Evidence of a two-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 tobacco necrosis virus (TNV) induced hypersensitive response and the TNV infection process. The first detectable effect of allopurinol treatment was a great reduction in the hypersensitive response seen as reduced collapse and extent of necrosis than in untreated controls. This was evaluated as reduction in electrolyte leakage from lesions in the leaves. Only 4-6 days of allopurinol treatment were needed to achieve this effect. With prolonged treatment (6-8 days), there was a second effect consisting of a strong reduction in the number of necrotic lesions produced with a basipetal (tip-to-base) intensity pattern. This second aspect 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 metabolic fate of allopurinol in leaves in different positions on the stem. The results obtained demonstrate that: (i) allopurinol which accumulated in leaves was oxidized to oxypurinol by host xanthine oxidase which was in turn inhibited, and consequently increasing amounts of xanthine accumulated in leaves, (ii) both allopurinol and oxypurinol were converted to their respective ribonucleosides. All these metabolic aspects connected with allopurinol metabolism in leaves, analogous with the reduced production of necrotic lesion had a basipetal (tip-to-base) intensity pattern. The present results are discussed considering that these two phenomena, i.e. inhibition of TNV-induced hypersensitive response and inhibition of the TNV infection process, are related to two metabolic mechanisms triggered by allopurinol treatment. The first depends on the inhibition of xanthine oxidase mediated superoxide production and the consequent general attenuation of the necrotic process. The second depends on the synthesis of both allopurinol and oxypurinol ribonucleosides which might directly or indirectly interfere with the process of TNV infection, and therefore be responsible for the reduction of the number of lesions in treated leaves.