Genetically Engineered Virus-Resistant Crops

By Stellos Tavantzis, Professor of Plant Pathology, University of Maine

SUMMARY

At present, plant viral diseases cause significant losses in crop quantity and quality. Naturally-occurring virus resistance genes have been introduced into commercial crop varieties by traditional plant breeding techniques. This is a proven approach but is limited by the availability of such genes and the many years of effort that this approach requires.

Genetic engineering has become a successful strategy for producing virus resistant crop plants since 1986. Several candidate genes from naturally resistant plants are now becoming available with which to produce transgenic plants. However, the most common approach to obtain resistance does not use resistance genes. Current commercial transgenic crop plants, such as NatureMark's New Leaf Y potato, express resistance to viral infection by inserting a gene that codes for a protein on the outside of the virus called the "coat protein". This mechanism of developing resistance to viral infection by expressing only a single viral protein is referred to as "pathogen-derived resistance."

A wide assortment of viral genes has been demonstrated to confer resistance against infection by that virus. This observation suggests that initiation of plant viral disease can be disrupted by a number of different processes. However, the precise mechanisms involved in disease resistance by transgenic plants are unknown. The use of genetic engineering to produce plants which are resistant to viral diseases has proven effective and is likely to be deployed widely in agriculture. For this reason, it is important to determine whether large-scale deployment of transgenic plants containing and expressing viral genes is safe for the environment, agriculture and human health. This paper reviews some of the issues scientists are studying in order to determine whether or not significant risks exist and, if they do, how they can be addressed.

Questions that have been posed include: 1) possible effects of virus-derived genes on the evolution of viruses; 2) possible effects of viral transgenes on the development of hybrid viral pathogens which can infect species that are currently not infected; 3) whether pollen from a transgenic plant can fertilize neighboring species; and 4) whether there is any effect of the viral gene product on human health or food safety.

We do know that the potential exists for the viral protein made by the transgenic plant to coat another virus and render it transmissible to other plants by insects. Mixed virus infections are common in nature. For example, a non-transmissible virus can become transmissible by aphids in the presence of a second co-infecting virus. This phenomenon of capturing the genetic material of one virus in the coat structure of another virus is called "dependent transmission." It has been shown that this process takes place in transgenic potato plants expressing the coat protein gene of the N strain of the potato virus Y (PVY) when these plants are infected with the O strain of PVY. It should be noted that since the viral hereditary material is not altered through this process, a "new" virus or a "new" viral disease does not result; the genetic information still makes only the proteins of the original virus. However, it is possible that the distribution of viruses in the environment might be altered by large-scale introduction of crops engineered with viral coat protein genes.

Research also indicates that recombination of another type of genetic material, known as RNA, has been an important natural process shaping the evolution of plant viruses containing this type of genetic material. The question arises as to whether RNA found in transgenic virus-resistant crops will be included in the RNA recombination processes, resulting in the development of new viruses. Such RNA recombination is not unique to transgenic plants, similar opportunities for RNA recombination already exist in nature in the cases of mixed viral infections. Survival of the recombined viruses appears to be limited in nature. Thus, the development of new, competitive plant viruses would be limited as well. However, deployment of virus-derived resistance genes presents us with new situations in terms of: 1) increasing the opportunities for RNA recombination, and 2) overcoming geographic rather than biological boundaries.

Another concern is that these virus-derived resistance genes could be transferred from genetically engineered crop plants to closely related weed species by pollen transfer. Such a transfer might result in a weed population with a competitive advantage because of its newly found virus resistance. Since some crop plants and wild relatives are able to cross fertilize, this possibility exists. However, this argument can be equally used for similar cases involving introduction of new virus-resistant varieties produced by traditional (non-transgenic) plant breeding methods.

The question as to whether virus-derived transgenes or their products pose any threat to human health is relatively easy to address. Plant tissues are virus infected more often than not. So, in this regard, coat protein-expressing transgenic plants do not present a novel situation. Governments and other organizations are currently assessing a more general question concerning genetically engineered crop plants that contain an antibiotic resistance gene. Whether eating produce containing these genes will affect antibiotic therapy, or whether these genes could spread from transgenic plants to bacteria are questions for which we have few answers at the present time.

For further information, contact:

Stellos Tavantzis, Professor of Plant Pathology
University of Maine
Biological Sciences
Phone: 581-2986
E-mail: stellos@maine.maine.edu

The University of Maine provides education on genetics and related topics as part of its Land Grant mission. Through courses and research with faculty members, students gain a thorough understanding of the science of genetics as well as its applications. Graduates qualify for jobs in the growing biotechnology industry and apply their skills in a variety of other occupations. As part of the university's research mission, UMaine scientists focus on basic genetic processes as well as those specifically relevant to agriculture, forestry, fisheries, wildlife and human health. Faculty collaborate with researchers at The Jackson Laboratory and the Maine Medical Research Center as well as federal laboratories and other universities.

Dr. Tavantzis Dr. Tavantzis studies plant-microbe interactions with the goal to understand how pathogens cause disease in plants, and to use this knowledge to develop environment-friendly strategies of managing plant diseases. Dr Tavantzis has received research funding from the US Department of Agriculture, the National Science Foundation, the Maine Potato Board and the Maine Center for Innovation in Biotechnology.