Resistance to heterologous isolates of tomato spotted wilt virus in transgenic tobacco expressing its nucleocapsid protein gene

The nucleocapsid protein (NP) gene of the lettuce isolate of tomato spotted wilt virus (TSWV-BL), from the L serogroup, was synthesized and cloned into a plant expression cassette using polymerase chain reaction. Transgenic tobacco plant lines were obtained via Agrobacterium tumefaciens-mediated leaf disk transformation, and their progenies were tested for their ability to resist infections by TSWV isolates belonging to the L and I serogroups. Nearly all transgenic RI plants were resistant to the homologous TSWV-BL isolate. High levels of resistance to the heterologous isolates of the L serogroup were found in plants that accumulated little, if any, NP; plants that accumulated high levels of NP showed the best resistance to infection by an isolate of the I serogroup (Begonia isolate). However, all of these plant lines were susceptible to infection by a distinct Brazilian isolate that belongs to neither the L nor the I serogroup, although the plants that accumulated high levels of the NP did display a delay in symptom development. These results demonstrate that a fairly broad spectrum of resistance can be obtained by the expression of a TSWV-BL NP gene and that there are limits to this spectrum. In addition, the finding that this spectrum of resistance is dependent on the levels to which the NP accumulates suggests that different mechanisms may exist that mediate these different resistance modes