plant cytokinesis: fission by fusion

Cytokinesis series

Plant cytokinesis: fission by fusionGerd Jurgens

ZMBP, Entwicklungsgenetik, Universitat Tubingen, Auf der Morgenstelle 3, 72076 Tubingen, Germany

Cytokinesis partitions the cytoplasm of a dividing cell.

Unlike yeast and animal cells, which form cleavage

furrows from the plasma membrane, cells in higher

plants make a new membrane independently of the

plasma membrane by homotypic fusion of vesicles. In

somatic cells, a plant-specific cytoskeletal array, called a

phragmoplast, is thought to deliver vesicles to the plane

of division. Vesicle fusion generates a membranous

network, the cell plate, which, by fusion of later-arriving

vesicles with its margin, expands towards the cell

periphery and eventually fuses with the plasma mem-

brane. In this review (part of the Cytokinesis series), I

describe recent studies addressing the mechanisms that

underlie cell-plate formation and the coordinated

dynamics of membrane fusion and cytoskeletal re-

organization during progression through cytokinesis.

Introduction

Two fundamentally different ways to divide a cell arerepresented by animals and yeast on one hand and higherplants on the other. Non-plant organisms initiate cyto-kinesis at the periphery of the cell. A contractile acto-myosin ring pulls in the plasma membrane, forming acleavage furrow that grows towards the centre of thedivision plane [1]. By contrast, higher plants do not makea contractile actomyosin ring, and the genome of Arabi-dopsis, which has been sequenced entirely, lacks genes

Prophase Late anaphase

PPB

N

G G

MZ

Ch

Sp

CDS

(a) (b) (

Figure 1. Somatic cytokinesis. (a) Prophase: The plane of cell division is determined by

(red) and actin filaments (green) that forms at the level of the nucleus (N) and marks the f

nucleate the formation of phragmoplast microtubules in the midzone (MZ) between the

consists of two antiparallel bundles each of microtubules (red) and actin filaments (green

nascent cell plate (nCP) is formed by homotypic fusion of vesicles (V). Golgi stacks (G) ac

microtubules causes the cell plate (CP) to expand and, eventually, fuse with the plasm

(Illustration adapted, with permission, from Ref. [54].)

Corresponding author: Jurgens, G. ([email protected]).Available online 2 April 2005

www.sciencedirect.com 0962-8924/$ - see front matter Q 2005 Elsevier Ltd. All rights reserved

that encode myosin II [2]. Instead, higher plants initiatecytokinesis by targeted secretion to the plane of celldivision. Membrane vesicles fuse to form one or moretransient membrane compartments that then fuse withthe plasma membrane of the parental cell to physicallyseparate the two daughter cells (Figure 1). This mode ofcytokinesis has long been known to occur in somatic cells,whereas cleavage furrows have been postulated for othertypes of plant cell [3]. However, recent studies indicatethat specialized modes of plant cytokinesis are variants ofsomatic cytokinesis.

Mechanisms of cytokinesis in plants have beenstudied predominantly in somatic cells, taking advan-tage of synchronizable BY-2 tobacco-cell cultures and ofArabidopsis mutants. Somatic cytokinesis is assisted bytwo plant-specific cytoskeletal arrays, the preprophaseband and the phragmoplast [4]. The preprophase band,which marks the future cortical-division site, appears inthe cell cortex late in G2 phase and disappears with thebreakdown of the nuclear envelope during prometa-phase (Figure 1). The phragmoplast originates in lateanaphase and is thought to play an essential role in thetargeted delivery of membrane vesicles to the plane ofdivision. Recent studies of membrane dynamics and itsinterplay with phragmoplast reorganization are begin-ning to reveal the mechanisms that underlie plantcytokinesis.

Review TRENDS in Cell Biology Vol.15 No.5 May 2005

TRENDS in Cell Biology

Cytokinesis

PHP

Ch

nCP

G

V

DN

DN

CP

G

VPHP

Telophasec) (d)

CPAMCPAM

a transient cortical preprophase band (PPB) of co-aligned bundles of microtubules

uture cortical division site (CDS). (b) Late anaphase: Remnants of the mitotic spindle

two sets of daughter chromosomes (Ch). (c) Telophase: The phragmoplast (PHP)

) whose plus-ends terminate in the cell-plate assembly matrix (CPAM) in which the

cumulate near the plane of division. (d) Cytokinesis: Lateral translocation of the PHP

a membrane at the cortical division site (arrows). DN, forming daughter nuclei.

. doi:10.1016/j.tcb.2005.03.005

http://www.sciencedirect.com


Setting the stage: formation of the phragmoplast as a

scaffold for vesicle delivery

At late anaphase, some kinetochore spindle microtubulesremain in the interzone between the two sets of daughterchromosomes and nucleate the formation of the phragmo-plast (Figure 1). Disassembly of the anaphase spindle andformation of the phragmoplast occur only if mitotic cyclinB1 has been degraded [5]. The phragmoplast contains twoantiparallel sets of co-aligned microtubules and actinfilaments. Whereas the plus ends of the microtubules arebelieved to overlap in the plane of division, the actinfilaments do not. However, recent evidence from electrontomography indicates that the antiparallel microtubulesterminate in a cell-plate assembly matrix without overlapof their plus ends [6]. The phragmoplast microtubules playa major role in cytokinesis, including the targeted deliveryof membrane vesicles during cell-plate formation andexpansion [6–8]. Vesicles are connected by rod-shapedlinkers to microtubules in cellularizing endospermwhereas both connected and unconnected vesicles are

Table 1. Cytokinesis-associated genes in Arabidopsis

Gene Protein class Locali

Membrane-associated functions

KN Syntaxin (Qa-SNARE) Divisio

SNAP33 SNAP25 homolog Cell pl

memb

NPSN11 Qb-SNARE Cell pl

KEU SM protein Not an

DRP1A (At5g42080) Dynamin-related GTPase Cell pl

DRP2A (At1g10290) Dynamin-related GTPase Golgi,

memb

Cytoskeleton-associated functions

TON2 or FS Regulatory subunit of pp2a Not an

TON1 Unknown Not an

TAN homolog (At3g05330) Microtubule-associated protein Spind

At5g55230 MAP65–1 All MT

PLE (At5g51600) MAP65–3 Spind

phrag

At2g38720 MAP65–5 Cell pl

At1g27920 MAP65–8 Phrag

surfac

At3g47690 AtEB1a All MT

At5g67270 AtEB1c Spind

TKRP125 homolog

(At2g36200)

Plus-end-directed bipolar kinesin Phrag

DcKRP120–2 homolog

(At3g45850)

Plus-end-directed bipolar kinesin Phrag

PAKRP1 (At4g14150) Phragmoplast-associated

kinesin-related protein

Interzo

phrag

PAKRP1L (At3g23670) Phragmoplast-associated


Interzo

phrag

PAKRP2 (At4g14330) Phragmoplast-associated


Phrag

ZWI or KCBP (At5g65930) Kinesin-like calmodulin-binding

protein

Phrag

GEM1 or MOR1 MAP215 family protein All MT

HIK or NACK1 Kinesin-related protein Phrag

TES or NACK2 Kinesin-related protein Not an

ANP1–3 or NPK1 MAP kinase kinase kinase Phrag

ANQ1 or NQK1 MAP kinase kinase Not an

ATM1 (At3g19960) Myosin VIII Matur

Other functions

CALS1 (At1g05570) Callose synthase 1 Cell pl

At4g32830 Aurora-like serine–threonine

kinase

Spind

www.sciencedirect.com

observed in somatic cytokinesis [6,7]. The role of the actinfilaments is less clear, but might involve guiding theexpanding cell plate to the cortical-division site [9].

The phragmoplast is stabilized by cross-linking micro-tubule-associated proteins (MAPs) and kinesin motormolecules, which counteract the dynamic instability ofmicrotubules (Table 1) [4]. MAPs have microtubule-bundling activity in vitro and co-localize with microtubulearrays in vivo. The most abundant group, named MAP65,is encoded by nine genes in Arabidopsis [10,11]. WhereasMAP65–1 associates with all microtubule arrays that areformed during the cell cycle [10], MAP65–3 [also known asPLEIADE (PLE)] localizes specifically to interzone spindlemicrotubules in anaphase and to the plus ends ofphragmoplast microtubules [12]. Analysis of ple mutants,which have cytokinesis defects in the seedling root [13],indicates that PLE stabilizes the phragmoplast micro-tubule array [12]. GEM1 (also known as MOR1), aMAP215 family protein that is essential for cytokinesisin haploid microspores, accumulates towards the plus

zation Function Refs

n plane, cell plate Vesicle fusion [28]

ate, plasma

rane

Vesicle fusion [29]

ate Vesicle fusion [31]

alyzed Vesicle fusion [32,33]

ate Membrane constriction [6,24]

cell plate, plasma

rane

Clathrin-coated vesicle budding [25]

alyzed Formation of preprophase band [49]

alyzed Formation of preprophase band [49]

le, phragmoplast Phragmoplast orientation? [50]

arrays MT-cross-linking protein [10,51]

le midzone,

moplast

MT cross-linking protein [12]

ate (plasmodesmata) MT cross-linking protein [51]

moplast, nuclear

e

MT cross-linking protein [51]

arrays MT end-binding protein [51,52]

le, phragmoplast MT end-binding protein,

regulation of cytokinesis?

[51]

moplast Phragmoplast stability [4]

moplast Phragmoplast stability? [4]

nal microtubules,

moplast

Phragmoplast stability [19]

nal microtubules,

moplast

Phragmoplast stability [20]

moplast Not analyzed [53]

moplast Phragmoplast stability? [22]

arrays MT-crosslinking protein, pollen

cytokinesis

[14]

moplast MT destabilization [36,37]

alyzed MT destabilization [44]

moplast MT destabilization [38]

alyzed MT destabilization [39]

e cell plate Cell-plate expansion? [51]

ate Callose deposition [24]

le, cell plate Regulation of cytokinesis? [51]


Review TRENDS in Cell Biology Vol.15 No.5 May 2005 279

ends of phragmoplast microtubules [14]. However, GEM1also localizes to other microtubule arrays and stabilizescortical microtubules at interphase [14,15]. TMBP200, thetobacco homolog of GEM1, has been isolated fromtelophase cells and shown to cross-bridge microtubules[16]. Thus, several MAPs stabilize phragmoplast micro-tubules, but their precise roles are still to be definedfunctionally.

The Arabidopsis genome encodes w60 kinesin-relatedmotor proteins (KRPs) [17], several of which localize to thephragmoplast and are implicated in its organization(Table 1) [4,18]. Bipolar KRPs such as tobacco TKRP125and carrot DcKRP120–2 are plus-end-directed motorsthat can slide antiparallel microtubules against eachother andmight, thus, compensate for microtubule growthat their plus ends (Figure 2) [4]. However, their preciserole in phragmoplast organization needs to be clarified ifthe plus ends of antiparallel microtubules do not overlap[6]. Other Arabidopsis KRPs such as PAKRP1 [19] and itshomolog PAKRP1L [20] form dimers and might maintainthe organization of the phragmoplast. These KRPs localizespecifically to the interzonal microtubules at anaphaseand to the plus ends of phragmoplast microtubules [19,20].The action of bipolar KRPs might be counteracted byminus-end-directed KRPs such as ATK1 [21] and thekinesin calmodulin-binding protein (KCBP) [22,23].KCBP appears to be regulated negatively by calcium,the concentration of which rises transiently duringcytokinesis [9,22]. In summary, both structural MAPsand KRPs maintain the dynamic stability of the micro-tubular phragmoplast perpendicular to the plane ofdivision. As a result, two solid antiparallel bundles of

MAP

KRP?

v

ccb

DRP

+

–

+

–

+

+

bKRP

MT

MT

–

–

α/β-tubulin

α/β-tubulin

Figure 2. Cell-plate assembly in the centre of the division plane during early cytokinesis.

motors (KRP?) along phragmoplast microtubules (MTs) to the cell plate assembly matrix

Constriction by dynamin-related protein 1a (DRP1a, magenta) generates fusion tubes. Aft

At the end of the early stage of cytokinesis, the phragmoplast consists of two solid bundle

face each other. Within each bundle, the aligned microtubules are crosslinked by micro

proteins (bKRPs) might maintain the organization of antiparallel MTs by counteractin

microtubules have not been demonstrated by electron microscopy to overlap with thei


microtubules assist in the formation of the incipient cellplate in the centre of the division plane.

Initiation of the cell plate by homotypic vesicle fusion

Phragmoplast microtubules terminate in a ribosome-free,cell-plate-assembly matrix in which Golgi-derived trans-port vesicles accumulate and membrane fusion is initiated[6]. Electron tomography reveals two types of vesicles,which differ in staining and size. The larger, light vesiclesappear to result from pair-wise fusion of smaller, darkvesicles [6]. Vesicle fusion generates hourglass-shapedintermediates and fusion tubes that, by further fusionevents, are transformed into a tubulo-vesicular networkcalled the cell plate (Figure 2). Dynamin-related proteins(DRPs), which form ‘dynamin rings’ that constrict thetubular membranes, prevent ballooning of the incipientmembrane compartment [7,24]. Several members of theDRP1 subfamily in Arabidopsis localize to the cell plate[25], and the drp1a drp1e double mutant displays defectsin cytokinesis [26]. In addition to constricting activity,DRP1a interacts with callose synthase, which spins outcallose into the lumen and, thus, further flattens theincipient cell plate [24]. At the end of the initial phase ofcytokinesis, the cell plate consists of a dense mesh oftubular membranes that span the width of the solidphragmoplast [6].

Membrane fusion is mediated by SNARE proteins thatassociate into a complex that forms a four-helical bundlebetween opposing membranes [27]. A major component ofhomotypic fusion during cytokinesis is a plant-specificsyntaxin (Qa-SNARE) named KNOLLE (KN), which wasidentified originally in mutants that accumulate unfused


+

–

–

1a

+

CPAM

SNAREcomplex

Golgi-derived membrane vesicles (v) might be delivered by putative kinesin-related

(CPAM) in which SNARE-complex-mediated homotypic vesicle fusions (right) occur.

er further fusion events, clathrin-coated buds (ccb) appear on cell-plate membranes.

s, each of microtubules and actin filaments (not shown), the plus-ends (C) of which

tubule-associated proteins (MAPs). Plus-end-directed bipolar kinesin-related motor

g their growth dynamics (left). It should be noted that antiparallel phragmoplast

r plus-ends [6].



PFSTNTVN

CCV

PMCW

CDS

CCB

E

G

CP

DRP1

DRP2a

R?

KRP?

AF

NACK1

MAP3KMAP

α/β-tubulin

v

MT

MT

α/β-tubulinα/β-tubulin

Vesicles

Fusion tubes TVN

Vesicles

TN

P

DeP

(a)

(b) NACK1MAP3K

(inactive)

(active)NACK1/MAP3K

CPAM

Figure 3. Cell-plate expansion and lateral translocation of the phragmoplast during

late cytokinesis. (a) Microtubules of the ring-shaped phragmoplast target Golgi (G)-

derived vesicles, which might be moved by putative kinesin-related motors (KRP?)

to the cell-plate assembly matrix (CPAM) at the margin of the expanding cell plate

(CP). Homotypic fusion of vesicles generates fusion tubes that are constricted by

dynamin-related proteins 1 (DRP1, magenta) and transformed successively into

tubulo-vesicular (TVN) and tubular networks (TN), and a planar fenestrated sheet

(PFS). Removal of membrane from the TVN occurs by means of clathrin-coated

buds (CCB), which are pinched off by dynamin-related protein 2a (DRP2a, green).

Clathrin-coated vesicles (CCV) fuse with endosomes (E), where sorting occurs,

possibly including recycling (R?) to the CP. CP expansion (long arrow) to the cortical

division site (CDS) at the plasma membrane (PM) and cell wall (CW) is guided by

actin filaments (AF). Image adapted, with permission, from Ref. [54]. (b) Interplay of

phragmoplast translocation and CP expansion (model). Microtubules (MT) that

polymerize (P) on the outer face of the phragmoplast ring target NACK1 and the

associated active mitogen-activated protein kinase kinase kinase (MAP3K) NPK1 as

well as G-derived vesicles to the CP assembly matrix (CPAM). Homotypic fusion of

vesicles generates fusion tubes. MAP3K signaling transforms fusion tubes into the

tubulo-vesicular network (TVN). This results in microtubule (MT) depolymerization

(DeP), which, in turn, inactivates MAP3K.


vesicles in the plane of division [28]. KN is expressedduring M phase only and localizes to the cell plate [28]. AKN-interacting SNAP25 homolog (Qb,c-SNARE) calledSNAP33 also localizes to the cell plate as well as to theplasma membrane [29]. Inactivating SNAP33 causes onlya minor defect in cytokinesis, possibly because of func-tional redundancy with two other homologs of SNAP25[29]. Although a VAMP (R-SNARE) that interacts with KNand SNAP33 to complete the SNARE complex has notbeen identified, several VAMP7-related proteins might becandidates [30]. Alternatively, the cytokinetic SNAREcomplex might be unusual and consist only of Q-SNAREs.In this case, it might contain the plant-specific Qb-SNARENPSN11, which also localizes to the cell plate andinteracts with KN [31]. Inactivation of NPSN11 causesno obvious defects, possibly because of functional redun-dancy with two closely related proteins [31].

Thus, the syntaxin KN is the only specific component ofthe cytokinetic SNARE complex that has been identified.To determine how syntaxin specificity in cytokinesisarises, several syntaxins have been expressed from theKN promoter and their subcellular localization and abilityto rescue a kn mutant analyzed [34]. The results indicatethat syntaxin specificity of cytokinesis is mediated by cell-cycle-regulated gene expression, protein targeting andprotein activity at the cell plate [34]. Finally, KN interactswith the SM-family protein KEULE (KEU) [32], whichwas identified originally in mutants that accumulateunfused vesicles in the plane of division [33]. AlthoughKEU seems to have an additional role in nonproliferatingtissues [32], the interaction between KEU and KN, andthe similarity in mutant phenotypes indicate that acti-vation of KN by KEUmight be necessary for vesicle fusionin cytokinesis.

Cell-plate expansion and phragmoplast reorganization

The initial assemblage of fusion tubes in the centre of thedivision plane undergoes a series morphological changesthat result in a tubulo-vesicular network [6]. At this stage,microtubules underneath the tubulo-vesicular networkdepolymerize, whereas those that flank the peripheralfusion tubes remain stable (Figure 3) [35]. Thus, theinitially solid phragmoplast is transformed into a ring-shaped array. Additional microtubules are then polymer-ized on its outer face. As a consequence, newly arrivingvesicles are targeted to the margin of the cell plate, whichexpands the assemblage of fusion tubes laterally. As fusiontubes near the inner face of the phragmoplast are trans-formed into a tubulo-vesicular network, the associatedmicrotubules depolymerize [35]. Repeated cycles of micro-tubule depolymerization on the inner face and micro-tubule polymerization on the outer face continuouslywiden the ring of phragmoplast microtubules until itreaches the cell cortex. This lateral translocation ofphragmoplast microtubules is arrested by treatmentwith taxol, which prevents microtubule depolymerization,and with brefeldin A (BFA), which inhibits vesicle traffick-ing [35]. These observations indicate that microtubuledepolymerization provides free tubulin heterodimers thatform new microtubules and that microtubule depolymer-ization requires vesicle trafficking to the plane of division.


Recent studies give an idea how these processes might belinked mechanistically.

Two orthologous plant-specific, kinesin-related proteins,HINKEL (HIK) in Arabidopsis and NPK1-activatingkinesin-like protein 1 (NACK1) in tobacco BY-2 cells,have comparable roles in phragmoplast dynamics duringcell-plate expansion [36,37]. In cytokinesis-defectivehik-mutant embryos, phragmoplast microtubules arestabilized beneath the expanding cell plate [36]. A domi-nant-negative form of NACK1 blocks expansion of the cellplate andstabilizes phragmoplastmicrotubulesbeneath thecell plate [37]. NACK1 activates the mitogen-activated



protein kinase kinase kinase (MAP3K) NPK1 and targetsNPK1 to the plus ends of phragmoplast microtubules [37].Overexpression of inactive NPK1 causes the same cyto-kinesis defects as expression of the dominant-negativeform of NACK1 [38]. Thus, NACK1 and HIK mediatespatial and temporal control of NPK1 signaling. NPK1activates the mitogen-activated protein kinase kinase(MAP2K) NQK1, which is also required for cell-plateexpansion [39]. Both NPK1 and NQK1 are inactivated bymicrotubule depolymerization [39], which indicates thatMAPK signaling is regulated by a negative-feedback loop.Why phragmoplast microtubules depolymerize more cen-trally, underneath the tubulo-vesicular network, but notat themargin of the expanding cell plate is not known. It isconceivable that MAPK signaling mediates membranereorganization, which might destabilize phragmoplastmicrotubules (Figure 3). Such a scenario would accountfor the lateral progression of cytokinesis by coordinatingthe translocation of phragmoplast microtubules withexpansion of the cell plate.

Cell-plate maturation and fusion with the parental

plasma membrane

The cell plate is transformed from a tubulo-vesicularnetwork via a tubular network into a planar, fenestratedsheet [6]. After the tubulo-vesicular network stage, calloseis deposited into the lumen of the cell plate, which isconcomitantly reduced in both volume and surface area.Clathrin-coated buds are associated with cell-plate mem-branes and clathrin-coated vesicles appear nearby, indi-cating that excess membrane material is removed by anendocytosis-related process [6]. This might involve cell-plate-localized dynamin-related protein DRP2a. DRP2a isrelated to dynamin, which mediates membrane scission inanimal cells [25]. Endocytosis assists in shaping the cellplate. In addition, membrane material removed from thecentre might be recycled, via endosomes, to the margin ofthe cell plate, to speed up cytokinesis. In dividing cells, thecytokinesis-specific syntaxin KN accumulates in recyclingendosomal compartments when vesicle trafficking isblocked by treatment with BFA [40]. However, recyclingof KN to the margin of the cell plate has not beendemonstrated conclusively.

Eventually, the margin of the expanding cell plate fuseswith the parental plasma membrane at the cortical-division site that is defined originally by the position ofthe preprophase band. This fusion triggers maturation ofthe cell plate into a stretch of plasma membrane andincludes the replacement of callose by cellulose. Accordingto the classical model, the entire margin of the fullyexpanded cell plate fuses simultaneously with the plasmamembrane. Recently, ‘polar cytokinesis’ has been observedin vacuolate cells [41]. In this, mitotic division starts off-centre, so that the cell plate fuses with the plasmamembrane on one side of the cell before expanding alongthe cortex to the opposite side of the cell. Asymmetricexpansion of the cell plate might account for theoccurrence of cell-wall stubs that are observed with eithergenetic or experimental interference with cell-plateformation [41]. Cell-wall stubs are also produced in knmutants that lack the cytokinesis-specific syntaxin [28],


which indicates that local membrane fusion with theplasma membrane involves different, unidentified fusionmachinery.

Different modes of plant cytokinesis: a common

underlying mechanism?

Somatic cytokinesis is the prototype of cytokinesis inhigher plants. However, other modes of cytokinesis occurin specialized cell types [3]. Recent studies have investi-gated two of these modes, endosperm cellularization andmale meiotic cytokinesis. The developing embryo issurrounded by a nutritive tissue called endosperm thatoriginates from a separate fertilization event. Earlydevelopment of the endosperm is characterized by nucleardivision cycles followed by cellularization. Thus, the largeendosperm cell is partitioned simultaneously into manysmall cells, which is somewhat akin to the cellularizationof the Drosophila blastoderm embryo. Endosperm cellu-larization, although superficially distinct, appears to be avariation of somatic cytokinesis. Microtubule arrays,termed mini-phragmoplasts, target membrane vesiclesto the planes of division, and the vesicles fuse to formtubular networks called mini-cell plates [7]. The sub-sequent expansion and maturation of mini-cell plates alsoresembles their counterparts in somatic cytokinesis [7].Moreover, many of the mutations that affect somaticcytokinesis also affect endosperm cellularization [42].

In male meiotic cytokinesis in Arabidopsis, the dividingcell partitions simultaneously into four haploid spores,assisted by radial microtubule arrays that emanate fromthe four nuclear envelopes [3]. Earlier studies haveindicated the involvement of cleavage furrows from theparental plasma membrane [3]. However, a recentelectron tomographic study has demonstrated that theearly phase is similar to somatic cytokinesis [43].Membrane vesicles are targeted along microtubules tothe planes of division and fuse with one another to formnumerous networks of tubular membranes that line upacross the division planes [43]. Subsequently, the plasmamembrane fuses successively with the tubular networks,progressing from the periphery to the centre of the cell[43]. Interestingly, loss-of-function mutations in NACK2in tobacco and TES, its ortholog in Arabidopsis, which arethe closest homolog of the cytokinesis-specific kinesinNACK1 (HIK in Arabidopsis), disrupt the radial micro-tubule arrays in male meiotic cytokinesis and result intetrasporous pollen grains [44]. This finding indicatesmechanistic similarities with somatic cytokinesis.

The similarities and differences between somaticcytokinesis and other specialized modes of cytokinesishave not been analyzed in detail [3]. However, thecytokinesis-specific kinesins HIK and TES have redun-dant functions in the production of viable gametes [45].Specifically, hik tes double-mutant gametophytes are notcellularized [45]. Haploid gametophytes are the earliestdevelopmental stage at which a mutant phenotype can beexpressed. It is, thus, conceivable that HIK and TES arealso required for female meiotic cytokinesis, but thiscannot be tested because the hikmutant is lethal [36]. Thetobacco orthologs of HIK and TES, NACK1 and NACK2,interact with the MAP3K NPK1, as discussed above [37].



In Arabidopsis, three ANP genes encode NPK1 homologsand, like hik tes double-mutants, anp1 anp2 anp3 triple-mutant gametophytes are nonfunctional [46]. Thus, aNACK–NPK1 pathway might be a common regulatorymodule in cytokinesis in higher plants.

Concluding remarks

Cytokinesis in higher plants has two distinctive featuresthat make it unique among eukaryotes: the absence ofan actomyosin-based cleavage furrow; and microtubule-assisted formation of membrane compartment(s) byhomotypic fusion of vesicles. Although the latter haslong been recognized for phragmoplast-assisted cell-plateformation in somatic cytokinesis, it was not knownwhether it also applied to other, superficially differentmodes of higher-plant cytokinesis. However, recent ultra-structural and genetic studies indicate that endospermcellularization and male meiotic cytokinesis are variantsof somatic cytokinesis and involve commonmechanisms. Iffemale meiotic cytokinesis and gametophytic cytokinesesalso follow the same pattern, as suggested by preliminarygenetic evidence, there will be no doubt that cytokinesis inhigher plants is fundamentally different from cytokinesisin non-plant organisms.

How did higher-plant cytokinesis evolve? Usually,phragmoplast-assisted cytokinesis occurs only in landplants. In some green algae, however, a phragmoplast-likearray of microtubules mediates completion of cytokinesisafter an actin-based cleavage furrow, which is initiated atthe plasma membrane, has progressed towards the centreof the division plane [47]. It is, thus, possible that thephragmoplast has become more elaborate in the higher-plant lineage at the expense of the original cleavage-basedmechanism of cytokinesis. By comparison, non-plantcytokinesis might have evolved differently, with thecleavage furrow supported by a contractile actomyosinring and the midbody microtubules only mediating thefinal step of cytokinesis. Midbody abscission requiresspecific SNARE-mediated membrane fusion that is dis-tinct from cleavage furrow ingrowth [48]. Whether thissuperficial resemblance to phragmoplast-assisted cell-plate formation of higher plants is a coincidence orwhether it reflects an ancient, common mechanismbetween higher plants and non-plant organisms remainsa challenge for the future.

Acknowledgements

I thank Ulrike Mayer, Katharina Steinborn and Dolf Weijers for helpfulcomments on the manuscript.

References

1 Balasubramanian, M.K. et al. (2004) Comparative analysis ofcytokinesis in budding yeast, fission yeast and animal cells. Curr.Biol. 14, R806–R818

2 Arabidopsis Genome Initiative. (2000) Analysis of the genomesequence of the flowering plant Arabidopsis thaliana. Nature 408,796–815

3 Otegui, M. and Staehelin, L.A. (2000) Cytokinesis in flowering plants:more than one way to divide a cell. Curr. Opin. Plant Biol. 3, 493–502

4 Lloyd, C. and Hussey, P. (2001) Microtubule-associated proteins inplants - why we need a MAP. Nat. Rev. Mol. Cell Biol. 2, 40–47


5 Weingartner, M. et al. (2004) Expression of a nondegradable cyclin B1affects plant development and leads to endomitosis by inhibiting theformation of a phragmoplast. Plant Cell 16, 643–657

6 Seguı-Simarro, J.M. et al. (2004) Electron tomographic analysis ofsomatic cell plate formation in meristematic cells of Arabidopsispreserved by high-pressure freezing. Plant Cell 16, 836–856

7 Otegui, M.S. et al. (2001) Three-dimensional analysis of syncytial-typecell plates during endosperm cellularization visualized by highresolution electron tomography. Plant Cell 13, 2033–2051

8 Steinborn, K. et al. (2002) The Arabidopsis PILZ group genes encodetubulin-folding cofactor orthologs required for cell division but not cellgrowth. Genes Dev. 16, 959–971

9 Hepler, P.K. et al. (2002) Roles for kinesin and myosin duringcytokinesis. Philos. Trans. R. Soc. Lond. B Biol. Sci. 357, 761–766

10 Smertenko, A. et al. (2000) A new class of microtubule-associatedproteins in plants. Nat. Cell Biol. 2, 750–753

11 Smertenko, A.P. et al. (2004) The Arabidopsis microtubule-associatedprotein AtMAP65-1: molecular analysis of its microtubule bundlingactivity. Plant Cell 16, 2035–2047

12 Muller, S. et al. (2004) The plant microtubule-associated proteinAtMAP65-3/PLE is essential for cytokinetic phragmoplast function.Curr. Biol. 14, 412–417

13 Muller, S. et al. (2002) Two new loci, PLEIADE and HYADE, implicateorgan-specific regulation of cytokinesis in Arabidopsis. Plant Physiol.130, 312–324

14 Twell, D. et al. (2002) MOR1/GEM1 has an essential role in the plant-specific cytokinetic phragmoplast. Nat. Cell Biol. 4, 711–714

15 Whittington, A.T. et al. (2001) MOR1 is essential for organizingcortical microtubules in plants. Nature 411, 610–613

16 Yasuhara, H. et al. (2002) TMBP200, a microtubule bundlingpolypeptide isolated from telophase tobacco BY-2 cells is a MOR1homologue. Plant Cell Physiol. 43, 595–603

17 Reddy, A.S. and Day, I.S. (2001) Kinesins in the Arabidopsis genome: acomparative analysis among eukaryotes. BMC Genomics 2, 2

18 Liu, B. and Lee, Y.R.J. (2001) Kinesin-related proteins in plantcytokinesis. J. Plant Growth Regul. 20, 141–150

19 Lee, Y.R. and Liu, B. (2000) Identification of a phragmoplast-associated kinesin-related protein in higher plants. Curr. Biol. 10,797–800

20 Pan, R. et al. (2004) Localization of two homologous Arabidopsiskinesin-related proteins in the phragmoplast. Planta 220, 156–164

21 Chen, C. et al. (2002) The Arabidopsis ATK1 gene is required forspindle morphogenesis in male meiosis. Development 129, 2401–2409

22 Vos, J.W. et al. (2000) The kinesin-like calmodulin binding protein isdifferentially involved in cell division. Plant Cell 12, 979–990

23 Vinogradova, M.V. et al. (2004) Crystal structure of kinesin regulatedby Ca2C-calmodulin. J. Biol. Chem. 279, 23504–23509

24 Hong, Z. et al. (2001) A cell plate-specific callose synthase and itsinteraction with phragmoplastin. Plant Cell 13, 755–768

25 Hong, Z. et al. (2003) Phragmoplastin dynamics: multiple forms,microtubule association and their roles in cell plate formation inplants. Plant Mol. Biol. 53, 297–312

26 Kang, B.H. et al. (2003) Members of the Arabidopsis dynamin-likegene family, ADL1, are essential for plant cytokinesis and polarizedcell growth. Plant Cell 15, 899–913

27 Jahn, R. et al. (2003) Membrane fusion. Cell 112, 519–53328 Lauber, M.H. et al. (1997) The Arabidopsis KNOLLE protein is a

cytokinesis-specific syntaxin. J. Cell Biol. 139, 1485–149329 Heese, M. et al. (2001) Functional characterization of the KNOLLE-

interacting t-SNARE AtSNAP33 and its role in plant cytokinesis.J. Cell Biol. 155, 239–249

30 Uemura, T. et al. (2004) Systematic analysis of SNARE molecules inArabidopsis: dissection of the post-Golgi network in plant cells. CellStruct. Funct. 29, 49–65

31 Zheng, H. et al. (2002) NPSN11 is a cell plate-associated SNAREprotein that interacts with the syntaxin KNOLLE. Plant Physiol. 129,530–539

32 Assaad, F.F. et al. (2001) The cytokinesis gene KEULE encodes a Sec1protein that binds the syntaxin KNOLLE. J. Cell Biol. 152, 531–543

33 Waizenegger, I. et al. (2000) The Arabidopsis KNOLLE and KEULEgenes interact to promote vesicle fusion during cytokinesis. Curr. Biol.10, 1371–1374



34 Muller, I. et al. (2003) Syntaxin specificity of cytokinesis inArabidopsis. Nat. Cell Biol. 5, 531–534

35 Nishihama, R. and Machida, Y. (2001) Expansion of the phragmoplastduring plant cytokinesis: a MAPK pathway may MAP it out. Curr.Opin. Plant Biol. 4, 507–512

36 Strompen, G. et al. (2002) The Arabidopsis HINKEL gene encodes akinesin-related protein involved in cytokinesis and is expressed in acell cycle-dependent manner. Curr. Biol. 12, 153–158

37 Nishihama, R. et al. (2002) Expansion of the cell plate in plantcytokinesis requires a kinesin-like protein/MAPKKK complex. Cell109, 87–99

38 Nishihama, R. et al. (2001) The NPK1 mitogen-activated proteinkinase kinase kinase is a regulator of cell-plate formation in plantcytokinesis. Genes Dev. 15, 352–363

39 Soyano, T. et al. (2003) NQK1/NtMEK1 is a MAPKK that acts in theNPK1 MAPKKK-mediated MAPK cascade and is required for plantcytokinesis. Genes Dev. 17, 1055–1067

40 Geldner, N. et al. (2001) Auxin-transport inhibitors block PIN1 cyclingand vesicle trafficking. Nature 413, 425–428

41 Cutler, S.R. and Ehrhardt, D.W. (2002) Polarized cytokinesis invacuolate cells of Arabidopsis. Proc. Natl. Acad. Sci. U. S. A. 99,2812–2817

42 Sørensen, M.B. et al. (2002) Cellularisation in the endosperm ofArabidopsis thaliana is coupled to mitosis and shares multiplecomponents with cytokinesis. Development 129, 5567–5576

43 Otegui, M.S. and Staehelin, L.A. (2004) Electron tomographic analysisof post-meiotic cytokinesis during pollen development in Arabidopsisthaliana. Planta 218, 501–515

44 Yang, C.Y. et al. (2003) TETRASPORE encodes a kinesin required formale meiotic cytokinesis in Arabidopsis. Plant J. 34, 229–240

Getting animated

Interested in the molecular cell biology of host–parasite interactions

in Parasitology, one of our companion TRENDS journals. The pictur

revealing the latest advances in understanding

MicrosporidBy C. Franzen [(2004)

http://archiv

Interaction ofBy E. Handman and D.V

http://archi


45 Tanaka, H. et al. (2004) The AtNACK1/HINKEL and STUD/

TETRASPORE/AtNACK2 genes, which encode functionally redun-dant kinesins, are essential for cytokinesis inArabidopsis.Genes Cells

9, 1199–121146 Krysan, P.J. et al. (2002) An Arabidopsis mitogen-activated protein

kinase kinase kinase gene family encodes essential positive regulatorsof cytokinesis. Plant Cell 14, 1109–1120

47 Sawitzky, H. and Grolig, F. (1995) Phragmoplast of the green algaSpirogyra is functionally distinct from the higher plant phragmoplast.J. Cell Biol. 130, 1359–1371

48 Low, S.H. et al. (2003) Syntaxin 2 and endobrevin are required for theterminal step of cytokinesis in mammalian cells. Dev. Cell 4, 753–759

49 Camilleri, C. et al. (2002) The Arabidopsis TONNEAU2 gene encodesa putative novel protein phosphatase 2A regulatory subunit essentialfor the control of the cortical cytoskeleton. Plant Cell 14, 833–845

50 Smith, L.G. et al. (2001) Tangled1: a microtubule binding proteinrequired for the spatial control of cytokinesis in maize. J. Cell Biol.152, 231–236

51 Van Damme, D. et al. (2004) Molecular dissection of plant cytokinesisand phragmoplast structure: a survey of GFP-tagged proteins. PlantJ. 40, 386–398

52 Chan, J. et al. (2003) EB1 reveals mobile microtubule nucleation sitesin Arabidopsis. Nat. Cell Biol. 5, 967–971

53 Lee, Y.R. et al. (2001) A novel plant kinesin-related protein specificallyassociates with the phragmoplast organelles. Plant Cell 13,2427–2439

54 Jurgens, G. (2004) Cytokinesis in higher plants. Annu. Rev. PlantBiol. (in press)

with parasites!

? Then take a look at the online animations produced by Trends

es below are snapshots from two of our collection of animations

parasite life cycles. Check them out today!

ia: how can they invade other cells?Trends Parasitol. 20, 10.1016/j.pt.2004.04.009]e.bmn.com/supp/part/franzen.html

Leishmania with the host macrophage.R. Bullen [(2002) Trends Parasitol. 18, 332–334]ve.bmn.com/supp/part/swf012.html


plant cytokinesis: fission by fusion

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