Abstract
Many animal and plant viruses rely on vectors for their transmission from host to host. Grapevine fanleaf virus (GFLV), a picorna-like virus from plants, is transmitted specifically by the ectoparasitic nematode Xiphinema index. The icosahedral capsid of GFLV, which consists of 60 identical coat protein subunits (CP), carries the determinants of this specificity. Here, we provide novel insight into GFLV transmission by nematodes through a comparative structural and functional analysis of two GFLV variants. We isolated a mutant GFLV strain (GFLV-TD) poorly transmissible by nematodes, and showed that the transmission defect is due to a glycine to aspartate mutation at position 297 (Gly297Asp) in the CP. We next determined the crystal structures of the wild-type GFLV strain F13 at 3.0 Å and of GFLV-TD at 2.7 Å resolution. The Gly297Asp mutation mapped to an exposed loop at the outer surface of the capsid and did not affect the conformation of the assembled capsid, nor of individual CP molecules. The loop is part of a positively charged pocket that includes a previously identified determinant of transmission. We propose that this pocket is a ligand-binding site with essential function in GFLV transmission by X. index. Our data suggest that perturbation of the electrostatic landscape of this pocket affects the interaction of the virion with specific receptors of the nematode's feeding apparatus, and thereby severely diminishes its transmission efficiency. These data provide a first structural insight into the interactions between a plant virus and a nematode vector.
Highlights
Efficient transmission from host to host by vectors is an important biological feature shared by many animal and plant viruses
After four decades of successive passages onto C. quinoa, the nematode transmission of varied Grapevine fanleaf virus (GFLV)-F13 inocula was assessed. This led to the identification of a variant poorly transmitted by X. index named GFLV-TD (Figure 1B)
In transmission assays (Figure S1), no difference in the ability of X. index to ingest GFLV-F13 and GFLV-TD was detected by RT-polymerase-chain reaction (PCR) after a monthly acquisition access period (AAP) (Figure 1C, top panel)
Summary
Efficient transmission from host to host by vectors is an important biological feature shared by many animal and plant viruses. Arthropods transmit many viruses to mammals and plants. Examples include highly pathogenic viruses such as Rift Valley fever virus, Dengue virus or Chikungunya virus, primarily transmitted to animals and humans by Aedes spp. mosquitoes [1,2], Tick-borne encephalitis virus transmitted by ticks [3] or Sharka/plum pox virus disease affecting stone fruits and vectored by aphids. Most plant viruses depend on vectors for their transmission. Non-enveloped viruses - the vast the majority of all plant viruses - are generally acquired by their vectors, but do not replicate in them [5,6,7,8]
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