Differential Roles of Arabidopsis Heterotrimeric G-Protein Subunits in Modulating Cell Division in Roots

Abstract

Signaling through heterotrimeric G proteins is conserved in diverse eukaryotes. Compared to vertebrates, the simpler repertoire of G-protein complex and accessory components in Arabidopsis (Arabidopsis thaliana) offers a unique advantage over all other multicellular, genetic-model systems for dissecting the mechanism of G-protein signal transduction. One of several biological processes that the G-protein complex regulates in Arabidopsis is cell division. We determined cell production rate in the primary root and the formation of lateral roots in Arabidopsis to define individually the types of modulatory roles of the respective G-protein a-and b-subunits, as well as the heterotrimer in cell division. The growth rate of the root is in part a consequence of cell cycle maintenance in the root apical meristem (RAM), while lateral root production requires meristem formation by founder pericycle cells. Thus, a comparison of these two parameters in various genetic backgrounds enabled dissection of the role of the G-protein subunits in modulation of cell division, both in maintenance and initiation. Cell production rates were determined for the RAM and lateral root formation in gpa1 (Arabidopsis G-protein a-subunit) and agb1 (Arabidopsis G-protein b-subunit) single and double mutants, and in transgenic lines overexpressing GPA1 or AGB1 in agb1 or gpa1 mutant backgrounds, respectively. We found in the RAM that the heterotrimeric complex acts as an attenuator of cell proliferation, whereas the GTP-bound form of the Ga-subunit's role is a positive modulator. In contrast, for the formation of lateral roots, the Gbg-dimer acts largely independently of the Ga-subunit to attenuate cell division. These results suggest that Arabidopsis heterotrimeric G-protein subunits have differential and opposing roles in the modulation of cell division in roots.

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

“…Association of the AGB1-GFP fusion with the plasma membrane is consistent with evidence that AGB1 is a peripheral, membrane-associated protein (Obrdlik et al 2000), and recent localization data using a yellow fluorescent protein-AGB1 fusion (Chen et al 2006a). Node-like structures in the plasma membrane similar to those formed by the AGB1-GFP fusion have been observed for GFP-tagged GCR1 (Humphrey and Botella 2001), and may represent caveolae, specialized plasma membrane structures that are highly enriched in signalling proteins including G-proteins and GPCRs (White and Anderson 2005).…”

“…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). In the root apical meristem, it was found that Ga is a positive modulator of cell division, while the Gbc dimer acts as a negative modulator by sequestering Ga in the heterotrimer complex.…”

“…In Arabidopsis, it has been established that AGB1 is a peripheral, membrane-bound protein, probably by association with a Gc subunit (Obrdlik et al 2000). Recently, green fluorescent protein (GFP) fusions of both GPA1 and AGB1 were localized to the plasma membrane in root cells, and preferentially at the cell plate in newly divided cells, suggesting a role in cytokinesis (Chen et al 2006a). No subcellular localization of AGG1 or AGG2 has been reported.…”

Arabidopsis extra‐large G proteins (XLGs) regulate root morphogenesis

“…Because growth phenotypes may vary with subtle differences in environmental conditions Pandey et al, 2006), gpa1-3, agb1-2, gpa1-4 agb1-2 double mutant and the xlg triple mutant were grown and assessed simultaneously for growth assays, even for established gpa1 and agb1 phenotypes. When grown in darkness under our particular conditions, plants harboring an agb1 mutation also had longer roots than wild type (as reported in lightgrown plants; Chen et al, 2006), although not as long as the xlg triple mutant ( Figure S6a). Root length of the gpa1-3 mutant was indistinguishable from wild type, as has also been previously reported for light-grown plants Pandey et al, 2006;Ullah et al, 2003).…”

The Arabidopsis G-Protein β-Subunit Is Required for Defense Response againstAgrobacterium tumefaciens

“…11,12) Previous studies have shown that the A. thaliana heterotrimeric G protein is involved in stress responses as well as in development. [13][14][15][16][17][18][19][20][21] Although many flg22-and elf18-induced responses have been studied, the components linking PAMP perception with early signaling events are still unknown. Here we show that in agb1-2 (AGB1 null mutation) mutants, ROS production triggered by flg22 or elf18 is significantly reduced and elf18-stimulated PTI against Agrobacterium tumefaciens is impaired.…”