Although signaling through heterotrimeric G proteins has been extensively studied in eukaryotes, there is little information about this important signaling pathway in plants. We observed that expression of GCR1, the gene encoding the only known (but still putative) G protein-coupled receptor of Arabidopsis thaliana, is modulated during the cell cycle and during plant development. Overexpression of GCR1 in tobacco (Nicotiana tabacum) BY-2 cells caused an increase in thymidine incorporation and in the mitotic index of aphidicolin synchronized cells. Overexpression of GCR1 in Arabidopsis caused two remarkable phenotypes: seed dormancy was abolished and time to flowering was reduced. Molecular markers of these two developmental processes (phosphatase PP2A and MYB65 in germination; LFY during flowering) were up-regulated in GCR1 overexpressors. These data are consistent with the hypothesis that GCR1 may be a regulator of the cell cycle and that this regulation underlies the developmental changes observed in the GCR1 transformants.