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In mammals, activation of heterotrimeric guanine nucleotide-binding protein (G protein)–coupled pathways transduces numerous neuroendocrine signals as well as visual, olfactory, and some gustatory inputs. Flowering plants also use G proteins in signal transduction. In contrast to the great diversity and combinatorial complexity of mammalian G protein heterotrimers, the model species Arabidopsis and rice typify a different situation in plants, in which only single canonical Gα and Gβ subunits are present and only two Gγ subunits have been identified. Nonetheless, null mutations of G protein components affect a wide variety of hormonal signaling pathways in plants. This review discusses the nature of these pleiotropic phenotypes, the puzzle of how so few heterotrimers can mediate so many diverse effects, and a potential role of heterotrimeric G proteins in allowing plants to adjust their phenotype in response to different environmental conditions (that is, phenotypic plasticity).