Isolation and eliciting activity of the Russian wheat aphid saliva in the resistance response of wheat
The comparative effect of elicitors in Russian wheat aphid (Diuraphis noxia, Kurdjumov, RWA) saliva on the inducible defence related responses in different wheat (Triticum aestivum L.) cultivars was investigated. The elicitors emanated from the South African RWA biotype 1 (RWASA1) and biotype 2 (RWASA2) saliva. Elicitors from RWASA1 and RWASA2 salivary material were intercellularly injected into wheat cultivars, Tugela (lacks resistance genes), Tugela Dn1 (Dn1 resistance gene) and PAN 3144 (Dn5 resistance gene) and the induced defence responses were measured. The accumulation of pathogenesis related (PR) proteins, peroxidase (PR9) and β-1,3-glucanase (PR2) involved in RWA resistance in wheat were used as indicators of the resistance response. Additionally, lipoxygenase (LOX) involved in the synthesis of lipid signals, jasmonates and other oxylipins, was also employed as a marker of resistance. Peroxidase, β-1,3-glucanase and LOX activities were determined spectrophometrically. The elicitors in RWASA1 saliva induced higher levels of peroxidase, β-1,3-glucanse and LOX activities in both Dn1 and Dn5 containing cultivars. However, the induced level of defence related enzyme activities was quantitatively higher in Dn1 than Dn5 containing cultivars. The purified elicitor active fractions from RWASA1 saliva also induced enhanced defense responses that were significantly higher in Dn1 than Dn5 containing cultivars. This differential induction of defence responses suggests a stronger recognition of RWASA1 elicitors by the Dn1 gene products. In contrast, elicitors in RWASA2 saliva interacted with the receptors in Dn5-containing wheat cultivars in a highly specific manner. The elicitors from crude saliva induced defence responses only in the Dn5, but not the Dn1 containing cultivar. The purer eliciting fractions, collected from the various chromatographic separations, also induced selective defence responses that were evident only in PAN 3144 (Dn5) Even though we suspect elicitors in the saliva of RWASA1 and RWASA2 to be distinct, they induced similar levels of defence related enzyme activities in the Dn5 containing cultivar. The elicitor active fractions purified from RWASA2 saliva induced similar responses in PAN 3144 (Dn5) as elicitors from crude saliva. This implies a strong capacity conferred by the Dn5 mediated resistance, to recognize distinct elicitors from the saliva of the two aphid biotypes. Polypeptide profiles of RWASA1 and RWASA2 crude and purified fractions of the salivary material, revealed expression of similar as well as distinct protein bands. These differences probably contributed to the disparity in the expression of defence related enzyme activities by saliva of RWASA1 and RWASA2 in wheat cultivars. Boiling of these fractions abolished the induced defence related activities, suggesting their proteinaceous character. Furthermore, the differential presence of carbohydrates and amino acids in elicitor active fractions originating from RWASA1 and RWASA2 saliva implies that in addition to proteins, carbohydrates or at least glycoproteins, could be one of active components in these inducing fractions. The findings from this study confirm that the defence related enzymes (peroxidase, β-1,3-glucanase and lipoxygenase) associated with RWA resistance in wheat, are induced by elicitors in RWA saliva. The study has further indicated differential effects of elicitors in RWASA1 and RWASA2 saliva in wheat cultivars expressing different resistance genes. Elicitors in RWASA2 saliva have overcome the resistance conferred by the Dn1 resistance gene, but are avirulent to cultivars containing the Dn5 gene. Furthermore, this Dn5 resistance gene, not only confers resistance after treatment with elicitors in RWASA2 saliva, but also in RWASA1 saliva.
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