G gamma 1+G gamma 2 not equal G beta: Heterotrimeric G protein G gamma-deficient mutants do not recapitulate all phenotypes of G beta-deficient mutants

Heterotrimeric G proteins are signaling molecules ubiquitous among all eukaryotes. The Arabidopsis (Arabidopsis thaliana) genome contains one G alpha (GPA1), one G beta (AGB1), and two G gamma subunit (AGG1 and AGG2) genes. The G beta requirement of a functional G gamma subunit for active signaling predicts that a mutant lacking both AGG1 and AGG2 proteins should phenotypically resemble mutants lacking AGB1 in all respects. We previously reported that G beta- and G gamma-deficient mutants coincide during plant pathogen interaction, lateral root development, gravitropic response, and some aspects of seed germination. Here, we report a number of phenotypic discrepancies between G beta- and G gamma-deficient mutants, including the double mutant lacking both G gamma subunits. While G beta-deficient mutants are hypersensitive to abscisic acid inhibition of seed germination and are hyposensitive to abscisic acid inhibition of stomatal opening and guard cell inward K(+) currents, none of the available G gamma-deficient mutants shows any deviation from the wild type in these responses, nor do they show the hypocotyl elongation and hook development defects that are characteristic of G beta-deficient mutants. In addition, striking discrepancies were observed in the aerial organs of G beta- versus G gamma-deficient mutants. In fact, none of the distinctive traits observed in G beta-deficient mutants (such as reduced size of cotyledons, leaves, flowers, and siliques) is present in any of the G gamma single and double mutants. Despite the considerable amount of phenotypic overlap between G beta- and G gamma-deficient mutants, confirming the tight relationship between G beta and G gamma subunits in plants, considering the significant differences reported here, we hypothesize the existence of new and as yet unknown elements in the heterotrimeric G protein signaling complex.