Post-transcriptional transgene silencing and consequent tospovirus resistance in transgenic lettuce are affected by transgene dosage and plant development

Transgenic lettuce plants expressing the nucleocapsid (N) protein gene of the lettuce isolate of tomato spotted wilt virus (TSWV-BL) were protected against TSWV isolates via transgenic hi protein when the protein accumulated at high levels or via an N transgene silencing mechanism activated by its overexpression. In a transgenic lettuce line, post-transcriptional gene silencing was activated at a relatively earlier developmental stage in homozygous than in hemizygous progenies. As a result, the homozygous progenies generally showed a uniform suppression of N protein accumulation and consequently high levels of virus resistance in all leaves of the silenced plants. In contrast, N protein accumulated at high levels in the lower leaves of the hemizygous progenies and at much reduced levels (due to transgene silencing) in the successive leaves, resulting in moderate levels of virus resistance. It was also observed that the timing of the hi transgene silencing in both homozygous and hemizygous plants was affected by environmental factors.