Inhibition of the development of leaf rust (Puccinia recondita) by treatment of wheat with allopurinol and production of a hypersensitive-like reaction in a compatible host

The effect of allopurinol [4-hydroxypyrazolo (3,4-d) pyrimidine], a purine analogue inhibitor of xanthine oxidase (XO) enzyme, was studied in the host-pathogen combination of Triticum aestivum-Puccinia recondita f.sp. tritici. Analysis of purines and pyrimidines in the allopurinol-treated wheat seedlings showed marked accumulation of xanthine, suggesting the in planta inhibition of XO activity. In the incompatible wheat-rust interaction application of allopurinol as a drench, even at the highest concentration (50 mu m), did not change the hypersensitive reaction phenotype; only the number of lesions was slightly reduced. Allopurinol treatment decreased the augmented rate of electrolyte leakage and lipid peroxidation associated with the hypersensitive response (HR), an effect probably related to the inhibition of rust development by allopurinol. By contrast, in the case of the compatible wheat-leaf-rust combination the reaction type was strongly affected. The formation of uredia and production of uredospores were diminished or completely inhibited depending on the concentration of allopurinol, which was applied either as a drench (3.125-50 mu m) or as a foliar spray (100-400 mu m) to plants grown in perlite. At the highest allopurinol concentration in the drench, the compatible reaction type changed to a hypersensitive-like necrotic reaction. Significant increases in electrolyte leakage and lipid peroxidation (characteristic of the HR) were found 4-6 days after infection in susceptible plants treated with allopurinol. Staining of leaf slices from allopurinol-treated and compatible rust-infected plants with Evans blue indicated cell death surrounding the pustules, while at this stage no cell death was detected in infected leaves without allopurinol treatment. The above results suggest that XO is not the main source of the generation of active oxygen species in wheat during the HR to leaf rust.