Histological Observations on Uromyces phaseoli and Puccinia recondita Infection in Allopurinol-treated Susceptible Plants

The development of rust after administering allopurinol, a specific inhibitor of xanthine oxidoreductase, via roots was studied at the histological level in leaves of susceptibleʹPinto 111ʹbean plants inoculated with Uromyces phaseoli andʹThatcherʹ,ʹMentanaʹandʹLeopardoʹwheat plants challenged with Puccinia recondita. A marked reduction and delay in fungal growth was observed in allopurinol-treated plants starting between 24 h and 48 h post-inoculation, i.e. after differentiation of the first haustoria (onset of the biotrophic plant-parasite relationship). Infection hyphae often grew twisted and convoluted in treated hosts, sometimes producing small, irregularly shaped colonies. Differentiation of subepidermal stromata in fungal colonies was delayed and restricted by the treatment and uredospore yield severely reduced. Allopurinol administration also tended to increase the proportion of haustoria which became embedded in thick translucent sheaths during the late stages of infection. These results support the view that plant xanthine oxidoreductase activity is necessary for biotrophic development of rust fungi and suggest that the inhibition of this enzyme, which impairs the pathogen metabolism, may favour some natural host responses to attack such as haustorial sheath formation.