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

Transgenic resistance

Transgenic resistance to plant viruses is an important technology for control of plant virus infection, which has been demonstrated for many model systems, as well as for the most important plant viruses, in terms of the costs of crop losses to disease, and also for many other plant viruses infecting various fruits and vegetables. Different approaches have been used over the last 28 years to confer resistance, to ascertain whether particular genes or RNAs are more efficient at generating resistance, and to take advantage of advances in the biology of RNA interference to generate more efficient and environmentally safer, novel "resistance genes." The approaches used have been based on expression of various viral proteins (mostly capsid protein but also replicase proteins, movement proteins, and to a much lesser extent, other viral proteins), RNAs [sense RNAs (translatable or not), antisense RNAs, satellite RNAs, defective-interfering RNAs, hairpin RNAs, and artificial microRNAs], nonviral genes (nucleases, antiviral inhibitors, and plantibodies), and host-derived resistance genes (dominant resistance genes and recessive resistance genes), and various factors involved in host defense responses. This review examines the above range of approaches used, the viruses that were tested, and the host species that have been examined for resistance, in many cases describing differences in results that were obtained for various systems developed in the last 20 years. We hope this compilation of experiences will aid those who are seeking to use this technology to provide resistance in yet other crops, where nature has not provided such.

Reciprocal sequence exchange between non-retro viruses and hosts leading to the appearance of new host phenotypes

Divergence among individuals of the same species may be linked to positional retrotransposition into different loci in different individuals. Here we add to recent reports indicating that individual variance occurs due to the integration of non-retroviral (potyviral) RNAs into the host genome via RNA recombination followed by retrotransposition. We report that in bees (Apis mellifera), approximately 30% of all tested populations carry a segment of a dicistrovirus in their genome and have thus become virus-resistant. Reciprocally, segments of host sequences have been found within defective-interfering-like sequences of a dicistrovirus. Similarly, host sequences were found fused to potyviral sequences, previously described integrated into their host genome. A potential, continuous RNA exchange leading to divergence is discussed.

Risk assessment of virus-resistant transgenic plants

Virus-resistant transgenic plants (VRTPs) hold the promise of enormous benefit for agriculture. However, over the past ten years, questions concerning the potential ecological impact of VRTPs have been raised. In some cases, detailed study of the mode of action of the resistance gene has made it possible to eliminate the source of potential risk, notably the possible effects of heterologous encapsidation on the transmission of viruses by their vectors. In other cases, the means of eliminating likely sources of risk have not yet been developed. When such residual risk still exists, the potential risks associated with the VRTP must be compared with those associated with nontransgenic plants so that risk assessment can fully play its role as part of an overall analysis of the advantages and disadvantages of practicable solutions to the problem solved by the VRTP.

Mutations in the coat protein gene of plum pox virus suppress particle assembly, heterologous encapsidation and complementation in transgenic plants of Nicotiana benthamiana

Two different motifs in the coat protein (CP) of Plum pox virus (PPV) (R(3015)Q(3016), D(3059)) were mutated by replacing the respective amino acids with others possessing different chemical properties. The mutated CP genes were introduced into an infectious full-length clone of PPV (p35PPV-NAT) to investigate their influence on systemic infection of transgenic wild-type PPV CP-expressing and non-transgenic plants of Nicotiana benthamiana. All mutants failed to establish systemic infections in non-transgenic N. benthamiana plants, but were complemented by intact CP in transgenic plants. Moreover, the CP-RQ-D mutant (carrying mutations in both the RQ and D motifs) was introduced into p35PPV-NAT engineered to express beta-glucuronidase (GUS) for direct observation of systemic movement and particle assembly in N. benthamiana leaves. GUS-staining revealed that the CP mutant (RQ-D) was restricted to initially infected cells without forming virions. Systemic movement and particle assembly were restored in CP-transgenic N. benthamiana plants. Finally, transgenic N. benthamiana plants were generated that expressed each of the three mutated CP genes. Homozygous T(2) lines were selected and tested for resistance to PPV. Immunogold labelling and electron microscopy revealed that heterologous encapsidation with challenging Chilli veinal mottle virus and Potato virus Y was suppressed in these lines. In addition, assembly mutants did not complement CP-defective p35PPV-NAT. The possible use of modified viral CP genes for the production of virus-resistant transgenic plants, thereby reducing the putative risks of heterologous encapsidation and complementation, is discussed.

Tobacco plants transformed with a modified coat protein of tomato mottle begomovirus show resistance to virus infection

ABSTRACT Tobacco plants (Nicotiana tabacum 'Xanthi') were transformed with a binary vector containing the coat protein gene of tomato mottle begomo-virus (ToMoV) modified by the deletion of 30 nucleotides in the 5' end. The R(1) generation was screened for resistance to ToMoV by inoculation with viruliferous whiteflies. Fifteen days after inoculation, symptom development was recorded weekly for up to 120 days using a visual scale, and ToMoV infection was confirmed by polymerase chain reaction and enzyme-linked immunosorbent assay. The response to high inoculation levels of ToMoV varied and ranged from susceptibility to immunity. The transgene transcript was detected by northern blot analysis; however, the transgene product could not be detected by protein blot analysis using antisera reactive with ToMoV coat protein. The lack of detection of the transgene product in resistant plants suggests that it is not involved in eliciting the resistance response and that resistance may be mediated by the transgene transcript.

Plum pox potyvirus resistance associated to transgene silencing that can be stabilized after different number of plant generations

Nicotiana benthamiana plants were transformed with a fragment of the plum pox potyvirus (PPV) genome that encodes the nuclear inclusion a (NIa) and b (NIb) proteins and the N-terminus of the capsid protein (NIa-NIb-CP). Lines transformed with this PPV genomic fragment harboring mutations in the GDD replicase-motif were also obtained. Plants of NIaDeltaV lines that carry a GDD to VDD mutation in the PPV transgene, were immune to PPV infection. The resistance was highly specific, since it was only partially overcome by a PPV strain different to that from which the transgene was derived, and no resistance was observed after inoculation with a second potyvirus. PPV was not able to replicate in protoplasts isolated from NIaDeltaV transgenic plants, indicating that the resistance was functional at the single cell level. Only a fraction of plants from lines transformed with the NIa-NIb-CP fragment harboring a GDD to ADD mutation (NIaDeltaA lines), were resistant to PPV infection. This same phenotype was observed in plants expressing the wild-type construction (NIaDelta), although the progeny of some non-infected plants seemed to be completely resistant to PPV, independently of the allelic status of the parental plant. In all cases, the resistance phenotype correlated positively with low levels of transgene mRNA accumulation, suggesting that it was mainly due to a gene silencing mechanism. Our results show that, although the transgene was not silenced in all R1 plants from some individual lines, a stable silenced status could be reached in the following generations.