RESISTANCE IN TRANSGENIC TOBACCO PLANTS EXPRESSING A NONSTRUCTURAL GENE SEQUENCE OF TOBACCO MOSAIC-VIRUS IS A CONSEQUENCE OF MARKEDLY REDUCED VIRUS-REPLICATION

Recently, tobacco plants transformed with the sequence encoding the putative nonstructural 54-kDa protein of tobacco mosaic virus (TMV) were found to be resistant to the virus (D. B. Golemboski, G. P. Lomonossoff, and M. Zaitlin, Proc. Natl. Acad. Sci. USA 87:6311-6315, 1990). As a first step in the dissection of the mechanism(s) underlying this novel form of genetically engineered resistance to TMV, studies with protoplasts were initiated. Resistance to strain U1 TMV was expressed in protoplasts derived from 54-kDa transgenic tobacco, showing that resistance was not a direct result of restricted virus movement. However, low levels of both (+) and (-) sense TMV-specific RNAs were detected in these protoplasts. Similar results were obtained with inoculated leaves of 54-kDa transgenic tobacco plants to which extremely high concentrations (0.5-1.0 mg/ml) of strain U1 TMV had been applied. These results indicated that expression of the 54-kDa protein gene sequence in transgenic tobacco plants does not confer absolute immunity to TMV but rather it protects the plant from subsequent spread of the virus by strongly inhibiting viral replication at the infection site.