Aphids are insect pests with major importance worldwide. By feeding from plant phloem, they directly withdraw nutrients. The feeding injury is often visible in the form of leaf rolling, chlorosis, necrosis or plant deformation. Their pest status is attributed also to vectoring of plant viruses. Aphid infestation on crop plants is currently managed with the use of pesticides, which may pose a threat to the environment. An alternative approach would be to identify and characterize genetic factors contributing to aphid resistance, as well as agents inducing resistance, with the intention to use this knowledge in breeding programs.

The aim of this thesis was to identify such resistance genes and characterize their involvement in plant-aphid interactions. This was accomplished using two approaches. Firstly, two putative aphid-resistance genes from barley, CI2c encoding a chymotrypsin inhibitor and LOX2.2 encoding a lipoxygenase, were transformed into Arabidopsis and/or barley and the effects of transformation were studied with regard to the performance of two aphid species. One was the specialist bird cherry-oat aphid (Rhopalosiphum padi L.), which is a pest on major cereals, and the other was a generalist, the green peach aphid (Myzus persicae Sulzer), which is a pest on plants belonging to more than 40 families. The effects of transformation were also studied concerning effects on the expression of other defense-related genes in the transgenic plants. Secondly, the effects of plant treatment with volatiles were studied with regard to induction of plant resistance, followed by tests with bird cherry-oat aphid on the treated plants.

The study of overexpression of CI2c showed that the gene product transiently reduced green peach aphid fecundity on transgenic Arabidopsis, but indirectly decreased this aphid’s avoidance of barley by suppressing defense. The transformation had no effects on bird cherry-oat aphids’ behaviour or fecundity. Overexpression of LOX2.2 was shown to affect expression of other genes regulated by jasmonic acid and decreased the short-term fecundity of both the bird cherry-oat aphid and the green peach aphid on barley. The study of volatile treatments supported the idea that resistance against aphids can be induced by application of volatiles. Several defense gene sequences were induced by application of methyl salicylate, methyl jasmonate and (Z)-3-hexen-1-ol. Of the three volatiles tested, methyl jasmonate showed the greatest potential as inducing agent, causing a short-term reduction in aphid fecundity.

To conclude, this thesis supports the ideas that the barley genes CI2c and LOX2.2 play a role in resistance against aphids and that moderate aphid resistance can be induced by external factors. Aphids may be directly affected by the gene product or there may be an indirect effect, caused by changes in the expression of other genes involved in plant defense. The observed negative effects on aphids were of moderate magnitude and it is proposed that acting individually, those genes are not likely to cause a strong negative effect, but they may contribute to provide resistance to aphids.