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The Role of Arabidopsis Heterotrimeric G-Protein Subunits in MLO2 Function and MAMP-Triggered Immunity

September 2013 , Volume 26 , Number  9
Pages  991 - 1,003

Justine Lorek,1,2 Thomas Griebel,1 Alan M. Jones,3 Hannah Kuhn,2 and Ralph Panstruga1,2

1Max Planck Institute for Plant Breeding Research, Carl-von-Linné-Weg 10, D-50829 Köln, Germany; 2RWTH Aachen University, Institute for Biology I, Unit of Plant Molecular Cell Biology, Worringerweg 1, D-52056 Aachen, Germany; 3University of North Carolina at Chapel Hill, Departments of Biology and Pharmacology, Chapel Hill, NC, U.S.A.

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Accepted 1 May 2013.

Heterotrimeric G-proteins, composed of Gα, Gβ, and Gγ subunits, regulate many fundamental processes in plants. In animals, ligand binding to seven transmembrane (7TM) cell surface receptors designated G-protein coupled receptors (GPCR) leads to heterotrimeric G-protein activation. Because the plant G-protein complex is constitutively active, the exact role of plant 7TM proteins in this process is unclear. Members of the mildew resistance locus O (MLO) family represent the best-characterized 7TM plant proteins. Although genetic ablation of either MLO2 or G-proteins alters susceptibility to pathogens in Arabidopsis thaliana, it is unknown whether G-proteins directly couple signaling through MLO2. Here, we exploited two well-documented phenotypes of mlo2 mutants, broad-spectrum powdery mildew resistance and spontaneous callose deposition in leaf mesophyll cells, to assess the relationship of MLO2 proteins to the G-protein complex. Although our data reveal modulation of antifungal defense responses by Gβ and Gγ subunits and a role for the Gγ1 subunit in mlo2-conditioned callose deposition, our findings overall are inconsistent with a role of MLO2 as a canonical GPCR. We discovered that mutants lacking the Gβ subunit show delayed accumulation of a subset of defense-associated genes following exposure to the microbe-associated molecular pattern flg22. Moreover, Gβ mutants were found to be hypersusceptible to spray inoculation with the bacterial pathogen Pseudomonas syringae.

© 2013 The American Phytopathological Society