Plant virus can influence the host selection behavior of its insect vector to facilitate the virus transmission [20, 21], such as the increased alate production in infected aphid vectors [19, 22]. Recently, the multi-trophic interactions among plant pathogens, insect vectors and plant hosts have expanded to involve a fourth party: bacterial symbionts harbored within the insect vector .
Shift in feeding preference of CMV-infected aphids facilitates the virus transmission
It is expected that most plant viruses can induce changes in plants that have positive effects for transmission by insect vectors . Here, CMV-infected aphids were attracted to the healthy plants rather than CMV-infected plants, and previous research on Aphis gossypii alates also showed the shift preference from CMV-infected to mock-inoculated plants . Besides, when aphids were exposed to CMV-infected plants, the probing behavior showed a sharp change over time . The increased number of CMV-infected aphids on healthy plants, in theory, will facilitate virus transmission in the field. However, the mechanism of aphid shifted preference mediated by non-persistent plant virus is largely unknown.
CMV infection reduced the relative abundance of B. aphidicola in M. persicae
Our RT-qPCR analysis showed that CMV infection reduced the relative abundance of B. aphidicola in M. persicae. In a parallel experiment, rifampicin treatment, not surprisingly, significantly reduced the B. aphidicola titer in M. persicae, which is consistent with previous findings [12, 15]. Although bacterial endosymbionts in M. persicae could be abundant and diverse, we only detected B. aphidicola following . None of the other symbionts, such as Rickettsia, Hamiltonella, Wolbachia, and Spiroplasma, were detected in this study. Interestingly, however, the level of reduction in B. aphidicola abundance in M. persicae infected with CMV was comparable to the aphids treated with the highest concentration of antibiotics (200 μg/mL rifampicin; Fig. 2). More importantly, it is intriguing that aphids with lower B. aphidicola abundance prefer healthy plants, which might be the contingency plan for these compromised aphids to choose between a challenged defense (infected plants) and an intact nutrition (healthy plants). Nevertheless, the eventual choice by aphids ultimately favor the transmission and spread of virus, an apparent incentive for virus during the co-evolution with its insect vector.
Reduced B. aphidicola titer in aphids leads to the quantitative changes in plant volatiles
Plants often respond to herbivore attack by releasing a specific blend of volatiles . Previous research showed that the volatiles emitted by CMV-infected plants appeared to be qualitatively similar to the blend emitted by healthy plants , which is in consistent with our results. Some terpenes typically repel arthropod herbivores, such as σ-cymene, α-pinene, β-pinene, and γ-terpinene [27-30], which is consistent with our results. σ-xylene, however, attracts aphids in our study, but deters whiteflies, Bemisia tabaci (Gennadius) . The discrepancy might depend on insect species. Mauck et al. (2010) showed that aphids were attracted to CMV-infected plants at first, and the viruliferous aphids emigrated from infected plants at a higher rate and exhibited reduced population growth when forced to feed on infected plants, suggesting that reductions in host palatability lead to the rapid dispersal of CMV-infected aphids . Here we found that CMV-infected plants released higher titer of attractive plant volatile, while after CMV-infected plants were infested by healthy aphids and B. aphidicola-decreased aphids, the repellent volatile was induced and the attractive volatiles were reduced, and the phenomena may also explain the rapid dispersal of CMV-infected aphids. After CMV-infected plants were infested by healthy aphids, the B. aphidicola is also reduced after aphid acquiring virus. Our results indicate that the reduction of a primary endosymbiont in aphids leads to a change of plant volatiles and subsequently drives the aphids away from the infect plants and settles on the healthy ones.
The mutualism between aphid, M. persicae, and its primary symbiont, B. aphidicola, is obligate, in which the partners cannot survive without the other. By reducing the abundance of B. aphidicola, the overall fitness of M. persicae is compromised. CMV is exploiting the situation by presenting a choice to its insect vector between the immunocompromised (infected) and nutritionally intact (healthy) host plants. In the meantime, the reduced B. aphidicola abundance quantitatively changes the plant volatile profiles to orient a choice by the insect vector favoring the transmission and spread of virus (Fig. 5).
During the transmission process of plant virus by insect vectors in a non-persistent manner, plant viruses are believed to be retained in the insect stylet. The debate in this field is that whether insect endosymbionts are involved in the process of non-persistent virus transmission. Based on this study, we proposed a simple model for B. aphidicola in the role of CMV transmission. This model exemplified the dynamics of aphids’ preference shift from CMV-infected plants to non-infected plants. The B. aphidicola abundance decrease in viruliferous aphids affected the plant volatile profiles, which result in the aphid preference shift from infected to healthy plants, and finally leads to the CMV outbreak (Fig. 5).