Tomato brown rugose fruit virus (ToBRFV) has been causing severe and widespread agricultural damage worldwide since its recent discovery. While related to tobacco mosaic virus, its structure and infection mechanisms are poorly understood. We performed topographic and nanomechanical analysis of ToBRFV with atomic force microscopy (AFM). We found that the virions are rod-shaped with a height and width of 9.03 ± 0.21 and 31.24 ± 1.27 nm, respectively, indicating that they are elastically deformed. Length was widely distributed (10–1500 nm) with a mode at 30.3 nm. The virion is thus a cylinder with an outer diameter of 22.1 nm. We detected a 22.7 ± 8.6 nm axial periodicity likely related to structural units. The virion has a Young modulus of 8.7 ± 4.3 MPa, a spring constant of 0.25 ± 0.12 N/m, and a rupture force (i.e., peak force at which the virion collapses) of 1.7 ± 0.66 nN. In the force curves, a step was seen at a height of 3.3 ± 0.8 nm, which we assign to the virion wall thickness. We also estimated wall thickness by predicting coat-protein structure with AlphaFold3, which yielded a protein with a length of 7.3 nm. Accordingly, the structural element of ToBRFv is a right circular cylinder with a height and diameter of ∼22 nm, and a wall thickness between 3.3 and 7.3 nm. Thus, considering the ToBRFV genomic length, at least 4–9 serially linked units are required to form the capsid and encapsulate the entire helically organized RNA genome. Fragmentation of ToBRFV into structural elements may result in easy release of the genome and thus efficient infection.
Puskás et al. (Sun,) studied this question.