Integration of host plant resistance and biological control: using Arabidopsis-insect interactions as a model system

Two main methods in sustainable pest control are host plant resistance and biological control. These methods have been developed in isolation. However, host plant characteristics can decisively affect the effectiveness of biological control agents, and therefore when altering plant characteristics in a breeding programme, the implications for biological control should be studied as well. Moreover, this may also provide the opportunity to breed for plants that are optimally compatible with biological control agents. Breeding for host plant resistance has a long history. However, in the past decades a new development was to use transgenes to generate plants resistant to pests and diseases. In more recent years also Plant genetic engineering makes it possible to transfer a foreign gene to a host plant to introduce resistance against insects or to produce a volatile which can attract the natural enemy of the pest after the host plant is wounded by insect herbivores. In this thesis I have modified direct and indirect defence to herbivores in Arabidopsis plants through a transgenic approach. In order to evaluate transgenic Arabidopsis with different genes that influence direct and indirect defence to the herbivore Plutella xylostella (diamondback moth, DBM). I investigated the effect of a protease inhibitor from mustard plants (Mustard Trypsin Inhibitor 2, MTI2). MTI-2 transgenic Arabidopsis did not affect the performance of a Dutch and a Chinese strain of DBM. My data show that the gut enzymes of DBM are insensitive to MTI2, which can be explained by the specific inactivation of MTI2. DBM has apparently developed MTI2 inactivation as a way to protect itself against this protease inhibitor. This makes ecological sense as MTI2 is part of the defense of its brassicaceous host plants. Terpenoids are among the plant volatiles involved in indirect defence of many plant species. I exploited the availability of a linalool synthase (LIS) gene to develop transgenic herbivore-inducible linalool-producing Arabidopsis plants and developed Arabidopsis plants with this linalool synthase gene under the protease inhibitor 2 (PI2) promoter from potato. As a result the linalool synthase was inducible by methyl jasmonate application, but not by P. xylostella infestation. To investigate a transgenic approach to combining direct and indirect defence, a gene encoding a Bt toxin and a gene encoding linalool synthase were integrated into Arabidopsis as a model. The data show that hybrid lines carrying Bt and LIS genes in Arabidopsis combine toxicity to the DBM larvae (due to Bt) and repellence to the adults of DBM (due to linalool). Moreover, the linalool emission also affected parasitoid behaviour when the plants were damaged. DBM-infested dual transgenic plants were more attractive to the parasitoid D. semiclausum than undamaged dual transgenic plants, but less attractive than DBM-infested untransformed plants. The percentage parasitization of DBM larvae on transgenic plants was not affected by the transgenes. In conclusion, my data provide interesting options for the development of transgenic crops that interfere with the biology of pests and enhance the effectiveness of the pest’s natural enemies. In this way transgenic crops that integrate host plant resistance with biological control may be developed.