Michael Vaisman, 
Hagit Hak, 
Tzahi Arazi, 
Ziv Spiegelman

Plant viruses cause systemic diseases that severely impair plant growth and development. While the accumulation of viruses in the root system has long been established, little is known as to how viruses affect root architecture. Here, we examined how the emerging tobamovirus, tomato brown rugose fruit virus (ToBRFV), alters root development in tomato. We found that ToBRFV and tobacco mosaic virus (TMV) both invaded root systems during the first week of infection. ToBRFV infection of tomato plants resulted in a significant decrease in root biomass and elongation, root-to-shoot ratio and a marked suppression of root branching. Mutation in RNA-dependent RNA polymerase 6 (RDR6) increased the susceptibility of tomato plants to ToBRFV, resulting in severe reduction of various root growth parameters including root branching. Viral root symptoms were associated with the accumulation of AUXIN RESPONSE FACTOR 10a (SlARF10a) transcript, a homolog of Arabidopsis ARF10, a known suppressor of lateral root development. Interestingly, loss-of function mutation in SlARF10a moderated the effect of ToBRFV on root branching. In contrast, downregulation of sly-miR160a, which targets SlARF10a, was associated with constitutive suppression root branching independent of viral infection. In addition, over-expression of a miRNA-insensitive mutant of SlARF10a mimicked the effect of ToBRFV on root development, suggesting a specific role for SlARF10a in ToBRFV-mediated suppression of root branching. Taken together, our results provide new insights into the impact of tobamoviruses on root development and the role of ARF10a in the suppression of root branching in tomato.