Transition from the vegetative phase to reproductive phase is a crucial process in the life cycle of higher plants. Although the molecular mechanisms of flowering regulation have been extensively characterized in a number of plant species, little is known regarding how the transition process initiates. Here, we show that the Rice Indeterminate 1 (RID1) gene acts as the master switch for the transition from the vegetative to reproductive phase. RID1 encodes a Cys-2/His-2-type zinc finger transcription factor that does not have an ortholog in Arabidopsis spp. A RID1 knockout (rid1), mutated by T-DNA insertion, never headed after growing for >500 days under a range of growth conditions and is thus referred to as a never-flowering phenotype. This mutation-suppressed expression of the genes is known to be involved in flowering regulation, especially in the Ehd1/Hd3a pathway and a series of RFT homologs. RID1 seems to be independent of the circadian clock. A model was proposed to place RID1 in the molecular pathways of flowering regulation in rice, for which there are two indispensable elements. In the first, RID1 is controlling the phase transition and initiation of floral induction. In the other, the

SummaryA binary GAL4-VP16-UAS transactivation system has been established in rice (Oryza sativa L.) in this study for the discovery of gene functions. This binary system consists of two types of transgenic lines, pattern lines and target lines. The pattern lines were produced by transformation of Zhonghua 11, a japonica cultivar, with a construct consisting of the transactivator gene GAL4-VP16 controlled by a minimal promoter and the GUSplus reporter controlled by the upstream activation sequence (UAS; cis-element to GAL4). Target lines were generated by transformation of Zhonghua 11 with constructs carrying the EGFP reporter and target genes of interest, both controlled by the UAS but in opposite directions. Hybrid plants were obtained by crossing target lines of 10 putative transcription factor genes from rice with six pattern lines showing expression in anther, stigma, palea, lemma and leaves. The EGFP and target genes perfectly co-expressed in hybrid plants with the same expression patterns as in the pattern lines. Various phenotypic changes, such as delayed flowering, multiple pistils, dwarfism, narrow and droopy leaves, reduced tillers, growth retardation and sterility, were induced as a result of the expression of the target genes. It is concluded that this transactivation system can provide a useful tool in rice to unveil latent functions of unknown or known genes.