GCR1 Can Act Independently of Heterotrimeric G-Protein in Response to Brassinosteroids and Gibberellins in Arabidopsis Seed Germination

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Abstract
Signal recognition by seven-transmembrane (7TM) cell-surface receptors is typically coupled by heterotrimeric G-proteins to downstream effectors in metazoan, fungal, and amoeboid cells. Some responses perceived by 7TM receptors in amoeboid cells and possibly in human cells can initiate downstream action independently of heterotrimeric G-proteins. Plants use heterotrimeric G-protein signaling in the regulation of growth and development, particularly in hormonal control of seed germination, but it is not yet clear which of these responses utilize a 7TM receptor. Arabidopsis GCR1 has a predicted 7TM-spanning domain and other features characteristic of 7TM receptors. Loss-of-function gcr1 mutants indicate that GCR1 plays a positive role in gibberellin- (GA) and brassinosteroid- (BR) regulated seed germination. The null mutants of GCR1 are less sensitive to GA and BR in seed germination. This phenotype is similar to that previously observed for transcript null mutants in the Gα-subunit, gpa1. However, the reduced sensitivities toward GA and BR in the single gcr1, gpa1, and agb1 (heterotrimeric G-protein β-subunit) mutants are additive or synergistic in the double and triple mutants. Thus, GCR1, unlike a typical 7TM receptor, apparently acts independently of the heterotrimeric G-protein in at least some aspects of seed germination, suggesting that this alternative mode of 7TM receptor action also functions in the plant kingdom.