Defective tobamovirus movement protein lacking wild-type phosphorylation sites can be complemented by substitutions found in revertants

We reported previously that the movement protein (MP) of tomato mosaic tobamovirus is phosphorylated, and we proposed that MP phosphorylation is important for viral pathogenesis. Experimental data indicated that phosphorylation enhances the stability of NIP in vivo and enables the protein to assume the correct intracellular location to perform its function. A mutant virus designated 37A238A was constructed; this virus lacked two serine residues within the NIP, which prevented its phosphorylation. In the present study, we inoculated plants with the 37A238A mutant, and as expected, it was unable to produce local lesions on the leaves. However, after an extended period, we found that lesions did occur, which were due to revertant viruses. Several revertants were isolated, and the genetic changes in their MPs were examined together with any changes in their in vivo characteristics. We found that reversion to virulence was associated first with increased NIP stability in infected cells and second with a shift in NIP intracellular localization over time. In one case, the revertant NIP was not phosphorylated in vivo, but it was functional.