Salicylic acid mediates resistance in the willow Salix viminalis against the gall midge Dasineura marginemtorquens.

Resistant willow (Salix viminalis L.) genotypes react with a rapid hypersensitive response (HR) when attacked by the gall midge Dasineura marginemtorquens Bremi. In general, infected plant genotypes or species that react hypersensitively to pathogens accumulate salicylic acid (SA) locally and systemically. Thus, pathogen-induced HRs are strongly associated with accumulation of SA. In this study, we tested the hypothesis that SA mediates resistance in S. viminalis against D. marginemtorquens. By means of HPLC, we found accumulation of SA in a resistant S. viminalis genotype, but not in a susceptible genotype one to two days after D. marginemtorquens egg hatch. To test whether this correlation between accumulation of SA and resistance is causal, we treated a normally susceptible S. viminalis genotype exogenously with SA and found a decreased larval survival frequency on treated shoots. Thus, the hypothesis that SA mediates resistance in the S. viminalis/D. marginemtorquens system was strongly supported. Interestingly, great similarity seems to exist between the biochemical signaling associated with pathogen-induced HRs and this gall-inducer induced HR.