The β-Subunit of the Arabidopsis G Protein Negatively Regulates Auxin-Induced Cell Division and Affects Multiple Developmental Processes[W]

Abstract

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 ␣ -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 G ␣ -subunit phenocopies the G ␤ mutants (auxin hypersensitivity), probably by sequestering the G ␤␥ -subunits, whereas overexpression of G ␤ reduces auxin sensitivity and a constitutively active (Q222L) mutant G ␣ behaves like the wild type. These data are consistent with a model in which G ␤␥ 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 G ␤ 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 G ␤ -subunit attenuates this auxin pathway upstream of the control of mRNA steady state levels.

Expression analysis and subcellular localization of the Arabidopsis thaliana G-protein β-subunit AGB1

“…Expression of the AGB1p::GUS fusion in the vascular and mesophyll tissue of seedling cotyledons in close proximity to carbohydrate reserves may reflect involvement of AGB1 in such a role. Another recurring feature of plant G-proteins is expression in meristematic regions, previously attributed to a role in regulation of cell division (Weiss et al 1993;Huang et al 1994) and since confirmed for both GPA1 and AGB1 (Ullah et al , 2003. Recently, a study using knockout mutants suggested different roles for Ga and Gbc in the regulation of cell proliferation in roots (Chen et al 2006a).…”

A Reevaluation of the Role of the Heterotrimeric G Protein in Coupling Light Responses in Arabidopsis

“…We replicated the design of published gain-offunction experiments using independently engineered inducible expression of native GPA1 (for description, see Ullah et al, 2003). In addition, we generated plants that constitutively express a constitutively active (Q222L mutant) GPA1 and plants that inducibly express AGB1.…”

“…Arabidopsis has a single gene encoding a canonical alpha subunit of a heterotrimeric G protein (GPA1; Ma, 1994), a single beta subunit (AGB1) and possibly two gamma subunits Botella, 2000, 2001). The modeled structure of the Arabidopsis heterotrimeric complex is robustly supported by fold recognition and compactability tests (Ullah et al, 2003).The etiolated and light-grown phenotypes of these mutants have been described extensively 2003). The main defect can be summarized as reduced control of cell division throughout development that manifests as fewer cells in many organs, altered lateral root formation, and altered apical dominance.…”

“…Arabidopsis has a single gene encoding a canonical alpha subunit of a heterotrimeric G protein (GPA1; Ma, 1994), a single beta subunit (AGB1) and possibly two gamma subunits Botella, 2000, 2001). The modeled structure of the Arabidopsis heterotrimeric complex is robustly supported by fold recognition and compactability tests (Ullah et al, 2003).…”

“…The etiolated and light-grown phenotypes of these mutants have been described extensively 2003). The main defect can be summarized as reduced control of cell division throughout development that manifests as fewer cells in many organs, altered lateral root formation, and altered apical dominance.…”