Attenuated isolates of cucumber mosaic virus produced by satellite RNA and cross protection between attenuated isolates and virulent ones.

Cucumber mosaic virus (CMV) has four major RNA segments, which are named RNA 1, 2, 3 and 4 in order of decreasing molecular weight. The largest three RNAs are essential for the manifestation of the infectivity of the virus, and RNA 4 is a subgenomic fragment of RNA 3. Recently, the existence of another low molecular weight satellite RNA, i.e. the fifth RNA, associated with the replication of CMV has been reported1-8). Kaper and Waterworth4) showed that CMV-S containing the satellite RNA (CARNA 5) induced lethal necrotic symptoms on tomato, and Takanami7) also reported striking changes in the symptoms on CMV-infected plants by the addition of the satellite RNA. Mossop and Francki6) showed that Sat-RNA attenuated symptoms of CMV on Rutgers tomato plants. In 1982, necrotic disease of tomato plants (Fig. 1) was observed in Ohno and Nanae, southern Hokkaido, and an isolate of CMV, designated as CMV-P(n), was found to be the causal agent of the disease9). A small RNA was detected among the RNAs of CMVP(n)10), and it had a satellite nature similar to that of CARNA 54) or satellite RNA7) previously reported. This satellite RNA which was isolated from CMV-P(n), and designated as (n)RNA 5, caused necrosis on tomato plants only when it was simultaneouly inoculated with satellite-free CMV. Another satellite RNA, designated (fl)RNA 5, was obtained from CMV-PF(fl) which induced fern-leaf symptoms on tomato plants10) (Fig. 2). In this case, however, the satellite RNA caused an attenuation of the symptoms on tomato plants induced by CMV when it was simultaneously inoculated with satellite-free CMV. The present paper dealt with the cross protection between the attenuated isolates containing (fl)RNA 5 and the virulent ones in order to determine whether the attenuated isolates can be used for controlling CMV on tomato plants.