Induced resistance in basket willow against a gall midge

Plants are defended against herbivores and pathogens by means of a vast array of mechanisms such as physical obstacles, chemical defences, phenological escape, and attraction of predators. Plant defence mechanisms may be present independently of the enemy attack, i.e. constitutive, or initiated by the activity of the enemy, thus referred to as induced resistance. This thesis focuses on induced resistance in basket willow (Salix viminalis, Salicaceae) against the leaf roller gall midge Dasineura marginemtorquens (Diptera: Cecidomyiidae). Observations from the field revealed an, for plant/insect systems, unusually great variation among S. viminalis genotypes in resistance to D. marginemtorquens. This great genotypic variation in combination with the fact that D. marginemtorquens is a leaf galler with a relatively sessile life style make this system especially suitable for studies on local resistance reactions in the plant to attack by the insect. We tested the hypothesis that the hypersensitive response (HR), a type of plant- programmed localised cell death, is involved in the resistance. We found support for the hypothesis. Local HR-like necroses were found in resistant but not in susceptible S. viminalis genotypes after larval attempts to initiate galls. The importance of defensive plant signalling transduction pathways was investigated in several experiments. We found that the HR-associated salicylic acid (SA)-dependent pathway mediates the resistance. In contrast, the jasmonic acid (JA)-dependent pathway does not seem to be important in this system either as a mediator of resistance or through suppression of the SA-dependent pathway (negative “cross-talk”). Furthermore, total peroxidase activity was associated with the resistance. As such, the resistance in S. viminalis to D. marginemtorquens seems to be of an HR-type associated with SA-dependent responses including induced peroxidase activity. Thus, great similarity exists between plant resistance in the S. viminalisID. marginemtorquens system and plant/pathogen systems with regards to activation of the HR, the SA-dependent pathway, and peroxidase activity.