A Study On The Underlying Mechanism Of Whitefly Interfering With Indirect Plant Defense Induced By Pieris Rapae

After being damaged by herbivorous insects,plants can rapidly synthesize and release complex volatile compounds,and use these volatiles to attract natural enemies of herbivorous insects to achieve the purpose of indirect defense.The current research on the release of volatiles from plants to attract natural enemy insects mainly focuses on the situation where plants are damaged by a single insect species.However,in nature or farmland ecosystems,plants often face damage from multiple insects,and the defense pathways induced by insects with different feeding guilds are also different.The interaction between different defense pathways induced by different insects is very likely to cause the volatiles released by plants to be different from those released by a single insect,thus affecting the indirect plant defense mediated by volatiles.Therefore,it is of great significance to elucidate the effects of simultaneous damage of multiple insects on the indirect defense of plants and their internal mechanisms,which is of great significance for the use of natural enemy insects for agricultural pest control.In this study,we used the sucking insect Bemisia tabaci,the chewing insect cabbage butterfly(Pieris rapae)larvae and Arabidopsis as the research model system.From the perspectives of insect behavior,chemical ecology,molecular biology,the effects of additional B.tabaci feeding on indirect plant defenses induced by cabbage butterfly larvae and the underlying mechanisms were studied.The specific research results are as follows:1.Effects of co-infestation by B.tabaci and P.rapae on host selection behavior of the parasitoid Cotesia rubeculaThe olfactory responses of C.rubecula towards the plant odors emitted from different treatments was observed by olfactometer experiments.The results showed that: when the feeding density of B.tabaci was 50 adults/plant,regardless of the feeding sequence,the wasps showed no selection preference neither to the odors from caterpillar-infested plants nor to the odors from plants co-infested by caterpillars and whiteflies.However,when the feeding density of B.tabaci was 100 adults/plant,regardless of the feeding sequence,the wasps showed a significant selection preferenceto the odors from caterpillar-infested plants.In addition,we used the application of exogenous salicylic acid(SA)to simulate whitefly feeding,and found that when the SA concentration was 0.5 mM,exogenous SA could interfere with the host selection behaviour of wasps regardless of the treatment sequence.The above results indicate that B.tabaci can interfere with the indirect plant defense(attracting the C.rubecula)induced by the cabbage butterfly,and its internal mechanism may be related to the SA pathway.2.Effects of co-infestation by B.tabaci and P.rapae on the volatile emission of Arabidopsis,and identification of the active volatile componentsThe dynamic headspace method was used to collect the volatiles from healthy,caterpillar-infested,B.tabaci(or exogenous SA)and caterpillar co-infested Arabidopsis,and GC-MS was used to analyze the composition and content of the volatile blends.The results showed that compared with the volatile components released by healthy Arabidopsis,feeding by caterpillar alone induced the enhanced emission of(Z)-3-hexenal,(Z)-2-penten-1-ol,(E)-3-hexen-1-ol,(Z)-β-ocimene,(E)-β-ocimene,(E)-β-farnesene,and TMTT,indicating that these volatile components may play a key role in caterpillar-induced indirect defense of Arabidopsis.Compared with the volatiles released from plants infested by caterpillars alone,the emission of(Z)-3-hexenal,(E)-3-hexen-1-ol,(E)-β-ocimene and TMTT from co-infested or co-treated plants was significantly reduced,indicating that B.tabaci may interfere with the release of these four volatiles to interfere with the host selection behavior of C.rubecula.To test this hypothesis,we artificially added(Z)-3-hexenal,(E)-3-hexen-1-ol,(E)-β-ocimene,TMTT to the odors from co-infested plants,to observe the changes of host selection behavior of the wasps.The results showed that:after supplementation with(Z)-3-hexenal or(E)-3-hexen-1-ol,the preference of the wasps to the odors from plants infested by caterpillars alone disappeared;while supplementation with(E)-β-ocimene or TMTT,the wasps still prefered the the odors from plants infested by caterpillars alone.This indicated that B.tabaci interfered with the host selection behavior of C.rubecula by inhibiting the emission of(Z)-3-hexenal and(E)-3-hexen-1-ol from plants infested by P.rapae.3.Molecular mechanism of B.tabaci interfering with indirect plant defense induced by P.rapae.Compared with the plants infested by caerpillars alone,the jasmonic acid(JA)-dependent genes LOX2 and OPR3,and the terpenoid synthase gene TPS3 in co-infested plants were significantly down-regulated after 1 and 3 days of herbivory damamge.The expression level of another terpenoid synthase gene TPS10 was significantly decreased only after 3 days of herbivory damamge.Similarly,the expression levels of genes LOX2,OPR3,TPS3 and TPS10 in plants co-treated with caterpillars and SA were significantly lower than those in plants infested by caterpillar alone after 1 and 3 days of treatment.In addition,we measured and compared the changes of endogenous JA and SA in plants infested by caterpillars alone and plants co-treated by whiteflies(or exogenous SA)and caterpillars.The results showed that the dose of endogenous JA in plants treated with whiteflies(or exogenous SA)and caterpillars was significantly decreased,while the dose of endogenous SA was significantly increased compared with that of plants infested by caterpillars alone.The above results indicate that,through inhibiting the JA-dependent defenses,B.tabaci interferes with the release of volatiles induced by P.rapae.Considering that the results of exogenous SA treatment were similar to those of whitefly treatment and the JA-SA interaction was mainly antagonistic,we speculated that B.tabaci may inhibit JA defense by activating SA.