Sequence analysis of five new field isolates demonstrates that the chain length of potato spindle tuber viroid (PSTVd) is not strictly conserved but as variable as in other viroids

The sequence analysis of five new field isolates of potato spindle tuber viroid (PSTVd) of different virulence revealed that the length of their RNA chain is not strictly conserved to 359 nucleotides (nts), as one could have inferred from the previously sequenced PSTVd strains. It was now found that the chain length is strain-specific like in the case of practically all other viroids, and that it may vary, so far, between 356 and 360 nts. Taking our previously sequenced and least virulent mild strain PSTVd KF6-M as standard, the new mild strains PSTVd WA-M and PSTVd F-M differ from it by one or two nts. The new intermediate-severe strains PSTVd F-IS and PSTV-F 88-IS differ from the standard mild strain by eight and nine nts, respectively, whereas the new severe-lethal strain PSTVd F-SL differs in seven nts. Most of these mutations are located within the virulence-modulating (VM) region and within the variable region (VR), and only in two strains a single mutation is found in the right terminal domain.