Grapevine fanleaf virus (GFLV) and Arabis mosaic virus (ArMV) from the genus Nepovirus, family Secoviridae, cause a severe degeneration of grapevines. GFLV and ArMV have a bipartite RNA genome and are transmitted specifically by the ectoparasitic nematodes Xiphinema index and Xiphinema diversicaudatum, respectively. The transmission specificity of both viruses maps to their respective RNA2-encoded coat protein (CP). To further delineate the GFLV CP determinants of transmission specificity, three-dimensional (3D) homology structure models of virions and CP subunits were constructed based on the crystal structure of Tobacco ringspot virus, the type member of the genus Nepovirus. The 3D models were examined to predict amino acids that are exposed at the external virion surface, highly conserved among GFLV isolates but divergent between GFLV and ArMV. Five short amino acid stretches that matched these topographical and sequence conservation criteria were selected and substituted in single and multiple combinations by their ArMV counterparts in a GFLV RNA2 cDNA clone. Among the 21 chimeric RNA2 molecules engineered, transcripts of only three of them induced systemic plant infection in the presence of GFLV RNA1. Nematode transmission assays of the three viable recombinant viruses showed that swapping a stretch of (i) 11 residues in the ␤B-␤C loop near the icosahedral 3-fold axis abolished transmission by X. index but was insufficient to restore transmission by X. diversicaudatum and (ii) 7 residues in the ␤E-␣B loop did not interfere with transmission by the two Xiphinema species. This study provides new insights into GFLV CP determinants of nematode transmission.

The viral determinants involved in the specific transmission of Grapevine fanleaf virus (GFLV) by its nematode vector Xiphinema index are located within the 513 C-terminal residues of the RNA2-encoded polyprotein, that is, the 9 C-terminal amino acids of the movement protein (2BMP) and contiguous 504 amino acids of the coat protein (2CCP) [Virology 291 (2001) 161]. To further delineate the viral determinants responsible for the specific spread, the four amino acids that are different within the 9 C-terminal 2BMP residues between GFLV and Arabis mosaic virus (ArMV), another nepovirus which is transmitted by Xiphinema diversicaudatum but not by X. index, were subjected to mutational analysis. Of the recombinant viruses derived from transcripts of GFLV RNA1 and RNA2 mutants that systemically infected herbaceous host plants, all with the 2CCP of GFLV were transmitted by X. index unlike none with the 2CCP of ArMV, regardless of the mutations within the 2BMP C-terminus. These results demonstrate that the coat protein is the sole viral determinant for the specific spread of GFLV by X. index.

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Grapevine fanleaf virus (GFLV) and arabis mosaic virus (ArMV) are nepoviruses responsible for grapevine degeneration. They are specifically transmitted from grapevine to grapevine by two distinct ectoparasitic dagger nematodes of the genus Xiphinema. GFLV and ArMV move from cell to cell as virions through tubules formed into plasmodesmata by the self-assembly of the viral movement protein. Five surface-exposed regions in the coat protein called R1 to R5, which differ between the two viruses, were previously defined and exchanged to test their involvement in virus transmission, leading to the identification of region R2 as a transmission determinant. Region R4 (amino acids 258 to 264) could not be tested in transmission due to its requirement for plant systemic infection. Here, we present a fine-tuning mutagenesis of the GFLV coat protein in and around region R4 that restored the virus movement and allowed its evaluation in transmission. We show that residues T258, M260, D261, and R301 play a crucial role in virus transmission, thus representing a new viral determinant of nematode transmission.Regions R3 (residues 207 to 210), R4 (residues 258 to 264), and R5 (residues 297 to 305) located in the jelly-roll domain B of the 2C CP protein were initially predicted to be exposed at the outer surface of the GFLV capsid from a GFLV 3D homology model derived from TRSV [27]. These regions differ between GFLV and ArMV and are described as motifs with possible function in encapsidation, movement, and transmission of GFLV. Following the obtention of the GFLV-F13 atomic structure at 3 Å resolution [21], we refined the structural features of these regions (Figure 1).Viruses 2019, 11, x FOR PEER REVIEW 6 of 23 Results Structural Environments of Regions R3, R4, and R5 of the GFLV 2C CPRegions R3 (residues 207 to 210), R4 (residues 258 to 264), and R5 (residues 297 to 305) located in the jelly-roll domain B of the 2C CP protein were initially predicted to be exposed at the outer surface of the GFLV capsid from a GFLV 3D homology model derived from TRSV [27]. These regions differ between GFLV and ArMV and are described as motifs with possible function in encapsidation, movement, and transmission of GFLV. Following the obtention of the GFLV-F13 atomic structure at 3 Å resolution [21], we refined the structural features of these regions (Figure 1).