Genetic Engineering and Mutation Breeding for Tolerance to Abiotic and Biotic Stresses: Science, Technology and Safety

Genetic engineering is often presented as a one-step, rapid solution to the improvement of stress tolerance in plants. While it may benefit from but not necessitate, the requirement for backcrossing for gene introgression, it does not reduce the requirement for field trials. The introduction of herbicide and pest resistance into plants has been an applied success and these characteristic singly and combined still dominate the applications for trials permits. However, they do not represent the complexity of the challenge for engineering for durable biotic and abiotic stress resistance. The dissection of stress responses in plants is showing high levels of complexity and redundancy at the perception, signalling and expression levels with cross regulation (cross talk) between stress pathways and over lapping functions between stress metabolites and stress proteins in different stresses. Stress metabolite engineering is complicated by a lack of knowledge of pathways and their regulation and poses the question of how metabolite fluxes between shared pathways can be controlled, indeed redundant homeostatic mechanisms may be discovered. In the case of stress proteins, there are limits on genes of known function that are available but perhaps more importantly is the issue of whether single or multiple gene transformations will confer stable resistance. There are technical limitations in multigene engineering but more important is the global character of stress responses. Some have argued that the solution lies in engineering for constitutive expression of stress pathways but this may confer a yield penalty and plants have evolved to rely on inducible responses. There is also the complication that at least some plant stress pathways are subject to reciprocal regulation i.e. the salicylic acid pathway for pathogen resistance may suppress the jasmonic acid pathway for pest resis-