Horizontal or poly­gene resistance is generally considered as a more stable resistance than vertical or mono­gene resistance. Since the role of genetic engineering is mainly restricted to vertical resistance strategies, its potential to contribute to durable plant resistance against pest and diseases would thus be limited. However, according to W.J. Stiekema, vertical resistance in combination with genetic engineering offers many old and new possibilities to achieve fast and cost­effective forms of durable resistance.

Crop protection can be achieved by the use of agro­chemicals, hygiene measures, biological control or plant resistance. The latter is the most attractive approach, because it is cost effective and combines benefits for growers, consumers and the environment. A disadvantage of many resistances is their pathogen specificity. Since usually more than one parasite is present, several resistances have to be introduced in a cultivar separately. This is a time consuming process and includes the risk of introducing unwanted traits, such as toxicity to the consumer. Potentially, genetic engineering can better control the characteristics of the newly developed variety, since it adds a known single gene to a known variety. Besides, genetic engineering usually is less time consuming and not limited by barriers between species.
A second disadvantage of resistance is potential instability. Pathogens often circumvent a specific resistance soon after its introduction. Many people relate too simplistically durability of resistances to the amount of genes that are involved in it. Vertical resistance, i.e. resistance that depends on one gene and thus can be handled easily by genetic engineering, is considered less durable than horizontal resistance. However, many examples exist of (non­transgenic) vertical, durable resistances, such as against potato virus A, X and Y in potato, against lettuce mosaic virus in lettuce, against the fungi Cladosporium cucumerinum and Corynespora melonis in cucumber, Fusarium oxysporum in cabbage, pea and tomato and against the nematodes Globodera rostochiensis in potato. An example of transgenic vertical, durable resistance is the transfer of a resistance gene against the beet cyst nematode from wild beet to sugar beet. Neglecting the potential of vertical resistance will result in an increased need for time and funds to obtain useful resistances.
What resistances are durable, can only be determined in retrospect. Since huge differences exist between pathogens in the ease with which they circumvent resistances, it is difficult to forecast the durability of (vertical) resistance in the field. In general, specialized parasitic fungal pathogens that are airborne or splashborne and consist of many strains are often capable of quickly circumventing vertical resistance. An example is the fungus Phytophthora infestans, which causes late blight in potato. Of this fungus, more than 10 races are already known that are capable of circumventing more than 10 potato resistance genes.

Does this imply that it is useless to target one or more single resistance genes to obtain resistance against late blight in potato? Through breeding and selection this is indeed the case. However, upon application of molecular biological tools there are good perspectives for the use of such genes. Due to worldwide research into the isolation and analysis of single resistance genes, knowledge accumulates rapidly on pathogens' capability of circumventing resistances. This knowledge will enable the development of new strategies to ensure durable types of resistance based on single genes.
One new strategy might be the transfer of a single resistance gene together with its avirulence gene. An avirulence gene is usually found in the pathogen and triggers the resistance reaction in the host. A specific resistance gene reacts to a specific avirulence gene. The new approach consists of the transfer of a resistance gene together with its related avirulence gene. This avirulence gene is introduced under the control of a promotor gene, that becomes active after a pathogen attack. If there is no pathogen attack, the resistance is not expressed. In such an approach the specific relationship between the resistance gene and the avirulence gene is beyond the control of the pathogen, and a broad and durable resistance may be obtained. This strategy is now under study for fungal and bacterial resistance in tomato and virus resistance in tobacco. It might also be applied to obtain resistance to late blight in potato once the resistance gene and avirulence gene have been isolated, and if the avirulence gene is exclusively activated by P. infestans.
Undoubtably, knowledge on resistance genes and resistance mechanisms will lead to further strategies to obtain durable vertical resistance. Besides isolation and analysis of vertical resistance genes of potato, research is being conducted on the specific location of the resistance genes on the plant genome. The aim is to relate genetic information on horizontal resistance with information on individual resistance genes. If vertical resistance and horizontal resistance could be related, new routes would emerge to obtain durable resistance against pest and diseases using vertical resistance. It may appear that the genetic context in which resistance genes are embedded is one of the factors that determine its durability. The new approaches towards durable vertical resistance will depend increasingly on the application of genetic engineering.
W.J. Stiekema

Head of the Department of Molecular Biology, Centre for Plant Breeding and Reproduction Research (CPRO-DLO), Wageningen, the Netherlands.