Resistance to TMV in transgenic plants results from interference with an early event in infection

Constitutive expression of the tobacco mosaic virus (TMV) coat protein (CP) gene in transgenic tobacco plants results in inhibition of disease symptom development following inoculation with TMV. Evidence is presented here that this protection is also observed in leaf mesophyll protoplasts isolated from these plants. Protoplasts were resistant to infection by TMV at concentrations of 10 microgram/ml to 1 mg/ml when introduced by either electroporation or polyethylene glycol-mediated inoculation. There was little protection against infection by TMV RNA and the protection was lost as the concentration of TMV RNA in the inoculum increased. When virus was incubated briefly at pH 8.0 prior to inoculation, protection broke down in a manner similar to that observed following RNA inoculation. Analogous results were obtained in experiments with whole plants. Because virus treated in this manner has presumably lost little or no CP, these results suggest that expression of the TMV CP gene in transgenic plant cells prevents TMV from uncoating. A model is presented for the mechanism of this blockage which relates these results to early events in TMV infection.