INCREASED VIRAL YIELD AND SYMPTOM SEVERITY RESULT FROM A SINGLE AMINO-ACID SUBSTITUTION IN THE TURNIP YELLOW MOSAIC-VIRUS MOVEMENT PROTEIN

Turnip yellow mosaic virus is a positive-strand RNA virus that produce s light green or yellow-preen mosaic symptoms in Chinese cabbage plant s. We have characterized a strain that produces nearly uniform yellow- preen chlorosis in systemically infected Chinese cabbage leaves. The i ncreased symptom severity is due to the single nucleotide substitution U1888-->C, which results in a tyrosine to histidine substitution in t he movement protein encoded by ORF-69. Coding by the overlapping ORF-2 O6 is not affected. The mutation results in fourfold higher accumulati ons of viral products in systemically infected Chinese cabbage leaves but does not affect viral replication in isolated protoplasts. These r esults suggest that the increased viral yield and symptom severity res ult from improved viral spread in the host plant. These effects were s pecific to Chinese cabbage, since neither viral yield nor symptoms in turnips were affected by the U1888-->C mutation.