The Rop GTPase Switch Controls Multiple Developmental Processes in Arabidopsis

Abstract

G proteins are universal molecular switches in eukaryotic signal transduction. The Arabidopsis genome sequence reveals no RAS small GTPase and only one or a few heterotrimeric G proteins, two predominant classes of signaling G proteins found in animals. In contrast, Arabidopsis possesses a unique family of 11 Rop GTPases that belong to the Rho family of small GTPases. Previous studies indicate that Rop controls actin-dependent pollen tube growth and H 2 O 2 -dependent defense responses. In this study, we tested the hypothesis that the Rop GTPase acts as a versatile molecular switch in signaling to multiple developmental processes in Arabidopsis. Immunolocalization using a general antibody against the Rop family proteins revealed a ubiquitous distribution of Rop proteins in all vegetative and reproductive tissues and cells in Arabidopsis. The cauliflower mosaic virus 35S promoter-directed expression of constitutively active GTP-bound rop2 (CA-rop2) and dominant negative GDP-bound rop2 (DN-rop2) mutant genes impacted many aspects of plant growth and development, including embryo development, seed dormancy, seedling development, lateral root initiation, morphogenesis of lateral organs in the shoot, shoot apical dominance and growth, phyllotaxis, and lateral organ orientation. The rop2 transgenic plants also displayed altered responses to the exogenous application of several hormones, such as abscisic acid-mediated seed dormancy, auxin-dependent lateral shoot initiation, and brassinolide-mediated hypocotyl elongation. CA-rop2 and DN-rop2 expression had opposite effects on most of the affected processes, supporting a direct signaling role for Rop in regulating these processes. Based on these observations and previous results, we propose that Rop2 and other members of the Rop family participate in multiple distinct signaling pathways that control plant growth, development, and responses to the environment.

Protein Geranylgeranyltransferase I Is Involved in Specific Aspects of Abscisic Acid and Auxin Signaling in Arabidopsis

“…This small GTP-binding protein has a consensus CSIL motif at its C terminus with an upstream polybasic domain, which qualifies it as a putative substrate of PGGT I. Rop2, which possesses a C-terminal CAFL motif and can be geranylgeranylated in vitro (D.N. Crowell, unpublished results), is also a negative regulator of ABA signaling (Li et al, 2001). Given this, it is surprising that ggb mutants exhibited no significant phenotype with respect to ABA inhibition of seed germination (Fig.…”

“…org/ptmdb/results.htm). These include eight Rop GTPases, which regulate actin organization (Fu et al, 2001(Fu et al, , 2002Gu et al, 2003), pollen tube tip growth (Lin et al, 1996;Fu et al, 2001;Gu et al, 2003), ABA and auxin signaling (Lemichez et al, 2001;Li et al, 2001;Tao et al, 2002), and other processes; AIG1, which is involved in RPS2-and avrRpt2-dependent responses to Pseudomonas infection (Reuber and Ausubel, 1996); Aux2-11, a negative regulator of auxin signaling (Wyatt et al, 1993;Caldelari et al, 2001;Dharmasiri and Estelle, 2002); and other proteins. Thus, it is surprising that era1, ggb, and plp phenotypes are not more severe.…”

The Plastidial 2-C-Methyl-d-Erythritol 4-Phosphate Pathway Provides the Isoprenyl Moiety for Protein Geranylgeranylation in Tobacco BY-2 Cells

“…The existence of farnesylated negative regulators of ABA signaling is inferred from the ABA hypersensitive phenotype of ENHANCED RESPONSE TO ABA1 (ERA1) mutants of Arabidopsis thaliana, which lack the b-subunit of PFT (Cutler et al, 1996;Pei et al, 1998). In addition, two geranylgeranylated proteins, ROP2 and ROP6, have been shown to be involved in negative regulation of ABA signaling (Lemichez et al, 2001;Li et al, 2001;Yang, 2002). Similarly, the existence of geranylgeranylated proteins involved in negative regulation of auxin signaling is inferred from the enhanced response of GERANYLGERANYL TRANSFERASE b (GGB) mutants, which lack the b-subunit of PGGT 1, to auxininduced lateral root initiation (Johnson et al, 2005).…”

Visualization and analysis of actin cytoskeleton organization in plants

“…Myosins are various motor proteins that enable organelle movement along the actin cytoskeleton and are essential for vesicular transport, assembly of transvacuolar actin cable, plasmodesmata pore adjustment, chloroplast positioning, and interaction between MTs and MFs (Šamaj, Peters, Volkmann, and Baluška, 2000;Meagher and Fecheimer, 2003;Grebe et al, 2003;Šamaj, Peters, Volkmann, and Baluška, 2006). Small GTP-binding proteins (GTPases) are also involved in the regulation of MF morphology in control of vesicular transport, polar growth and the development of complex cell morphology (Valster et al, 2000;Li et al, 2001;Vernoud et al, 2003;Berken et al, 2005;Li, Xu, J., Xu, Z., and Xue, 2005) as well as in reproduction development (Kawashima et al, 2014). ROPs convert external signals to the microfilament branching state by interaction with ARP2/3 (Klyachko, 2004;Xu and Scheres, 2005;Hussey, Ketelaar, and Deeks, 2006;Nagawa et al, 2012).…”