Genetic variability of potato spindle tuber viroid RNA replicon

The genetic continuity of the potato spindle tuber viroid (PSTVd) genome wa s analysed after infection of tomato plants with cloned cDNAs of parental s trains. During the six weeks of the experiment, several new sequence varian ts appeared. The sequence variants detected in the progeny population induc ed sequence-specific disease symptoms. The PSTVd genome therefore follows t he pattern expected for typical pseudo-strains propagating in plants as a p opulation of similar sequences. Assessing further the replicon continuity, a PSTVd cDNA mutant with a deletion in the central conserved region was con structed and proven to be non-infectious. Surprisingly, in a sub-population of potato transformants expressing the same deleted PSTVd RNA an infectiou s viroid was detected. This suggests specific transcript conversion followe d by recovery of the full-length pathogen genome.