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Saliva fractions from South African Russian wheat aphid biotypes induce differential defence responses in wheat

The Russian wheat aphid (RWA), Diuraphis noxia (Kurdjomov, 1913), is a notorious pest that reduces yield in wheat. Nevertheless, the source of eliciting activity during RWA–wheat interaction has not been established. This paper reports on the isolation of eliciting activity in aphid saliva that is capable of inducing differential defence responses in various wheat cultivars. Aphid saliva discharged into an artificial diet by the South African RWA biotypes (RWASA1 and RWASA2) was collected and purified by C18 reversed phase and size exclusion chromatography. Intercellular injection of RWASA1, but not RWASA2, saliva fractions into wheat leaves selectively induced higher activities of defence-related enzymes (lipoxygenase, peroxidase and β-1,3-glucanase) only in the RWASA1-resistant cultivar ‘Tugela Dn1’. No defence responses were induced in the near-isogenic susceptible ‘Tugela’. Heat-treatment deactivated these fractions, suggesting their proteinaceous nature. Identification of eliciting activity in aphid saliva forms the basis for further characterisation and identification of elicitors in aphid saliva, which will help in unravelling the RWA resistance mechanism of wheat during RWA–wheat interactions.