Differential effects of satellite RNA on the accumulation of cucumber mosaic virus RNAs and their encoded proteins in tobacco vs zucchini squash with two strains of CMV helper virus.

The presence of cucumber mosaic virus (CMV) satellite RNA usually reduces the yield of accumulated helper virus, although more so in solanaceous than in cucurbit hosts. The accumulation of viral RNA and viral-encoded proteins of two strains of CMV (Fny- and Sny-) known to differ in their ability to support satellite RNA in zucchini squash was examined in squash and tobacco to determine the effect of satellite RNA on the accumulation of viral-associated components. In the absence of satellite RNA, Fny- and Sny-CMV showed similar levels of accumulation of RNA at 7 days postinoculation (p.i.), but by 14 days p.i. the Fny-CMV RNAs accumulated to lower levels than did both strains at 7 days p.i., in either host. The levels of accumulated Sny-CMV-encoded proteins were higher than those encoded by Fny-CMV in tobacco, but not squash plants, at 7 days p.i. At 14 days p.i., for Fny-CMV vs Sny-CMV, there were differences in the levels of accumulation of most CMV-encoded proteins in both hosts, more exacerbated in tobacco vs squash. The effect of satellite RNA was to intensify these differences; that is, by 7 days p.i., satellite RNA reduced the accumulation of Fny-CMV RNAs 1 and 2 and their encoded proteins in both tobacco and squash but had little or no effect on the accumulation of Sny-CMV RNAs or encoded proteins. By 14 days p.i., the levels of accumulation of all Fny-CMV RNAs and encoded proteins were severely reduced in both hosts, and the levels of accumulation of Sny-CMV RNAs 1 and 2 and their encoded proteins were also reduced in tobacco, but not squash. Sny-CMV did not support satellite RNA accumulation in squash plants or protoplasts. Satellite RNA did not appear to have a direct effect on the movement of either CMV strain. Rather, accumulation studies in tobacco protoplasts indicated that the difference in response of Fny-CMV vs Sny-CMV to satellite RNA in tobacco was due to the extent to which satellite RNA affected the levels of RNA 1, and to a lesser extent RNA 2, and their encoded proteins, 1a and 2a, both components of the CMV replicase.