| Summary: | Bacterial production of indole-3-acetic acid (IAA) and its effects on plant growth have been frequently studied but there have been few studies on the ecology of IAA-degrading bacteria. In this study, among eight endophytic bacterial strains previously isolated from the same sweet potato sample including two IAA producers, <i>Klebsiella</i> sp. Sal 1 and <i>Enterobacter</i> sp. Sal 3, all of the strains showed IAA-degrading ability to some extent. <i>Herbaspirillum</i> sp. Sal 6 had the highest activity for IAA and tryptophan. When the IAA producers and the degrader were co-cultured in tryptophan-amended N<sup>+</sup>MR liquid medium, the concentrations of IAA decreased. Inoculation with <i>Klebsiella</i> sp. Sal 1, the highest IAA producer among the test strains, increased fresh root weight of tomato and radish, but the effect decreased by co-inoculation with IAA-degrading <i>Herbaspirillum</i> sp. Sal 6. Since both strains colonized plant parts at high populations, it was likely that the IAA degrader decreased IAA levels in the plants by degrading IAA and/or its precursor tryptophan. When IAA-producing biofertilizers are used, interactions with IAA degraders in plants should be considered.
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