Phenotypically dissimilar greenhouse isolates from a Polish collection
of potato spindle tuber viroid (PSTVd) were analysed. Partially purif
ied PSTVd genomic RNAs from severe, intermediate and mild isolates was
reverse transcribed and the resulting cDNAs enzymatically amplified.
Abutting-primer PCR (Ab-P PCR) technology was used to obtain, in a sin
gle step, infectious full-length PSTVd cDNA monomers and these were se
quenced. The mild isolate was found to be composed of a unique molecul
ar variant (M), closely related to previously described PSTVd mild iso
lates. In the intermediate isolate, three variants, i2, i3 and i4, wer
e detected. The severe isolate was found to be a mixture containing at
least four molecular variants: s23, s27, i4 and i2. Infection of test
plants with plasmids carrying monomeric cDNAs corresponding to each o
f the cloned variants confirmed that they are infectious. In addition,
variant M produced mild symptoms, variants i2, i3, i4 intermediate sy
mptoms and variants s23 and s27 severe symptoms. Therefore, the diseas
e symptoms produced by a mixture are determined by the severe variants
, masking the presence of milder ones. All the variants detected (exce
pt i2 which is identical to previously described PSTVd-DI) represent n
ovel PSTVd sequences with point mutations located in the V and/or P do
mains. In particular, variants s23 and i4 represent shorter (358 nucle
otides) versions of the PSTVd genome.