Silicon-mediated Indirect Defense Of Cucumis Sativus L.to Helicoverpa Armigera(H(?)bner)

Silicon(Si)plays an important role in the biological resistance of plants to herbivores.The effects of silicon addition on constitutive plant defenses have previously been investigated and include enhanced plant mechanical strength and leaf toughness.The impact of silicon on induced plant defense is not as well studied.In recent years,studies on the effect of silicon have been found to extend to indirect induced plant defenses(those that operate via natural enemy attraction)including the release of plant volatiles to attract natural enemies.The effect of silicon on plant defense system was explored in a tri-trophic system of cucumber,Cucumis sativus L.;the herbivore,Helicoverpa armigera(Hiibner)and the parasitoid,Microplitis mediator(Halidag).Cucumber plants were grown hydroponically with and without silicon supplement(+/-Si).Silicon levels were studied in different plant parts.The attraction response of the parasitoid M.mediator to cucumber plants with 1,3,6 and 12 hours after damage by H.armigera was measured by Y-tube olfactometer.Gas chromatography-mass spectrometry(GC-MS)was used to identify volatiles of cucumber plants from each treatment.Cucumber plants in the 6 and 12 hours treatments were used for transcriptome sequencing analysis to preliminarily explore the role of silicon in the regulation of plant defense pathways.In the +Si treatment,the silicon was significantly accumulated in the leaves and stems of cucumber plants,especially in the leaves.Silicon content in the leaves of +Si cucumber plants was 233.33 ± 16.67mg/kg,which is 7 times of the silicon content(33.33 ± 0.88 mg/kg)in the stems of +Si cucumber plants and 11.67 times of the silicon content(20.00 ±10.00mg/kg)in the leaves of-Si cucumber plants.Without silicon treatment,the cucumber plants with 3 hours,6 hours and 12 hours post damage by H.armigera could have an attraction to M.mediator compared with control plants.While with silicon treatment,the cucumber plants withl hour,3 hours,6 hours and 12 hours post damage by H.armigera had a significant attraction response to M.mediator compared with control plants.Under the same damaged time,+Si cucumber plants with3 hours post damage by H.armigera was no more attractive to M.mediator than-Si cucumber plants with the same time post damage by H.armigera;while M.mediator was more likely to the+Si cucumber plants with 6 hours and 12 hours post damage by H.armigera than-Si cucumber plants with the same time post damage by H.armigera.Therefore the addition of silicon shortened the plant response time to herbivore attack and promoted plant indirect plant defense by attracting the parasitoid M.mediator.The volatiles produced by cucumber plants included alcohols,terpenes,esters,ketones,aldehydes,phenols and alkanes and significant differences in the volatile constituent were discovered between +Si and-Si plants as well as between plants under different herbivory duration.There was a difference in the composition and content of volatiles produced by cucumber plants under different treatments.Cucumber plants with silicon that were damaged for 1 hour released more types and higher amounts of alkane compared with-Si cucumber plants.After 3 hours of herbivory,+Si cucumber plants began to produce p-chlorobenzene sulfinic acid;and also increased the amounts of dodecane;but decreased the amounts of longifolene compared with herbivore damaged-Si cucumber plants.After 6 hours of herbivory +Si cucumber plants increased the amounts of 1,4-cineole and methylnaphthalene;but decreased the amounts of acetaldehyde,a-pinene,camphene,β-pinene,methylheptenone,D-limonene,eucalyptol and butylated hydroxytoluene compared with herbivore damaged-Si cucumber plants.After 12 hours of herbivory,+Si cucumber plants increased the amounts of naphthalene,p-chlorobenzene sulfinic acid,4-dodecanone,butyl acetate and various alkanes;but decreased the amounts of 4,7-dimethylundecane compared with herbivore damaged-Si cucumber plants.The volatiles of +Si cucumber plants with 1 hour,3 hours,6 hours,and 12 hours post damage by H.armigera showed a trend of increasing,holding,decreasing and increasing the total content compared with-Si cucumber plants separately.Transcriptome data analysis showed that the addition of Si affected the expression of a large number of cucumber genes.Compared with healthy +Si cucumber,using KEGG(Kyoto Encyclopedia of Genes and Genomes)pathway enrichment analysis,it was found that silicon affected the expression of genes in the jasmonate signal pathway.Silicon could also regulate genes associated with the synthesis of volatiles,such as terpene synthase gene TPS in +Si cucumber plants with 6 hours,and 12 hours post damage by H.armigera.Thus,silicon can regulate the release of herbivore-induced plant volatiles by inducing the expression of volatile-synthesis genes in cucumber plants.