Regulation of resistance and susceptibility in wheat-powdery mildew pathosystem with exogenous cytokinins.

Dose-response relationship between resistance of wheat seedlings (Triticum aestivum, cultivar Zarya) to Erysiphe graminis f. sp. tritici Marchal. (Syn. Blumeria graminis), a causal organism of wheat powdery mildew and exogenous zeatin has been investigated. Two-week-old seedlings were inoculated with the pathogen. Zeatin or zeatinriboside were added to the nutrient solution immediately after inoculation. The dose-response curve of cytokinin in the most cases was multiphasic, with peaks of increased susceptibility occurring at 0.25-1.5 and 1.5-9microM cytokinin, separated by a region of increased resistance at 0.5-3microM cytokinin. The change in mineral nutrition or simultaneous treatment with thidiazuron revealed alterations of the dose-response curve ranging from a curve with maximum of resistance to a curve with maximum of susceptibility. Both multiphase nature of dose-response and its variability were proposed as possible explanations for earlier observed discrepancies in experimental data on modification of disease resistance by cytokinins. A mathematical model for two metabolic processes with substrate inhibition connected in-series was suggested to explain the multiphase dose-response. In this model, the product of the first reaction was used as substrate for the second reaction. Numerical experiments showed the changes in the shape of dose-response curve with changes in parameters dependent of cytokinin metabolism.