Molecular determinants influencing the inheritance of transgenic virus resistance in segregating tobacco families transformed with the nucleocapsid gene of tomato spotted wilt virus

The successful introgression of transgenic virus resistance to control tomato spotted wilt virus (TSWV) epidemics relies on the stable inheritance of this type of resistance. We developed five segregating tobacco families transformed with sense or antisense copies of the TSWV nucleocapsid gene to determine the usefulness of TSWV-resistant transgenic parents in a breeding program, and to define the molecular determinants (transgene dosage, transcript and protein accumulation) controlling the expression of resistance. Three generations per family were evaluated: (1) R-3 or R-4 progeny derived from resistant transgenic parents used in crosses, (2) F-1 hybrids from crosses between the resistant transgenic parents and untransformed tobacco, and (3) F-2 progeny derived from resistant F-1 plants. Analysis of two of these families revealed that resistance was correlated with either two insertions in hemizygous plants or a single homozygous insertion. In contrast, in two other families, there was no correlation between the number of insertions and the resistance response, but rather resistance was correlated with a single, actively transcribed insertion. Transgene protein accumulation was not detected in any of the plants analyzed, indicating that resistance is probably RNA-mediated. As inheritance of the transgene does not always result in the expression of resistance, special care may be necessary when using transgenic resistant parents in a breeding population.