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Cytokinesis-Defective Mutants of Arabidopsis
Rosi Söllner, Gerti Glässer, Gehard Wanner, Chris R. Somerville, Gerd Jürgens, and Farhah F. Assaad*
Genetics and Microbiology Institute, Ludwig Maximillian University, Maria Ward Strasse 1a, 80638 Munich,
Germany (R.S., G.G., F.F.A.); Botany Institute, Ludwig Maximillian University, Menzingerstrasse 67, 80638
Munich, Germany (G.W.); Carnegie Institution, Department of Plant Biology, 260 Panama Street, Stanford,
California 94305 (C.R.S.); and Lehrstuhl für Entwicklungsgenetik, Universität Tübingen, D–72076 Tübingen,
Germany (G.J.)
We have identified mutations in six previously uncharacterized genes of Arabidopsis, named club, bublina, massue, rod,
bloated, and bims, that are required for cytokinesis. The mutants are seedling lethal, have morphological abnormalities, and
are characterized by cell wall stubs, gapped walls, and multinucleate cells. In these and other respects, the new mutants are
phenotypically similar to knolle, keule, hinkel, and pleiade mutants. The mutants display a gradient of stomatal phenotypes,
correlating roughly with the severity of their cytokinesis defect. Similarly, the extent to which the different mutant lines were
capable of growing in tissue culture correlated well with the severity of the cytokinesis defect. Phenotypic analysis of the
novel and previously characterized loci indicated that the secondary consequences of a primary defect in cytokinesis include
anomalies in body organization, organ number, and cellular differentiation, as well as organ fusions and perturbations of
the nuclear cycle. Two of the 10 loci are required for both cytokinesis and root hair morphogenesis. The results have
implications for the identification of novel cytokinesis genes and highlight the mechanistic similarity between cytokinesis
and root hair morphogenesis, two processes that result in a rapid deposition of new cell walls via polarized secretion.
During the process of cell division, cells undergo
four key transitions: entry into S phase (DNA replication), entry into mitosis, exit from mitosis, and the
onset and execution of cytokinesis, the partitioning of
the cytoplasm after nuclear division. The key regulators of entry into S phase and mitosis appear
largely conserved among plant, yeast, and animal
cells (Assaad, 2001b). By contrast, the last phase of
the cell cycle seems different in plants as compared
with fungi and animals (Assaad, 2001b). Polo kinases
and septins and the CDC15 protein, which is required for mitotic exit and/or cytokinesis, are
present in all sequenced eukaryotes with the notable
exception of Arabidopsis (Song and Lee, 2001; Assaad, 2001b). Localization and phosphorylation studies, and the study of dominant negative mutants
have highlighted the importance of a mitogenactivated protein kinase cascade in the regulation of
plant cytokinesis (Bögre et al., 1999; Nishihama et al.,
2001).
Plant cytokinesis has many unique features and
can be considered as a form of polarized secretion
1
This work was supported by a European Union Biotechnology
Program Framework IV grant (to G.J.), by the U.S. Department of
Energy (grant no. DOE–FG02– 00ER20133 to C.R.S.), and by the
Deutsche Forschungsgemeinschaft (grant no. AS110/2–1 to
F.F.A.). F.F.A. was supported by a long-term European Molecular
Biology Organization fellowship and an Hochschulsonderprogram stipend from the University of Munich.
* Corresponding author; e-mail [email protected];
fax 650 –325– 6857.
Article, publication date, and citation information can be found
at www.plantphysiol.org/cgi/doi/10.1104/pp.004184.
(Assaad, 2001a). At the end of anaphase, Golgiderived secretory vesicles carrying cell wall materials
are transported to the equator of a dividing cell.
Fusion of these vesicles gives rise to a membranebound compartment, the cell plate. The cell plate
expands until it reaches the division site on the
mother cell wall (Ehrhardt and Cutler, 2002). Once
this attachment has taken place, the cell plate undergoes a complex process of maturation during which
callose is replaced by cellulose and pectin (Samuels et
al., 1995, and refs. therein). Two plant-specific cytoskeletal arrays of microtubules and actin filaments,
the preprophase band and the phragmoplast, play
central roles in the orientation and expansion of the
cell plate and in the execution of cytokinesis (for
review, see Otegui and Staehelin, 2000; Sylvester,
2000; Assaad, 2001a). It follows from this brief description that genes implicated in membrane and
cytoskeletal dynamics, vesicle trafficking, and cell
wall biogenesis will impact plant cytokinesis.
Relatively few genes involved in cytokinesis have
been identified by mutation in plants. tso1, stud, tardy
asynchronous meiosis, tetraspore, and sidecar pollen specifically affect cytokinesis in floral organs or during
pollen development (Chen and McCormick, 1996;
Hülskamp et al., 1997; Spielman et al., 1997; Hauser
et al., 2000; Song et al., 2000; Magnard et al., 2001).
Genes required for cytokinesis in somatic plant cells
seem to be partially distinct from those required
during gametophytic development (Lauber et al.,
1997; Otegui and Staehelin, 2000; Assaad et al., 2001)
and fall into two classes. Genes in the first class are
required for the proper orientation of the plane of
division, and genes in the second class are required
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Söllner et al.
for the execution of cytokinesis. fas/tonneau, discordia,
and tangled (for review, see Sylvester, 2000; Smith,
2001) mutants are implicated in regulating the plane
of division. The cyd mutants of pea (Pisum sativum)
and Arabidopsis, and the KNOLLE, KEULE, and
HINKEL genes of Arabidopsis are required for the
execution of cytokinesis (Liu et al., 1995; Assaad et
al., 1996; Lukowitz et al., 1996; Yang et al., 1999;
Strompen et al., 2002). We focus here on the second
class of mutants. Light and electron microscopy
showed that dividing cells in cyd, cyd1, knolle, and
keule mutants are often multinucleate, with gapped
or incomplete cross walls, which defines these mutants as cytokinesis-defective (Liu et al., 1995; Assaad
et al., 1996; Lukowitz et al., 1996; Yang et al., 1999).
The multinucleate cells are invariably enlarged (Assaad et al., 1996) and account for the rough surface
and bloated appearance of these cytokinesis mutants.
The KNOLLE, HINKEL, and KEULE genes have been
cloned. HINKEL encodes a plant-specific kinesinrelated protein required for the reorganization of
phragmoplast microtubules during cell-plate formation (Strompen et al., 2002). KNOLLE encodes a novel
cytokinesis-specific syntaxin (Lukowitz et al., 1996;
Lauber et al., 1997). Thus, HINKEL and KNOLLE
represent two distinct subclasses of cytokinesisspecific mutants, affecting cytoskeletal and membrane dynamics, respectively. KEULE encodes a key
regulator of vesicle trafficking, a Sec1 protein, that
interacts genetically and biochemically with the syntaxin KNOLLE (Waizenegger et al., 2000; Assaad et
al., 2001). In both keule and knolle mutants, vesicles
are transported to the equator of a dividing cell but
do not fuse (Waizenegger et al., 2000). A biochemical
and reverse genetic approach has implicated an additional vesicle trafficking gene, SNAP33, in plant
cytokinesis (Heese et al., 2001).
Cell wall stubs and radial swelling are characteristic of cell wall mutants such as korrigan, procuste,
rsw1/rms, and cyt1 (Nickle and Meinke, 1998; Fagard
et al., 2000; Zuo et al., 2000; Lukowitz et al., 2001).
Cytokinesis defects have also been described in
sterol-defective fackel mutants (Schrick et al., 2000)
and in titan or pilz mutants, characterized by the
incidence of giant nuclei (Liu and Meinke, 1998;
Mayer et al., 1999). Because mutations affecting cytokinesis, nuclear content, sterol biosynthesis, and
cell wall biogenesis share a number of common phenotypic features, an important question is to what
extent one can distinguish between mutations affecting these diverse pathways based on phenotypic
analysis. We monitor embryo, seedling, stomatal,
root hair, trichome, and postembryonic development
in the novel as well as previously characterized
cytokinesis-defective mutants and address the following questions: By what criteria do we define a
mutant as being predominantly cytokinesis defective? Are cytokinesis-defective mutants impaired in
cellular processes other than cytokinesis? What are
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the developmental consequences of a cytokinesis defect? And what phenotypes may be used in attempting to isolate novel cytokinesis-defective loci?
RESULTS AND DISCUSSION
Isolation and Initial Characterization of the Novel
Cytokinesis-Defective Mutants
A number of novel cytokinesis-defective mutants
were uncovered in a large-scale screen for mutations
affecting seedling body organization (Mayer et al.,
1991, 1999; Nacry et al., 2000). Three of the loci identified in this screen, KNOLLE, KEULE, and HINKEL,
have since been characterized in detail (Assaad et al.,
1996, 2001; Lukowitz et al., 1996; Lauber et al., 1997;
Strompen et al., 2002). The collection of cytokinesisdefective mutants includes four knolle alleles, 16 keule
alleles, one hinkel allele, and one pleiade allele (see
“Materials and Methods”). pleiade mutants were
identified in a screen for mutations affecting root
morphogenesis (M.T. Hauser, personal communication; see also Hauser et al., 1997). In addition to the
above-mentioned mutants, six mutant lines were isolated that resembled knolle mutants, and seven mutant lines were isolated that resembled keule mutants
but were not in the KEULE, KNOLLE, HINKEL, or
PLEIADE complementation groups (U. Mayer and
F.F. Aassad, unpublished data). Although keule and
knolle mutants are indistinguishable at a cellular
level, cytokinesis defects are more severe in knolle
mutants (see Table II). As a result, knolle mutants are
tuber-like and keule mutants club-like in shape,
whence their names. The six knolle-like lines have
been described elsewhere (Nacry et al., 2000). In this
study, we focus on the seven keule-like lines and
compare them with cytokinesis-specific mutants such
as hinkel and knolle identified in this screen. Data on
keule, knolle, hinkel, or pleiade are presented where
unpublished or as reference points.
Complementation analysis together with mapping
showed that the seven keule-like mutant lines represent six novel loci (Table I; “Materials and MethTable I. Six novel cytokinesis-defective loci
Mapping was carried out with PCR-based molecular markers, with
sufficient resolution to map adjacent loci to distinct genetic intervals.
Complementation tests were then performed between each of the
uncharacterized mutants and any nearby keule-like mutants. The six
loci are distributed on four of the five Arabidopsis chromosomes,
with a cluster, including KNOLLE, KEULE, and HINKEL on the upper
arm of chromosome I. See “Materials and Methods” for further
details. * Corresponds to a rough map location.
Locus
Lines (Allele)
Map Location
CLUB
BUBLINA
MASSUE
ROD
BLOATED
BIMS
U57 (1)
R3-27 (1)
G88 (1), MNN (2)
S100 (1)
U119 (1)
S302 (1)
V/109
I/29
II/66
IV/⬃65*
IV/92
I/12
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Plant Physiol. Vol. 129, 2002
Cytokinesis-Defective Mutants of Arabidopsis
ods”). We have named these loci club, bublina, massue,
rod, bloated, and bims, based on the bloated and rodor club-shaped appearance of mutant seedlings (Fig.
1). All of the keule-like lines depicted in Figure 1 lack
functional shoot and root meristems and carry recessive, seedling-lethal mutations. Nonetheless, clearing
preparations show that the apical-basal and radial
organization of tissue layers is generally conserved in
mutant seedlings. This is illustrated by the massue-2
mutant in Figure 1I. The vascular strands often show
interruptions, misalignment and deviation from the
wild type with respect to the number and width of
strands per bundle, as illustrated by the bims mutant
in Figure 1J. In some instances, short vascular strands
are found adjacent to the centrally located vascular
bundles (see bloated mutant in Fig. 1K). Thus, whereas
the overall organization and differentiation of tissue
layers is normal, patches of cells acquire the fate of an
adjacent layer or cell row, as has been described for
keule and knolle (Assaad et al., 1996; Lukowitz et al.,
1996). The seedling phenotypes were variable within
most lines, ranging from stout seedlings with reduced cotyledons as shown for club, bublina, and bims
(Fig. 1, B, C, and G) to seedlings with well-defined
cotyledons and a stunted root as shown for massue-2,
rod, and bloated (Fig. 1, D–F).
Cytokinesis versus Cell Wall Defects
Figure 1. Bloated seedlings lacking functional root and shoot meristems. All seedlings have a rough surface layer and consist of
cotyledons and a hypocotyl. Though the basal area has root-like
properties, roots are absent in keule, club, bublina, and bims mutants
(A, B, C, and G); stunted in massue and bloated mutants (D and F);
and reduced in rod mutants (E). At the shoot apex, true leaves are
absent or stunted, though these may develop upon transfer to tissue
culture (see Fig. 5, F and G). The seedling phenotype is variable in all
of the lines, with the exception of club and bims mutants for which
the range of phenotypes is fairly narrow. The seedlings shown represent the median within the respective ranges of phenotypes, although massue mutants (D) often have considerably stronger and
bloated mutants (F) considerably weaker phenotypes than those
shown here. A clearing preparation shows that the apical basal body
plan and the radial organization of tissue layers are generally conserved (I). The pattern elements of the seedling are shown in I and/or
H; along the apical-basal axis, these include the cotyledons, hypocotyl, and root; and along the radial axis they include the epidermis,
ground tissue, and vascular bundle. The vascular strands often show
interruptions, misalignment, and deviation from the wild type with
respect to the number and width of strands per bundle (arrow in J). In
some instances, short vascular strands are found adjacent to the
centrally located vascular bundles (arrow in K). Note the long root
Plant Physiol. Vol. 129, 2002
To determine whether the keule-like mutants were
cytokinesis defective, we examined histochemically
stained sections of mutant embryos and seedlings by
light microscopy. Cytokinesis-defective mutants are
typically characterized by the presence of cell wall
stubs in dividing cells. Therefore, we sectioned dividing tissues from globular to heart stage embryos
and/or the apical meristems of seedlings. Cell wall
stubs, gapped cell walls, and multinucleate cells were
found in all of the keule-like mutant lines in these
tissues (Fig. 2). The stubs and incomplete walls
stained with the cell wall-specific periodic acid schiff
stain, as shown in Figure 2G.
In the case of keule and knolle, cytokinesis defects
have been shown to arise as of the first division of the
zygote (Assaad et al., 1996; Lukowitz et al., 1996). An
analysis of clearing preparations and histological sections of dermatogen-torpedo stage embryos (for a
description of Arabidopsis embryogenesis, see Jürgens and Mayer, 1994) showed that cytokinesis defects occur before the two-cell stage in hinkel and
massue embryos, before the dermatogen stage in
pleiade, and before the globular stage in bublina and
hairs at the basal end of the massue-1 mutant in I. The mutant lines
shown are as indicated above each panel, and massue-1 is shown in
I, bims is shown in J, and bloated is shown in K. keule allele MM125
is shown as reference in A. a, Apical meristem, first true leaf primordia; c, cotyledons; e, epidermis; g, ground tissue; h, hypocotyl; rh,
root hairs; r, root; and v, vascular bundle. Bars ⫽ 200 ␮m.
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Söllner et al.
bloated embryos (Fig. 2B; data not shown). By contrast, cytokinesis defects were rarer and appeared to
occur later in bims embryos (Fig. 2K), which have a
relatively narrow range of phenotypes. In club mutants, early divisions of the true leaf primordia and
apical meristem are affected (Fig. 2, C and D). The
first appearance of cytokinesis defects during development seems to be stochastically determined and
may contribute to the great variability of embryo and
seedling phenotypes characteristic of cytokinesisdefective mutants.
Cell wall-defective mutants such as procuste, knopf,
rsw1/rms, and korrigan share two features in common
with cytokinesis-defective mutants such as keule: cell
wall stubs and radial expansion of affected tissues
such as the root (Fagard et al., 2000; Peng et al., 2000;
Zuo et al., 2000; Boisson et al., 2001; Gillmor et al.,
2002). In cytokinesis-defective mutants, stubs are of-
Figure 2. Histological sections of embryos or seedlings revealing cell wall stubs and multinucleate cells. Histological
sections were stained with toluidine blue, which stains the nuclei and cell wall, with the exception of that in G, which is
stained with the cell wall-specific periodic acid schiff stain. Toluidine blue-stained nuclei appear as vacuolate structures
with dark, round- or doughnut-shaped nucleoli. A, B, E, G, I, J, K, and L are embryo sections; C, D, F, and H are seedling
sections. In C and D, an apical meristem is shown. B, This section depicts the basal area of a torpedo stage pleaide embryo,
representing a severely affected hypocotyl and root primordium; in the same mutant, the cotyledons appear unaffected (not
shown); the multinucleate cell shown encompasses more than one-half of the mutant embryo, as can be seen by comparison
with the small outer cells in the same section, which suggests that the cytokinesis defects occurred early during embryogenesis; note the large nuclei with multiple nucleoli (black spots designated by arrowhead). F, Severely affected surface layer
of a bublina cotyledon. H, Cotyledon cells adjacent to a true leaf primordium show the difference in cell size between
expanded (lower cell with cytokinesis defects) and meristematic (upper left) cells in a rod seedling. Arrows point to cell wall
stubs, small white arrow heads to multinucleate cells; larger, white arrow heads point to a cell wall gap in H and to a
metaphase plate in J. In some instances, there are two juxtaposed wall stubs on either side of the cell (see black arrowhead
in J), but for the most part, the cell wall stubs are anchored to the mother cell wall on one side of the cell, with no stub on
the other side (see arrows in D, E, G, H, and K as well as text). Bars are 50 ␮m in A, C, F, I, and L; 20 ␮m in B, E, G, J, and
K; and 10 ␮m in D. H and K are the same scale.
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Plant Physiol. Vol. 129, 2002
Cytokinesis-Defective Mutants of Arabidopsis
ten asymmetric, attached to the parent cell wall on
one side of the cell, with no stub on the opposing side
(Figs. 2 and 3B; Assaad et al., 1996). A fourdimensional analysis of cytokinesis shows that the
cell plate often anchors on one side of the cell at an
early stage of cytokinesis, then grows across the cell
in a highly polarized fashion (Ehrhardt and Cutler,
2002). This provides a simple explanation for cell
wall stubs as resulting from normal polar cytokinesis
caught at early stages (Ehrhardt and Cutler, 2002).
Cell wall gaps, consisting of two opposing stubs, as
seen in procuste (Fagard et al., 2000), could arise upon
cell expansion as the disruption of a mechanically
compromised cell wall. Mechanically compromised
cell walls are, in fact, the expected outcome of mutations at the CYT1, KORRIGAN, and KNOPF loci,
which result in considerably reduced cellulose levels
(Lukowitz et al., 2000; Boisson et al., 2001; His et al.,
2001; Gillmor et al., 2002). Incomplete cell walls are
observed in both vacuolated/expanded and nonvacuolated cells in the collection of keule-like lines
presented here, and in cyd1 mutants (Figs. 2 and 3;
Yang et al., 1999). By contrast, incomplete walls are
observed later in development and only in vacuolated cells in cell wall mutants (Nickle and Meinke,
1998; Fagard et al., 2000; Zuo et al., 2000). We conclude that, to distinguish between cytokinesis and
cell wall-defective mutants, it is helpful to monitor
populations of rapidly dividing cells, as found in
early embryos or in meristems.
Nuclear Defects in Cytokinesis-Defective
Mutants and in titan and pilz Mutants
In histological sections of embryos, interphase nuclei appear as vacuolate structures with darkly staining, round or doughnut-shaped nucleoli (Fig. 3B).
Light and electron microscopy of mutant embryos
revealed enlarged nuclei, often containing multiple
nucleoli (as illustrated in Figs. 2B and 3A). This has
been observed in all the novel mutant lines, as well as
in knolle, keule, and pleiade mutants (see Fig. 2, A, B, J,
and K; Assaad et al., 1996). Mitotic figures are also
more frequent in the mutant lines than in the wild
type, as has been reported for keule (Fig. 2J; Assaad et
al., 1996). The nuclear phenotypes observed in
cytokinesis-defective mutants, large nuclei with multiple nucleoli, are distinct from the giant nuclei observed in titan/pilz mutants (Liu and Meinke, 1998;
Mayer et al., 1999) and are, in all likelihood, a secondary consequence of a primary defect in cytokinesis. Presumably as a result of increased DNA content,
the mutant seedlings have larger cells than the wildtype seedlings, and there are fewer cells per file in the
cotyledons and hypocotyl (compare bublina and
wild-type seedlings in Fig. 3, C and D). The number
of cell files, however, appears roughly conserved.
It is well known that disrupting the nuclear cycle
blocks cytokinesis, and titan/pilz mutants could most
Plant Physiol. Vol. 129, 2002
Figure 3. Cytokinesis-defective mutants have enlarged nuclei with
multiple nucleoli, and enlarged cells. A and B are electron micrographs of embryos and C and D are histological seedling sections. A,
Nucleus with two nucleoli (n), resembling a nuclear fusion, in a
keule embryo. B, Wild-type nuclei (N) with single nucleoli (n) in a
wild-type embryo. Note the difference in size between the nuclei in
A and B, shown at the same scale. C, bublina seedling. D, Wild-type
seedling, showing hypocotyl and apical meristem with the true leaf
primordia; the cotyledons and root are not included due to the
slightly tangential angle of the section. The bublina seedling shown
in B has fewer cells per file than the wild type, but a roughly
conserved number of cell files. The cells are larger than in the wild
type (compare C and D, shown at the same scale). The arrowhead
points to an irregularly shaped bloated surface cell, and the arrow to
a cell wall stub. Bar is 5 ␮m in A for A and B and 100 ␮m in D for
C and D.
simply be classified as having a perturbed nuclear
cycle resulting in aborted cytokinesis (Mayer et al.,
1999; McElver et al., 2000; Nacry et al., 2000; Nigg,
2001). It is less clear, however, what role aborted cytokinesis has on the nuclear cycle (Balasubramanian
et al., 2000). The incidence in cytokinesis-defective
mutants of multiple nuclei within a single cell shows
that new nuclear cycles are initiated even if cytokinesis is incomplete. Whereas multinucleate cells are
found early on in development, mature cells often
have single, enlarged nuclei with high DNA content
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Söllner et al.
(Assaad et al., 1996). It is not clear whether these
enlarged nuclei arise by endoreduplication or by the
fusion of multiple nuclei present in the same cell.
Cytokinesis-Defective versus Sterol-Deficient or
Membrane Mutants
There is considerable overlap between the ranges
of phenotypes observed in the sterol-deficient fackel
mutants and in those seen here. keule mutants are
typically club-like in shape, with two reduced cotyledons and a bloated hypocotyl. Strong alleles of
keule segregate rod, oval, or ball-shaped seedlings.
fackel seedlings are stout, often lacking a hypocotyl,
and have a highly disorganized apex with supernumerary apices and cotyledons. Histological sections
of fackel mutants reveal cytokinesis defects, namely,
the presence of cell wall stubs and multinucleate cells
in dividing embryonic cells (Schrick et al., 2000).
Cytokinesis-defective mutant lines conversely segregate a small percentage of fackel-like seedlings, which
may have twinned as opposed to single vascular
bundles (Table II; Fig. 4, E and F), as described for
fackel (Jang et al., 2000; Schrick et al., 2000). In addition to these defects, the keule-like lines segregate a
small percentage of cup-shaped, pin-shaped, or fused
cotyledons, as well as mono- or tricotyledonous seedlings (Table II; Fig. 4, A–D), as described in fackel
(Jang et al., 2000; Schrick et al., 2000).
Whereas 2% to 9% of cytokinesis-defective mutant
seedlings have a grossly aberrant body organization
(Table II), 73% to 100% of fackel seedlings are severely
perturbed in their body organization, depending on
allele strengths. Thus, mutations at the FACKEL locus
have a strong effect on body organization but a weak
effect on cytokinesis (see also Schrick et al., 2000).
The cytokinesis mutants described above conversely
have a strong effect on cytokinesis without grossly
affecting the organization of the body plan.
Sterols determine the fluidity and permeability of
plant membranes. By altering the sterol metabolism
of the plant cell, mutations at the FACKEL locus may
impact membrane dynamics, and, thereby, have an
effect on cytokinesis in much the same way as mutations at the knolle and keule loci. Sterols are also the
precursors of steroid hormones such as brassinosteroids. Thus, it is not surprising that mutations at
the FACKEL locus are more pleiotropic in their effects
than the cytokinesis defectives. We conclude that the
difference between fackel mutants and cytokinesis defectives can be recognized by virtue of the differential
effect of such mutations on body organization as
compared with cytokinesis.
Defining Cytokinesis-Defective Mutants
Cytokinesis-defective lines have a number of additional phenotypes such as organ fusions, anomalies
in cellular differentiation, anomalies in organ num6 of 13
ber and/or in body organization, and perturbations
of the nuclear cycle. We infer that most of these gross
phenotypes are secondary consequences (direct or
indirect) of the cytokinesis defect. We have provisionally defined mutants as being cytokinesis defective if they have cell wall stubs, gapped walls, and
multiple nuclei in dividing as opposed to expanding
cells. In contrast to cell wall-defective mutants, these
defects tend to occur early during embryo development, as of the first division of the zygote. Furthermore, the body plan is in general conserved.
By these criteria, all of the keule-like loci described
here seem to be predominantly defective in cytokinesis, with the possible exception of BIMS. BIMS
mutants have four features in common with fackel
mutants: a reduced hypocotyl, the production of
stout and reduced rosette leaves in tissue culture, a
weak cytokinesis defect that may occur later in development, and a severely reduced etiolation response (F.F. Assaad, unpublished data; Schrick et al.,
2000; Assaad et al., 2001). Yet bims mutants do not
have the gross aberrations in body organization that
characterize fackel mutants. It may be useful to examine whether mutations at the BIMS locus affect membrane dynamics, as in fackel mutants, yet without
impacting the biosynthesis of crucial signaling and
growth molecules such as brassinosteroids.
Postembryonic Processes in the
Cytokinesis-Defective Lines
We monitored two postembryonic processes particularly sensitive to perturbations in the execution of
cytokinesis: stomatal development and the ability of
organ primordia to develop in tissue culture. Stomatal development requires a series of well-defined
divisions. Asymmetric divisions in the epidermis define the guard mother cell, which divides symmetrically to form a ventral wall. Thereafter, a pore (Fig.
5D) is formed by a separation of the two guard cells
at the middle lamella, and through extensive and
localized remodeling of the ventral wall (Zhao and
Sack, 1999).
As judged by light and confocal microscopy, the
cytokinesis-defective lines displayed a gradient of
stomatal phenotypes, correlating roughly with the
severity of their cytokinesis defect. The epidermis of
knolle seedlings was so severely perturbed that we
could not recognize stomatal precursors. In keule mutants, guard mother cells could be recognized, but
they fail to form a ventral wall (Fig. 5C). In massue
and bublina mutants, some stomata resemble those
seen in keule, in others ventral wall formation was
initiated but incomplete (data not shown). Incomplete ventral wall formation was also observed in
bims mutants (data not shown). In bublina and pleiade
mutants, some stomata had pores attached to a single
side of the mother cell, suggesting that ventral wall
formation might have been nearly but not fully com-
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Plant Physiol. Vol. 129, 2002
Cytokinesis-Defective Mutants of Arabidopsis
Table II. A comparison of 10 cytokinesis-defective loci
Strength of cytokinesis
defecta
knolle
pleiade
⫹⫹⫹⫹⫹⫹
⫹⫹⫹⫹⫹⫹
Root
keule
⫹⫹⫹⫹⫹
hinkel
club
⫹⫹⫹⫹
⫹⫹⫹⫹
bublina
⫹⫹⫹
massue
⫹⫹⫹
⫹⫹
Shoota
Extent of growth in
cultureb
None
None
Root
⫹
Root
bloated
bims
⫹⫹
⫹
⫹⫹⫹⫹⫹
⫹⫹⫹⫹
Stout
leaves
as in
fackel
⫹⫹
Shoota
None
None
None
⫹
⫹⫹
⫹⫹⫹
Shoot
Stomata
Guard mother cellc
Ventral walld
Pore formatione
rod
⫹⫹⫹⫹⫹
Root
⫹⫹⫹⫹
Shoot
⫺
⫹
⫹, i
⫺/⫹
⫹
⫺
⫹
⫹
⫹
⫹
⫹
⫹
⫺/⫹, i
⫺/⫹
⫹
⫺/⫹, i
⫺/⫹
⫹
⫹
⫺/⫹
⫹
⫺/⫹
⫺/⫹
⫹
⫹, i
⫺/⫹
Long root hairs
Trichomesf
Yes
nd
Yes
Yes
No
nd
Yes
Yes
No
nd
Yes
Yes
Yes
Yes
Yes
Yes
Yes
Yes
Yes
Yes
Segregation ratio
% Mutantg
25%
25%
20%–24%h
20%
20%
15%
24%
16%
20%
19%
3%
4%
3%
0%
4%–9%g
22%–23%
4%
5%
0%
1%
6%
2%
2%–7%
4%
0%
2%
3%
3%
3%
0%
4%
145
5%
81
0
112
12%
105
5%
79
4%
51
1%
250
6%
122
31%
120
5%
109
Body organization
% Fackligi
% Monocot or fused
cotyledons
% Tricot
Total no. of mutants
a
The strength of the cytokinesis defect was measured by the frequency with which cell wall stubs and multinucleate cells occurred in
histological sections and by the size of multinucleate cells. For pleiade, strong cytokinesis defect and no growth in culture on hypocotyl and root,
b
weak cytokinesis defect and good growth on RIM for shoot (as described by Hauser et al. [1997]); converse for rod.
Ability to produce true
leaves on root-inducing medium. Lines such as bublina and massue, which died on RIM after elaborating their first true leaves, were nonetheless
c
d
able to develop bloated, reduced leaves on SIM.
Presence (⫹) or absence (⫺) of guard mother cells.
Ventral wall formation (⫹); i,
e
incomplete wall formation; ⫺/⫹, some guard mother cells form ventral wall, others not.
Presence (⫹) or absence (⫺) of well defined
f
stomatal pores.
Trichomes on true leaves in seedlings grown on root-inducing (or shoot-inducing) medium. Some have abnormal branching
g
patterns. nd, Not determined, because no true leaves developed.
These percentages do not take aborted embryos/seeds into account, even
though the cytokinesis defects could cause a small percentage of severe embryo lethality. Non-germinated seedlings were dissected out of their
seed coats for phenotyping. Percentages based on at least 100 seedlings total. The reduced segregation ratios in bublina, rod, and bims mutants
h
i
may be indicative of a gametophytic effect.
Strong alleles versus weak alleles, or vice versa.
fackel-like in appearance, abnormal body
organization.
plete, as has been shown in cyd1 mutants (Yang et al.,
1999). Nonetheless, club, hinkel, bublina, pleiade, rod,
bims, and bloated seedlings were capable of forming
stomata with well-developed pores, attached to the
mother cell by the ventral wall at both ends (e.g. Fig.
5, A and B). The gradient of stomatal phenotypes
described here has been well documented in cyd1
mutants (Yang et al., 1999).
Some lines (bloated, rod, hinkel, massue, bims, and
bublina) showed variable degrees of growth in tissue
culture (Table II; Fig. 5, F and G), whereas others
including keule, knolle, and club could only be propagated as calli (Table II; Fig. 5E). On hormonesupplemented tissue culture media, some mutant
seedlings produced true leaves, which were often
bloated and irregular in appearance (e.g. Fig. 5G), or
had irregular margins with more pronounced dentation as seen in cyd1 (Yang et al., 1999). In all instances,
Plant Physiol. Vol. 129, 2002
the shoots that developed were abnormal. The results
suggest that the KEULE-like loci are required not
only during embryogenesis, but for cytokinesis in
somatic cells throughout the vegetative life cycle.
Whether or not the novel loci are required for cytokinesis in reproductive organs or during gametophytic development remains to be determined. In
general, the ability to grow in tissue culture appeared
to be correlated with the strength of the cytokinesis
defect. Thus, lines that had severe cytokinesis defects
were incapable of growth in tissue culture and failed
to develop normal stomata.
Tip Growth Processes in Cytokinesis-Defective Mutants
We considered the possibility that, in addition to
their role in cytokinesis, the keule-like loci might be
perturbed in other functions requiring extensive ves-
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Söllner et al.
Figure 4. Abnormal body organization in keulelike mutants. A, Pin-shaped cotyledons. B, Cupshaped cotyledons. C, Monocotyledonous seedling. D, Tricotyledonous seedling. E, keule
mutant that looks like fackel. F, Clearing preparation of keule mutant with hypocotyl deletion,
with two vascular bundles (v) emerging from the
basal area, as in fackel mutants. G, fackel mutant with a hypocotyl, somewhat like keule in
appearance. H, Classical fackel mutant, showing central deletion and abnormal body organization. Bars ⫽ 200 ␮m.
icle traffic. We investigated root hair growth and
trichome morphogenesis whenever true leaves developed upon transfer to tissue culture medium. These
are tip growth processes that, like cytokinesis, result
in a rapid deposition of new cell walls via polarized
secretion. An analysis of the root hairs in mutant
seedlings shows that wherever the basal part of the
seedling is strongly affected, as occurs in knolle,
hinkel, bublina, and pleaide mutants, the root hairs are
often multinucleate, swollen and even branched (Fig.
6E). The variability of the root hair phenotype is
evident in a comparison of two bublina seedlings (Fig.
6, E and F). These defects have not been observed in
fackel mutants, and may be indirect consequences of
the nuclear and cell differentiation defects seen in the
mutants (see above), with a variable environmental
component related to germination and humidity. Although the majority of the lines, including knolle,
hinkel, pleiade, bims, and bublina are capable of growing long root hairs (Figs. 1I and 6, A and C and F–I),
keule and club mutants appear incapable of doing so.
Thus, unless these mutants are kept for weeks on
tissue culture medium, their root hairs are invariably
stunted, radially swollen, and branched (Fig. 6, D
and E). The defects seen in keule and club mutants,
including bulbous bases and a crooked appearance,
are typical of those seen in root hair mutants defective in tip growth, such as cow1, cen1, and cen3 (Fig.
6, J and K; Parker et al., 2000), and are, therefore,
likely to represent a tip growth defect.
A number of mutants affected in root hair morphogenesis also affect trichome morphogenesis. We were
Figure 5. A gradient of stomatal and postembryonic phenotypes. A through D, surface cells or
seedling cotyledons. A and B, Histological sections. C and D, Scanning electron micrographs.
Despite their heavily perturbed surface layer (A),
club mutants are capable of complete ventral
wall formation and form stomata with welldefined pores (B). B depicts the stomatal complex in A. By contrast, guard mother cells in
keule mutants are incapable of ventral wall formation (C). E and F, Growth in tissue culture. E,
keule explant on shoot-inducing medium (SIM),
showing callus-like outgrowth. F, A bublina
seedling transferred to SIM develops some true
leaves, which show signs of being cytokinesis
defective. G, rod seedling transferred to rootinducing medium shows near wild-type development, yet the leaves are irregular in shape and
margins and have bloated cells. The trichomes
(arrow head) have abnormal branching patterns.
H, wild-type explant on SIM. Bar in A is 50 ␮m.
8 of 13
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Plant Physiol. Vol. 129, 2002
Cytokinesis-Defective Mutants of Arabidopsis
Figure 6. Root hair defects in keule-like mutants. Long root hairs are seen in knolle, hinkel,
bublina, bloated, and bims mutants (A, C, F, G,
H, and I). C and H, Long root hairs on one side
of a hinkel or bloated mutant, stunted ones on
the other side in contact with the agar on the
plate. F, Long root hairs in bublina mutants
(compare with E). E, Binucleate, bloated root
hair in bublina seedling. B and D, stunted,
bloated, crooked, and/or branched root hairs in
keule and club mutants. Note the bulbous bases
and crooked root hairs in club mutants, which
resemble the defects seen in root hair mutants
affected in tip growth, such as cen3 (J and K). n,
Nucleus. Root hairs in rod mutants are often
somewhat shorter than in the wild type (G), yet
rod mutants are capable of growing long root
hairs. Arrows designate branched root hair in B,
bulbous bases in J, and crooked hairs in K. Bars
are 100 ␮m. With the exception of E, all panels
are the same scale as in B.
unable to monitor trichome morphogenesis in keule
and club mutants because these do not develop true
leaves upon transfer to tissue-culture medium. In the
majority of the other lines, which did develop true
leaves, we noticed that the leaves developed
trichomes but with abnormal branching patterns (Table II). Thus, whereas a wild-type trichome has three
branches, rod and bloated trichomes often had one to
five branches, as has been described in cyd1 mutants
(Yang et al., 1999). These branching defects are likely
to be a secondary consequence of the cytokinesis
defect.
Absent, stunted, radially swollen, and branched
root hairs are invariably seen in keule and club muPlant Physiol. Vol. 129, 2002
tants, yet all of the other cytokinesis-defective mutants are capable of growing long root hairs (Table II;
Fig. 6). We conclude that the root hair defect is not a
secondary consequence of the cytokinesis defect. A
detailed analysis of keule mutants at the electron microscope level has not shown morphological defects
in any of the endomembrane systems such as the
endoplasmic reticulum or Golgi, nor does secretion
seem to be impaired (Assaad et al., 1996). Also, pollen tubes, which require tip growth, appear unaffected in KEULE mutants (Assaad et al., 1996).
Similarly, club mutants do not appear to affect
gametophytic development (Table II). Thus, it seems
that KEULE and CLUB are not pleiotropic in their
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9 of 13
Söllner et al.
effects, but are required specifically for two distinct
yet related processes.
Both cytokinesis and root hair morphogenesis involve the rapid deposition of new walls via polarized
secretion (Miller et al., 1997; Assaad, 2001a; Ryan et
al., 2001). In both instances, vesicle trafficking is
tightly regulated, with respect to cell cycle cues in the
case of cytokinesis and of developmental, hormonal,
and environmental signals in the case of root hair
morphogenesis. In the average cell, new cell walls are
laid down in roughly 50 min (Ehrhardt and Cutler,
2002), and root hairs grow at a rate of 100 ␮m h⫺1
(Schiefelbein et al., 1992). Thus, during both cytokinesis and root hair morphogenesis, a very large
amount of cell wall material and new membrane
must be brought to a specific site in a short amount of
time. KEULE encodes a key regulator of vesicle trafficking and has been shown to affect vesicle fusion at
the cell plate (Waizenegger et al., 2000; Assaad et al.,
2001). In light of the phenotypic similarity between
KEULE and CLUB, it will be interesting to see if
CLUB also plays a role in polarized secretion during
cytokinesis and root hair morphogenesis.
Identifying New Cytokinesis Mutants of Arabidopsis
Our mechanistic understanding of plant cytokinesis has been limited by the paucity of genes that have
been implicated in this process. It is apparent from
the fact that we have only single alleles for five of the
keule-like loci that saturation mutagenesis has not
been achieved, and further searches for keule-like
mutants may well uncover additional loci. Given the
complexity of cytokinesis in plants and the large and
ever increasing number of genes implicated in polarized secretion in yeast and animal cells, one would
expect at least 100 genes to be implicated in this
process in the plant cell. Yet, we estimate by extrapolation from the available mutant collections (see
also Nacry et al., 2000) that the number of loci mutating to knolle or keule-like phenotypes will not exceed 20 to 30 loci in total. Based on the severe embryo
lethality of knolle keule double mutants (as opposed to
the seedling lethality of keule and knolle; Waizenegger
et al., 2000), it has been assumed that cytokinesis
genes might mutate to phenotypes more severe than
observed in either keule or knolle. Yet few cytokinesis
genes have been shown to mutate to embryo lethality
(Nacry et al., 2000). The possibility that a subset of
the genes required for cytokinesis in somatic plant
cells might mutate to gametophytic lethality remains
to be explored. The novel mutant loci described here
all have phenotypes weaker than those seen in keule,
and reverse genetic analyses as well as analysis of the
cyd1 mutant suggest that cytokinesis-defective loci of
Arabidopsis can be viable and have subtle phenotypes (Yang et al., 1999; Heese et al., 2001). Taken
together, these considerations point to the importance of looking not for more severe embryo-lethal
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phenotypes, but for more subtle cytokinesis defects
in viable mutants. Viable mutants will be easier to
isolate, but more difficult to recognize as cytokinesis
defective. In this respect, it is noteworthy that, upon
propagation in tissue culture, keule-like mutants
share a number of features with the viable cyd1 mutant, including leaves with irregular margins, organ
fusions, abnormal trichome branching patterns,
anomalies in organ number, and stomata in which
ventral wall formation is not always complete (Yang
et al., 1999). These common features may facilitate
the identification of novel cytokinesis-defective
mutants.
CONCLUSION
We have provisionally defined mutants as being
cytokinesis defective if they have cell wall stubs,
gapped walls, and multiple nuclei in dividing as
opposed to expanding cells. These defects tend to
occur early during embryo development and the
body plan is in general conserved. In addition,
cytokinesis-defective mutants are characterized by
the incidence of enlarged nuclei with multiple nucleoli and enlarged cells.
A phenotypic analysis of the novel and previously
characterized cytokinesis-defective mutants has lead
to three conclusions. First, developmental consequences of a defect in cytokinesis include anomalies
in cell differentiation, organ number, and body organization, as well as organ fusions. Second, of the 10
cytokinesis-defective loci compared in this study,
two were found to be required for root hair morphogenesis. Thus, certain aspects of cytokinesis and root
hair morphogenesis, two processes that result in a
rapid deposition of new cell walls via polarized secretion, may be regulated by the same molecules.
Third, by monitoring embryo, seedling, stomatal,
and postembryonic development, we found that the
cytokinesis-defective mutants present a continuous
range of phenotypes linking seedling-lethal mutations to viable ones. All six loci described here have
phenotypes weaker than those observed in keule mutants, which in turn have weaker phenotypes than
seen in knolle mutants. The results stress the importance of looking for more subtle rather than more
severe phenotypes in further screens for novel
cytokinesis-defective loci.
MATERIALS AND METHODS
Lines and Genetic Methods
Lines G67, G88, G235, R3-27, T286, S100, S302, U1-15,
U57, and U119 were identified in a screen of ethyl
methanesulfonate-mutagenized Landsberg erecta described
by Mayer et al. (1991) and grouped as keule-like based on
their seedling phenotypes (U. Mayer, personal communication). Line G235 is in the HINKEL complementation
group (W. Lukowitz, personal communication) and line
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Plant Physiol. Vol. 129, 2002
Cytokinesis-Defective Mutants of Arabidopsis
T286 was shown to be allelic to pleiade; both lines were used
for phenotypic analysis in this study. In addition, lines
U1-15 and G67 were shown to be allelic to keule, validating
the keule-like designation for the mutant collection. The
hinkel and pleiade alleles described here have the same
overall appearance as the alleles found in other screens for
embryo or seedling lethal mutations (Strompen et al., 2002;
W. Lukowitz, personal communication). It does not seem,
therefore, that our screening criteria have selected for rare
alleles of known loci such as keule, hinkel, and pleaide. One
should note, however, that the pleaide alleles uncovered in
seedling or embryo-lethal screens are stronger than the
viable alleles of pleaide isolated in root morphogenesis
screens (Hauser et al., 1997). Line AP297, identified by W.
Lukowitz and U. Mayer in a screen of x-ray mutagenized
Landsberg erecta (Mayer et al., 1999), was shown to be
allelic to knolle and was used in this study in light of its
relatively high germination efficiency; AP297 corresponds
to a weak allele of knolle, with a phenotype intermediate
between stereotypical knolle and keule mutants. The
massue-2 allele, line MNN, was isolated by M. Hülskamp
from an ethyl methanesulfonate population of Landsberg
erecta. Previously described keule alleles MM125, T282, and
R227 and knolle were used in this study (Assaad et al., 1996;
Lukowitz et al., 1996). Alleles of pleiade and fackel and the
root hair-defective mutants cen3-1 and cen3-2 were kindly
provided by W. Lukowitz and C. Grierson, respectively. To
eliminate potential second-site mutations, the mutant lines
were self-fertilized four to 10 times and outcrossed to wild
type (Landsberg erecta and/or Columbia). Phenotypes
were scored in the progeny of healthy, fully fertile,
greenhouse-grown plants.
Seedling Phenotype and Tissue Culture
To observe mutant seedlings, seeds were surface sterilized with 5% (w/v) calcium hypochlorite for 20 min at room
temperature, rinsed, sown on Murashige and Skoog medium, and placed at 4°C for 3 to 4 d and then at 22°C at 16-h
light/8-h night cycles for 5 to 6 d. Dark-germinated seedlings were treated in the same way but petri dishes were
wrapped with aluminum foil. To test the ability to grow in
tissue culture, seedlings were transferred to SIM (0.15 mg
L⫺1 indole-3-acetic acid; 6-(␥,␥-dimethylallylamino) purine
5 mg L⫺1) or root-inducing medium (1 mg L⫺1 indolebutyric acid). Tissue culture media contained 0.2% (w/v)
phytagel (gellum gum), Murashige and Skoog salts, 0.5 g
L⫺1 MES, B5 vitamins, and 0.4% (w/v) Glc, and the final pH
was adjusted to 5.7 with KOH. For dark-field and phase
contrast microscopy, seedlings were cleared as described
(Mayer et al., 1993).
Light and Electron Microscopy and Image Analysis
Samples for light and electron microscopy were prepared as described (Assaad et al., 1996). The cell wallspecific periodic acid schiff staining method for histological sections was as described by Nickle and Meinke (1998).
The light microscopes used were an Axiophot (Zeiss, Jena,
Plant Physiol. Vol. 129, 2002
Germany) and a DMB fluorescence, phase contrast and
DIC microscope (Leica Microsystems, Wetzlar, Germany).
The electron microscopes were a Zeiss EM912 and an
S-4100 field emission scanning electron microscope (Hitachi Software Engineering, Yokohama, Japan). All images
were processed with Photoshop and/or Illustrator software (Adobe Systems, Mountain View, CA).
Root Hair Analysis and Confocal Imaging
Root hairs were stained with propidium iodide at a
concentration of 10 ␮g mL⫺1 in Murashige and Skoog salts
and mounted on cover slips. Confocal imaging was performed using an MRC1024 laser scanning confocal head
(Bio-Rad, Hercules, CA) mounted on a Diaphot 200 inverted microscope (Nikon, Tokyo). The objective used was
a 20⫻ Nikon PlanApo water immersion (Technical Instruments, San Francisco); excitation, 568 nm; emission, 585 nm
long pass filter. Three-dimensional reconstructions of
image stacks were generated with Lasersharp software
(Bio-Rad) or NIH Image (Wayne Rasband, National Institute of Mental Health, Bethesda, MD).
Mapping and Complementation
For mapping, the mutant lines in the Landsberg background were crossed to wild-type Columbia. The lines
were mapped with respect to the simple sequence length
polymorphisms and to the cleaved amplified polymorphisms PHYA, NCC1, m59, and g2395. Primer sequences,
polymorphisms, and precise map locations for all the molecular markers can be found at http://www.Arabidopsis.
org. With the exception of the ROD locus, bulk segregant
analysis on pools of mutant seedlings was followed by the
phenotyping and genotyping of 48 to 300 individual F2
plants for linked markers flanking the mutant locus, as
described (Lukowitz et al., 2000).
Complementation analysis between the mutants presented here as well as to known cytokinesis-defective mutants was carried out based on similarity of map location.
ACKNOWLEDGMENTS
F.F.A. is especially grateful to Ulli Mayer for making the
mutant lines available for this analysis and thanks Regine
Kahmann and August Böck for their support. Many thanks
to Ramon Torres Ruiz, Thomas Berleth, and Simon Misera
for their contribution to the seedling-lethal screen and to
Martin Hülskamp for the massue-2 allele. Claire Grierson
kindly provided root hair mutants affected in tip growth.
Steffi Cubash, Heddy Bendjaballah, Yoann Huet, Frank
Coutand, Eric Le Gouille, and Natalia Kalinina helped with
sectioning or mapping. Thanks to Wolfgang Lukowitz for
stimulating discussions, for sharing unpublished information and mutant lines, and for useful suggestions on the
manuscript. Thanks to Dave Ehrhardt for help with imaging, useful discussions, and critical evaluation of the
manuscript.
Received February 11, 2002; accepted March 18, 2002.
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Söllner et al.
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