Overexpression of the IAGLU gene from maize (ZmIAAGLU) in Arabidopsis thaliana, under the control of the CaMV 35S promoter, inhibited root but not hypocotyl growth of seedlings in four different transgenic lines. Although hypocotyl growth of seedlings and inflorescence growth of mature plants was not affected, the leaves of mature plants were smaller and more curled as compared to wild-type and empty vector transformed plants. The rosette diameter in transgenic lines with higher ZmIAGLU expression was also smaller compared to the wild type. Free indole-3-acetic acid (IAA) levels in the transgenic plants were comparable to the wild type, even though a decrease in free IAA levels might be expected from overexpression of an IAA-conjugate-forming enzyme. IAA-glucose levels, however, were increased in transgenic lines compared to the wild type, indicating that the ZmIAGLU gene product is active in these plants. In addition, three different 35SZmIAGLU lines showed less inhibition of root growth when cultivated on increasing concentrations of IAA but not indole-3-butyric acid (IBA) and 2,4-dichlorophenoxyacetic acid (2,4-D). Feeding IAA to transgenic lines resulted in increased IAA-glucose synthesis, whereas the levels of IAA-aspartate and IAA-glutamine formed were reduced compared to the wild type. Our results show that IAA homeostasis can be altered by heterologous overexpression of a conjugate-forming gene from maize.