Carbohydrates Stimulate Ethylene Production in Tobacco Leaf Discs : II. Sites of Stimulation in the Ethylene Biosynthesis Pathway.

Galactose, sucrose, and glucose (50 millimolar) applied to tobacco leaf discs (Nicotiana tabacum L. cv ;Xanthi') during a prolonged incubation (5-6 d) markedly stimulated ethylene production which, in turn, could be inhibited by aminoethoxyvinylglycine (2-amino-4-(2'-aminoethoxy)-trans-3-butenoic acid) (AVG) or Co(2+) ions. These three tested sugars also stimulated the conversion of l-[3,4-(14)C]methionine to [(14)C]1-amino-cyclopropane-1-carboxylic acid (ACC) and to [(14)C]ethylene, thus indicating that the carbohydrates-stimulated ethylene production proceeds from methionine via the ACC pathway. Sucrose concentrations above 25 mm considerably enhanced ACC-dependent ethylene production, and this enhancement was related to the increased respiratory carbon dioxide. However, sucrose by itself could directly promote the step of ACC conversion to ethylene, since low sucrose concentrations (1-25 mm) enhanced ACC-dependent ethylene production also in the presence of 15% CO(2).The data suggest that the stimulation of ethylene production by sugars in tobacco leaf discs results from enhancement of ACC formation as well as from the conversion of ACC to ethylene, when both steps could be involved in regulation of ethylene biosynthesis.