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Heterologous encapsidation and recombination in transgenic plants containing viral sequences

Summary

Very good resistance toward viruses can be achieved with genetically modified plants that express viral sequences. The safety of these plants was examined, and two mechanisms which may lead to the appearance of new viral diseases were identified:

• Heterologous encapsidation, in plants accumulating coat proteins
• Recombination

In natural situation, heterologous encapsidation may modify the type of transmission of the viruses and provoke an increase of the severity of disease symptoms.

With transgenic plants a new situation occurs, because:

• The coat protein may be produced in plants which are not hosts f the virus,
• The coat protein gene may be modified,
• The frequency of heterologous encapsidation may be increased in some virus families.

However, the effects of a heterologous encapsidation are only transient, and are thus considered of low risk. Moreover, simple measures can limit the possibility of heterologous encapsidation in transgenic plants.

The recombination is a natural mechanism which can cause a modification of the genetic material of the viruses. In natural situation, examples of recombination which occurred in the past were observed. In transgenic plants, recombination was only observed under high selection pressure.

A new situation may be associated to the genetically modified plants, because:

• The transgenic viral sequences may be introduced in non-host plants,
• The viral sequences may be modified.

The effects of a recombination are permanents, but the risks specific to the transgenic plants are still poorly known. More research is needed to study these plants.

The perceived risks of heterologous encapsidation or recombination can be reduced in transgenic plants, by following one or several recommendations:

• Introduction of viral sequences in natural host plants of the virus only.
• Modification of the coat protein genes to eliminate the sites for insect transmission, for encapsidation or assembly.
• Favor use of short transgenic sequences.
• Favor strategies using RNA-mediated protection.
• Avoiding 3'- and 5'-non-translated regions.

Conclusion:

The transgenic plants are indispensable to manage some viral diseases. The perceived risks at the present time are low in comparison to the potential benefits. Moreover, they can be reduced by adopting simple measures during the conception of the strategy.