Regulation of Ethylene Biosynthesis in Virus-Infected Tobacco Leaves : I. DETERMINATION OF THE ROLE OF METHIONINE AS THE PRECURSOR OF ETHYLENE.

The hypersensitive reaction of Samsun NN tobacco leaves to tobacco mosaic virus (TMV) was accompanied by a large increase in ethylene production, just before necrotic local lesions became visible. Normal and virus-induced ethylene production were both largely inhibited by 0.1 millimolar aminoethoxyvinylglycine indicating that methionine is a main ethylene precursor.The contribution of methionine to ethylene production was estimated by labeling leaves with l-[U-(14)C]methionine and comparing the specific activities of methionine within and ethylene produced by the leaf. When taken up through the petiole, methionine was largely retained in the veins, leading to production of ethylene with a far higher specific activity in the veins than in the interveinal tissue. After TMV infection, ethylene production increased only in the interveinal tissue, resulting in a decrease in specific activity of the ethylene produced. In the interveinal tissue, the specific radioactivity of the ethylene was lower than expected if methionine were the only precursor. After labeling by vacuum infiltration, the specific activities of the ethylene produced by water- and TMV-inoculated leaves were both identical and in accordance with the specific radioactivity of methionine. Inasmuch as the content of 1-aminocyclopropane-1-carboxylic acid was increased severalfold two days after TMV infection, methionine can be considered to be the only ethylene precursor in healthy and in TMV-infected tobacco leaves.The increase in ethylene production after TMV-infection was not accompanied by an increased concentration of free methionine within the leaf. Compartmentation of methionine does not appear to be a regulating factor since labeled methionine supplied to the leaf by vacuum infiltration is equilibrated very rapidly with any methionine pool within the leaf cells.