Ratio of mutated versus wild-type coat protein sequences in Pepino mosaic virus determines the nature and severity of yellowing symptoms on tomato plants.

Recently, Pepino mosaic virus (PepMV) infections causing severe yellowing symptoms in tomato plants have been reported in glasshouse tomato crops. When studying this phenomenon in commercial glasshouses, two different types of yellowing symptoms, occurring in adjacent plants, were distinguished: interveinal leaf yellowing and yellow mosaics. After several weeks, the interveinal leaf yellowing symptoms gradually disappeared and the plant heads became green again, with yellow mosaic patterns on the leaves as an intermediate stage. The sequencing of multiple isolates causing interveinal leaf yellowing identified two point mutations, occurring in positions 155 and 166 of the coat protein (CP), as unique to the yellowing pathotype. Site-directed mutagenesis of infectious clones confirmed that both CP mutations are determinants of the interveinal leaf yellowing symptoms. Sequencing of CP clones from plants or plant parts with the yellow mosaic symptoms resulted in a mixture of wild-type and mutated sequences, whereas sequencing of CP clones from the green heads of recovered plants resulted in only wild-type sequences. Yellow mosaic symptoms could be reproduced by inoculation of an artificial 1:1 mixture of RNA transcripts from the wild-type and mutated infectious clones. These results show that the ratio of mutated versus wild-type sequences can determine the nature and severity of symptom development. The gradual recovery of the plants, which coincides with the disappearance of the yellowing mutations, suggests that selection pressure acts to the advantage of the wild-type virus. Experiments with wild-type and mutated infectious clones showed that reverse mutation events from mutant to wild-type occur and that the wild-type virus does not have a replicative advantage over the mutant. These results suggest that reverse mutation events occur, with subsequent selection pressure acting in favour of the wild-type virus in the growing plant parts, possibly related to a lower long-distance movement efficiency of the mutant.