Long-term impact of genetically modified plants in their response to pathogens

There are three main concerns about the long-term impact of engineering genes for disease resistance in transgenic plants: the resistance of the plants to pathogen attack once they are grown in large scale, the development of resistance by the pathogen and the possible phenotypic alterations of the plant. The application of biotechnology in agriculture has had great success in the generation of commercially useful insect- and virus-resistant crops (Dunwell 2000). However, the first commercially available Bt-expressing cotton crop (grown in 1996) showed mixed success (Dempsey et al. 1998). Clearly, detailed and sufficiently extensive studies about the large-scale agronomic performance of each new variety grown in different conditions are necessary. However, until the new variety is grown on a large scale in the appropriate place, one may not determine precisely the actual resistance to pathogen attack.

Resistant plants can impose a selective pressure that results in development of resistance in the pathogen, which is not an uncommon event (Tabashnik et al. 2000). There exists an antagonistic coevolution between plants and pathogens that is constantly selecting for genotypes that can overcome the other’s defences (Stahl and Bishop 2000). Clearly, the introduction of a single transgene, therefore, may not be sufficient to achieve durable and broad-spectrum disease resistance. A combination of transgenic strategies will be needed to ensure durability of resistance. By combining several methods for pest control, one may reduce the probability that any of these methods will soon become obsolete as a result of adaptation by the pest or disease causing agent. The increasing availability of resistance genes is allowing the generation of transgenic plants with resistance to various pests. However, since constitutive expression of certain R and Avr genes may have deleterious effects on the plant (Honée et al. 1995), a judicious choice of the genes to be transferred, combined with detailed molecular studies, will be necessary to achieve durable resistance together with an optimal field performance (Rommens and Kishore 2000). Furthermore, multiple R genes may compete for the signalling components during a mixed infection, thereby interfering with the activation of defences (Bent 1996).

There still remains a series of questions that need to be addressed concerning the response to pathogens of the engineered plants. For instance, what is the likelihood that plants engineered for disease resistance will provide an environment for the development of a novel pathogen that exhibits an increased host range or is resistant to currently available control methods? How durable will the engineered resistance prove to be once crops are grown on a large scale? Will the different R genes, cloned and those to be cloned, prove to be functional in heterologous plant species?