ABA-Dependent and -Independent G-Protein Signaling in Arabidopsis Roots Revealed through an iTRAQ Proteomics Approach

Heterotrimeric G-proteins are important signal transducers in all eukaryotes. The plant hormone abscisic acid (ABA) has emerged as a key regulator of G-protein-mediated signaling pathways in plants. ABA-regulation of G-protein signaling involves both conventional and novel mechanisms. We have utilized the null mutant of the Arabidopsis G-protein alpha subunit gpa1 to evaluate to what extent ABA-dependent changes in the proteome are regulated by G-proteins. We used Arabidopsis root tissue as both ABA and G-proteins, individually and in combination, affect root growth and development. We identified 720 proteins, of which 42 showed GPA1-dependent and 74 showed ABA-dependent abundance changes. A majority of ABA-regulated proteins were also GPA1-dependent. Our data provide insight into how tissue specificity might be achieved in ABA-regulated G-protein signaling. A number of proteins related to ER body formation and intracellular trafficking were altered in gpa1 mutant, suggesting a novel role for GPA1 in these pathways. A potential link between ABA metabolism and ABA signaling was also revealed: The comparison of protein abundance changes in the absence of ABA offers clues to the role of GPA1 in ABA-independent signaling pathways, for example, regulation of cell division. These findings substantially contribute to our knowledge of G-protein signaling Mechanisms in plants.