Investigations On Interactions Of Bemisa Tabaci-geminiviruses-plants And The Underlying Plant Defensive Mechanisms

The whitefly Bemisia tabaci and geminiviruses it transmits are important agricultural pests worldwide, which have caused huge loss to many crops such as cotton, tomato and cassava. Outbreaks of B. tabaci populations were often associated with epidemics of geminivirus diseases; it is assumed that the tripartite interactions among B. tabaci, geminiviruses and host plants may play an important role in the epidemics of virus diseases and rapid spread of the whitefly. To assess the role of vector-virus-plant interactions in the epidemics of tomato yellow leaf curl virus (TYLCV) and the invasive process of invasive B. tabaci biotypes, we investigated the interactions of tomato-mediated interactions between TYLCV and the invasive B/Q biotype as well as the indigenous ZHJ2 biotype whiteflies. In view of the ecological importance of plant-mediated whitefly-begomovirus indirect mutualism, we also investigated the plant physiological mechanisms of the indirect mutualism between the B biotype B. tabaci and tomato yellow leaf curl China virus (TYLCCNV) as mediated by their shared host plant tomato, with a focus on the defense-related plant characteristics. The results are summarized as follows:(1) Acquisition, retention and transmission of TYLCV by the invasive B/Q biotype or indigenous ZHJ2 biotype whitefliesFor all whitefly biotypes, TYLCV DNA was detected following a 30-min acquisition access period (AAP) to infected leaves. The percentage of adults with viral DNA increased with the length of AAP and reached 100%after a 10-12 h AAP. Following acquisition viruliferous B, Q and ZHJ2 adults retained TYLCV DNA for their entire life. Transmission was achieved with one B/Q adult per plant at the frequency of 50-55%, which rose to 100%with 10 insects per plant. In contrast, transmission of the virus was not observed with one ZHJ2 adult per plant in the experiments, and the transmission frequency rose to only 30-45%when whitefly adults increased to 5-10 per plant. (2) Performance of the Q biotype and ZHJ2 biotype whiteflies on TYLCV-infected or uninfected tomato plantsThe Q biotype whitefly performed better than the ZHJ2 biotype on either uninfected or virus-infected tomato plants. However, virus-infection of host plants did not, or only marginally affected, the performance of either biotype of whiteflies in terms of fecundity, longevity, survival, development and population increase. Likewise, association of the vectors with TYLCV did not exert an effect on fecundity and longevity of the Q or ZHJ2 biotype whiteflies on cotton, a non-host of TYLCV.(3) Performance of the B biotype whitefly on TYLCCNV infected tobacco plants as affected by the length of time interval following virus inoculationThe fecundity of B biotype whitefly on TYLCCNV-infected plants at 15,30 or 45 days post virus inoculation were all significantly higher than that on healthy control plants; the whiteflies deposited significantly fewer eggs on virus-infected plants 45 days post virus inoculation than that on virus-infected plants 15 or 30 days post virus inoculation. Similarly, survival of B biotype whitefly on virus-infected plants 15,30 and 45 days post virus inoculation were all significantly higher than that on healthy control plants; the survival of whiteflies on virus-infected plants 45 days post virus inoculation was significantly lower than that on virus-infected plants 15 days post virus inoculation. Mean numbers of eggs per adult-day on virus-infected plants 15 or 30 days post virus inoculation were significantly higher than those on healthy control plants; however, no difference was found between the daily fecundity on virus-infected plants 45 days post virus inoculation and that on healthy control plants.(4) Comparison of nicotine concentrations between TYLCCNV-infected and uninfected tobacco plantsConcentrations of nicotine in virus-infected plants 15 and 30 days post virus inoculation were 2 and 1.5 times higher than those in healthy control plants, respectively. However, concentration of nicotine in virus-infected plants 45 days post virus inoculation was similar to that in healthy control plants.(5) Influence of TYLCCNV DNA A and TYLCCNV DNAβinfection on the suitability of tobacco plants to B biotype whitefly Numbers of nymphs and adults of B biotype whitefly on TYLCCNV DNA A infected plants were similar to those on healthy plants, which were significantly lower than that on TYLCCNV DNA A+βinfected plants.(6) Influence of jasmonic acid and salicylic acid on the suitability of tobacco plants to B biotype whiteflyThe JA concentration in virus-infected plants was approximately 50%of that in uninfected plants; while the SA concentration in virus-infected plants was slightly higher than that in uninfected plants. The B biotype whitefly performed much better on methyl salicylate (MeSA) treated plants than on control or methyl jasmonate (MeJA) treated plants. Mean numbers of eggs deposited on MeSA treated plants were 2.3 and 5.7 times higher than those on control plants and MeJA treated plants. Similarly, adult survival on MeSA treated plants was 2.8 and 5.8 times higher than that on control and MeJA treated plants.The invasive B and Q biotypes of B. tabaci were able to transmit TYLCV more efficiently than was the indigenous ZHJ2 biotype, and Q biotype performed better than ZHJ2 on either uninfected or virus-infected tomato plants. Thus, the Q biotype whitefly may make up the major source of TYLCV vectors and facilitate the virus epidemics. With further invasion by the Q biotype and its associated TYLCV and possibly other begomoviruses, control of begomovirus diseases may present a great challenge in the production of tomato and possibly other crops in the years to come.The performance of B biotype whitefly was significantly affected by the symptom development of the virus-infected plants. Moreover, without the pathogenesis factor DNAβ, the suitability of TYLCCNV DNA A infected plants did not differ significantly from that of healthy plants, which indicated that DNAβmay play an important role in the indirect interaction between whitefly and virus. The JA concentration in virus-infected plants was significantly lower than that in healthy plant, and exogenous application of SA induced similar improvement of plant suitability to the B biotype whitefly to that induced by virus-infection. Thus, it is likely that virus-infection of plants causes suppression of the JA pathway, which may contribute to the improvement of performance of invasive biotype whiteflies on virus-infected plants. The suppression of JA pathway may be brought about by the down-regulation of JA response genes mediated by the elevated expression of SA genes or directly by action of the virus DNAβ.