Transgenic tobacco expressing tobacco streak virus or mutated alfalfa mosaic virus coat protein does not cross-protect against alfalfa mosaic virus infection.

Transgenic tobacco plants expressing the coat protein (CP) genes of tobacco streak virus (TSV) and alfalfa mosaic virus (AIMV) were used in studies on cross-protection and genome activation. Plants expressing the TSV CP gene were highly resistant to infection with TSV nucleoproteins but were susceptible to infection with AIMV nucleoproteins. Moreover, these plants could be infected with a mixture of AIMV RNAs 1, 2, and 3 in contrast to the nontransformed control plants. This demonstrates that the endogenously produced TSV CP is able to activate the AIMV genome but does not cross-protect against this virus. Conversely, it was shown that plants expressing the AIMV CP gene did not resist TSV infection. Transgenic tobacco plants transformed with an AIMV CP gene with a frame-shift mutation in the reading frame were found to accumulate viral transcripts to a level similar to that obtained in plants expressing a wild-type AIMV CP gene. However, these plants did not produce detectable amounts of viral protein and showed no resistance to infection with AIMV nucleoproteins in contrast to transgenic plants accumulating wild-type AIMV CP. This demonstrates that it is the CP that is responsible for cross-protection in transgenic plants and not the chimeric CP mRNA.