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 seed lings showed marked accumulation of xanthine, suggesting the in planta inhi bition of XO activity. In the incompatible wheat-rust interaction applicati on of allopurinol as a drench, even at the highest concentration (50 mu m), did not change the hypersensitive reaction phenotype; only the number of l esions was slightly reduced. Allopurinol treatment decreased the augmented rate of electrolyte leakage and lipid peroxidation associated with the hype rsensitive response (HR), an effect probably related to the inhibition of r ust development by allopurinol. By contrast, in the case of the compatible wheat-leaf-rust combination the reaction type was strongly affected. The fo rmation of uredia and production of uredospores were diminished or complete ly inhibited depending on the concentration of allopurinol, which was appli ed either as a drench (3.125-50 mu m) or as a foliar spray (100-400 mu m) t o plants grown in perlite. At the highest allopurinol concentration in the drench, the compatible reaction type changed to a hypersensitive-like necro tic reaction. Significant increases in electrolyte leakage and lipid peroxi dation (characteristic of the HR) were found 4-6 days after infection in su sceptible plants treated with allopurinol. Staining of leaf slices from all opurinol-treated and compatible rust-infected plants with Evans blue indica ted cell death surrounding the pustules, while at this stage no cell death was detected in infected leaves without allopurinol treatment. The above re sults suggest that XO is not the main source of the generation of active ox ygen species in wheat during the HR to leaf rust.