Induction of a Highly Specific Antiviral State in Transgenic Plants: Implications for Regulation of Gene Expression and Virus Resistance.

Abstract

Transgenic tobacco plants expressing either a full-length form of the tobacco etch virus (TEV) coat protein or a form truncated at the N terminus of the TEV coat protein were initially susceptible to TEV infection, and typlcal systemic symptoms developed. However, 3 to 5 weeks after a TEV infection was established, transgenlc plants "recovered" from the TEV infection, and new stem and leaf tissue emerged symptom and virus free. A TEV-resistant state was induced ln the recovered tissue. The resistance was virus speclflc. Recovered plant tissue could not be lnfected wlth TEV, but was susceptlble to the closely related virus, potato virus Y. The resistance phenotype was functional at the slngle-cell leve1 because protoplasts from recovered transgenic tissue did not support TEV repllcation. Surprlslngly, steady state transgene mRNA levels in recovered tissue were 12-to 22-fold less than transgene mRNA levels in uninoculated transgenic tissue of the same developmental stage. However, nuclear run-off studles suggested that transgene transcription rates in recovered and uninoculated plants were similar. We propose that the resistant state and reduced steady state levels of transgene transcript accumulation are mediated at the cellular leve1 by a cytoplasmic actlvity that targets speclfic RNA sequences for inactlvation.

Chalcone synthase cosuppression phenotypes in petunia flowers: comparison of sense vs. antisense constructs and single-copy vs. complex T-DNA sequences

“…Also, comparisons of white sector size in patterns inherited from single-copy transgenotes versus dispersed-repeat transgenotes suggest a dosage effect for Chs sense cosuppression, i.e., multiple copies produce more transcripts than do single copies and so exceed a threshold at which transcript turnover is triggered, as proposed elsewhere [1, 11,15]. It is important to note that multiple copies then have two distinguishable effects, one being the dosage effect leading to white flowers and the other being the paramutation-like effect leading to vein-based patterns.…”

Analysis of transgenic tobacco plants expressing a truncated form of a potyvirus coat protein nucleotide sequence

“…A model has been proposed to explain the mode of action and suggests a cellular, cytoplasmic activity is induced by the over-expression of the transgene. This activity is responsible for highly selected RNA elimination and may be mediated via a protein or nucleic acid [20]. The characteristics of RNAmediated resistance were observed with the ACl18 plants.…”

“…The results from a typical inoculation series are presented in Table 2 never observed to support TEV replication. The remaining lines were susceptible to TEV, although a recovery phenotype, which has been described for FL transgenic plants [20], aphids. Virus concentration in each leaf was estimated using DAS-ELISA.…”

Post-transcriptional gene-silencing and RNA interference: genetic immunity, mechanisms and applications

“…34 The ®rst indication that cosuppression operated via a mechanism that had evolved for virus-resistance came from the discovery that when transgenes expressing viral components were introduced into plants some lines were found to be virus resistant. 35,36 Subsequently, the use of viral replicon-based cDNAs as expression vectors (Amplicon 2 ) has been shown to result in reliable cosuppression. 37 The Amplicon 2 vector is an expression cassette containing the entire potato virus X (PVX) cDNA except the coat-protein gene is replaced by a target endogene (Fig 6(a)).…”