Hypersensitive responses (HR) play a major role in plant resistance to pathogens. Much less is known about HR as a resistance mechanism against insects. Great genotypic variation in resistance against a galling insect Dasineura marginemtorquens has been documented in basket willow, Salix viminalis. On resistant willow genotypes 100% of neonate larvae die, in contrast to susceptible genotypes where survival is close to 100%. Surprisingly, females do not discriminate against resistant genotypes but lay as many eggs on resistant as on susceptible genotypes. Symptoms considered to be part of the HR expression, i.e. rapid cell death and accumulation of salicylic acid, are associated with resistance in some S. viminalis genotypes. In other genotypes, however, larvae die without any visible plant symptoms. Genotypes with and without visible symptoms were compared with respect to induction of hydrogen peroxide, a marker of HR. It was found that hydrogen peroxide was only induced in genotypes with HR symptoms although resistance was also complete (i.e. larvae died) in genotypes without symptoms. (See Höglund et al., pp. 3215–3222.)
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