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Auxin signal transduction in plants depends on regulation by short-lived nuclear plant proteins called auxin/indole-3-acetic acid (Aux/IAA) proteins. The OsIAA9, which is highly and rapidly induced by auxin, encodes the entire Aux/IAA domain characteristic of the Aux/IAA family in rice. The promoter region of OsIAA9 contains several cis-elements. Analysis of POsIAA9:GUS transgenic plants indicates a specific expression of OsIAA9 in roots and seedling shoots, especially lateral roots and root tips. Based on real-time PCR, the expression of OsIAA9 was induced by multiple hormones and abiotic stresses. Furthermore, ectopic overexpression of OsIAA9 in rice led to fewer crown and lateral roots and reduced the inhibition of root elongation by auxin. These observations indicate that OsIAA9 was a negative regulator of auxin-regulated root growth.