Diversity of Heterotrimeric G proteins in Plant Innate Immunity – Evidence of a G protein Code

Abstract

Heterotrimeric G proteins are one of the most conserved eukaryotic signaling components that are responsible for transducing ligand‐recognition signals from transmembrane receptors. Upon ligand recognition, the heterotrimeric G protein complex divides into two functional components: the GTP‐bound Gα‐subunit and the Gβγ dimer. However, it remains unclear how G proteins decode receptor signals and activate appropriate effectors. This is particularly true for innate immunity, where G proteins have long been recognized to play a role. By contrast to the 40 subunits characterized in humans, the model plant Arabidopsis thaliana (A. thaliana) has only one Gα, one Gβ (AGB1), and three Gγ subunits characterized to date. This small repertoire of subunits offers a simplified system to study the role of G proteins in innate immunity. Using sequence homology to guide our functional genomic screens, we identified a number of putative Gβ subunits involved in the activation of different immune responses (e.g. cell wall reinforcement) upon the recognition of specific microbe‐associated ligands, known collectively as MAMPs. We will present a case that one of the candidates, AGB2, is a novel Gβ subunit, and that like AGB1, the known Gβ subunit, is involved in transducing receptor signals for fungal vs. bacterial pathogens. More specifically, both AGB1 and AGB2 are involved in activating the bacterial MAMP‐elicited lignin deposition response as well as activating the fungal MAMP‐elicited callose deposition response. Furthermore, we will describe the mechanism for their antagonistic and additive interactions for each defense response as well as present a model on how the variability of subunits would allow G proteins to be assigned to different signal transduction effector function (defense response) for the same ligand‐receptor pair, and to couple different ligand‐receptor pairs to the same defense response.