Heterotrimeric G proteins, consisting of Gα, Gβ and Gγ subunits, play important roles in plant development and cell signaling. In Arabidopsis, in addition to one prototypical G protein a subunit gene, GPA1, there are three extra-large G proteins, XLG1, XLG2, and XLG3 of largely unknown function. Yeast two-hybrid library screening and in vitro protein pull-down assays revealed that XLG2 interacts with the nuclear protein RELATED TO VERNALIZATION1 (RTV1). A mutant XLG2 that lacks GTP binding does not interact with RTV1, suggesting the dependence of this protein interaction on the G-protein cycle. Electrophoretic mobility shift assays show that RTV1 binds to DNA in vitro in a non-sequence specific manner and that GTP-bound XLG2 promotes the DNA binding activity of RTV1. Overexpression of RTV1 results in early flowering. Combined overexpression of XLG2 and RTV1 enhances this early flowering phenotype, and elevates expression of the floral pathway integrator genes, FT and SOC1, but does not repress expression of the floral repressor, FLC. Chromatin immunoprecipitation assays show that XLG2 increases RTV1 binding to FT and SOC1 promoters. Thus, a Ca2+-dependent Gprotein, XLG2, promotes RTV1 DNA binding activity for a subset of floral integrator genes, and contributes to floral transition.