Silvana Moreno

PCDU

WS 15/16

Origins of polarity

Outline_____________________

• Introduction

• Polarity systems in plants

• Signaling mechanisms behind cell polarity formation

• Summary

• References

Introduction

Assymetry in cellshape, protein

distributions and cell functions.

The development, growth and

morphogenesis inevery stage of the

life cycle.

Tied to developmental

and environmental

regulation

Formation of cell polarity

A general signaling mechanism forcell polarity formation.Provide spatial information forfeedback regulation

Subfamilies of the Rho family small GTPases.11 members of the Arabidopsis Rho-relatedGTPase from plants (ROP) family.Representative members of the Cdc42, Rac,and Rho subfamilies.

Yang, 2008

Rho GTPasesRho-family small GTPases have been shown as central regulators of polarity in different multicellular organisms by modulating polarized trafficking.

In plants, a self-organizing mechanism based on ROPs (Rho-like small GTPases from plants) has emerged as a common mechanism for symmetry breaking to generate polar or local PM domains.

Control tip growth, pavement cell morphogenesis, secondary cell wall patterning in xylem cells, and PIN polarization.

Diversity of shapes of polarized cells

Nelson, 2003

Structural Organization of Budding Yeast and Polarized Epithelial Cells

Dubrin, D & J, Nelson, 1996

Polarity systems in plants

Tip growth model

• Polar growth occurs in alleukaryotic kingdoms and is requiredfor the generation ofhighly elongated tubular cells.

• Pollen tube and root hair (rapid tipgrowth)

• To deliver sperm to the ovule, the PT must extend rapidly and directionally by navigating through many tissues.

• Nonessential, many morphological mutants and concerned genes have been identified.

Yang, 2008

Pistil with growing pol len tubes

polarized cellular s tructure of pollen tubes

Morphogenesis of epidermal cells

Model for the study of planar cellpolarity and/or the polarity of diffusegrowth.

• Trichomes are nonessential cells,and their morphology is easilyobserved.

• The development of pavement cellsis used as a model for signalingnetwork regulating.

Yang, 2008

Epidermal cells

Model for asymmetric distribution and polarity cell division

• Model for asymmetric distributionof molecules within a cell: The polarlocalization of PINs, which directsauxin flow and produces auxingradients

• Model for the polarity of celldivision: Guard cell differentiation

Yang, 2008

Signaling mechanisms behind cell polarity formation

The pavement cell model

Polarity involving cell – cellcoordination

The development of pavement cells(PC).

SI: Near-square PC initials first elongateslightly to form near-rectangular cells.

SII: Produce alternating small bumpsand indentations, generating cells withmultiple shallow lobes and indentations

SIII: Reiterative outgrowing andindenting continue, producing highlylobed interlocking PC that often containsecondary lobes.

Yang, 2008

Dynamic and oscillating apical cap of active ROP1Active ROP1 reporter in a tobacco pollen tube. Localization of GFP‐RIC4 is restricted to the apical region of the PM.

Oscillation of PM-localized GFP-RIC4C.

Models for positive feedback–mediated lateral propagation of the apical ROP1 activity and for negative feedback–mediatedglobal ROP1 downregulation by the REN1 GTPase–activating protein (RhoGAP) and lateral inhibition by cortical microtubules (MT) and phospholipase C (PLC).

CDPK, calcium-dependent protein kinase; F-actin, actin microfilaments; RIC, ROP-interactive Cdc42/Rac-interactive binding (CRIB) motif–containing protein.

Pollen tube model

Yang, 2008

PIN-Formed protein model

Secondary transporters acting in the efflux of the plant signal molecule auxin from cells.

PIN genes are found exclusively in the genomes of multicellular plants

Regulate asymmetric auxin distribution in multiple developmental processes, including embryogenesis, organogenesis, tissue differentiation and tropic responses.

Structure with amino- and carboxy-terminal hydrophobic, membrane-spanning domains separated by a central hydrophilic domain.

The hydrophilic domain is more divergent and it determines eight groups within the protein family.

The activity of PIN proteins is regulated at multiple levels, including transcription, protein stability, subcellular localization and transport activity.

Polar localization of proteins

•PINOID (PID) ser/thrprotein kinase and its antagonist, PP2AA phosphatase, are critical for the control of PIN polarity.

PIN polarity

•Partially colocalizewith PIN1 in the PM.

•The signaling most likely occurs there.

PID and PP2AA •Component in this

signaling pathway(s). • Interacts with

phosphorylates, and activates PID.

PDK1

Summary

• Cell polarity is portrayed in a wide variety of shapes and functions in single-cell organisms and cells in multi-cell tissues

• Cell polarity signaling has been studied through several model systems, including single-cell systems such asPT and root hairs and multicellular systems such as PC and PIN protein localization in roots.

• As in all other eukaryotic cells, the plant subfamily of conserved Rho GTPases, ROP, is the central regulator ofcell polarity signaling.

• ROP is mediated by both conserved and plant-specific regulators, and it controls the dynamics andorganization ofboth F-actin and MT through plant-specific effector proteins such as RICs.

• In jigsaw-puzzle-shaped PC, the complex polarity of cell expansion is controlled by ROP2 activation oflocalized F-actin in the lobing domain, which counteracts RIC1 activation of MT arrangement in the indentingdomain.

• Auxin may be a common polarizing signal, activating ROP signaling pathways in its control of cell polarity.

• Polar localization of PINs is mediated by a cascade of PDK1 and PID ser/thr kinases, whereas asymmetric celldivision may be controlled by an RLK-MAPK cascade signaling mechanism. Both of these kinase signalingcascades may be linked to ROP GTPase.

References

• Baluska,F; Salaj, J; Mathur, J; Braun, M; Jasper, F; Samaj, J; Chua, N; Barlow, P & D, Volkmann. 2000.Root Hair Formation: F-Actin-Dependent Tip Growth Is Initiated by Local Assembly of ProfilinSupportedF-Actin Meshworks Accumulated within Expansin-Enriched Bulges. Developmental Biology 227, 618–632.

• Dhonukshe, P. 2011. Mechanistic Framework for Establishment, Maintenance, and Alteration of CellPolarity in Plants.

• Nelson, W. 2003. Adaptation of core mechanisms to generate cell polarity. NATURE | VOL 422.

• Rademacher, E & Offringa, R. 2012. Evolutionary adaptations of plant AGC kinases: from light signalingto cell polarity regulation. doi: 10.3389/fpls.2012.00250.

• Yang, Z. 2008. Cell polarity signaling in Arabidopsis. Annu. Rev. Cell Dev. Biol. 2008. 24:551–75

• Yang, Z & I, Lavagi. 2012. Spatial control of plasma membrane domains: ROP GTPase-based symmetrybreaking. Science Direct. 15:601–607

Thank you for your attention!