Influence Of Host Plants And Vector Symbionts On Transmission Of Southern Rice Black-streaked Dwarf Virus And The Related Mechanisms

Southern rice black-streaked dwarf virus,SRBSDV,is one of the most destructive rice virus in Asian countries.White-backed planthopper(WBPH),Sogatella furcifera(Hemiptera: Delphacidae),the only vector of SRBSDV,transmits the virus in a persistent,circulative and propagative manner.To elucidate the epidemiology of SRBSDV as well as the factors influencing on its transmission by WBPH,this thesis investigated the influence of host plant species,induced plants defense response in rice,vector symbionts on SRBSDV transmission and the related mechanisms.The results are as follows:1.Plant viruses are mostly transmitted by sucking insects via their piercing behaviors,which may differ due to host plant species and their developmental stages.We characterized the transmission of southern rice black-streaked dwarf virus(SRBSDV),by the planthopper vector S.furcifera,between rice and corn plants of varying developmental stages.SRBSDV was transmitted from infected rice to uninfected corn plants as efficiently as its transmission between rice plants,while was acquired by S.furcifera nymphs at a much lower rate from infected corn plants than from infected rice plants.We also recorded a high mortality of S.furcifera nymphs on corn plants.It is evident that young stages of both the virus donor and recipient plants added to the transmission efficiency of SRBSDV from rice to corn plants.Feeding behaviors of the vector recorded by electrical penetration graph showed that phloem sap ingestion,the behavioral event that is linked with plant virus acquisition,was impaired on corn plants,which accounts for the high mortality of and low virus acquisition by S.furcifera nymphs on corn plants.Our results reveal an asymmetric spread of SRBSDV between its two host plants and the underlying behavioral mechanism,which is of significance for assessing SRBSDV transmission risks and field epidemiology,and for developing integrated management approaches for SRBSDV disease.2.Herbivorous attack induces plant defenses.There is evidence that some pests suppress these defenses by interfering with signaling pathways.We here report that infestation by the white-backed planthopper,S.furcifera,induces defense responses in rice and infection of the SRBSDV in the planthoppers suppresses the induced plant defenses.Salicylic acid(SA)levels in rice plants infested for 72 h by viruliferous or nonviruliferous planthoppers generally increased over those in the uninfested plants,while jasmonic acid(JA)levels generally decreased in the infested plants.The increase in SA was less while the decrease in JA was more in the viruliferous insect-infested plants than in the nonviruliferous insect-infested plants at both 48 and 72 h post infestation.The phytohormone levels corresponded to the patterns of relative expression levels of SA-marker genes(ICS1 and NPR1)and JA-marker gene(AOS2)in the plant treatments.The planthoppers performed better on the control plants than on the infested plants and were of marginally high performance on the plants previously attacked by viruliferous planthoppers in comparison with the plants previously attacked by nonviruliferous insects.Our results indicate that the virus plays a role in partially suppressing the plant defenses induced by the planthopper.These findings provide a new perspective on plant-virus-vector interactions.3.Influence of damage by vector and non-vector on induced plant defense responses in SRBSDV-infected and uninfected rice plants.SA and JA levels and the relative expression of the concerned genes were measured in SRBSDV-infected and uninfected rice plants that were damaged by the vector WBPH and the non-vector BPH.The results showed that,regardless of damage by the planthoppers,levels of both SA and JA in SRBSDV-infected plants were significantly higher than those in uninfected plants.When infested by WBPH or co-infested by WBPH and BPH,the infected plants harbored higher SA levels than the uninfected plants,while SA levels did not differ between infected and uninfected plants that were infested by BPH alone.The phytohormone levels corresponded to the patterns of relative expression levels of SA-marker genes(ICS1)and JA-marker gene(AOS2)in the different plant treatments.Therefore,SRBSDV infection can activate the induced systemic resistance and systemically acquired resistance in rice plants.In contrast to the non-vector BPH,the vector WBPH can trigger the SA pathway to a higher extent in infected than in uninfected plants,while infestation by the vector and the non-vector shows no significant influence on the JA pathway.4.Influence of vector symbionts on acquisition and propagation of SRBSDV in the vector.By multi-locus sequence typing,the study determined that the laboratory WBPH population was infected by B group Wolbachia,w Stri.One WBPH population was obtained through feeding WBPH with each of rifampicin and penicillin G so that the symbionts Wolbachia and Cardinium were eliminated in the treated WBPH.Nymphs(3-4 instars)of the two populations and an un-treated control population were tested for virus acquisition.After the virus circulative phase in WBPH,detection showed that virus acquisition in males did not differ between the three populations,while females of the rifampicin-treated population acquired SRBSDV at a lower rate than females of the control population.Virus titers were further detected dynamically in the three WBPH populations after they were fed on infected plants for 48 h and then maintained on uninfected plants for 0,3,6,and 9 days.Virus titers in the control population were higher than those in the penicillin G-treated population immediately after their feeding on the infected plants,and were higher than those in the rifampicin-treated population at 6 days after feeding on the infected plants.Until 9 days after feeding on the infected plants,virus titers showed no difference between the three populations.The results indicate that symbionts play a role in increasing virus acquisition in females but not in males,and virus titers tend to be similar in the treated and control populations with increase in the time after virus acquisition.