Gγ1+Gγ2+Gγ3=Gβ: The search for heterotrimeric G-protein γ subunits in Arabidopsis is over

In Arabidopsis, heterotrimeric G-proteins consist of one Gα (GPA1), one Gβ (AGB1) and three Gγ (AGG1, AGG2 and AGG3) subunits. Gβ and Gγ subunits function as obligate heterodimers, therefore any phenotypes observed in Gβ-deficient mutants should be apparent in Gγ-deficient mutants. Nevertheless, the first two Gγ subunits discovered failed to explain many of the phenotypes shown by the agb1 mutants in Arabidopsis, prompting the search for additional Gγ subunits. The recent discovery of an additional, although quite atypical, Gγ subunit in Arabidopsis (AGG3) has helped to complete the picture and explains almost all of the missing agb1 'orphan' phenotypes. There is nevertheless still one unexplained phenotype, the reduction in rosette size reported for agb1, that has not been observed in any of the individual agg mutants or the double agg1agg2 mutant. We have now created a triple gamma mutant (agg1agg2agg3) in Arabidopsis and show that it recapitulates the remaining 'orphan' agb1 phenotypes. Triple agg1agg2agg3 mutants show the reduction in rosette size previously observed in agb1 mutants. In addition we show that small differences in flower and silique size observed between agb1 and agg3 mutants are also accounted for by the triple agg1agg2agg3 mutant. Our results strongly suggest that there are no additional members of the G-protein family remaining to be discovered in Arabidopsis.