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Plant Cell Advance Online Publication
Published on April 27, 2007; 10.1105/tpc.107.050096

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Received January 8, 2007
Returned for revision March 22, 2007
Accepted April 10, 2007

Heterotrimeric G Protein {gamma} Subunits Provide Functional Selectivity in G{beta}{gamma} Dimer Signaling in Arabidopsis

Yuri Trusov 1, James Edward Rookes 1, Kimberley Tilbrook 1, David Chakravorty 1, Michael Glenn Mason 1, David Anderson 1, Jin-Gui Chen 2, Alan M. Jones 2, and José Ramón Botella 1*

1 Plant Genetic Engineering Laboratory, Department of Botany, School of Integrative Biology, University of Queensland, Brisbane, Queensland 4072, Australia
2 Departments of Biology and Pharmacology, University of North Carolina, Chapel Hill, North Carolina 27599-3280

* To whom correspondence should be addressed. E-mail: j.botella{at}uq.edu.au.

The Arabidopsis thaliana heterotrimeric G protein complex is encoded by single canonical G{alpha} and G{beta} subunit genes and two G{gamma} subunit genes (AGG1 and AGG2), raising the possibility that the two potential G protein complexes mediate different cellular processes. Mutants with reduced expression of one or both G{gamma} genes revealed specialized roles for each G{gamma} subunit. AGG1-deficient mutants, but not AGG2-deficient mutants, showed impaired resistance against necrotrophic pathogens, reduced induction of the plant defensin gene PDF1.2, and decreased sensitivity to methyl jasmonate. By contrast, both AGG1- and AGG2-deficient mutants were hypersensitive to auxin-mediated induction of lateral roots, suggesting that G{beta}{gamma}1 and G{beta}{gamma}2 synergistically inhibit auxin-dependent lateral root initiation. However, the involvement of each G{gamma} subunit in this root response differs, with G{beta}{gamma}1 acting within the central cylinder, attenuating acropetally transported auxin signaling, while G{beta}{gamma}2 affects the action of basipetal auxin and graviresponsiveness within the epidermis and/or cortex. This selectivity also operates in the hypocotyl. Selectivity in G{beta}{gamma} signaling was also found in other known AGB1-mediated pathways. agg1 mutants were hypersensitive to glucose and the osmotic agent mannitol during seed germination, while agg2 mutants were only affected by glucose. We show that both G{gamma} subunits form functional G{beta}{gamma} dimers and that each provides functional selectivity to the plant heterotrimeric G proteins, revealing a mechanism underlying the complexity of G protein-mediated signaling in plants.




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