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Heterotrimeric G Protein Subunits Provide Functional Selectivity in Gß Dimer Signaling in Arabidopsis^[OA]

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

² To whom correspondence should be addressed. E-mail j.botella{at}uq.edu.au; fax 61-7-33651699.

The Arabidopsis thaliana heterotrimeric G protein complex is encoded by single canonical G and Gß subunit genes and two G 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 genes revealed specialized roles for each G 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 Gß1 and Gß2 synergistically inhibit auxin-dependent lateral root initiation. However, the involvement of each G subunit in this root response differs, with Gß1 acting within the central cylinder, attenuating acropetally transported auxin signaling, while Gß2 affects the action of basipetal auxin and graviresponsiveness within the epidermis and/or cortex. This selectivity also operates in the hypocotyl. Selectivity in Gß 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 subunits form functional Gß 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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