An early auxin-induced gene was isolated from rice (Oryza sativa L. subsp. japonica cv Nihonmasari) coleoptiles by a fluorescent-labeled differential display screen. The full-length gene contains conserved domains characteristic for the cytochrome P450 superfamily. This gene, designated as CYP87A3, was weakly expressed in dark-grown coleoptiles but was up-regulated rapidly and transiently when coleoptile segments were incubated in 5 m indole-3-acetic acid. This induction by auxin could not be suppressed by cycloheximide. Depletion of segments from endogenous auxin reduced the amount of CYP87A3 transcripts. The CYP87A3 transcript level was rapidly, although transiently, up-regulated in response to light as well. The observed pattern of gene regulation might indicate a role in the suppression of auxin-induced coleoptile growth. The role of CYP87A3 is discussed with respect to auxin signaling in the regulation of coleoptile growth.Coleoptiles represent a classical model system to study the control of growth-related signaling, because they grow exclusively by cell elongation (Wada, 1961) and respond to environmental signals. The plant hormone auxin plays a central role in the control of coleoptile elongation. As a matter of fact, auxin was discovered as a powerful growth substance that is produced mainly in the coleoptile tip, migrates basipetally, and is crucial for the signaldependent regulation of growth (Went, 1928).Although the importance of basipetal auxin transport has been confirmed by a wealth of data collected over several decades (for review, see Masuda et al., 1998), the role of auxin in the signal-dependent growth response appears to be complex. As shown by recent data, auxin distribution depends on several intrinsic activities, mainly synthesis, conjugation/deconjugation, and catabolism (for review, see Normanly, 1997). Moreover, changes in auxin transport between neighboring tissues and/or local concentrations can influence growth. For instance, a correlation between growth inhibition and a decrease in the level of extractable indole-3-acetic acid (IAA) could be demonstrated in the epidermis of irradiated pea (Pisum sativum) seedlings (Behringer and Davies, 1992). In addition, sensitivity/responsiveness of the target tissue to auxin have to be considered (Trewavas, 1981). Because of this complexity, the events underlying signaling mediating auxin-triggered cell elongation are still far from being understood.Growing evidence has appeared concerning regulation of auxin action at the level of gene expression (for example, expressional control of primary auxinresponsive genes, such as members of GH3 or Aux/ IAA family; see Leyser, 2002; Hagen and Guilfoyle, 2002; Nakazawa et al., 2001). From the expression pattern of such genes, their role in signaling as well as their putative function have been inferred. We therefore ventured to isolate auxin-regulated gene(s) related to auxin-induced elongation. Using fluorescent differential display (FDD; Kuno et al., 2000), we screened for a subset of auxin-induced...