Producing Disease-Resistant Plants

Roger N. Beachy, professor of biology, Washington University, St. Louis, MO.

Diseases caused by a variety of pathogens inflict enormous losses to crops each year. Control of diseases has historically relied on several approaches, including application of chemicals to control the spread of disease-inciting agents (bacteria or fungi) or their insect vectors, removing diseased material from the field to reduce the source of inoculum, maintaining good cultural practices, and incorporating genes for disease resistance. The latter is, by far, the most environmentally and, generally, economically sound way to control the spread of disease. It is also the most limiting approach, however, since disease organisms far outnumber the genes for resistance so far identified.

The search for resistant genes involves screening cultivars or species of plants to identify individuals that exhibit resistance to infection, replication, or spread of a pathogen. If the resistance trait is the result of expression of a single gene or genetic locus, Plant breeders begin the task of introducing the resistance trait into a cultivar having desirable agronomic traits that will ultimately be released to the farmer. The plant-breeding Process usually requires more than 5 Years and considerable evaluation of Progeny to eliminate plants that contain undesirable traits in addition to the disease resistance trait.

Search for Disease Resistance Genes

Isolating a Gene Based on Its Function. With the advent of the techniques of recombinant DNA and the transformation of plants, research scientists began to explore alternate approaches for producing disease-resistant plants. In the first and most favored approach, fundamental research is carried out on host pathogen interactions to determine the molecular basis that enables one plant to be resistant to a pathogen, while another is susceptible. In this approach, which is more or less standard, although difficult, researchers attempt to identify and isolate a specific gene or genes expressed in the resistant plant but absent or not expressed in a susceptible plant.

Scientists in a number of laboratories, primarily in the United States and in Europe, have taken this approach to study resistance to viral, bacterial, and fungal diseases. This work has led to the identification of genes in the production of phytoalexins, a class of chemicals that can act as part of a plant's defense reaction. It remains to be demonstrated that phytoalexins alone are capable of stopping pathogen invasion. More likely, resistance to disease is the result of a number of different processes of which a physical barrier or a chemical reaction may be only a part.

In some disease interactions, the resistant but not the susceptible plant produces one or more gene products of unknown function that may play a role in the resistance reaction. In one example, that of resistance of a single variety of cowpea to cowpea mosaic virus, resistant cells apparently block a specific step in virus replication according to G. Bruening, University of California, Davis. Isolating and characterizing this and other gene products that block steps in disease development may ultimately lead to their isolation and transfer to a susceptible plant, in this way conferring resistance.

Isolating a Gene of Unknown Function. The second method to isolate a gene for disease resistance in the absence of knowing the function of the gene involves an approach known as gene tagging. Gene tagging can theoretically be carried out in any plant species that contains a well-characterized system of transposable elements.

Also known as jumping genes, transposable elements have the capacity to move from one position on a chromosome to another. Such elements have been isolated and characterized from several plant species, including maize. A scenario for tagging a gene for resistance to stalk rot disease of maize is to start with a maize line resistant to the disease and to sexually cross this plant with a maize line containing a transposable element such as AC or Mu.

Seeds are collected from this cross, planted, and taken to flowering. After self-pollinating these plants, progeny contain both the gene for resistance and the transposable element. When each seedling is inoculated with the pathogen, all will be resistant unless the transposable element has inactivated the resistance gene, in which case the plant will be susceptible. The resistance gene can then be isolated by virtue of its being "tagged" with the well-characterized transposable element. Transferring the resistance gene to a susceptible plant may confer disease resistance to the recipient.

Although gene tagging has been proposed, it has not yet yielded a disease resistance gene. A primary reason for the lack of success to date may reflect the large size of the maize genome, and the unlikelihood that the element will insert into a resistance gene. It is also possible that the element will not insert into the target gene because it lacks the DNA sequences needed for such insertion.

Modern approaches to producing disease-resistant plants require the use of a variety of techniques. At Washington University in St. Louis, Missouri, a visiting scholar carries out an antibody reaction to determine if a transformed plant is expressing a "foreign gene."

Disease Resistance by Transferring a Specific Gene

A third approach to produce disease-resistant plants is to generate plants that are cross-protected against virus infection. In the classical sense, cross-protection refers to the condition in which a plant infected by a mild strain of a virus is somewhat resistant to infection by a severe strain of the virus. The result of cross-protection is that, while the infection by mild strain of virus may depress the yield potential of the crop, it is substantially better than if a severe virus infection had spread through the crop.

A number of hypotheses have been proposed to explain cross-protection, but the molecular mechanism remains unknown. R. I. Hamilton suggested, in a 1980 article, that expression of viral genes in transformed cells might induce a protective response by the plants. In other words, expression of a single virus gene in cells might trigger the cross-protection mechanism, engendering disease resistance to transformed cells and plants.

Recent research results from my laboratory in collaboration with scientists at Monsanto Company, St. Louis, MO have shown this to be correct. We produced transformed tobacco and tomato plants that express the coat protein gene of tobacco mosaic virus (TMV). Transformed plants and their progeny are resistant to infection by TMV.

This result, reported in Science , May 9, 1986, is the first example of producing plants that exhibit resistance to virus infection as the result of a transfer of a single gene. It can be anticipated that this approach will be applied to a variety of plant viruses that invade a number of different types of plants.

The mechanisms that cause the tobacco and tomato plants to be resistant to TMV infection have not been identified, and may involve the direct interaction of the product of the introduced gene with the infecting virus, or a defense response of the host triggered by expression of the introduced gene, or both. Substantial research effort will be needed to identify the mechanism and the degree of efficacy of this resistance in agriculture.

The success of these gene transfer experiments in producing virus-resistant plants lends hope that a similar approach might be used to induce resistance reactions against bacterial and fungal pathogens.

For many years plant pathologists, such as Prof. J. Kuc, University of Kentucky, have shown that when the lower leaf of some plants is treated with an extract of a pathogen, a disease resistance reaction is induced in upper leaves of the plant. If the inducing molecule is identified, it may be possible to genetically engineer plants to be permanently induced to resistance.

Although this approach currently is only an hypothesis, it should be tested. But a great deal of work must precede such an experiment, since, to date, the identity of the inducing molecule remains to be elucidated.

Long-Term Basic Research Commitment Needed

There is a high potential payoff for plant agriculture in the 21st century if disease-resistant plants can be generated either by classical approaches or by the new gene transfer technologies. Application of the new techniques to plant disease resistance, however, requires indepth understanding of how and why plants are susceptible or resistant. This can only be accomplished by a strong, broadly based and long-term commitment to a program of basic research in government, industry, and public and private university research laboratories.