Heterotrimeric G Protein γ Subunits Provide Functional Selectivity in Gβγ Dimer Signaling in Arabidopsis

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Abstract
The Arabidopsis thaliana heterotrimeric G protein complex is encoded by single canonical Gα and Gβ subunit genes and two Gγ subunit genes (AGG1 and AGG2), raising the possibility that the two potential G protein complexes mediate different cellular processes. Mutants with reduced expression of one or both Gγ genes revealed specialized roles for each Gγ subunit. AGG1-deficient mutants, but not AGG2-deficient mutants, showed impaired resistance against necrotrophic pathogens, reduced induction of the plant defensin gene PDF1.2, and decreased sensitivity to methyl jasmonate. By contrast, both AGG1- and AGG2-deficient mutants were hypersensitive to auxin-mediated induction of lateral roots, suggesting that Gβγ1 and Gβγ2 synergistically inhibit auxin-dependent lateral root initiation. However, the involvement of each Gγ subunit in this root response differs, with Gβγ1 acting within the central cylinder, attenuating acropetally transported auxin signaling, while Gβγ2 affects the action of basipetal auxin and graviresponsiveness within the epidermis and/or cortex. This selectivity also operates in the hypocotyl. Selectivity in Gβγ signaling was also found in other known AGB1-mediated pathways. agg1 mutants were hypersensitive to glucose and the osmotic agent mannitol during seed germination, while agg2 mutants were only affected by glucose. We show that both Gγ subunits form functional Gβγ dimers and that each provides functional selectivity to the plant heterotrimeric G proteins, revealing a mechanism underlying the complexity of G protein–mediated signaling in plants.