The possible involvement of polyamines in the tobacco mosaic virus (TMV)-induced hypersensitive reaction (HR) in Nicotiana tabacum (L.) cv. Samsun NN, which bears the N resistance gene, was investigated. Concentrations of free and conjugated polyamines, as well as the activities of their biosynthetic enzymes ornithine decarboxylase (ODC) and arginine decarboxylase (ADC), were analysed in the necrotic area (I, internal zone) and in the two concentric zones surrounding it (M, middle zone; E, external zone) during the progression of the HR (1, 3 and 5 d from TMV inoculation). Results were compared with those obtained in control plants (mock-inoculated) and with the pattern in susceptible TMV-infected tobacco plants.

In the HR, concentration increased towards zone I for free putrescine and spermidine. Highest concentrations in zones M and I occurred on day 3. Conjugated polyamines similarly showed an increasing concentration gradient towards zone I, with maximum accumulation on day 5. ODC and ADC activities also showed an increasing gradient towards zone I that could be related to the increase in free and conjugated polyamines. By contrast, in control plants concentrations of free and conjugated polyamines decreased from day 1 to day 5, ODC activity increased slightly, whereas ADC activity did not show significant changes. In the TMV-infected susceptible tobacco plants, polyamines tended to remain in the free state instead of being conjugated as infection progressed. These results suggest that polyamines could play a role in mechanisms of resistance to biotic stress.