Rop GTPases: Polarity and Cell Shape in Plants

Abstract

Similar to Rho GTPases in animals and fungi, Rho of plant (ROP) GTPases, which are active in the guanosine triphosphate‐bound state and inactive in the guanosine diphosphate‐bound state, serve as molecular switches in plants. Although the general function is conserved, the upstream regulators and downstream effectors appear to be plant specific. In Arabidopsis thaliana, a gene family of 11 ROPs regulates a wide range of processes among which their role in polarity and cell shape is best studied. Growth and nongrowth regions are defined by ROPs through the differential activation of downstream effectors that, in turn, control the local function of actin and microtubules. ROPs are involved in regulatory feedback loops that mediate tissue‐specific cues to individual growth behaviours of single cells.

Key Concepts:

  • The general function of ROPs GTPases in plants is conserved.

  • The upstream and downstream effectors of ROPs are plant specific.

  • ROPs regulate cell shape through the organisation of actin and microtubules.

  • ROPs regulate cell morphogenesis by differential activation of downstream effectors.

  • ROPs regulate the size of growth areas through regulatory feedback loops.

Keywords: actin; microtubules; cell shape; polarity; ROP GTPase

Figure 1.

Cycle between active membrane bound and inactive cytoplasmic RhoGTPases. The black bar represents the plasma membrane to which the RhoGTPase bound through its prenylated C‐terminus indicated by the black zigzag line. Binding of GEF catalyses the exchange of GDP with GTP, leading to membrane recruitment and binding to downstream effector proteins. Binding to GAP leads to a GTP exchange with GDP and, in turn, to an association with GDI that stabilises the inactive state.

Figure 2.

Comparison of actin and MT configurations and their roles in morphogenesis among different model cell systems. The first column shows schematic presentations of three model cell types. Arrows indicate the growth direction, and the regions in which growth takes place are indicated in blue. The second column indicates where cortical fine F actin (red lines) is located and how cortical MTs (green lines) are arranged. In general, cortical fine F actin is required for cell growth at sites of growth. In contrast, well‐ordered cortical MTs control the direction or the polarity of cell growth by restricting growth. The last column shows the effect of MT and actin drugs.

Figure 3.

Regulation scheme of ROP1‐dependent growth control. The figure highlights the differential regulation of the downstream effectors RIC3, RIC4 and ICR1 and the feedback loops leading to a self‐maintained size control of the growth region. Arrows represent positive interactions and blunted bars negative interactions.

Figure 4.

Regulation scheme of ROP‐dependent coordination of growth and nongrowth regions in pavement cell morphogenesis. The figure highlights how the activation of ROPs leads to actin‐dependent growth at the side of active ROPs and MT bundling in the neighbourhood. Arrows represent positive interactions and blunted bars negative interactions.

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Further Reading

Craddock C, Lavagi I and Yang Z (2012) New insights into Rho signaling from plant ROP/Rac GTPases. Trends in Cell Biology 22(9): 492–501.

Lorek J, Panstruga R and Huckelhoven R (2010) The role of seventransmembrane domain MLO proteins, heterotrimeric G‐proteins, and monomeric RAC/ROPs in plant defense. In: Yalovsky S, Baluska F and Jones A (eds) Integrated G Protein Signaling in Plants, pp. 197–220. Berlin: Springer‐Verlag.

Wu HM, Hazak O, Cheung AY and Yalovsky S (2011) RAC/ROP GTPases and auxin signaling. Plant Cell 23(4): 1208–1218.

Yang ZB and Lavagi I (2012) Spatial control of plasma membrane domains: ROP GTPase‐based symmetry breaking. Current Opinion in Plant Biology 15(6): 601–607.

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Uhrig, Joachim F, and Hülskamp, Martin(Mar 2014) Rop GTPases: Polarity and Cell Shape in Plants. In: eLS. John Wiley & Sons Ltd, Chichester. http://www.els.net [doi: 10.1002/9780470015902.a0023758]