Cooperation of Ethylene and Auxin in the Growth Regulation of Rice Coleoptile Segments

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Abstract
Elongation of coleoptile segments, having or not having a tip, excised from rice (Oryza sativa L. cv. Sasanishiki) seedlings was promoted by exogenous ethylene above 0.3 μl l−1 as well as by IAA above 0.1 μM. Ethylene production of decapitated segments was stimulated by IAA above 1.0μM, and this was strongly inhibited by 1.0 μM AVG. AVG inhibited the IAA-stimulated elongation of the decapitated segment with a 4 h lag period, and this was completely recovered by ethylene applied at the concentration of 0.03 μl l−1, which had no effect on elongation without exogenous IAA. The effects of IAA and ethylene on elongation were additive. These facts show that ethylene produced in response to IAA promotes rice coleoptile elongation in concert with IAA, probably by prolonging the possible duration of the IAA-stimulated elongation, but that they act independently of each other. Moreover, AVG strongly inhibited the endogenous growth of coleoptile segments with tips and this effect was nullified by the exogenous application of 0.03 μl l−1 ethylene. These data imply that the elongation of intact rice coleoptiles may be regulated cooperatively by endogenous ethylene and auxin in the same manner as found in the IAA-stimulated elongation of the decapitated coleoptile segments.