Tomato Brown Rugose Fruit Virus Isolate Containing A134T Amino Acid Substitution in the Movement Protein Enhanced Symptom Severity in Tomato Plants.

Tomato brown rugose fruit virus (ToBRFV) is a seedborne, soilborne, mechanically transmitted tobamovirus infecting tomato plants worldwide. Tobamovirus-contaminated soil occurring after a growth cycle of infected crops constitutes a primary source of infection for newly planted seedlings, severely affecting crops under the common practice of monoculture farming. For our studies of interrelationships between ToBRFV-infected plants and virus preservation in soil, we have grown ToBRFV-inoculated wild tomato species for 5 months and analyzed ToBRFV preservation in soil after plant removal. Soil virion RNA subjected to a high-throughput sequencing analysis revealed the presence of a single isolate with 99.92% similarity to wildtype (WT) ToBRFV. The isolate recovered from the soil contained one nonsynonymous mutation at the movement protein resulting in A134T amino acid substitution. ToBRFV-A134T was stable in Tm-2²-resistant tomato plants (e.g., cv. Ikram), and in coinoculation experiments with ToBRFV-WT, both isolates were present. The effect of ToBRFV-A134T on tomato plants was studied in comparison with ToBRFV-WT infections. Whereas ToBRFV-WT-infected plants showed mottling-mosaic leaves occasionally associated with shoestring-like symptoms, ToBRFV-A134T-infected plants' leaves showed severe shoestring-like symptoms and unique fern-like leaflets on shoestring-like leaf backbones. Analyses of genes involved in shoestring-like symptoms in Tm-2²-resistant tomato plants by RT-qPCR revealed differential relative expression of RNA-dependent RNA polymerase 6b, Dicer-like 4, and Dicer-like 2b in ToBRFV-WT-infected Ikram plants compared with ToBRFV-A134T-infected plants. The results may indicate that different mechanisms are involved in the regulation of severe shoestring-like symptoms induced by ToBRFV-A134T compared with symptoms induced by ToBRFV-WT.