Articles
ALTERNATIVES TO THE USE OF VIRUS COAT PROTEIN FOR ENGINEERING VIRUS RESISTANCE IN PLANTS
Practical use of cross protection has been used extensively in tomato, citrus and papaya as a means of protecting plantings from infection by severe virus strains (Matthews, 1991). With developments in molecular biology and an increased understanding of the mechanisms by which Agrobacterium tumefaciens transfers genes to plant genomes, the tools to provide protection against virus infection using a portion of viral genomes became a reality (Bevan, 1985).
Since the first demonstration that virus coat protein expressed in plants provides some level of resistance (Powell- Abel et al., 1986) this approach has been used to develop resistance to a large number of different viruses (Beachy et al., 1990). However, there are reasons to look at alternatives to coat protein-mediated resistance (CPMR). First, it is likely that strains able to circumvent resistance will evolve, particularly as CPMR becomes widely used in crop protection.
It can be expected that multiple protection strategies will be necessary to realize the maximum potential for virus-free transgenic crops when they are subjected to field conditions. (Scholthof et al., 1993). Second, viruses are very efficient in terms of utilizing the host’s genes and cellular synthesis. systems.
All viral genes tend to be essential and unique to pathogenic functions.
This means that nearly any viral gene may be a suitable target for the genetic engineering of pathogen-derived resistance (Sanford and Johnstone, 1985). Third, CPMR usually provides resistance to low levels of inoculum.
In most cases this is sufficient, however, insect or nematode vectors may provide enough inoculum at the cellular level (not at the leaf or whole plant level) to overcome the resistance and establish an infection.
Fourth, heterologous encapsidation as demonstrated for potyviruses may alter the ecology and epidemiology of some viruses (Farinelli et al., 1992; Lecoq et al., 1993). Fifth, with the possibility of recombination as demonstrated for DNA (Gal et al., 1991) and for RNA plant viruses (Greene and Allison, 1994) it may be undesirable to use the coat protein gene as a source of resistance.
In cases where the coat protein is involved in transmission recombination of the gene to a different virus could alter its epidemiology and ecology.
When one considers these points it seems imperative that we look at alternative strategies to enhance the usefulness of engineered resistance.
Also, an alternative strategy may provide either better resistance or perhaps a more durable form of resistance.
Another consideration is the risk associated with the use of various strategies for engineering resistance.
This is an area that needs more research so that risk assessment is based on scientific data rather than unsupported assumptions.
The recent demonstration of recombination between a transgene and a disabled virus (Greene and Allison, 1994) raises many questions about the risk associated with use of viral genes to develop resistance.
It would be desirable to see a study using a viral transgene