Regulation of root-wave response by extra large and conventional G proteins in Arabidopsis thaliana

Heterotrimeric G proteins composed of α, β and γ subunits regulate a number of fundamental processes concerned with growth and development in plants. In addition to the canonical heterotrimeric G proteins, plants also contain a small family of extra large G proteins (XLGs) that show significant similarity to the G-protein α subunit in their C-terminal regions. In this paper we show that one of the three XLG genes, XLG3, and the Gβ subunit (AGB1) of the Arabidopsis G-protein heterotrimer are specifically involved in the regulation of a subset of root morphological and growth responses. Based on analysis of T-DNA insertional mutant phenotypes, XLG3 and AGB1 each positively regulate root waving and root skewing. Since these responses are regulated by physical as well as physiological cues, we assessed the roles of AGB1 and XLG3 in gravitropism, thigmotropism and hormonal responses. Our data show that mutants lacking either XLG3 or AGB1 genes are hypersensitive to ethylene and show growth responses consistent with alterations in auxin transport, while maintaining an essentially wild-type response to the physical cues of gravity and touch. These results suggest that XLG3 and AGB1 proteins regulate the hormonal determinants of root-waving and root-skewing responses in plants and possibly interact in a tissue-specific or signal-specific manner. Because plants harboring knockout mutations in the Ga subunit gene, GPA1, exhibit wild-type root waving and skewing, our results may indicate that the AGB1 subunit functions in these processes without formation of a classic Gαβ γ heterotrimer.