Studies On The Indirect Defense Response Of Wheat Plant Induced By Exogenous Methyl Salicylate Application

Wheat is the main grain crop in China.Wheat aphid,one of the major insect pest of wheat,is mainly controlled by chemical pesticides.Studies have shown that the plant elicitors can induce plant defense against pests.The herbivore-induced plant volatile(HIPV)methyl salicylate(MeSA),as a signaling hormone involved in plant pest defense,is used to recruit natural enemies to protect wheat and other crops.However,the defense response and its mechanism remain largely unknown.Here,the headspace volatiles of wheat plants were collected by Porapak Q,identified and analyzed by gas chromatography-mass spectrometry(GC-MS)and principal component analysis(PCA),and then gas chromatography with electroantennographic detection(GC-EAD)was used to screen the active components of Aphidius gifuensis.Finally,the electroantennogram(EAG)and Y-tube bioassay were performed to test the physiological and behavioral responses of A.gifuensis to the active volatile compounds(p-Cymene,m-Diethylbenzene,Carveol,Linalool)that identified by using GCEAD.This study aims to clarify the indirect defense response and its mechanism induced by MeSA in wheat,and provide support for green prevention and control of wheat aphids.The main results are as follows:1.The results of GC-MS and PCA showed that induction of MeSA caused qualitative differences in volatile emission between untreated and treated plants,and the volatile categories reached the maximum after 24 h treated by MeSA.Among the 20 identified volatile compounds,13 volatile compounds were identified induced by MeSA and seven volatile compounds(2,5-Dimethyl-1-hepten-4-ol,p-Cymene,Carveol,Levomenthol,Linalool,Tetradecane and β-Acorenol)were unique to 1 mmol/L.PCA showed that these changes were mainly related to Carveol,Linalool,m-Diethylbenzene,p-Cymene,Nonanal,D-limonene and 6-methyl-5-Hepten-2-one.2.The relative contents of volatile compounds were analyzed by GC-MS.The MeSA emissions was decreased at the beginning and then dramatically increased.This may have opened up the de novo synthesis pathway of MeSA.3.Four active compounds m-Diethylbenzene,p-Cymene,Carveol and Linalool were identified in GC-EAD assay.The results of EAG assay showed that the female A.gifuensis showed strong EAG reaction to the tested pure compounds.The EAG values of cymene at 0.1,1,10 μg/μL were significantly higher than those of the control,and the EAG values of mDiethylbenzene and Carveol at the concentration of 1 and 10 μg/μL were significantly higher than those of the control.The EAG values of Linalool at 0.01,0.1,1 μg/μL were significantly higher than those of the control.4.In the olfactory test,all the four active compounds in EAG assay showed significant attraction to female A.gifuensis at a certain concentration,among which p-Cycmene and mdiethylbenzene showed significant attraction to A.gifuensis at 1 μg/μL.Carveol at 0.1 μg/μL and Linalool at 0.1 μg/μL and 0.01 μg/μL showed significant attraction to female A.gifuensis.In this study,we explore the mechanism of indirect defense induced by exogenous application of MeSA in wheat.The induced active volatiles were identified and analyzed.The results showed that exogenous MeSA application could induce wheat plants to produce indirect defense response by attracting parasitic natural enemies.This study not only provides a theoretical basis for the application of exogenous MeSA in the field for wheat aphid control,but provides a basis for the development of green prevention and control strategies and new technologies for wheat pests.