Transgenically expressed cucumber mosaic virus RNA 1 simultaneously complements replication of cucumber mosaic virus RNAs 2 and 3 and confers resistance to systemic infection.

Tobacco plants transformed with a cDNA copy of RNA 1 of the Fny strain of cucumber mosaic virus (CMV) promoted the asymptomatic accumulation of inoculated viral RNAs 2 and 3, which could be detected in noninoculated leaves, suggesting that the transgene also permitted viral long-distance movement. Typical symptoms of infection appeared later and correlated with the appearance of viral RNA 1 regenerated from the transgenic mRNA. Although all R0-generation plants were susceptible to Fny-CMV, one line displaying variable susceptibility to the virus in R1-and R2-generations led to selected R3-generation lines with systemic resistance to Fny-CMV. In the inoculated leaves of resistant plants, a dramatic decrease in the accumulation of viral RNA 1 was observed, relative to susceptible plants. No viral RNAs were detected in noninoculated leaves of the resistant plants, but such leaves were susceptible to infection. Furthermore, these leaves could sustain replication of inoculated CMV RNAs 2 and 3, indicating that a complete transgene-silencing had not been induced. Although a transgene-mediated, CMV RNA 1-suppression occurred in the inoculated leaf of resistant plants, the absence of a complete systemically acquired silencing suggests the existence of additional interferences with viral infection that prevented systemic infection by viral RNAs 2 and 3.