Type B Heterotrimeric G Protein γ-Subunit Regulates Auxin and ABA Signaling in Tomato

Heterotrimeric G proteins composed of alpha, beta, and gamma subunits are central signal transducers mediating the cellular response to multiple stimuli in most eukaryotes. G gamma subunits provide proper cellular localization and functional specificity to the heterotrimer complex. Plant G gamma subunits, divided into three structurally distinct types, are more diverse than their animal counterparts. Type B G gamma subunits, lacking a carboxyl-terminal isoprenylation motif, are found only in flowering plants. We present the functional characterization of type B G gamma subunit (SlGGB1) in tomato (Solanum lycopersicum). We show that SlGGB1 is the most abundant G gamma subunit in tomato and strongly interacts with the G beta subunit. Importantly, the green fluorescent protein-SlGGB1 fusion protein as well as the carboxyl-terminal yellow fluorescent protein-SlGGB1/amino-terminal yellow fluorescent protein-G beta heterodimer were localized in the plasma membrane, nucleus, and cytoplasm. RNA interference-mediated silencing of SlGGB1 resulted in smaller seeds, higher number of lateral roots, and pointy fruits. The silenced lines were hypersensitive to exogenous auxin, while levels of endogenous auxins were lower or similar to those of the wild type. SlGGB1-silenced plants also showed strong hyposensitivity to abscisic acid (ABA) during seed germination but not in other related assays. Transcriptome analysis of the transgenic seeds revealed abnormal expression of genes involved in ABA sensing, signaling, and response. We conclude that the type B G gamma subunit SlGGB1 mediates auxin and ABA signaling in tomato.