Single Amino Acid Change at Two Conserved Residues in Tomato Brown Rugose Fruit Virus Coat Protein Compromises Virion Assembly and Viral Systemic Infection

Tomato brown rugose fruit virus (ToBRFV; Tobamovirus fructirugosum) is a recently discovered new species in the genus Tobamovirus that causes the ongoing pandemic and threatens tomato production worldwide. In this study, we determined the complete genome sequences of three ToBRFV isolates collected from greenhouse tomato plants in Canada in 2019, analyzed their possible phylogenetic relationships and developed corresponding full-length infectious cDNA clones. Using the ToBRFV infectious clone, we generated gene-specific mutants via site-directed mutagenesis, followed by infection assay. We confirmed that both movement protein (MP) and coat protein (CP) are indispensable for ToBRFV long-distance movement. Moreover, we found that alanine substitution of the amino acid D89 or R114, both of which are highly conserved among tobamoviral CPs, compromised ToBRFV systemic infection. Confocal and electron microscopy analyses further revealed that either D89A or R114A substitution disrupted CP self-interactions and virion assembly. Additionally, we demonstrated that the ToBRFV 126 kDa replicase has RNA silencing suppression activity. Taken together, our data contribute to a better understanding of ToBRFV-encoded proteins at the molecular level and the ToBRFV full-length infectious cDNA clones developed from this study are a useful tool to facilitate ToBRFV research.