Pathogenicity regulation by satellite RNAs of cucumber mosaic virus: minor nucleotide sequence changes alter host responses.

Six satellite RNAs of cucumber mosaic virus (CMV) could be differentiated on the basis of symptom expression they elicit in tomato and tobacco, and all but two could be differentiated by gel electrophoretic migration. Three of the satellite RNAs (B2-sat, G-sat, and WL1-sat RNA) ameliorated the symptoms induced by CMV on tomato, whereas three others (B1-sat, B3-sat, and WL2-sat RNA) induced chlorosis on tomato, the extent and nature of which was CMV-strain dependent. By contrast, B2-sat RNA induced chlorosis in tobacco, whereas WL1-sat and G-sat RNAs did not. Thus, the symptoms observed were dependent on the host species, the particular satellite RNA, and also the strain of helper virus, suggesting that a complex association of at least three factors is involved in symptom elicitation. Comparisons of the nucleotide sequences of pairs of satellite RNAs inducing the various chlorotic responses suggest that only a few nucleotide changes in specific domains are required for the elicitation of different host responses.