A single base pair in the right terminal domain of tomato planta macho viroid is a virulence determinant factor on tomato.

Tomato planta macho viroid (TPMVd), including isolates previously designated as Mexican papita viroid (MPVd), causes serious disease on tomatoes in North America. Two predominant variants, sharing 93.8% sequence identity, incited distinct severe (MPVd-S) or mild (MPVd-M) symptoms on tomato. To identify virulence determinant factor, a series of chimeric infectious clones were generated using synthetic DNA approach to progressively replace each structural domain between the two variants. In bioassays on tomato 'Rutgers', three chimeras containing Terminal Left and Pathogenicity (MPVd-H1), Central (MPVd-H2), or Variable (MPVd-H3) of MPVd-S, incited mild to intermediate symptoms. However, a chimera containing Terminal Right (T) of MPVd-S (MPVd-H4) incited severe symptoms. Only one base-pair mutation in the T domain between MPVd-M (U:A) and MPVd-S (G:C) was identified. A reciprocal mutant (MPVd-H5) rendered the chimeric viroid mild on tomato. This single base-pair in the T domain was determined as the virulence determinant factor for TPMVd.