Auxin and Ethylene in Hairy Root Cultures of Hyoscyamus muticus

Treatment of normal and Agrobacterium rhizogenes-transformed root cultures of Hyoscyamus muticus with three different auxins (IAA, IBA, NAA) revealed that the response varied considerably between auxins, between transformed and normal roots and depending on the parameter examined (length of primary root, number and length of lateral roots). While IBA reduced primary root elongation only at the highest concentration tested, the other two auxins were strongly inhibitory at all concentrations in both root lines. All three auxins promoted branching in normal roots (except 2.5 μM IAA) but not in transformed roots where they were generally without effect (IBA) or inhibitory. In normal roots, which produced less ethylene than transformed roots, the auxin stimulation of ethylene formation was more pronounced than in transformed roots. The ethylene precursor ACC and its conjugated form MACC were undetectable in both root lines but accumulated considerably when NAA was supplied. The ethylene synthesis inhibitor AVG had, at high concentrations, a drastic effect on root growth and development while silver thiosulphate, an ethylene antagonist, did not. The latter compound, however, resulted in no or very limited root hair formation in transformed and normal roots, respectively.