Lesions and virus accumulation in inoculated transgenic tobacco plants expressing the coat protein gene of tobacco mosaic virus.

The objective of this work was to identify steps in virus infection which were inhibited in transgenic tobacco plants (Nicotiana tabacum cv. Xanthi) that express the coat protein (CP) gene of the U, strain of tobacco mosaic virus (TMV). These plants were shown to be protected against disease development after inoculation with U,-TMV (P. Powell Abel, R. S. Nelson, B. De, N. Hoffman, S. G. Rogers, R. T. Fraley, and R. N. Beachy, 1986, Science 232, 738-743). Experiments were also conducted to allow comparison between the protection observed for the transgenic plants and cross-protection. In addition to protection against U1-TMV, the CP-expressing transgenic plants were protected against symptom development of infection after inoculation with a severe TMV strain, PV230, a strain which is immunologically related to the U1 strain. The numbers of chlorotic lesions produced on inoculated leaves of CID-expressing transgenic Xanthi plants infected with PV230 were 30%-or-less of those on leaves of control plants. Likewise necrotic lesion numbers produced on inoculated leaves of CP-expressing transgenic Xanthi 'nc' plants infected with U1 were 5%-or-less of those on leaves of control plants. Virus accumulation in the inoculated leaves of the CP-expressing Xanthi plants was substantially lower than that in leaves of control plants and thus correlated well with the lesion numbers. These results indicate that the delay in disease development includes prevention of virus accumulation in the inoculated leaves. Furthermore, there was a substantial reduction in accumulation of virus in the first leaf above the inoculated leaves in transgenic plants compared with control plants. Inoculation with viral RNA rather than virus largely overcame the protection, leading to the conclusion that the presence of the CID on virus particles in the challenge inoculum was necessary for maximum protection. As shown by these studies, expression of the TMV-CP coding sequence in transgenic plants mimics several of the characteristics of classical cross-protection previously reported by other researchers. We therefore refer to the protection observed for the transgenic plants as "genetically engineered cross-protection."