Minor differences between nucleotide sequences of mild and severe strains of potato spindle tuber viroid

Abstract

VIROIDS are the smallest replicating pathogenic agents known¹. Composed entirely of RNA, they have genome sizes in the range of 330–380 nucleotides², 10 times smaller than the smallest bacteriophage of Escherichia coli³. Although there is no evidence for the existence of a protective protein coat for viroids, they infect a wide variety of plants and in many produce severe disease symptoms^1,4. The molecular mechanisms by which viroids replicate and interact with their hosts are not yet understood. In its most severe form, the disease^5,6 caused by potato spindle tuber viroid (PSTV) causes general stunting of potato plant growth, deformity of the upper foliage, and production of disfigured potatoes⁵. Mild strains of PSTV which produce barely detectable symptoms have also been isolated⁷. Furthermore, plants infected by mild strains are somehow protected from developing symptoms following subsequent inoculation with severe strains^8,9. This biological protection phenomenon has been called cross-protection⁷. As the only known component of viroids is RNA, mild and severe strains must differ in nucleotide sequence. The study described here aimed to determine the extent of this difference.