Nontarget and ecological effects of transgenically altered disease resistance in crops -possible effects on the mycorrhizal symbiosis

The ability to increase crop disease resistance by using transgenic (TG) means has recently been demonstrated for several crops. The current TG procedures alter the temporal expression of transgene pathogenesis-related (PR) proteins, so that the usually inducible PR proteins are expressed constitutively in the foreign host. The constitutive expression of the transgene PR protein chitinase is believed to increase the host's nonspecific basic resistance to pathogens. A potential nontarget effect of constitutively expressing chitinase may be a decrease in the activity of beneficial microbes, especially vesicular-arbuscular mycorrhizal fungi. The decrease in activity of mycorrhizal fungi is related to reduced susceptibility of TG plant roots to colonization by these fungi, which is in turn associated with lysis of fungal cell walls by the constitutively expressed chitinase. An argument is presented that use of TG means to alter the temporal expression of PR proteins ignores a legacy of past evolutionary trade-offs in vascular plants. A major nontarget effect of expressing transgene chitinase is a reduction in the susceptibility of roots to colonization by mycorrhizal fungi. This reduction in mycorrhizal susceptibility occurs without alteration of the mycorrhizal dependence of the host on symbiont-supplied nutrients. Data are presented in support of this contention that demonstrate a strong negative association between host pathogen resistance and mycorrhizal colonization. An ecological consequence of reducing mycorrhizal colonization is a decrease in the soil's mycorrhizal propagule reserve that diminishes the next crop's production, especially under low-input cropping practices. A further consequence that has both ecological and evolutionary outcomes is the escape of the transgene for improved pathogen resistance into wild populations. By increasing a crop's disease resistance by TG means, we may inadvertently be creating a ‘super weed’ when the TG plant or the transgene escapes into wild relatives through hybridization. Hybridization of wild relatives with TG plants would be especially relevant for crops, such as sugar beet, rapeseed, and many modern cereal cultivars that have close relatives in the wild but have a relatively low requirement for symbiont supplied nutrients or are nondependent.