Prospects for genetic engineering of insect resistance in forest trees

Abstract

Gene transfer and recombinant DNA methods provide opportunities for enhancing insect resistance of forest trees by importing genes from other species and by manipulating native genes to create novel forms of resistance. Current opportunities for enhancing insect resistance include insertion of the toxin gene from the bacterium Bacillus thuringiensis and transfer of proteinase inhibitor genes from other plant species. Work is under way in a number of laboratories throughout the world to insert Bacillus thuringiensis toxin genes into forest tree species. Other strategies, such as the manipulation and transfer of chitinase genes, lectin genes, baculovirus genes, and genes encoding enzymes involved in the production of novel secondary compounds, also hold promise but require more information before their likelihood of success can be judged. Use of genetically engineered, resistant trees should be environmentally safer than controlling insect pests with insecticides. This is primarily because engineered trees affect only species that feed on them, and even then will generally be harmful to only a limited number of insect taxa. The main environmental risk associated with the use of engineered trees is that insects may counter-evolve to overcome their resistance. This would be particularly significant, for example, if counter-evolution to the Bacillus thuringiensis toxin gene in trees also precluded the use of insecticides containing Bacillus thuringiensis. We argue, however, that the risks of serious counter-evolution can be reduced to an acceptable level by maintaining genetic diversity in the forest, using multiple genes for resistance, and employing forest management practices that mitigate the potential for counter-evolution. Genetically engineered resistance should be more effective than the spraying of insecticides because the toxins are delivered as soon as insects begin feeding, can be produced continuously, and are delivered within tissues — thereby contacting insects that are difficult to reach with exterior sprays. We hypothesize that the improved nutrition of coniferous trees provided by intensive forest management should allow them to make heavier investments in novel nitrogen-based defensive compounds such as proteins and alkaloids, and that genetic engineering can help to take advantage of this opportunity. The greatest limitations to the current use of genetic engineering to improve insect resistance of trees are: insufficient knowledge of the molecular biology of insect development, insect pathogenesis, and tree defenses against insects; inefficient systems for insertion of genes into large numbers of tree genotypes; inability to produce sterile trees, necessary to prevent the release of engineered genes into natural or feral populations; concerns about insect counter-evolution to overcome the effects of engineered resistance genes; a lack of public understanding of the true benefits and risks of genetic engineering. In the short term, the greatest benefits from recombinant DNA and genetic engineering technology will be to provide new avenues for understanding tree-insect interactions, and thus new options for combating insect pests.