Heterotrimeric G-proteins in plant development

Signaling through heterotrimeric G-proteins (G-proteins) is a conserved mechanism found in all eukaryotes. In plants, the repertoire of G-protein signaling complex is much simpler than in metazoans. Specifically, the genome of the model plant, Arabidopsis, encodes only one canonical Galpha, one Gbeta, and two Ggamma subunits. Similarly, only one Regulator of G-protein Signaling (RGS) protein is encoded by the Arabidopsis genome, and no bona fide G-protein-coupled receptor (GPCR) together with its ligand has been unequivocally identified. Nonetheless, several proteins, including AtPIRIN1, PLDa 1, PD1, and THF1, have been shown to physically interact with the Arabidopsis heterotrimeric G-protein alpha subunit (GPA1), and are potential downstream effectors for GPA1. The smaller repertoire of the heterotrimeric G-protein complex in plants offers a unique advantage over its counterpart in mammals for dissecting their roles in development. The analyses of loss-of-function alleles and gain-of-function transgenic lines of G-protein subunits and signaling components suggest that the G-proteins play regulatory roles in multiple developmental processes ranging from seed germination and early seedling development to root development and organ shape determination. Future studies are expected to reveal more components of the heterotrimeric G-protein signal transduction pathways, and to identify the mechanisms by which G-proteins regulate phenotypic and developmental plasticity.