Mutational Analysis of Viroid Pathogenicity: Tomato Apical Stunt Viroid. Robert A. Owens. Microbiology and Plant Pathology Laboratory, Plant Sciences Institute, USDA-ARS Beltsville, MD 20705 U.S.A. . MPMI 3:374-380. Accepted 16 July 1990. This article is in the public domain and not copyrightable. It may be freely reprinted with customary crediting of the source. The American Phytopathological Society, 1990.

A series of nucleotide substitutions within the pathogenicity domain of tomato apical stunt viroid have been evaluated for their effects upon infectivity and symptom expression. None of the 12 A→G substitutions and one C→U substitution that were examined abolished infectivity in a whole plant bioassay, and the resulting progeny were characterized by nucleotide sequence analysis of cDNAs amplified by the polymerase chain reaction. Four of the 13 substitutions gave rise to altered progeny, but the patterns of sequence changes observed were unexpectedly complex. Mutations that did not rapidly revert to the wild-type sequence are located near the right border of the pathogenicity domain, a region which shows considerable natural sequence variability. None had a detectable effect upon symptom expression. The ability to observe viroid sequence evolution in vivo may provide insight into the molecular interactions responsible for viroid host range and symptom formation.