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

Abstract

Summary 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 N 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 N transgene silencing in both homozygous and hemizygous plants was affected by environmental factors.

Resistance in plants transformed with the P1 or P3 gene of tobacco vein mottling potyvirus.

“…4), where the silencing is triggered simultaneously both in inoculated and non-inoculated plants, and seems to occur at a certain developmental stage. A similar example of development-associated silencing has been reported recently for a viral transgene, the tospoviral N gene (Pang et al, 1996) and there are also examples of this phenomenon for non-viral transgenes (Elmayan & Vaucheret, 1996 ;Kunz et al, 1996). TVMV infection of our P3 lines may have an overlapping role resulting in a final degree of gene silencing that is more complete in inoculated than uninoculated plants (Fig.…”

Pathogen‐derived resistance targeted against the negative‐strand RNA of tobacco mosaic virus: RNA strand‐specific gene silencing?

“…An additional and particularly distinctive feature is the strand polarity of the RNA target in the 54K plants, as revealed by the transient gene expression assay (Figure 6). In previously described examples of homology-dependent resistance, it can be inferred from the associated transgene silencing phenotype that the sense RNA is a target of the mechanism (English et al, 1996;Guo and García, 1977;de Haan et al, 1992;Lindbo et al, 1993;Mueller et al, 1995;Pang et al, 1996;Prins et al, 1996;Rubino and Russo, 1995;Sijen et al, 1996aSijen et al, , 1996bSmith et al, 1994;Tenllado et al, 1995Tenllado et al, , 1996van der Vlugt et al, 1992).…”

“…Virus resistance associated with post-transcriptional gene silencing has also been described in transgenic plants expressing virus-derived sequences from regions other than the replicase open-reading frame. In all of these examples the resistance is dependent upon extreme sequence similarity between the sense RNA products of the transgene and the inoculated virus (de Haan et al, 1992;English et al, 1996;Guo and García, 1977;Lindbo et al, 1993;Mueller et al, 1995;Pang et al, 1996;Prins et al, 1996;Rubino and Russo, 1995;Sijen et al, 1996aSijen et al, , 1996bSmith et al, 1994;Tenllado et al, 1995Tenllado et al, , 1996van der Vlugt et al, 1992). In this paper we use the term 'homology-dependent resistance' when referring to the examples in which the mechanism is related to gene silencing.…”

Transgenic accumulation of two plant virus coat proteins on a single self-processing polypeptide.

“…There are examples, however, in which transgenic proteins do not accumulate, or they do so to low levels, yet significant levels of protection are observed. In some of these examples, non-translatable gene transcripts confer protection (De Haan et al, 1992 ;Lindbo et al, 1992 a, b ;Van der Vlugt et al, 1992), in some cases as a result of posttranscriptional transgene silencing (Lindbo et al, 1993 ;Mueller et al, 1995 ;Pang et al, 1996).…”