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This is a proposal to provide one component of an ongoing multi-laboratory effort to understand how closteroviruses evolved to have specific interactions with different insects. This virus group has members that are vectored in a semi-persistent manner by different families of insects: aphids, whiteflies, and mealybugs. Yet each virus-insect interaction is precise. This group of viruses has thread-like flexuous virions encapsidated primarily by a major coat protein, but one end of the virion is encapsidated by a diverged copy of the coat protein. In addition, several other viral proteins are associated with virion preparations. Specifically, we will focus on Citrus tristeza virus (CTV), the largest and most complex of the Closteroviridae, which is transmitted by several species of aphids. Our first expectations were that efficiency of transmission would be due to the interactions between the virion or helper factor and the aphid. However, recent evidence suggests that the ability of the virus to move in citrus trees, the ability to establish a systemic infection after being injected into the tree by the aphid, might also be a component of transmissibility. From a practical standpoint, the ability to manipulate the transmissibility of CTV is a necessary component of management strategies, particularly in the event of introduction of severe stem-pitting isolates into the US, whether by accident or bioterrorism.
NON-TECHNICAL SUMMARY: Citrus tristeza virus (CTV) is a severe pathogen of citrus, and Florida already has the most efficient vector, the brown citrus aphid, introduced in 1995. The introduction of severe stem-pitting isolates, whether done by accident or by bioterrorism, could destroy the economic viability of the U.S. citrus industries. This potential disaster demands that we have management procedures in place. One management procedure, which we are in the process of developing, is to custom genetically-engineer protecting strains to prevent superinfection by severe isolates. One desirable requirement for cross-protecting strains is the lack of transmissibility by aphids to other citrus varieties that might be susceptible to the cross-protecting virus. Experiments proposed here should allow engineering protecting isolates that will not be spread by aphids.
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APPROACH: We propose to examine factors that facilitate transmission of CTV by aphids by two different approaches. One is to identify viral sequences that are involved in efficiency of transmission. Keeping in mind that multiple factors might facilitate transmissibility and that different isolates might have different combinations of these factors, we will do this for several independent isolates of virus chosen because of their transmission characteristics. An advantage of this approach is that it does not require any assumptions about how those sequences function to effect transmission - as virion components, as proteins that bridge the virion to the aphid (helper factors), as movement factors, or some combinations of these. A parallel approach is to examine localization and movement of CTV in donor trees before aphid transmission and in the small receptor trees after aphid transmission to determine whether there is a relationship between ability of the virus to move and efficiency of aphid transmission.