| The whitefly Bemisia tabaci is one of the most important invasive crop pests in the past two decades in China.One of the main reasons that they can cause serious damage to crops is that when plants are infesed by whiteflies,plants mobilize salicylic acid(SA)-dependent defenses,which mainly target pathogens.In contrast,jasmonic acid(JA)-dependent defenses are gradually suppressed in whitefly-infested plants,the down-regulation of JA defenses make plants more susceptible to insects,including whiteflies.Recent studies have shown that greenhouse whitefly-induced tomato plant volatiles can elicit anti-disease defenses in neighboring tomato plants.This reminds us that B.tabaci might possibly,through mainpulating plant-plant communication,interfere with the defenses of neighbouring plants.This speculation provides us with a new idea for studying the mechanism of rapid invasion by whiteflies.More understanding of the new mechanism that whitefly interferes with host plant defenses will provide us with necessary theoretic basis of field pest management strategies of whiteflies in future.In this study,we used B.tabaci,bacterium Pseudomonas syringae pv.tomato DC3000(Pst DC3000),and tomato as model system.We exposed the neighbouring healthy tomato plants to the volatiles emitted from B.tabaci-infested or Pst DC3000-infected plants,and sebsequently compared the dynamic changes of anti-herbivore or anti-disease defense reponses in exposed neighbouring plant when challenged by B.tabaci or Pst DC3000.Next,we analyzed the volatile compounds emitted from whitefly-infested and bacteria-infected tomato plants,and examined the changes in defense responses and anti-herbivore resistances of tomato plants that had been pre-exposed to synthetic volatile compounds induced by whitefly and bacteria.Taken together,we identified the effective volatile compounds that play a crucial role in manipulating plant-plant communication by whitefly.The main results are listed as follow: 1.Effects of exposure to B.tabaci-induced toamto volatiles on anti-herbivore resistance and defense responses of neighbouring plantsFirstly,we measured the number of egg laid by female adults and the development rate of nymphs reared on tomato plants that had been exposed to the volatiles emitted either from undamaged or B.tabaci-infested tomato plants.Our results showed that the number of eggs laid by females on plants pre-exposed to volatiles from whitefly-infested plants(regardless of the duration of whitefly feeding)did not significantly differ from that laid by females on plants pre-exposed to volatiles from undamaged plants.However,exposure to the volatiles from plants infested with whiteflies for 5 or 7 days significantly accelerated the nymphal development.These results indicate that exposure to volatiles from whitefly-infested plants could enhance the performance of nymphs,but have no effects on adult fecundity,on neighbouring plants.Next,to evaluate the effects of volatiles emitted by B.tabaci-infested plants on the defense reponses in neighbouring plants,we quantified the levels of endogenous JA or SA and the transcript levels of defense-related genes in neighbouring plants before and after feeding by B.tabaci adults.We found exposed plants that were not infested with whiteflies showed no differences in JA and SA levels when we compared plants exposed to volatiles from uninfested plants and those exposed to volatiles from whitefly-infested plants,the relative expression levels of LoxD,PI-I,PI-II,Mi-1.2,PR-1a and PR-1b also had no significant difference.However,upon infestation with whiteflies for 12 or 24 h,plants exposed to volatiles from whitefly-infested plants showed a significant increase in SA levels.In contrast,plants exposed to volatiles from whitefly-infested plants showed a significant decrease in JA levels,and LoxD,PI-I,PI-II,Mi-1.2 showed significant suppression,but PR-1a and PR-1b showed stronger induction.Taken together,these results imply that exposure to the volatiles from whitefly-infested plants causes an overall suppression of JA-dependent defense responses in tomato plants,which is only evident when the exposed plants are challenged by whiteflies,this priming effect renders the plants more suitable as hosts for whiteflies.2.Effects of exposure to Pst DC3000-induced tomato volatiles on anti-disease or anti-herbivore resistance and defense responses of neighbouring plants.We examined the resistance to pathogen and the development rate of nymphs on plants that had been exposed to volatiles from undamaged or Pst DC3000-infected tomato plants.The results showed that bacterial titers inoculated on plants exposed to volatiles from pathogen-infected plants did not significantly differ differ from that of bacterial titers on plants exposed to volatiles from undamaged plants when they were spray-inoculated at 0h.However,exposure to the volatiles from plants infected with pathogen for 48 h significantly reduced bacterial titers and accelerated the nymph development.These results indicate that exposure to to volatiles from pathogen-infected plants could enhance the resistance to pathogen and the performance of nymphs on neighbouring plants.Next,to evaluate the effects of volatiles emitted by Pst DC3000-infected plants on the defense reponses in neighbouring plants,we quantified the levels of endogenous JA or SA and the transcript levels of insect or pathogens resistance defense-related genes in neighbouring plants after infecting by pathogen.When we compared control and treatment plants,we found that treatment plants showed a significant increase in SA levels and an early increased expression of SA-related genes PR-1a,PR-1b after inoculation at 0h,but no differences in JA levels and JA-related genes PI-I,PI-II.When control and treatment plants were spray-inoculated with Pst DC3000 for 24 h,treatment plants showed no differences in SA and SA-related genes PR-1a,PR-1b,but JA levels and the expression of the JA-related genes PI-I,PI-II were significantly suppressed.In conclusion,we thought that volatiles from Pst DC3000-infected plants can directly activate SA defense response of neighbouring plants,which also decrease its resistance to whiteflies,these results were similar with the results of plants that had been exposed to the volatiles emitted either from B.tabaci-infested tomato plants.Taken together,these results imply that B.tabaci can manipulate defense responses in neighbouring tomato plants may be adaptive to the harmful geminiviruses.3.Identifying the effective volatile compounds involved in the manipulation of plant-plant communication by whitefly.We first quantitatively analyze the volatiles from healthy,B.tabaci-infested and Pst DC3000-infected tomato plants at different time.Quantitative analysis showed that the amounts of volatiles released from plants infested with B.tabaci for 1 or 3 d,respectively,did not significantly differ from uninfested plants,but significant increases in the emission of the monoterpenes ?-myrcene and ?-cymene,and of the sesquiterpene ?-caryophylleneonly started 5 days after initial infestation.Besides,volatiles from plants infested with B.tabaci for 7 days,significant increases in the emission of the ?-myrcene and ?-caryophylleneonly.Being different from these,after Pst DC3000-infected tomato plants for 24 h,the bacterium induced the release of several compounds that were not found in the other treatments,namely(Z)-3-hexenol,1-hexanol,methyl salicylate,and ?-cubebene,and the bacterial infection had the most dramatic effect and resulted in increases in the release of virtually all of the detected volatiles when plants were infected by Pst DC3000 for 48 h.Next,plants were exposed to a realistic dose of ?-myrcene or ?-cymene or ?-caryophyllene compounds for 24 h,comparing with control plants that exposed to solvent or ?-pinene for 24 h,exposure to the volatiles from ?-myrcene and ?-caryophyllene significantly accelerated the nymph development.Finally,we analyzed the levels of endogenous JA or SA and the transcript levels of defense-related genes in control exposed plants before and after feeding by B.tabaci adults.The results showed that without subsequent B.tabaci infestation,exposure to ?-cymene,?-myrcene or ?-caryophyllene did not affect the accumulation of JA or SA level and the expression of LoxD,PI-I,PI-II,and Mi-1.2.But upon infestation,plants exposed to ?-myrcene or ?-caryophyllene showed a significant increase in SA level and a significant decrease in JA level,and the four anti-herbivore defense genes were suppressed.In conclusion,these results indicate that exposure to ?-myrcene and ?-caryophyllene can not directly activate defense responses,but can prime SA induction and JA suppression,in neighbouring plants.These results were similar with the results of plants that had been exposed to the volatiles from B.tabaci-infested tomato plants.Taken together,B.tabaci can suppress neighbouring plants defenses through manipulating the enhanced emission of ?-myrcene and ?-caryophyllene from infested tomato plants.This could be one of the important reasons why whiteflies could rapidly spread in field and cause serious damage on crop production. |
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