The relationship between stimulated ethylene production and symptom expression in virus-infected tobacco leaves

The regulation of ethylene biosynthesis was studied in Samsun and Samsun NN tobacco (Nicotiana tabacum L.), infected with tobacco mosaic virus (TMV) strains W U1 or Holmes' Ribgrass (HR), or with tobacco necrosis virus (TNV). Stimulation of ethylene production upon infection was not primarily determined by the genetic constitution of the host plant, nor by the infecting virus, but appeared to be related to symptom expression and virus localization. A hypersensitive reaction (Samsun NN infected with TMV W U1 (20 °C) or TNV, and Samsun infected with TMV HR or TNV), was accompanied by a sharp rise in the production, and accumulation of the ethylene precursor, 1-aminocyclopropane-1-carboxylic acid (ACC), followed by rapid evolution of ethylene when the capacity to convert ACC to ethylene had increased. None of these changes occurred in systemically reacting plants (TMV W U1-infected Samsun NN at 30 °C or Samsun at 20 °C). These observations indicate that the increase in ethylene production during a hypersensitive reaction depends primarily on an increase in tissue production of ACC. Both the increase in ACC production and its accumulation were restricted to the cells surrounding the necrotic areas. Consequently, virus-stimulated ethylene production also occurred locally at the site where the virus is actively localized. After primary infection of hypersensitively reacting plants, the capacity for converting ACC to ethylene was increased systemically. Challenge inoculation did not lead to an accumulation of ACC because it was converted immediately to ethylene. This rapid local ethylene production may be responsible for limiting lesion enlargement in tobacco leaves exhibiting systemic acquired resistance. Ethylene production commenced 2 h after shifting TMV W U1-infected Samsun NN from 30 to 20 °G, 6 h before necrosis developed. Thus, the increase in ethylene production is not determined by the onset of necrosis, but by a much earlier event occurring in the plant-virus interaction at 20 °C.