Heterotrimeric G-Protein-Coupled Signaling in Higher Plants

Heterotrimeric G proteins are key signaling elements in eukaryotes. The fundamental building blocks of this pathway, the Gα, Gβ, and Gγ subunits, are encoded in plant genomes, as are regulator of G-protein signaling (RGS) proteins, and candidate seven-transmembrane (7TM) G-protein-coupled receptors (GPCRs). However, plants are distinguished fromother metazoans by having far fewer genes encoding these functions: for example, the genome of the model plant species Arabidopsis thaliana encodes single canonical Gα and Gβ subunits, two Gγ subunits, one RGS protein (which, unlike animal RGS proteins, contains a 7TM domain), and manyfewer candidateGPCRsthan mammalian genomes. Nevertheless, genetic approaches have demonstrated the importance of heterotrimeric G-protein signaling in a wide diversity of responses that are fundamental to plant growth and survival, including cell division, ion channel regulation, responses to most of the major plant hormones, and aspects of light signaling, oxidative stress, and pathogen response. These studies have also demonstrated that, similar to the situation in other eukaryotes, some responses are primarily mediated by the Gα subunit and others by the Gβ subunit (βγ dimer). The role that a given G-protein component plays in a given signaling process can differ between different plant cell types, as illustrated most thoroughly for regulation of cell division and hormonal response. These results imply that different plant cell types may employ different upstream and downstream proteins to couple with the heterotrimeric subunits. However, to date, only a few proteins have been shown to physically interact with plant G-protein subunits, and this is a fertile area for future research.