Histological observations on Uromyces phaseoli and Puccinia recondita infection in allopurinol-treated susceptible plants

The development of rust after administering allopurinol, a specific inhibit or of xanthine oxidoreductase, via roots was studied at the histological le vel in leaves of susceptible 'Pinto 111' bean plants inoculated with Uromyc es phaseoli and 'Thatcher', 'Mentana' and 'Leopardo' wheat plants challenge d with Puccinia recondita. A marked reduction and delay in fungal growth wa s observed in allopurinol-treated plants starting between 24 h and 48 h pos t-inoculation, i.e, after differentiation of the first haustoria (onset of the biotrophic plant-parasite relationship). Infection hyphae often grew tw isted and convoluted in treated hosts, sometimes producing small, irregular ly shaped colonies. Differentiation of subepidermal stromata in fungal colo nies was delayed and restricted by the treatment and uredospore yield sever ely reduced. Allopurinol administration also tended to increase the proport ion of haustoria which became embedded in thick translucent sheaths during the late stages of infection. These results support the view that plant xan thine oxidoreductase activity is necessary for biotrophic development of ru st fungi and suggest that the inhibition of this enzyme, which impairs the pathogen metabolism may favour some natural host responses to attack such a s haustorial sheath formation.