Download Arabidopsis thaliana, a plant model organism for

Journal of Experimental Botany, Vol. 62, No. 1, pp. 89–97, 2011
doi:10.1093/jxb/erq278 Advance Access publication 2 September, 2010
REVIEW PAPER
Arabidopsis thaliana, a plant model organism for the
neuronal microtubule cytoskeleton?
John Gardiner* and Jan Marc
The School of Biological Sciences, The University of Sydney 2006, Australia
* To whom correspondence should be addressed. E-mail: [email protected]
Received 5 July 2010; Revised 24 July 2010; Accepted 17 August 2010
Abstract
The microtubule cytoskeleton is an important component of both neuronal cells and plant cells. While there are large
differences in the function of microtubules between the two groups of organisms, for example plants coordinate the
ordered deposition of cellulose through the microtubule cytoskeleton, there are also some notable similarities. It is
suggested that Arabidopsis thaliana, with its superior availability of knockout lines, may be a suitable model
organism for some aspects of the neuronal microtubule cytoskeleton. Some cellular processes that involve the
neuronal microtubule cytoskeleton including neurotransmitter signalling and neurotrophic support may have
homologous processes in plant cells. A number of microtubule-associated proteins (MAPs) are conserved, including
katanin, EB1, CLASP, spastin, gephyrin, CRIPT, Atlastin/RHD3, and ELP3. As a demonstration of the usefulness of
a plant model system for neuronal biology, an analysis of plant tubulin-binding proteins was used to show that
Charcot–Marie–Tooth disease type 2D and spinal muscular atrophy may be due to microtubule dysfunction and
suggest that indeed the plant microtubule cytoskeleton may be particularly similar to that of motor neurons as both
are heavily reliant upon motor proteins.
Key words: Arabidopsis thaliana, microtubule, motor neuron, neuroscience.
Introduction
Microtubules in both plants and animals are composed of
heterodimers of a- and b-tubulin which bind a molecule of
GTP. GTP bound to a-tubulin is stable while GTP bound
to b-tubulin may be hydrolysed to GDP. A microtubule
with GDP-tubulin is prone to depolymerization, and there
is generally a cap of GTP-tubulin protecting the microtubule from disassembly. Many proteins interact with microtubules (microtubule-associated proteins or MAPs) and are
involved in critical functions such as microtubule growth,
stabilization, destabilization, and interaction with other
cellular organelles (Wade, 2009). While some MAPs are
specific to either plants or animals, some are found in both
groups of organisms. The neuronal microtubule cytoskeleton plays a key role in many cellular processes including
protein transport, neurite extension, and cell division
(Jaglin and Chelly, 2009), whereas in plants it coordinates
the deposition of cellulose microfibrils in the cell wall
and thus regulates plant growth and development, and
plays a key role in responses to hormones, pathogens, and
environmental stress (Buschmann and Lloyd, 2008).
The conservation of actin-interacting proteins between
plants and neurons has been discussed, including the possibility
that they form ‘plant synapses’ between cells (Baluška et al.,
2005). Here the similarities between the plant microtubule
cytoskeleton and the neuronal microtubule cytoskeleton are
discussed. Although the two are only distantly related in
evolutionary terms, they share many features, thus suggesting that Arabidopsis thaliana L. may indeed be a useful
model organism for neurons. Although it may seem that
Arabidopsis, an organism made up of many cells, might not
Abbreviations: BDNF, brain-derived neurotrophic factor; CLASP, CLIP-associated protein; CLIP, cytoplasmic linker protein; CMT, Charcot–Marie–Tooth; CRIPT,
cysteine-rich PDZ-binding protein; EB1, end-binding 1; elo, elongata; ELP3, elongator protein 3; MAP, microtubule-associated protein; NT, neurotrophin; PSD,
postsynaptic density; RHD3, root hair defective 3; SAM, S-adenosylmethionine; +TIPs, microtubule plus end-binding proteins; TOG, tumour overexpressed; Trk,
tyrosine receptor kinase.
ª The Author [2010]. Published by Oxford University Press [on behalf of the Society for Experimental Biology]. All rights reserved.
For Permissions, please e-mail: [email protected]
90 | Gardiner and Marc
reflect the function of individual neurons which are highly
specialized cells, neural tissues made up of many neurons
function as an entity possibly in much the same way as
a plant. It is also possible that within each plant cell there
are microtubule-based domains which are each analogous
to a single neuron (see discussion of EB1 below).
Similarities between Arabidopsis and neurons are
emphazised at the levels of both sequence similarity and
functionality. Many MAPs are conserved and it is to be
expected that, as a result of this, functionality will also be
conserved. Arabidopsis has several advantages over animal
cells as a model organism, including short generation time
and particularly the availability of knockout lines for
virtually every gene. Indeed it is suggested that an analysis
of plant tubulin-binding proteins offers a novel insight into
Charcot–Marie Tooth (CMT) disease and spinal muscular
atrophy. In addition, knowledge of the microtubule cytoskeleton from neuroscience studies will continue to prove
useful in plant biology.
Here the functional similarities of the microtubule
cytoskeleton between plants and neurons are first examined,
in particular neurotransmitter function and neurotrophic
support. Conserved MAPs and their function as cytoplasmic MAPs and scaffolding MAPs are then discussed. Next
comes an examination of the similarities between motor
neurons and plants, focusing again on the MAPs involved,
before insights into motor neuron diseases revealed through
analysis of plant tubulin-binding proteins are discussed.
inhibited by 6,7-dinitroquinoxaline-2,3 dione (DNQX), a
competitive inhibitor of animal glutamate receptors
(GLRs). Interestingly, molecular modelling shows that
glycine and not glutamate is likely to be the natural ligand
for most plant GLRs (Dubos et al., 2003). Here it appears
that glycine may promote transverse orientation of microtubules and increase cell elongation rates in a manner
similar to that of serotonin in plants.
Serotonin in neurons is a key neurotransmitter and is
a target for drugs used to treat depression and other
illnesses. The drugs target serotonin breakdown (monoamine oxidase A inhibitors) or serotonin reuptake from the
synapse. In neurons, serotonin is released from axons into
the synaptic gap via exocytosis and diffuses to dendritic
spines where it binds to various serotonin receptors and
activates intracellular second messenger cascades. Serotonin
may play a key role in promoting microtubule stability in
neurons by increasing levels of the microtubule-stabilizing
protein MAP2 (Whitaker-Azmitia et al., 1995). Serotonin
has been found in >42 plant species and it appears to play
key roles in growth and development. It stimulates root
growth, the growth of the hook of oat coleoptiles, and the
germination of radish seed and pollen of Hippeastrum
hybridum (Kang et al., 2007). Surprisingly there have been
no studies on the role of serotonin in A. thaliana. The role
of serotonin in stimulating root elongation and coleoptile
growth suggests that it could promote ordering of cortical
microtubule arrays, perhaps by modulating the level of
a MAP as in neurons.
Neuronal processes involving microtubules
that may have homologous processes in
plants
Neurotrophic support and the microtubule cytoskeleton
Neurotransmitters and the microtubule cytoskeleton
Glutamate is an important neurotransmitter in neurons and
modulates the function of the microtubule cytoskeleton.
Activation of glutamate receptors 1 and 5 increases MAP2
phosphorylation (Llansola et al., 2005), which would
destabilize microtubules, and glutamate also promotes
calpain-mediated proteolysis of MAP2 (Minger et al.,
1998). Arabidopsis also possesses glutamate as a signalling
molecule, and glutamate receptor proteins are encoded in
the Arabidopsis genome. In Arabidopsis, glutamate causes
microtubule depolymerization and membrane depolarization, and both of these effects are blocked by a specific
antagonist of ionotropic glutamate receptors, 2-amino5-phosphopentanoate. Aluminium toxicity involves damage
to the microtubule cytoskeleton, which also appears to be
mediated via efflux of a glutamate-like ligand and its
binding to a glutamate receptor (Sivaguru et al., 2003).
Thus glutamate acts as a microtubule-disrupting agent in
both neurons and Arabidopsis cells.
Glycine acts as a signalling molecule in plants. Glutamate
and glycine synergistically control ligand-mediated gating of
calcium and hypocotyl elongation in plants. Transient
increases in cytosolic calcium and hypocotyl elongation were
Neurotrophic support is a key process leading to survival of
neurons, and is dependent upon an intact microtubule
cytoskeleton. Neurotrophins, or neurotrophic factors, can
up-regulate antioxidant proteins and antioxidant processes
in neuronal cells (Gardiner et al., 2009). Neurotrophins
include brain-derived neurotrophic factor (BDNF), neurotrophin-3 (NT3), neurotrophin-4/5 (NT4/5), and nerve
growth factor (NGF). These proteins are generally viewed
as target-derived survival factors that guide neuronal axons
to the target tissue. The terminals of the axons bear receptor
tyrosine kinases (TrkA, TrkB, TrkC, and the p75 neurotrophin receptor p75NTR) that bind the neurotrophins. The
neurotophins are then endocytosed (Kuruvilla et al., 2004;
Hibbert et al., 2006) and trafficked by motor proteins along
microtubules to the cell body (Gauthier et al., 2004), where
they induce gene expression (Roberts et al., 2006). BDNF is
also trafficked in the opposite direction (Butowt and von
Bartheld, 2007). There is a positive feedback loop in
operation, with greater transport of BDNF and other
neurotrophic factors along microtubules leading to greater
expression of BDNF and other neurotrophic factors, and
hence increased neuronal survival. While it appears that
neurotrophic support is basically a process found only in
neurons, it is worth noting that Arabidopsis eukaryotic
initiation factors eIF3 and eIF4 share a motif with BDNF
(E-x-L-x-x-D-x-x-V-x-P-x-E-E-N). Both eIF3 and eIF4
Microtubules in plants and neurons | 91
associate with microtubules (Chuong et al., 2004), as does
BDNF via motor proteins and in neurons. In addition,
BDNF induces translation initiation processes involving
eukaryotic initiation factors (Takei et al., 2004). Thus there
may be some similarities in function between BDNF and
plant eukaryotic initiation factors, possibly involving the
microtubule cytoskeleton.
Tubulin isoforms
Arabidopsis, despite its small genome, possesses more
tubulin isoforms that in mammalian systems. In the mouse,
five a-tubulin genes and six b-tubulin genes are functional
(Villasante et al., 1986; Wang et al., 1986), while in
Arabidopsis at least six a-tubulin genes (Kopczak et al.,
1992) and nine b-tubulin genes are expressed (Snustad et al.,
1992). In both systems different tubulin isoforms are
expressed in different tissues (Villasante et al., 1986; Ludwig
et al., 1988), apparently reflecting a fine-tuning of tubulin
function during development.
Cytoplasmic microtubule-associated
proteins conserved between plants and
neurons
Katanin
Katanin is a microtubule-severing ATPase. In neurons the
formation of interstitial axonal branches involves the
severing of microtubules at sites where new branches form.
Katanin plays a key role in enhancing branch formation
through the severing of microtubules (Qiang et al., 2010).
Katanin in both neuronal and plant cells acts to cut long
stationary microtubules into mobile short pieces which can
be reorganized by means of treadmilling (Baas et al., 2005).
Indeed it has been proposed that the main activity of
Arabidopsis katanin is to free cortical microtubules, generating motile microtubules that are incorporated into bundles (Stoppin-Mellet et al., 2006), thus playing a key role in
the organization of the plant microtubule cytoskeleton and
cell wall deposition (Burk et al. 2001).
CLASP
CLIP-associated (CLASP) proteins are found in neurons
where they act as microtubule plus end-binding proteins
(+TIPs) (Lee et al., 2004). CLASP is also found in
Arabidopsis where it modulates microtubule–cortex interactions. In CLASP mutants, deviations from normal linear
polymerization trajectories lead to increased bundling of
microtubules and hyperparallel cortical microtubules in
clasp-1 leaf epidermis (Ambrose and Wasteneys, 2008). In
neurons, CLASP proteins also interact with actin filaments
(Tsvetkov et al., 2007), but this has yet to be shown in plant
cells. That CLASP may act as a microtubule–actin crosslinker is exciting since there is known to be interaction
between microtubules and actin filaments in plants in-
cluding Arabidopsis (Collings et al., 2006), but the proteins
responsible have been elusive.
EB1
EB1 +TIPs were first identified in animals. In neurons they
play important roles in targeting the F-actin-binding protein drebrin in a process required for neuritogenesis
(Geraldo et al., 2008), targeting Kv1 voltage-gated K+
channels (Gu et al., 2006), complementing MAP1B during
axonogenesis (Jiménez-Mateos et al., 2005), and labelling
distal ends of microtubules in neurons (Stepanova et al.,
2003). Arabidopsis possesses three EB1-like proteins. Arabidopsis EB1a–green fluorescent protein (GFP) fusion protein
labels microtubule plus ends as a comet but also marks
minus ends at sites from which microtubules can grow and
shrink. The minus-end nucleation sites are mobile, supporting the idea of a flexible centrosome in plants (Chan et al.,
2003). Arabidopsis EB1 localizes to microtubule plus ends
but also to motile tubulovesicular membrane networks
surrounding organelles, similarly to +TIPs such as CLIP170 (Mathur et al., 2003). Using EB1–GFP-labelled cells
revealed that the majority of plant cortical microtubules in
ordered arrays face the same direction, with selective
stabilization of dynamic cortical microtubules playing a predominant role in their self-organization. Microtubule arrays
appear to be organized into smaller domains at the plasma
membrane, with microtubules in each domain all facing the
same way (Dixit et al., 2006). This suggests that plant
microtubule arrays are more similar to axonal microtubules,
where polymerization takes place almost exclusively in an
anterograde direction, rather than dendrites where both
anterograde and retrograde polymerization takes place
(Kollins et al., 2009). This idea is strengthened by the
observation that EB1 protein levels are increased during
axon but not dendrite formation in neuroblastoma cells
(Morrison et al., 2002) and by the high constitutive
expression of EB1 proteins during plant cell development
(Bisgrove et al., 2008).
Tubulin–tyrosine ligase like
The Arabidopsis genome possesses one homologue of
tubulin–tyrosine ligase (Gardiner and Marc, 2003). This
tubulin–tyrosine ligase like (TTLL) protein may either
catalyse the addition of tyrosine to tubulin (Smertenko
et al., 1997) or catalyse the polyglutamylation of tubulin, as
do other TTLL proteins (Janke et al., 2008). In neurons,
tubulin tyrosination navigates kinesin-1 away from dendritic microtubules to detyrosinated axonal microtubules
(Konishi and Setou, 2009), and tubulin post-translational
modifications may play a similar role in plants.
MAP kinase
MAP kinase (MAPK) associates with microtubules in the
brain where it integrates signal transduction cascades
(Morishima-Kawasima and Kosik, 1996). Similarly, MAPK4
of Arabidopsis is necessary for overall microtubule
92 | Gardiner and Marc
organization, with a homozygous mutant demonstrating
abnormal cell growth such as branching of root hairs and
swelling of epidermal cells (Beck et al., 2010).
Scaffolding MAPs conserved between
neurons and plants
Gephyrin
Gephyrin is a postsynaptic density protein that plays a key
role in maintaining a high concentration of inhibitory
glycine receptors juxtaposed to presynatic releasing sites
(Zita et al., 2007) through its association with the microtubule cytoskeleton (Prior et al., 1992). Arabidopsis possesses
a gephyrin homologue which is crucial for the formation of
the molybdenum cofactor (MoCo) and which also associates with the actin cytoskeleton, as does its mammalian
homologue (Schwarz et al., 2000). Indeed due to the ability
of gephyrin to rescue a universal MoCo-deficient mutation
in plants, gephyrin was identified as the gene affected in
a patient with symptoms typical of MoCo deficiency
(Reiss et al., 2001). It is not known as yet whether the
Arabidopsis gephyrin homologue plays a role in microtubule
organization and receptor clustering.
CRIPT
CRIPT is a postsynaptic protein that binds selectively to the
third PDZ domain of PSD-95. In neurons, CRIPT binds
directly to microtubules, thus linking PSD-95 to the
microtubule cytoskeleton. Disruption of the CRIPT–
PSD-95 interaction with a PDZ3-specific peptide prevents
the association of PSD-95 with microtubules and inhibited
the synaptic clustering of PSD-95, chapsyn-110/PSD-93,
and GKAP (a PSD-95-binding protein), but the number of
synapses and synaptic clustering of N-methyl-D-adenosine
(NMDA) receptors was unaffected (Passafaro et al., 1999).
The Arabidopsis homologue of CRIPT (At1g61780), which
is 66% identical to CRIPT, may also localize to microtubules. Arabidopsis CRIPT possesses the consensus PDZbinding C-terminal sequence (x-S/T-x-V-COOH). PDZ
domain proteins play key roles as scaffolds for a variety of
receptor proteins in neurons (Neff et al., 2009). There
appears to be only one PDZ protein in the Arabidopsis
genome, GAN or At1g55480, which is involved in drought
and salinity tolerance. Thus there may be an interesting link
between the microtubule cytoskeleton, receptor-based
signalling, and environmental stress.
MAPs crucial for motor neuron function
which are conserved in plants
Kinesins
A number of MAPs which are crucial for motor neuron
development and function are also crucial for plant growth
and development. This suggests a certain similarity be-
tween plant cells and motor neurons. Motor neurons are
particularly sensitive to disruption of microtubule-based
protein trafficking, although axonal transport also plays
a part in other neurological diseases such as Alzheimer’s
disease and Huntington’s disease (Holzbaur, 2004). Plant
cells are also highly dependent upon motor proteins
(particularly kinesins) and indeed Arabidopsis possesses
a grand total of 61 kinesins (Lee and Liu, 2004). It is not
clear why plants have so many kinesins, but this does
indicate that any perturbation of the microtubule cytoskeleton and kinesin function is likely to have severe consequences for plant growth and development, as with motor
neurons. The large number of kinesins in Arabidopsis
has perhaps enabled the divergence of highly specialized
proteins with unusual functions. For example, two kinesinlike proteins KAC1 and KAC2 appear not to bind microtubules but rather actin during chloroplast movement
(Suetsugu et al., 2010). In comparison, neuronal kinesins
appear to be more constrained in their function, generally
transporting cargoes along microtubules (Goldstein et al.,
2008).
Atlastin/ROOT HAIR DEFECTIVE 3 and spastin
Hereditary spastic paraplegia (HSP) is an inherited neurological disorder characterized by progressive spasticity and
weakness of the lower extremities. The most common form
of early-onset HSP is caused by mutation in the human
gene encoding the dynamin family GTPase Atlastin.
Atlastin is required for normal growth of muscles and
synapses at the neuromuscular junction and correct endoplasmic reticulum and Golgi morphogenesis. It appears
to act with the microtubule-severing protein spastin to
disassemble microtubules in muscles, with the microtubuledestabilizing drug vinblastine alleviating synapse and
muscle defects in mutant Drosophila (Lee et al., 2009). In
Arabidopsis, the homologue of Atlastin is ROOT HAIR
DEFECTIVE 3 (RHD3). Loss of function of RHD3 in an
Arabidopsis mutant results in an almost complete loss of the
normal epidermal cell file rotation, root skewing, and
waving on hard agar surfaces. The mutant does not respond
to low doses of the microtubule-depolymerizing drug
propyzamide (Yuen et al., 2005), indicating that RHD3 also
destabilizes microtubules in Arabidopsis and that its loss of
function will in fact stabilize microtubules. Indeed the
microtubule-stabilizing drug taxol causes wavy root hairs
(Carol and Dolan, 2002), as seen in the RHD3 mutant
(Schiefelbein and Somerville, 1990). RHD3 loss of function
also alters vacuole enlargement (Galway et al., 1997) and is
required for cell wall biosynthesis and actin organization
(Hu et al., 2003). There are two Arabidopsis proteins with
a high degree of homology to spastin, indicating that
RHD3 may interact with these proteins, as in animals.
Interestingly a mutation in spastin can cause amyotrophic
lateral sclerosis (Münch et al., 2008), and mutations
cause HSP (Errico et al., 2002), again linking a MAP
conserved between plants and humans to motor neuron
function.
Microtubules in plants and neurons | 93
Elongator protein 3
Acetylated tubulin may be important in both plant cells and
motor neurons. In Nicotiana tabacum, acetylated tubulin
was detected in the generative cell of pollen tubes and in
kinetochore fibres, in polar spindle regions, and in the cell
plate of the phragmoplast during generative cell division
(Aström, 1992). Acetylated tubulin was also found in leaves
of Zea mays L. but not in roots (Wang et al., 2004). The
tubulin acetylase ELP3, a component of the Elongator
complex, is present in both plant cells. In A. thaliana the
elongata mutants elo1, elo2, and elo3 have decreased leaf
and primary root growth due to reduced cell proliferation
(Nelissen et al., 2010). To date no studies on tubulin
acetylation have been performed in Arabidopsis so it is
unknown whether these growth defects are associated with
a reduction in acetylated tubulin. However, it is likely that
ELP3 acts as a tubulin acetylase in plants since ELP3 binds
to ab-tubulin heterodimers (Chuong et al., 2004; Gardiner
et al., 2007).
In mammalian systems, ELP3 plays various roles. It is
important for zygotic paternal genome demethylation
through its radical S-adenosylmethionine (SAM) domain
(Okada et al., 2010). It is important for the migration and
branching of projection neurons, probably due to its
a-tubulin acetylase activity (Creppe et al., 2009). a-Tubulin
acetylation is of key importance in neurons as it regulates
the binding of kinesin motor proteins to microtubules, and
hence controls the transport of proteins and organelles
within the cell (Reed et al., 2006). It may also play a role in
the regulation of the actin cytoskeleton and mitochondria in
human cells (Johansen et al., 2008; Barton et al., 2009).
ELP3 variants are associated with motor neuron degeneration in amyotrophic lateral sclerosis, with risk-associated
ELP3 genotypes correlated with reduced brain ELP3
expression (Simpson et al., 2009). Thus, here, Arabidopsis
may serve as a model system for studies of the aetiology of
motor neuron disease. The function of the Elongator
complex appears to be conserved between plants and
animals in at least some respects, for example in its role as
an a-tubulin acetylase. It seems likely that the presence of
acetylated tubulin in the mitotic spindle of plant cells
may act to promote the binding of motor proteins
during cell division and that this may prove to be a useful
model system for understanding the role of ELP3 in
neuronal cells.
Tubulin folding cofactor E
Tubulin folding cofactor E (TBCE) is highly conserved
between animals and plants. In animals it binds microtubules and proteasomes and protects against misfolded
protein stress (Voloshin et al., 2010). There is an antagonistic interaction between TBCE and the microtubule-severing
protein spastin and indeed TBCE appears to be a tubulinpolymerizing protein. A mouse Tbce mutation causes
progressive motor neuropathy (Jin et al., 2009). In
Arabidopsis, the TBCE homologue (a PILZ group protein)
is embryo lethal when deleted, with plants lacking microtubules entirely (Steinborn et al., 2002). Thus here again
a MAP necessary for motor neuron function is also
required for proper development of Arabidopsis.
Charcot–Marie–Tooth disease, aminoacyl-tRNA
synthetases, and microtubules
A mutation in the protein dynamin, which is involved in
endocytic vesicle formation, leads to CMT disease. It has
been recently shown that dynamin also participates in
microtubule dynamics, with Dyn2 required for proper
dynamic instability of microtubules (Tanabe and Takei,
2009). Cells expressing the 551Delta3 mutant protein show
an increase in acetylated tubulin, a marker of microtubule
stability, and depletion of dyn2 leads to accumulation of
stable microtubules. Microtubule-based organelle transport
is impaired in cells expressing the mutant protein, with
a decrease in the formation of mature Golgi complexes
(Tanabe and Takei, 2009). A mutation in the motor domain
of the kinesin KIF1B leads to CMT disease type 2A, again
showing that disruption of microtubule-based protein
trafficking can cause human motor neuron neuropathy
(Zhao et al., 2001). In addition, a model of the X-linked
form of CMT disease, which is associated with mutations in
the connexin32 gene, shows a depletion of axonal microtubules (Sahenk and Chen, 1998). Microtubule-based transport in the longitudinal bands of Cajal may permit
intermodal Schwann cells to lengthen in response to axonal
growth, ensuring rapid nerve impulse transmission (Court
et al., 2004).
Mutations in aminoacyl-tRNA synthetases cause motor
neuron disease. Mutations in glycyl-tRNA synthetase cause
CMT disease type 2D and distal spinal muscular atrophy
type V in humans, motor and sensory neuropathy in mice,
and defects in terminal arborization of axons and dendrites
in the fruit fly. A study of tubulin-binding proteins in
Arabidopsis found that methionyl-, glycyl-, threonyl-, and
multifunctional aminoacyl-synthetase all bind to ab-tubulin
(Chuong et al., 2004). A previous study found that yeast
valyl- and lysyl-tRNA synthetases bind to microtubules and
cause stabilization and bundling of microtubules without
the intervention of an extraneous bundling protein (Melki
et al., 1991). This evidence suggests that aminoacyl-tRNA
synthetases may play a key role in microtubule function,
possibly acting as MAPs that bundle and stabilize microtubules not only in yeast and plants but also in the animal
nervous system. It is suggested that diseases caused by
mutations in these aminoacyl-tRNA synthetases may be, at
least in part, due to disruption of this microtubule-centric
function. This idea is supported by a model of dominantintermediate CMT disease, which is caused by a mutation in
tyrosyl-tRNA synthetase, in Drosophila which found that
the phenotype is not due to a reduction in aminoacylation
activity, but either a gain of function of or interference with
a function of the wild-type protein (Storkebaum et al.,
2009).
94 | Gardiner and Marc
Tudor domain proteins and motor neuron disease
References
Similarly, a tudor domain protein has been found to bind
ab-tubulin in Arabidopsis (Chuong et al., 2004). The motor
neuron disease spinal muscular atrophy is caused by a mutation
in SMN protein, which also possesses a tudor domain. Thus
here, as well, the homologue of an Arabidopsis tubulin-binding
protein is implicated in motor neuron disease. Indeed the Xtr
(Xenopus tudor repeat) protein plays a key role in microtubule
assembly during cleavage in Xenopus laevis (Hiyoshi et al.,
2005).
Ambrose JC, Wasteneys GO. 2008. CLASP modulates
microtubule–cortex interaction during self-organization of
acentrosomal microtubules. Molecular Biology of the Cell 19,
4730–4737.
Novel plant MAPs with human homologues
A number of plant MAPs have human homologues that
have not as yet been shown to have a function in neurons.
However, if plant cells really do have such a strong
similarity to neurons, particularly motor neurons, this
suggests that it may be worth exploring whether neurons
also use these proteins. MOR1 (microtubule organization 1)
is a homologue of the human protein Dis1/TOG and plays
a key role in organizing plant cortical microtubules
(Whittington et al., 2001). It is unknown whether it plays
a role in the organization of microtubules in neurons but is
expressed in oligodendrocytes and neurons and associates
with microtubules (Kosturko et al., 2005). In plants,
a phospholipase D has been found to play a role in the
attachment of microtubules to the plasma membrane
(Gardiner et al., 2001; Dhonukshe et al., 2003) and
a phospholipase A2 also plays a key role in microtubule
organization (Gardiner et al., 2008). Again it is not known
whether phospholipases regulate microtubule attachment to
membranes in neurons. Other conserved proteins in the
PILZ group of genes—tubulin-folding cofactors C and D as
well as associated small G-protein Arl2 (Steinborn et al.,
2002)—may participate in the regulation of motor neuron
microtubules.
Conclusions
It has been demonstrated here that there are many
structural and functional similarities between the neuronal
microtubule cytoskeleton and the plant microtubule cytoskeleton. In the past many insights from studies of the
animal cytoskeleton have proven useful in plant biology,
and doubtless this will continue into the future. Recent
developments in plant cell biology including the development of resources such as the SALK T-DNA knockout
lines mean that plant biology is increasingly in the position
of being able to supply useful insights into animal biology,
particularly neuronal biology. It is worth noting that both
consciousness, the product of the neuronal microtubule
cytoskeleton, and plant growth and development, the
product of the plant microtubule cytoskeleton, are at heart
anti-entropic processes. However, it appears that motor
neurons may be the closest homologue of plant cells in the
animal nervous system as both are highly dependent upon
motor proteins for correct development and function.
Aström H. 1992. Acetylated alpha-tubulin in the pollen tube
microtubules. Cell Biology International Reports 16, 871–881.
Baas PW, Karabay A, Qiang L. 2005. Microtubules cut and run.
Trends in Cell Biology 15, 518–524.
Baluška F, Volkmann D, Menzel D. 2005. Plant synapses:
actin-based domains for cell-to-cell communication. Trends in Plant
Science 10, 106–111.
Barton D, Braet F, Marc J, Overall R, Gardiner J. 2009.
ELP3 localises to mitochondria and actin-rich domains at edges of
HeLa cells. Neuroscience Letters 455, 60–64.
} ller J, Menzel D, Samaj J. 2010. Arabidopsis
Beck M, Komis G, Mu
homologs of nucleus- and phragmoplast-localized kinase 2 and 3 and
mitogen-activated protein kinase 4 are essential for microtubule
organization. The Plant Cell 22, 75–771.
Bisgrove SR, Lee Y-R J, Liu B, Peters NT, Kropf DL. 2008.
The microtubule plus-end binding protein EB1 functions in root
responses to touch and gravity signals in Arabidopsis. The Plant Cell
20, 396–410.
Burk DH, Liu B, Zhong R, Morrison WH, Ye ZH. 2001. A
katanin-like protein regulates normal cell wall biosynthesis and cell
elongation. The Plant Cell 13, 807–827.
Buschmann H, Lloyd CW. 2008. Arabidopsis mutants and the
network of microtubule-associated functions. Molecular Plant 1,
888–898.
Butowt R, von Bartheld CS. 2007. Conventional kinesin-I
motors participate in the anterograde axonal transport of
neurotrophins in the visual system. Journal of Neuroscience Research
85, 2546–2556.
Carol RJ, Dolan L. 2002. Building a hair: tip growth in Arabidopsis
thaliana root hairs. Philosophical Transactions of the Royal Society B:
Biological Science 357, 815–821.
Chan J, Calder GM, Doonan JH, Lloyd CW. 2003. EB1 reveals
mobile microtubule nucleation sites in Arabidopsis. Nature Cell Biology
5, 967–971.
Collings DA, Lill AW, Himmelspach R, Wasteneys GO. 2006.
Hypersensitivity to cytoskeletal antagonists demonstrates
microtubule–microfilament cross-talk in the control of root elongation
in Arabidopsis thaliana. New Phytologist 170, 275–290.
Creppe C, Malinouskaya L, Volvert ML, et al. 2009. Elongator
controls the migration and differentiation of cortical neurons through
acetylation of alpha-tubulin. Cell 136, 551–564.
Chuong SDX, Good AG, Taylor GJ, Freeman MC,
Moorhead GBG, Muench DG. 2004. Large-scale identification of
tubulin-binding proteins provides insight on subcellular trafficking,
metabolic channelling, and signalling in plant cells. Molecular and
Cellular Proteomics 3, 970–983.
Court FA, Sherman DL, Pratt T, Garry EM, Ribchester RR,
Cottrell DF, Fleetwood-Walker SM, Brophy PJ. 2004. Restricted
Microtubules in plants and neurons | 95
growth of Schwann cells lacking Cajal bands slows conduction in
myelinated nerves. Nature 431, 191–195.
Holzbaur EL. 2004. Motor neurons rely on motor proteins. Trends in
Cell Biology 14, 233–240.
Dhonukshe P, Laxalt AM, Goedhart J, Gadella TWJ, Munnik T.
2003. Phospholipase D activation correlates with microtubule
reorganization in living plant cells. The Plant Cell 15, 2666–2679.
Hu Y, Zhong R, Morrison 3rd WH, Ye ZH. 2003. The Arabidopsis
RHD3 gene is required for cell wall biosynthesis and actin organization.
Planta 217, 912–921.
Dixit R, Chang E, Cyr R. 2006. Establishment of polarity during
organization of the acentrosomal plant cortical microtubule array.
Molecular Biology of the Cell 17, 1298–1305.
Jaglin XH, Chelly J. 2009. Tubulin-related cortical dysgeneses:
microtubule dysfunction underlying neuronal migration defects. Trends
in Genetics 25, 555–566.
Dubos C, Huggins D, Grant GH, Knight MR, Campbell MM.
2003. A role for glycine in the gating of plant NMDA-like receptors.
The Plant Journal 35, 800–810.
Janke C, Rogowski K, van Dijk J. 2008. Polyglutamylation:
a fine-regulator of protein function? ‘Protein modifications: beyond the
usual suspects’. Review Series. EMBO Reports 9, 636–641.
Errico A, Ballabio A, Rugarli EI. 2002. Spastin, the protein mutated
in autosomal dominant hereditary spastic paraplegia, is involved in
microtubule dynamics. Human Molecular Genetics 11, 153–163.
Jiménez-Mateos EM, Paglini G, González-Billault C, Cáceres A,
Avila J. 2005. End binding protein-1 (EB1) complements
microtubule-associated protein-1B during axonogenesis. Journal of
Neuroscience Resesarch 80, 350–359.
Galway ME, Heckman JW Jr, Schiefelbein JW. 1997. Growth and
ultrastrucure of Arabidopsis root hairs: the rhd3 mutation alters
vacuole enlargement and tip growth. Planta 201, 209–218.
Gardiner J, Andreeva Z, Barton D, Ritchie A, Overall R, Marc J.
2008. The phospholipase A inhibitor, aristolochic acid, disrupts cortical
microtubule arrays and root growth in Arabidopsis. Plant Biology
(Stuttgart) 10, 725–731.
Gardiner J, Barton D, Marc J, Overall R. 2007. Potential role of
tubulin acetylation and microtubule-based protein trafficking in familial
dysautonomia. Traffic 8, 1145–1149.
Gardiner J, Barton D, Overall R, Marc J. 2009. Neurotrophic
support and oxidative stress: converging effects in the normal and
diseased nervous system. Neuroscientist 15, 47–61.
Gardiner JC, Harper JD, Weerakoon ND, Collings DA, Ritchie S,
Gilroy S, Cyr RJ, Marc J. 2001. A 90-kD phospholipase D from
tobacco binds to microtubules and the plasma membrane. The Plant
Cell 13, 2143–2158.
Gardiner J, Marc J. 2003. Putative microtubule-associated proteins
from the Arabidopsis genome. Protoplasma 222, 61–74.
Gauthier LR, Charrin BC, Borrell-Pages M, et al. 2004. Huntingtin
controls neurotrophic support and survival of neurons by enhancing
BDNF vesicular transport along microtubules. Cell 118, 127–138.
Geraldo S, Khanzada UK, Parsons M, Chilton JK, GordonWeeks PR. 2008. Targeting of F-actin-binding protein drebrin by the
microtubule plus-tip protein EB3 is required for neuritogenesis. Nature
Cell Biology 10, 1181–1189.
Goldstein AY, Wang X, Schwarz TL. 2008. Axonal transport and
the delivery of pre-synaptic components. Current Opinion in
Neurobiology 18, 495–503.
Gu C, Zhou W, Puthenveedu MA, Xu M, Jan YN, Jan LY. 2006.
The microtubule plus-end tracking protein EB1 is required for Kv1
voltage-gated K+ channel aonal targeting. Neuron 52, 803–816.
Hibbert AP, Kramer BM, Miller FD, Kaplan DR. 2006. The
localization, trafficking and retrograde transport of BDNF bound to
p75NTR in sympathetic neurons. Molecular and Cellular Neuroscience
32, 387–402.
Hiyoshi M, Nakajo N, Abe S-I, Takamune K. 2005. Involvement of
Xtr (Xenopus tudor repeat) in microtubule assembly around nucleus
and karyokinesis during cleavage in Xenopus laevis. Development,
Growth and Differentiation 47, 109–117.
Jin S, Pan L, Liu Z, Wang Q, Xu Z, Zhang YQ. 2009. Drosophila
tubulin-specific chaperone E functions at neuromuscular synapses
and is required for microtubule network formation. Development 136,
1571–1581.
Johansen LD, Naumanen T, Knudsen A, et al. 2008. IKAP
localizes to membrane ruffles with filamin A and regulates actin
cytoskeleton organization and cell migration. Journal of Cell Science
121, 854–864.
Kang S, Kang K, Lee K, Back K. 2007. Characterization of
tryptamine 5-hydroxylase and serotonin synthesis in rice plants.
Plant Cell Reports 26, 2009–2015.
Kollins KM, Bell RL, Butts M, Withers GS. 2009. Dendrites differ
from axons in patterns of microtubule stability and polymerization
during development. Neural Development 4, 26.
Konishi Y, Setou M. 2009. Tubulin tyrosination navigates the kinesin-1
motor domain to axons. Nature Neuroscience 12, 559–567.
Kopcazk SD, Haas NA, Hussey PJ, Silflow CD, Snustad DP.
1992. The small genome of Arabidopsis contains at least six
expressed alpha-tubulin genes. The Plant Cell 4, 539–547.
Kosturko LD, Maggipinto MJ, D’Sa C, Carson JH, Barbarese E.
2005. The microtubule-associated protein tumor overexpressed
gene binds to the RNA trafficking protein heterogeneous
nuclear ribonucleoprotein A2. Molecular Biology of the Cell 16,
1938–1947.
Kuruvilla R, Zweifel LS, Glebova NO, Lonze BE, Valdez G, Ye H,
Ginty DD. 2004. A neurotrophin signaling cascade coordinates
sympathetic neuron development through differential control of TrkA
trafficking and retrograde signalling. Cell 118, 243–55.
Lee H, Engel U, Rusch J, Scherrer S, Sheard K, Van Vactor D.
2004. The microtubule plus end tracking protein Orbit/MAST/CLASP
acts downstream of the tyrosine kinase Abl in mediating axon
guidance. Neuron 42, 913–926.
Lee M, Paik SK, Lee MJ, et al. 2009. Drosophila Atlastin regulates
the stability of muscle microtubules and is required for synapse
development. Developmental Biology 330, 250–262.
Lee YR, Liu B. 2004. Cytoskeletal motors in Arabidopsis. Sixty-one
kinesins and seventeen myosins. Plant Physiology 136, 3877–3883.
Llansola M, Erceg S, Felipo V. 2005. Chronic exposure to ammonia
alters the modulation of phosphorylation of microtubule-associated
96 | Gardiner and Marc
protein 2 by metabotropic glutamate receptors 1 and 5 in cerebellar
neurons in culture. Neuroscience 133, 185–191.
Ludwig SR, Oppenheimer DG, Silflow CD, Snustad DP. 1988.
The a1-tubulin gene of Arabidopsis thaliana: primary stricture and
preferential expression in flowers. Plant Molecular Biology 10, 311–321.
} lskamp M.
Mathur J, Mathur N, Kernebeck B, Srinivas BP, Hu
2003. A novel localization pattern for EB1-like protein links microtubule
dynamics to endomembrane organization. Current Biology 13,
1991–1997.
Melki R, Kerjan P, Waller JP, Carlier MF, Pantaloni D. 1991.
Interaction of microtubule-associated proteins with microtubules: yeast
lysyl- and valyl-tRNA synthetases and tau 218–235 synthetic peptide
as model systems. Biochemistry 30, 11536–11545.
Minger SL, Geddes JW, Holtz ML, Craddock SD, Whitehart SW,
Siman RG, Pettigrew LC. 1998. Glutamate receptor antagonists
inhibit calpain-mediated cytosksletal proteolysis in focal cerebral
ischemia. Brain Research 810, 181–199.
Morishima-Kawashima M, Kosik KS. 1996. The pool of map
kinase associated with microtubules is small but constitutively active.
Molecular Biology of the Cell 7, 893–905.
Morrison EE, Moncur PM, Askham JM. 2002. EB1 identifies sites
of microtubule polymerisation during neurite development. Brain
Research and Molecular Brain Research 98, 145–152.
} nch C, Rolfs A, Meyer T. 2008. Heterozygous S44L missense
Mu
change of the spastin gene in amyotrophic lateral sclerosis.
Amyotrophic Lateral Sclerosis 9, 251–253.
Neff 3rd RA, Gomez-Varela D, Fernandes CC, Berg DK. 2009.
Potsynaptic scaffolds for nicotinic receptors on neurons.
Acta Pharmacologica Sinica 30, 694–701.
Nelissen H, De Groeve S, Fleury D, et al. 2010. Plant Elongator
regulates auxin-related genes during RNA polymerase II transcription
elongation. Proceedings of the National Academy of Sciences, USA
107, 1678–1683.
Okada Y, Yamagata K, Hong K, Wakayama T, Zhang Y. 2010.
A role for the elongator complex in zygotic paternal genome
demethylation. Nature 463, 554–558.
Qiang L, Yu W, Liu M, Solowska JM, Baas PW. 2010. Basic
fibroblast growth factor elicits formation of interstitial axonal branches
via enhanced severing of microtubules. Molecular Biology of the Cell
21, 334–344.
Passafaro M, Sala C, Niethammer M, Sheng M. 1999. Microtubule
binding by CRIPT and its potential role in the synaptic clustering of
PSD-95. Nature Neuroscience 2, 1063–1069.
Prior P, Schmitt B, Grenningloh G, et al. 1992. Primary structure
and alternative splice variants of gephyrin, a putative glycine
receptor–tubulin linker protein. Neuron 8, 1161–1170.
Roberts DS, Hu Y, Lind I, Brooks-Kaya AR, Russek SJ. 2006.
Brain-derived neurotrophic factor. BDNF-induced synthesis of early
growth response factor 3, Egr3, controls the levels of type A GABA
receptor a4 subunits in hippocampal neurons. Journal of Biological
Chemistry 281, 29431–29435.
Sahenk Z, Chen L. 1998. Abnormalities in the axonal cytoskeleton
induced by a connexin32 mutation in nerve xenografts. Journal of
Neuroscience Research 51, 174–184.
Schiefelbein JW, Somerville C. 1990. Genetic control of root hair
development in Arabidopsis thaliana. The Plant Cell 2, 235–243.
Schwarz G, Schulze J, Bittner F, Eilers T, Kuper J, Nerlich A,
Brinkmann H, Mendel RR. 2000. The molybdenum cofactor
biosynthetic protein Cnx1 complements molybdite-repairable mutants,
transfers molybdenum to the metal binding pterin, and is associated
with the cytoskeleton. The Plant Cell 12, 2455–2472.
Simpson CL, Lemmens R, Miskiewicz K, et al. 2009. Variants of
the elongator protein 3. ELP3. gene are associated with motor neuron
degeneration. Human Molecular Genetics 18, 472–481.
Sivaguru M, Pike S, Gassmann W, Baskin TI. 2003. Aluminium
rapidly depolymerizes cortical microtubules and depolarizes the
plasma membrane: evidence that these responses are mediated by
a glutamate receptor. Plant and Cell Physiology 44, 667–675.
Smertenko A, Blume Y, Viklický V, Opatrný Dráber P. 1997.
Post-translational modifications and multiple tubulin isoforms in
Nicotiana tabacum L. cells. Planta 201, 349–358.
Snustad DP, Haas NA, Kopczak SD, Silflow CD. 1992. The small
genome of Arabidopsis contains at least six expressed alpha-tubulin
genes. The Plant Cell 4, 39–547.
Steinborn K, Maulbetsch C, Priester B, et al. 2002.
The Arabidopsis PILZ group genes encode tubulin-folding cofactor
orthologs required for cell division but not cell growth. Genes and
Development 16, 959–971.
Stepanova T, Slemmer J, Hoogenraad CC, Lansbergen G,
Dortland B, De Zeeuw CI, Grosveld F, van Cappellen G,
Akhmanova A, Galjart N. 2003. Visualization of microtubule growth
in cultured neurons via the use of EB3–GFP (end-binding protein
3–green fluorescent protein). Journal of Neuroscience 23, 2655–2664.
Stoppin-Mellet Gaillard J, Vantard M. 2006. Katanin’s severing
activity favours bundling of cortical microtubules in plants. The Plant
Journal 46, 1009–1017.
Storkebaum E, Leitão-Goncxalves R, Godenschwege T, et al.
2009. Dominant mutations in tyrosyl-tRNA synthetase gene
recapitulate in Drosophila features of human Charcot–Marie–Tooth
neuropathy. Proceedings of the National Academy of Sciences, USA
106, 11782–11787.
Reed NA, Cai D, Blasius TL, Jih GT, Meyhofer E, Gaertig J,
Verhey KJ. 2006. Microtubule acetylation promotes kinesin-1 binding
and transport. Current Biology 16, 2166–2172.
Suetsugu N, Yamada N, Kagawa T, Yonekura H, Uyeda TQ,
Kadota A, Wada M. 2010. Two kinesin-like proteins mediate actinbased chloroplast movement in Arabidopsis. Proceedings of the
National Academy of Sciences, USA 107, 8860–8865.
Reiss J, Gross-Hardt S, Christensen E, Schmidt P, Mendel RR,
Schwarz G. 2001. A mutation in the gene for the neurotransmitter
receptor-clustering protein gephyrin causes a novel form of
molybdenum cofactor deficiency. American Journal of Human
Genetics 68, 208–213.
Takei N, Inamura N, Kawamura M, Namba H, Hara K,
Yonezawa K, Nawa H. 2004. Brain-derived neurotrophic factor
induces mammalian target of rapamycin-dependent local activation of
translation machinery and protein synthesis in neuronal dendrites.
Journal of Neuroscience 24, 9760–9769.
Microtubules in plants and neurons | 97
Tanabe K, Takei K. 2009. Dynamic instability of microtubules
requires dynamin 2 and is impaired in a Charcot–Marie–Tooth mutant.
Journal of Cell Biology 185, 939–948.
Wang W, Vignani R, Scali M, Sensi E, Cresti M. 2004.
Post-translational modifications of alpha-tubulin in Zea mays L. are
highly tissue specific. Planta 218, 460–465.
Tsvetkov AS, Samsonov A, Akhmanova A, Galjart N, Popov SV.
2007. Microtubule-binding proteins CLASP1 and CLASP2 interact
with actin filaments. Cell Motility and the Cytoskeleton 64, 519–530.
Whitaker-Azmitia PM, Borella A, Raio N. 1995. Serotonin depletion
in the adult rat causes loss of the dendritic marker MPA-2. A new
animal model of schizophrenia? Neuropsychopharmacology 12,
269–272.
Voloshin O, Gocheva Y, Gutnick M, Movshovich N, Bakhrat A,
Baranes-Bachar K, Bar-Zvi D, Parvan R, Gheber L, Raveh D.
2010. Tubulin chaperone E binds microtubules and proteasomes and
protects against misfolded protein stress. Cellular and Molecular Life
Sciences 67, 2025–2038.
Villasante A, Wang D, Dobner P, Dolph P, Lewis SA, Cowan NJ.
1986. Six mouse a-tubulin mRNAs encode five distinct isotypes:
testis-specific expression of two sister genes. Molecular and Cellular
Biology 6, 2409–2419.
Whittington AT, Vugrek O, Wei KJ, Hasenbein NG, Sugimoto K,
Rashbrooke MC, Wasteneys GO. 2001. MOR1 is essential for
organizing cortical microtubules in plants. Nature 411, 610–613.
Yuen CY, Sedbrook JC, Perrin RM, Carroll KL, Masson PH.
2005. Loss-of-function mutations of ROOT HAIR DEFECTIVE3
suppress root waving, skewing, and epidermal cell file rotation in
Arabidopsis. Plant Physiology 138, 701–714.
Wade RH. 2009. On and around microtubules: an overview.
Molecular Biotechnology 43, 177–191.
Zhao C, Takita J, Tanaka Y, et al. 2001. Charcot–Marie–Tooth
disease type 2A caused by mutation in a microtubule motor
KIF1Bbeta. Cell 105, 587–597.
Wang D, Villasante A, Lewis SA, Cowan NJ. 1986.
The mammalian b-tubulin repertoire: hematopoietic expression of
a novel, heterologous b-tubulin isotype. Journal of Cell Biology 103,
1903–1910.
Zita MM, Marchionni I, Bottos E, Righi M, Del Sal G,
Cherubini E, Zacchi P. 2007. Post-phosphorylation prolyl
isomerisation of gephyrin represents a mechanism to modulate glycine
receptors function. EMBO Journal 26, 1761–1771.