Extra-Large G Protein 2 Promotes Floral induction by Activation of the DNA Binding-Protein RTV1

Heterotrimeric G proteins play important roles in plant development and cell signaling. In Arabidopsis, in addition to one prototypical G protein a subunit, GPA1, there are three extra-large G proteins, XLG1, XLG2, and XLG3 of largely unknown function. Each XLG has a C-terminal Ga-like region and a ~400 amino acid N-terminal extension. Yeast two-hybrid library screening and in vitro protein pull-down assays revealed that XLG2 interacts with the nuclear protein RELATED TO VERNALIZATION1 (RTV1). Electrophoretic mobility shift assays show that RTV1 binds to DNA in vitro in a non-sequence specific manner and that 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, XLG2 promotes RTV1 DNA binding activity for a subset of floral integrator genes, and contributes to floral transition.