OsARF1, an auxin response factor from rice, is auxin-regulated and classifies as a primary auxin responsive gene

We screened for auxin-induced genes with an expression correlated to the auxin-induced growth response from rice coleoptiles by fluorescent differential display. A rice homologue of the auxin response factor (ARF) family of transcriptional regulators, OsARF1, was identified. An OsARF1:GFP fusion protein was localized to the nucleus. Steady-state levels of OsARF1 mRNA correlated positively with auxin-dependent differential growth: gravitropic stimulation enhanced the amount of OsARF1 transcript in the lower, faster-growing flank accompanied by a decrease in the upper flank of gravitropically stimulated rice coleoptiles. Exogenous auxin up-regulated the steady-state level of OsARF1 mRNA within 15-30 min. This up-regulation is independent of de novo protein synthesis. Thus, OsARF1 is the first ARF that classifies as an early auxin-responsive gene. The observed auxin-dependent regulation comprises a new level of regulation in auxin-induced gene expression and is discussed as a possible feedback mechanism in plant growth control.