L MeSA or the 0.5 mmol/L MeSA plant extracts.

Figure 3

Principal Component Analysis (PCA) of the 20 volatile organic compounds (VOCs) emit-ted from wheat plant. The numbers represented the chemical compounds were as showed in Table 1. Scatter plots visualize the location of each collected sample on each PC with the percentage of explained variation in parentheses, whereas vectors (blue line) visualize the loadings for each VOC.

Figure 4

Heatmap clustering showing the abundance (in decreasing color intensity) of the most discriminant VOCs across replicates of wheat plants. Alcohols (20, 13, 10, 15, 2), aldehyde (12, 14, 17, 11), hydrocarbon (8, 14, 1, 7, 6, 18, 9, 3, 19, 4), keto (5), ester (16). The numbers represented the chemical compounds were as showed in Table 1.

Figure 5

Coupled GC-EAD analysis showing antennal responses of female Aphidius gifuensis to volatile organic compound samples collected from wheat plants that were treated with MeSA (1 mmol/L). Upper trace: antennal response, lower trace: FID response. The EAD-active VOCs for female A. gifuensis were identified as: m-Diethyl-benzene, p-Cymene, Carveol and Linalool.

Figure 6

Absolute EAG response [mean ± SE (μV), n = 6] of female Aphidius gifuensis antenna to 10 μL of synthetic chemical. (a) P-cymene, (b) Carveol, (c) M-Diethylbenzenel. Different letters indicate that values differ statistically at p < 0.05 (ANOVA, followed by LSD test).

Figure 7

Behavioral responses of Aphidius gifuensis female adults (n = 20) to 10 μL of synthetic chem-icals at different concentrations (0.01–10 μg/μL) in olfactometer bioassays: (a) p-Cymene, (b) Carveol, (c) m-Diethylbenzenel.

Figure 8

Responses of female Aphidius gifuensis (n = 20) in a Y-tube olfactometer to the single com-pound against mixture. The mixture was prepared from 1 μg/μL p-Cymene, 0.1 μg/μL Carveol and 1 μg/μL m-Diethyl-benzene, the concentration of the single compound in the mixture was kept con-stant. Recipe for 1000 μL of mixture: 1.16 μL p-Cymene, 1.16 μL m-Diethyl-benzene, 0.104 μL Carveol, 997.58 μL n-hexane.