XANTHINE-OXIDASE ACTIVITY IN THE SUSCEPTIBLE AND HYPERSENSITIVE RESPONSES OF TOBACCO-LEAVES TO TOBACCO MOSAIC-VIRUS INFECTION

A superoxide-producing xanthine oxidoreductase was isolated and quanti fied after polyacrylamide disc gel electrophoresis of tobacco leaf ext racts. The results obtained indicate that, like uricase activity, a sl ight increase in tobacco xanthine oxidase activity takes place in the susceptible interaction with tobacco mosaic virus (TMV). In contrast, out of three hypersensitive tobacco cultivars tested, only two showed the same slight increase in activity during the late stage of hypersen sitive response. Allopurinol [4-hydroxypyrazolo(3,4-d)pyrimidine] a sp ecific and potent in vitro and in vivo inhibitor of xanthine oxidoredu ctase, applied to tobacco plants by root absorption, starting about 8 days before the inoculation, did nor affect the hypersensitive respons e but weakened the hypersensitivity-linked virus localization and prom oted the movement of a certain amount of TMV particles and/or virus re lated material from necrotic lesions which induced systemic necrotic s ymptoms in uninoculated leaves. However, due to the inefficacy of allo purinol in preventing necrotic lesion development, all results are con sistent with the hypothesis that xanthine oxidoreductase, the first en zyme in purine oxidative degradation, plays only a secondary role duri ng induction of primary hypersensitive cell death in TMV infected toba cco leaves.