The conserved aromatic residue W122 is a determinant of potyviral coat protein stability, replication, and cell-to-cell movement in plants

Construction of full-length cDNA infectious clones of Chilli veinal mottle virus

Chilli veinal mottle virus (ChiVMV), a member of the genus Potyvirus in the family Potyviridae, causes severe diseases and poses a great threat to solanaceous crops. Reverse genetics technology is an efficient tool to facilitate the study of virus biology and pathogenicity. However, the construction of an infectious cDNA clone of ChiVMV is yet to be reported. In this study, full-length cDNA infectious clones of ChiVMV and GFP-tagged ChiVMV were constructed using yeast homologous recombination for the first time. These infectious clones were able to successfully infect host plants (Nicotiana benthamiana, Nicotiana tabacum and Solanum lycopersicum) by Agrobacterium-mediated infiltration and cause vein banding and leaf curling symptoms. Mutations were introduced to pChiVMV-GFP to investigate the role of key amino acids in ChiVMV 6K2. The results showed that substitution mutants of leucine (L^{9, 11}) to alanine acid (A), tryptophan (W¹⁵) to alanine acid (A), and glycine (G^{29, 33}) to valine acid (V) reduced the viral accumulation and the mutant clones were unable to induce the symptoms in N. benthamiana plants. Taken together, these infectious clones we developed will be effective tools for future studies of the function of viral factors encoded by ChiVMV and the interactions between ChiVMV and its different host plants.

A Predicted Stem Loop in Coat Protein-Coding Sequence of Tobacco Vein Banding Mosaic Virus Is Required for Efficient Replication

Potyviral coat protein (CP) is involved in the replication and movement of potyviruses. However, little information is available on the roles of CP-coding sequence in potyviral infection. Here, we introduced synonymous substitutions to the codon C574G575C576 coding conserved residue arginine at position 192 (R192) of tobacco vein banding mosaic virus (TVBMV) CP. Substitution of the codon C574G575C576 to A574G575A576 or A574G575G576, but not C574G575A576, C574G575T576, or C574G575G576, reduced the replication, cell-to-cell movement, and accumulation of TVBMV in Nicotiana benthamiana plants, suggesting that C574 was critical for replication of TVBMV. Nucleotides 531 to 576 of the TVBMV CP-coding sequence were predicted to form a stem-loop structure, in which four consecutive C-G base pairs (C576-G531, C532-G575, C574-G533, and C534-G573) were located at the stem. Synonymous substitutions of R178-codon C532G533C534 to A532G533A534 and A532G533G534, but not C532G533A534, C532G533T534, or C532G533G534, reduced the replication levels, cell-to-cell, and systemic movement of TVBMV, suggesting that C532 was critical for TVBMV replication. Synonymous substitutions disrupting base pairs C576-G531 and C534-G573 did not affect viral accumulation. After three serial-passage inoculations, the accumulation of spontaneous mutant viruses was restored, and codons A532G533A534, A532G533G534, A574G575A576, or A574G575G576 of mutants were each separately changed to C532G533A534, C532G533G534, C574G575A576, or C574G575G576. Synonymous mutation of R178 and R192 also reduced viral accumulation in N. tabacum plants. Therefore, we concluded that the two consecutive C532-G575 and C574-G533 base pairs played critical roles in TVBMV replication via maintaining the stability of the stem-loop structures formed by nucleotides 531 to 576 of the CP-coding sequence.

Identification of genetic determinants of tomato brown rugose fruit virus that enable infection of plants harbouring the Tm-22 resistance gene

Tomato cultivars containing the Tm-22 resistance gene have been widely known to resist tobacco mosaic virus (TMV) and tomato mosaic virus. Tomato brown rugose fruit virus (ToBRFV), a new emerging tobamovirus, can infect tomato plants carrying the Tm-22 gene. However, the virulence determinant of ToBRFV that overcomes the resistance conferred by the Tm-22 gene remains unclear. In this study, we substituted the movement protein (MP) encoding sequences between ToBRFV and TMV infectious clones and conducted infectivity assays. The results showed that MP was the virulence determinant for ToBRFV to infect Tm-22 transgenic Nicotiana benthamiana plants and Tm-22 -carrying tomato plants. A TMV MP chimera with amino acid residues 60-186 of ToBRFV MP failed to induce hypersensitive cell death in the leaves of Tm-22 transgenic N. benthamiana plants. Chimeric TMV containing residues 60-186 of ToBRFV MP could, but chimeric ToBRFV containing 61-187 residues of TMV MP failed to infect Tm-22 transgenic N. benthamiana plants, indicating that 60-186 residues of MP were important for ToBRFV to overcome Tm-22 gene-mediated resistance. Further analysis showed that six amino acid residues, H67 , N125 , K129 , A134 , I147 , and I168 of ToBRFV MP, were critical in overcoming Tm-22 -mediated resistance in transgenic N. benthamiana plants and tomato plants. These results increase our understanding of the mechanism by which ToBRFV overcomes Tm-22 -mediated resistance.

Pepper Mottle Virus and Its Host Interactions: Current State of Knowledge

Pepper mottle virus (PepMoV) is a destructive pathogen that infects various solanaceous plants, including pepper, bell pepper, potato, and tomato. In this review, we summarize what is known about the molecular characteristics of PepMoV and its interactions with host plants. Comparisons of symptom variations caused by PepMoV isolates in plant hosts indicates a possible relationship between symptom development and genetic variation. Researchers have investigated the PepMoV–plant pathosystem to identify effective and durable genes that confer resistance to the pathogen. As a result, several recessive pvr or dominant Pvr resistance genes that confer resistance to PepMoV in pepper have been characterized. On the other hand, the molecular mechanisms underlying the interaction between these resistance genes and PepMoV-encoded genes remain largely unknown. Our understanding of the molecular interactions between PepMoV and host plants should be increased by reverse genetic approaches and comprehensive transcriptomic analyses of both the virus and the host genes.

Residues R192 and K225 in RNA-Binding Pocket of Tobacco Vein Banding Mosaic Virus CP Control Virus Cell-to-Cell Movement and Replication

Potyviruses move to neighboring cells in the form of virus particles or a coat protein (CP)-containing ribonucleoprotein complex. However, the precise roles of RNA-binding residues in potyviral CP in viral cell-to-cell movement remain to be elucidated. In this study, we predicted the three-dimensional model of tobacco vein banding mosaic virus (TVBMV)-encoded CP and found nine residues presumably located in the CP RNA-binding pocket. Substitutions of the two basic residues at positions 192 and 225 (R192 and K225) with either alanine, cysteine, or glutamic acid abolished TVBMV cell-to-cell and systemic movement in Nicotiana benthamiana plants. These substitutions also reduced the replication of the mutant viruses. Results from the electrophoretic mobility shift assay showed that the RNA-binding activity of mutant CPs derived from R192 or K225 substitutions was significantly lower than that of wild-type CP. Analysis of purified virus particles showed that mutant viruses with R192 or K225 substitutions formed RNA-free virus-like particles. Mutations of R192 and K225 did not change the CP plasmodesmata localization. The wild-type TVBMV CP could rescue the deficient cell-to-cell movement of mutant viruses. Moreover, deletion of any of the other seven residues also abolished TVBMV cell-to-cell movement and reduced the CP RNA-binding activity. The corresponding nine residues in watermelon mosaic virus CP were also found to play essential roles in virus cell-to-cell movement. In conclusion, residues R192 and K225 in the CP RNA-binding pocket are critical for viral RNA binding and affect both virus replication and cell-to-cell movement.[Formula: see text] Copyright © 2021 The Author(s). This is an open access article distributed under the CC BY-NC-ND 4.0 International license.