ROPs in the spotlight of plant signal transduction

Abstract

Small guanine nucleotide binding proteins of the Rho family called ROP play a crucial role as regulators of signal transduction in plants. They participate in pathways that influence growth and development, and the adaptation of plants to various environmental situations. As members of the Ras superfamily, ROPs function as molecular switches cycling between a GDP-bound 'off' and a GTP-bound 'on' state in a strictly regulated manner. Latest research provided fascinating new insights into ROP regulation by novel guanine nucleotide exchange factors, unconventional GTPase activating proteins, and guanine nucleotide dissociation inhibitors, which apparently organize localized ROP activation. Important progress has also been made concerning signaling components upstream and downstream of the ROP cycle involving receptor-like serine/threonine kinases and effectors that can manipulate cytoskeletal dynamics, intracellular calcium levels, H2O2 production and further cellular targets. This review outlines the fast developing knowledge on ROP GTPases highlighting their specific features, regulation and roles in a cellular signaling context.

The Arabidopsis ROP-activated receptor-like cytoplasmic kinase RLCK VI_A3 is involved in control of basal resistance to powdery mildew and trichome branching

“…A function of the VI subfamily of RLCKs as RAC/ROP downstream signaling effectors in plants was supported by RAC/ROP GTPase-dependent activation of RLCKs in Arabidopsis, Medicago trunculata (M. trunculata) and barley (Hordeum vulgare L) (Dorjgotov et al 2009;Huesmann et al 2012). RAC/ROP proteins regulate processes like cell development, hormone signaling, cytoskeleton rearrangement and plant disease resistance or susceptibility via various downstream effector proteins including RLCKs (Berken 2006;Nibau et al 2006). RAC/ROPs act as molecular switches, transducing extracellular signals into intracellular responses by shuttling between an inactive GDP-bound and an activated GTP-bound state.…”

Cloning and Characterization of a Gene Encoding Rac/Rop-Like Monomeric Guanosine 5'-Triphosphate-Binding Protein from Scoparia dulcis

“…4,5) It has been assumed that each member of the Rap/Rop subfamily is functionally distinct and plays specific roles, respectively. [6][7][8][9] It is well known 10) that jasmonic acid and its ester, methyl jasmonate (MJ), functions as the messenger molecule in various plant defense responses such as phytoalexin production and hypersensitive cell death. We previously demonstrated 11,12) that the activation of Ca 2ϩ -cascade in plant cells is an essential event to induce defensive secondary metabolism, and calmodulin (CAM), a well known Ca 2ϩ -modulator protein, functions as the key component in these processes.…”

Characterization of ROP GTPase-activated Arabidopsis receptor-like cytoplasmic kinases (RLCK class VI_A)

“…More interestingly, cognate Ras proteins have not been identified in plants which leaves the Rho family into the center of attention for small G protein regulated signaling events in these organisms (Berken 2006).…”

“…The activity of the ROP switch governs a variety of processes in the life cycle of a plant like cell polarity, growth and morphogenesis, cytokinesis, hormone signaling and plant-microbe interactions Berken 2006;Nibau et al 2006;Yang and Fu 2007). Moreover, it was found that ROPs are involved in endo-and exocytotic processes and membrane recycling (Bloch et al 2005;, abiotic stress responses (BaxterBurrell et al 2002;Xin et al 2005), meristem maintenance Trotochaud et al 1999), xylogenesis (Ko et al 2006;Brembu et al 2005), and microspore development (Chan and Pauls 2007) as well.…”

“…Moreover, plants do not have cognate RasGTPases although these proteins are also key elements of MAPK-mediated mitogen signaling in other eukaryotes (Dunn et al 2005). It is generally believed therefore that ROP GTPases, as the only signaling type small GTPases in plants with combined Rho and Ras functions, should be linked to kinase cascades in a presently unknown way (Berken 2006;Yang and Fu 2007). In our laboratory a previous research showed that ROPs interact with and activates RLCKs in Medicago (Dorjgotov et al 2009), and the aim of this work was to characterize the function of corresponding RLCK group in Arabidopsis.…”