Nucleocapsid gene-mediated transgenic resistance provides protection against Tomato spotted wilt virus epidemics in the field

Transformation of plants with the nucleocapsid (N) gene of Tomato sported w ilt tospovirus (TSWV) provides resistance to disease development; however, information is lacking on the response of plants to natural inoculum in the field. Three tobacco cultivars were transformed with the N gene of a dahli a isolate of TSWV (TSWV-D), and plants were evaluated over several generati ons in the greenhouse. The resistant phenotype was more frequently observed in 'Burley 21' than in 'KY-14' or 'K-326', but highly resistant 'Burley 21 ' transgenic lines were resistant to only 44% of the heterologous TSWV isol ates tested. Advanced generation (R-3 and R-4) transgenic resistant lines o f 'Burley 21' and a 'K-326' F-1 hybrid containing the N genes of two TSWV i solates were evaluated in the field near Tifton, GA, where TSWV is endemic. Disease development was monitored by symptom expression and enzyme-linked immunosorbent assay (ELISA) analysis. Whereas incidence of TSWV infection i n 'Burley 21' susceptible controls was 20% in 1996 and 62% in 1997, the mea n incidence in transgenic lines was reduced to 4 and 31%, respectively. Thr ee transgenic 'Burley 21' lines were identified that had significantly lowe r incidence of disease than susceptible controls over the two years of the study. In addition, the rate of disease increase at the onset of the 1997 e pidemic was reduced for all the 'Burley 21' transgenic lines compared with the susceptible controls. The 'K-326' F-1 hybrid was as susceptible as the 'K326' nontransformed control. ELISA analysis demonstrated that symptomless plants from the most resistant 'Burley 21' transgenic lines accumulated de tectable nucleocapsid protein, whereas symptomless plants from more suscept ible lines did not. We conclude that transgenic resistance to TSWV is effec tive in reducing incidence of the disease in the field, and that accumulati on of transgene protein may be important in broad-spectrum resistance.