EVIDENCE OF A 2-FOLD MECHANISM RESPONSIBLE FOR THE INHIBITION BY ALLOPURINOL OF THE HYPERSENSITIVE RESPONSE INDUCED IN TOBACCO BY TOBACCO NECROSIS VIRUS

Allopurinol applied to tobacco plant roots interferes strongly with to bacco necrosis virus (TNV) induced hypersensitive response and the TNV infection process. The first detectable effect of allopurinol treatme nt was a great reduction in the hypersensitive response seen as reduce d collapse and extent necrosis than in untreated controls. This was ev aluated as reduction in electrolyte leakage from lesions in the leaves . Only 4-6 days of allopurinol treatment were needed to achieve this e ffect. With prolonged treatment (6-8 days), there was a second effect consisting of a strong reduction in the number of necrotic lesions pro duced with a basipetal (tip-to-base) intensity pattern. This second as pect was related to a lack of TNV antigen production and therefore to the inhibition of the TNV infection process. To explain the metabolic basis for these phenomena, a study was pursued to establish the metabo lic fate of allopurinol in leaves in different positions on the stem. The results obtained demonstrate that: (i) allopurinol which accumulat ed in leaves was oxidized to oxypurinol by host xanthine oxidase which was in turn inhibited, and consequently increasing amounts of xanthin e accumulated in leaves; (ii) both allopurinol and oxypurinol were con verted to their respective ribonucleosides. All these metabolic aspect s connected with allopurinol metabolism in leaves, analogous with the reduced production of necrotic lesion had a basipetal (tip-to-base) in tensity pattern. The present results are discussed considering that th ese two phenomena, i.e. inhibition of TNV-induced hypersensitive respo nse and inhibition of the TNV infection process, are related to two me tabolic mechanisms triggered by allopurinol treatment. The first depen ds on the inhibition of xanthine oxidase mediated superoxide productio n and the consequent general attenuation of the necrotic process. The second depends on the synthesis of both allopurinol and oxypurinol rib onucleosides which might directly or indirectly interfere with the pro cess of TNV infection, and therefore be responsible for the reduction of the number of lesions in treated leaves. (C) 1996 Academic Press Li mited