Microbial-based mechanisms of defense are especially important because plants lack genetic resistance to many common pathogens and insects, especially soilborne organisms. Suppressive soils are the best examples of indigenous micro-organisms protecting plants against pests. Natural instances of pathogen and insect suppression have been rich sources of micro-organisms for development into BCAs. Although the use of biocontrol technology remains only a small fraction of that of chemical pesticides, the number of new biocontrol agents and their performance continues to increase. However, inconsistent performance and narrow spectra of activity are issues that must be resolved before the use of biocontrol technology can reach its full potential as an integral part of sustainable agriculture in the 21st century. BCAs have been engineered to colonise better, tolerate stress better, perform more consistently and effectively, and have a broader spectrum of activity than their wild-type progenitors. All biocontrol mechanisms have been targeted for improvement: competition/pre-emptive exclusion, parasitism/predation, induction of systemic resistance and antibiosis/toxin production. A very wide variety of genetic approaches have been used to engineer BCAs and they can be grouped into three categories: deletion or mutation of existing genes, alteration of gene regulation and introduction of heterologous genes. When new genes are introduced into a BCA, they can influence the expression of biocontrol traits already present. Competitiveness of the transgenic BCA as compared to the parental strain can depend on the host crop. Current micro-organisms of choice for development as commercial BCAs (Bacillus, Trichoderma, Beauveria and Metarhizium spp.) will probably be the microbes of choice for future development as transgenic BCAs and Pseudomonas will continue to be an important research tool.

Understanding the biogeography of potential transgenes (i.e. those encoding antibiotics and toxins) and their role in nature should lessen concerns about the commercial use of recombinant BCAs. Future research should continue to focus on the development of novel engineered BCAs but broader field testing is needed for engineered agents that have been constructed during the last 25 years and are known to have enhanced activity. During the last 15 years, there has been much greater research emphasis on transgenic plants than transgenic microbes for pest control.

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