Ethylene plays multiple nonprimary roles in modulating the gravitropic response in tomato

Ethylene is known to interact with auxin in regulating stem growth, and yet evidence for the role of ethylene in tropic responses is contradictory. Ou r analysis of four mutants of tomato (Lycopersicon esculentum) altered in t heir response to gravity, auxin, and/or ethylene revealed concentration-dep endent modulation of shoot gravitropism by ethylene. Ethylene inhibitors re duce wild-type gravicurvature, and extremely low (0.0005-0.001 mu L L-1) et hylene concentrations can restore the reduced gravitropic response of the a uxin-resistant dgt (diageotropica) mutant to wild-type levels. Slightly hig her concentrations of ethylene inhibit the gravitropic response of all but the ethylene-insensitive nr (never-ripe) mutant. The gravitropic responses of nr and the constitutive-response mutant epi (epinastic) are slightly and significantly delayed, respectively, but otherwise normal. The reversal of shoot gravicurvature by red light in the Iz-2(lazy-2) mutant is not affect ed by ethylene, Taken together, these data indicate that, although ethylene does not play a primary role in the gravitropic response of tomato, low le vels of ethylene are necessary for a full gravitropic response, and moderat e levels of the hormone specifically inhibit gravicurvature in a manner dif ferent from ethylene inhibition of overall growth.