Auxin-Induced Ethylene Production as Related to Auxin Metabolism in Leaf Discs of Tobacco and Sugar Beet

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Abstract
Exogenously supplied IAA stimulated ethylene production in tobacco leaf discs but not in those of sugar beet. The stimulatory effect of IAA in tobacco was relatively small during the first 24 h of incubation but became greater during the next 24 h. Leaf discs of these 2 spp. metabolized [1-14C]IAA differently. The rate of decarboxylation in sugar beet discs was much higher than in tobacco. The latter contained much less free IAA but a markedly higher level of IAA conjugates. The major conjugate in the sugar beet extracts was indole-3-acetylaspartic acid, whereas tobacco extracts contained mainly 3 polar IAA conjugates which were not found in the sugar beet extracts. The accumulation of the unidentified conjugates corresponded with the rise of ethylene production in the tobacco leaf discs. Reapplication of all the extracted IAA conjugates resulted in a great stimulation of ethylene production by tobacco leaf discs which was accompanied by decarboxylation of the IAA conjugates. The results suggest that in tobacco IAA-treated leaf discs the IAA conjugates could stimulate ethylene production by a slow release of free IAA. The inability of the exogenously supplied IAA to stimulate ethylene production in the sugar beet leaf discs was not due to a deficiency of free IAA within the tissue but rather to the lack of responsiveness of this tissue to IAA, probably because of an autoinhibitory mechanism existing in the sugar beet leaf discs.