A reevaluation of the role of the heterotrimeric G protein in coupling light responses in Arabidopsis

Differential responses of G-protein Arabidopsis thaliana mutants to ozone

•  Ground-level ozone (O₃ ) curtails agricultural production in many regions worldwide. However, the etiology of O₃ toxicity remains unclear. Activated oxygen species appear to inflict biochemical lesions and propagate defense responses that compound plant injury. Because some plant defense responses involve membrane-delimited GTPases (G proteins), we evaluated the O₃ sensitivity of Arabidopsis mutants altered in the heterotrimeric G-protein pathway. •  Eight genotypes were treated with a range of O₃ concentrations (0, 100, 175 and 250 nmol mol^-1 ) for 13 d in controlled environment chambers. •  After treatment with O₃ , the epinasty typically observed for wild type leaves did not occur in mutant plants lacking the alpha subunit of the G-protein complex (gpa1). O₃ -induced suppression of leaf chlorophyll levels and leaf mass per unit leaf area were less for gpa1 mutants and were not due to differences in O₃ flux. •  There was a positive correlation between the lack of a G-protein alpha subunit and decreased O₃ sensitivity. Our results suggest that a heterotrimeric G-protein is critically involved in the expression of O₃ effects in plants.

GCR1 can act independently of heterotrimeric G-protein in response to brassinosteroids and gibberellins in Arabidopsis seed germination

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 Galpha-subunit, gpa1. However, the reduced sensitivities toward GA and BR in the single gcr1, gpa1, and agb1 (heterotrimeric G-protein beta-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.

A Seven-Transmembrane RGS Protein That Modulates Plant Cell Proliferation

G protein-coupled receptors (GPCRs) at the cell surface activate heterotrimeric G proteins by inducing the G protein alpha (Galpha) subunit to exchange guanosine diphosphate for guanosine triphosphate. Regulators of G protein signaling (RGS) proteins accelerate the deactivation of Galpha subunits to reduce GPCR signaling. Here we identified an RGS protein (AtRGS1) in Arabidopsis that has a predicted structure similar to a GPCR as well as an RGS box with GTPase accelerating activity. Expression of AtRGS1 complemented the pheromone supersensitivity phenotype of a yeast RGS mutant, sst2Delta. Loss of AtRGS1 increased the activity of the Arabidopsis Galpha subunit, resulting in increased cell elongation in hypocotyls in darkness and increased cell production in roots grown in light. These findings suggest that AtRGS1 is a critical modulator of plant cell proliferation.

The beta-subunit of the Arabidopsis G protein negatively regulates auxin-induced cell division and affects multiple developmental processes

Plant cells respond to low concentrations of auxin by cell expansion, and at a slightly higher concentration, these cells divide. Previous work revealed that null mutants of the alpha-subunit of a putative heterotrimeric G protein (GPA1) have reduced cell division. Here, we show that this prototypical G protein complex acts mechanistically by controlling auxin sensitivity toward cell division. Loss-of-function G protein mutants have altered auxin-mediated cell division throughout development, especially during the auxin-induced formation of lateral and adventitious root primordia. Ectopic expression of the wild-type Galpha-subunit phenocopies the Gbeta mutants (auxin hypersensitivity), probably by sequestering the Gbetagamma-subunits, whereas overexpression of Gbeta reduces auxin sensitivity and a constitutively active (Q222L) mutant Galpha behaves like the wild type. These data are consistent with a model in which Gbetagamma acts as a negative regulator of auxin-induced cell division. Accordingly, basal repression of approximately one-third of the identified auxin-regulated genes (47 of 150 upregulated genes among 8300 quantitated) is lost in the Gbeta transcript-null mutant. Included among these are genes that encode proteins proposed to control cell division in root primordia formation as well as several novel genes. These results suggest that although auxin-regulated cell division is not coupled directly by a G protein, the Gbeta-subunit attenuates this auxin pathway upstream of the control of mRNA steady state levels.