The genome structures of a large number of viruses transmitted by olpidium and plasmodiophorid vectors have been determined. The viruses are highly diverse, belonging to 12 genera in at least 4 families. Plasmodiophorids are now classified as protists rather than true fungi. This finding, along with the recognition of the great variety of viruses transmitted by olpidium and plasmodiophorid vectors, will likely lead to an elaboration of the details of in vitro and in vivo transmission mechanisms. Recent progress in elucidating the interaction between Cucumber necrosis virus (CNV) and its zoospore vector suggests that specific sites on the capsid as well as on the zoospore are involved in transmission. Moreover, some features of CNV/zoospore attachment are similar to poliovirus/host cell interactions, suggesting evolutionary conservation of functional features of plant and animal virus capsids.
Little is currently known regarding the specific interactions that govern transmission of plant viruses by their vectors. A cucumber necrosis virus (CNV) variant (LL5) deficient in fungal transmissibility has been isolated from mechanically passaged CNV and characterized. Although LL5 accumulates to wild-type (WT) levels, is capable of rapid systemic infection, and produces stable, highly infectious particles, it is only inefficiently transmitted by Olpidium bornovanus zoospores. The LL5 coat protein (CP) gene was amplified by RT-PCR and cloned in place of the WT CNV CP gene in an infectious CNV cDNA clone. Particles derived from this construct also failed to be efficiently transmitted. The LL5 CP gene was sequenced and found to contain two amino acid substitutions relative to WT CNV CP. One substitution (Phe to Cys) occurred in the arm region and another (Glu to Lys) in the shell domain. These amino acid changes were separately introduced into the WT CNV genome through in vitro mutagenesis and it was found that the Glu to Lys change in the LL5 CP shell domain is largely responsible for the loss of transmissibility. In vitro binding assays were developed to determine if the defect in transmissibility was due to a defect in binding zoospores. LL5 particles were found to bind less efficiently than WT CNV. Furthermore, the nontransmissible tomato bushy stunt virus did not detectably bind zoospores. These binding studies suggest that the specificity of CNV transmission by O. bornovanus occurs through specific recognition of a putative zoospore receptor.
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