The Plant Cell, Vol. 7, 1625-1634, October 1995

Symptom Attenuation by a Normally Virulent Satellite RNA of Turnip Crinkle Virus Is Associated with the Coat Protein Open Reading Frame

Qingzhong Kong*, Jong-Won Oh#, and Anne E. Simon*

*Department of Biochemistry and Molecular Biology and Program in Molecular and Cellular Biology, University of Massachusetts, Amherst, Massachusetts 01003

#Department of Microbiology, University of Massachusetts, Amherst, Massachusetts 01003

Many satellite RNAs (sat-RNAs) can attenuate or intensify the symptoms produced by their helper virus. Sat-RNA C, associated with turnip crinkle virus (TCV), was previously found to intensify the symptoms of TCV on all plants in which TCV produced visible symptoms. However, when the coat protein open reading frame (ORF) of TCV was precisely exchanged with that of cardamine chlorotic fleck virus, sat-RNA C attenuated the moderate symptoms of the chimeric virus when Arabidopsis plants were coinoculated with the chimeric virus. Symptom attenuation was correlated with a reduction in viral RNA levels in inoculated and uninoculated leaves. In protoplasts, the presence of sat-RNA C resulted in a reduction of ~70% in the chimeric viral genomic RNA at 44 hr postinoculation, whereas the sat-RNA was consistently amplified to higher levels by the chimeric virus than by wild-type TCV. TCV with a deletion of the coat protein ORF also resulted in a similar increase in sat-RNA C levels in protoplasts, indicating that the TCV coat protein, or its ORF, downregulates the synthesis of sat-RNA C. These results suggest that the coat protein or its ORF is a viral determinant for symptom modulation by sat-RNA C, and symptom attenuation is at least partly due to inhibition of virus accumulation.