ASGSB 1998 Annual Meeting Abstracts

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ETHYLENE PLAYS MULTIPLE ROLES IN MODULATING THE GRAVITROPIC RESPONSE. A. Madlung, F. Behringer and T.L. Lomax. Oregon State University, Department of Botany and Plant Pathology and Center for Gene Research and Biotechnology, Corvallis, OR, 97331

To explore novel aspects of the mechanism regulating gravitropic signal transduction, we have used four mutants of tomato (Lycopersicon esculentum Mill.) which are altered in their response to either gravity, auxin, or ethylene. Ethylene is known to interact with auxin in regulating stem growth, yet evidence on the role of ethylene in tropic responses is contradictory. We find that the wild-type gravitropic response of tomato is influenced by ethylene in a concentration-dependent manner, with decreased graviresponsiveness at moderate levels which do not inhibit overall growth, and complete inhibition of curvature at growth-inhibiting high concentrations. A requirement for ethylene is indicated by the fact that ethylene inhibitors dramatically reduce wild-type gravicurvature at concentrations where overall growth is not affected. The reduced gravitropic response of the auxin-insensitive diageotropica (dgt) mutant can be restored to wild-type levels by extremely low ethylene concentrations, which confirms that low levels of ethylene are necessary for a full gravitropic response. In contrast, the ethylene-insensitive never-ripe (nr) mutant shows a slightly delayed but otherwise normal gravitropic response in ambient air and no reduction in gravicurvature in an ethylene atmosphere. The ethylene-overproducing/constitutive response mutant epinastic (epi) exhibits a reduced gravitropic response in both ambient air and ethylene. The reversal of gravicurvature by red light in the lazy-2 (lz-2) mutant does not appear to be affected by either ethylene or ethylene inhibitors, indicating that ethylene does not play a role in the direction of the gravitropic response. Taken together, these data indicate that while low levels of ethylene are necessary for a full gravitropic response, moderate levels of the hormone inhibit gravicurvature in a specific manner which does not inhibit overall growth. (Supported by NASA award No. NAGW-3716 to T.L.L.)

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