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Journal of Experimental Botany, Vol. 65, No. 10, pp. 2617–2632, 2014
doi:10.1093/jxb/ert363 Advance Access publication 9 November, 2013
Review paper
Auxin and the ubiquitin pathway. Two players–one target: the
cell cycle in action
Juan C. del Pozo* and Concepción Manzano
Centro de Biotecnología y Genómica de Plantas (CBGP) INIA-UPM. Instituto Nacional de Investigación y Tecnología Agraria y
Alimentaria. Campus de Montegancedo, Pozuelo de Alarcón, 28223 Madrid, Spain
* To whom correspondence should be addressed. E-mail: [email protected]
Received 21 June 2013; Revised 7 October 2013; Accepted 7 October 2013
Abstract
Plants are sessile organisms that have to adapt their growth to the surrounding environment. Concomitant with this
adaptation capability, they have adopted a post-embryonic development characterized by continuous growth and
differentiation abilities. Constant growth is based on the potential of stem cells to divide almost incessantly and on
a precise balance between cell division and cell differentiation. This balance is influenced by environmental conditions and by the genetic information of the cell. Among the internal cues, the cross-talk between different hormonal
signalling pathways is essential to control this division/differentiation equilibrium. Auxin, one of the most important
plant hormones, regulates cell division and differentiation, among many other processes. Amazing advances in auxin
signal transduction at the molecular level have been reported, but how this signalling is connected to the cell cycle
is, so far, not well known. Auxin signalling involves the auxin-dependent degradation of transcription repressors by
F-box-containing E3 ligases of ubiquitin. Recently, SKP2A, another F-box protein, was shown to bind auxin and to
target cell-cycle repressors for proteolysis, representing a novel mechanism that links auxin to cell division. In this
review, a general vision of what is already known and the most recent advances on how auxin signalling connects to
cell division and the role of the ubiquitin pathway in plant cell cycle will be covered.
Key words: APC/S, auxin, plant cell cycle, SCF, SKP2A, TIR1, ubiquitin.
Introduction
Plants are multicellular organisms that are remarkably complex and with an extraordinary developmental plasticity. Due
to their sessile style of life, plants are obliged to adapt their
growth to the surrounding environment. The post-embryonic
developmental programme of plants is extremely flexible and
offers the peculiarity of an incessant ability to grow, a circumstance that requires a highly synchronized balance between
cell division and cell differentiation rates. This almost perpetual growth potential relies on the ability of stem cells, a
specific cell type located in meristems, to divide and later on
to acquire the specific cell fate to form new tissues and organs
(Brukhin, and Morozova, 2011). The cell cycle is a critical
process that is under the control of strict and often overlapping regulatory systems that aim to ensure the successful production of progeny cells. It is regulated through a number of
different supervisory mechanisms, with phosphorylation and
ubiquitin-dependent degradation of key regulatory proteins
being the most relevant. The majority of core genes pertaining to the cell cycle of other eukaryotes have already been
identified in plants (Vandepoele et al., 2002). Genomic annotations and recent molecular data have shown that cell-cycle
regulation may be highly conserved in plants. To understand,
at the mechanistic level, how plants regulate cell division, we
think it is useful to make comparisons to other model systems,
Abbreviations: APC/C, anaphase-promoting complex/cyclosome; ARF, auxin-response factor; Aux/IAA, auxin/indole-3-acetic acid; AuxRE, auxin-response
elements; CKI, cyclin kinase inhibitors; PCNA, proliferating cell nuclear antigen; Rb, Retinoblastoma; UPS, ubiquitin–proteasome system.
© The Author 2013. Published by Oxford University Press on behalf of the Society for Experimental Biology. All rights reserved.
For permissions, please email: [email protected]
2618 | del Pozo and Manzano
in which more advances have been made, to take advantage
of this information and fill some gaps in our understanding of the plant cell cycle. During this review, information
from other model systems, basically mammals and yeast, is
mentioned.
Precise changes in protein levels are critical to drive the
correct entry and transition between different phases of the
cell cycle and the integrity of genomes. Ubiquitin–proteasome-mediated proteolysis is a specific system that ensures a
rapid and highly precise mechanism to regulate the activity
of many proteins. Similar to other eukaryotes, ubiquitinmediated degradation of plant cell-cycle core proteins plays a
critical role in the sequence of events leading to cell division
(Genschik and Criqui, 2007; Ito, 2013). The balance between
division and differentiation is influenced by the surrounding
environment and by the genetic material with clues largely
regulated by different phytohormones. Among them, auxin, a
Greek-derived term (auxein) meaning ‘to grow’, is one of the
most prominent hormones of plant development and plays a
pivotal role controlling plant growth and differentiation, integrating genetic internal clues to external stimuli responses.
Auxin is sensed by a co-receptor system that involves an E3
ligase of ubiquitin [transport inhibitor response 1/auxin signalling F-boxes (TIR1/AFBs)] and repressor targets, which
are ubiquitinated and degraded through the proteasome to
release the auxin signalling (Gray et al., 2001; Dharmasiri
et al., 2005; Peer, 2013). This hormone-sensing system based
on E3 ligases seems to function in regulating the plant cell
cycle as well. S-phase kinase-associated protein 2 (SKP2A),
an F-box protein that forms part of the SCFSKP2A complex,
binds to auxin to control the stability of cell-cycle transcription factors (Jurado et al., 2010; Peer, 2013).
The ubiquitin–proteasome system (UPS)
Ubiquitin is a small peptide that is attached to target proteins in a sequential enzymatic cascade that involves E1 (activating), E2 (conjugating), and E3 (ligase) enzyme activities
(for more details, see reviews by Santner and Estelle, 2010;
Olsen and Lima, 2013; Strieter and Korasick, 2012; Hoeller
and Dikic, 2013; Skaar et al., 2013). In the final step, target
proteins are specifically recruited by the E3 ligase to the proximity of E2, which loads the ubiquitin moiety, via a covalent
bond, to a lysine in the target. The term ‘ubiquitination’ comprises different ways of ubiquitin conjugation, as targets can
be mono-ubiquitinated in one position or in several positions,
or poly-ubiquitinated by different structural ubiquitin polymers, which in turn encode different signals (Li and Ye, 2008).
The structural differences of poly-ubiquitin chains depend on
the lysine used to attach the successive ubiquitin molecules.
The best known ubiquitin mark is the poly-ubiquitination
that targets proteins for degradation through the proteasome.
This selective degradation of proteins by the UPS is an irreversible and highly precise mechanism that ensures the complete elimination of protein activity. Genomic studies have
revealed that plant genomes encode a large number of components of the UPS system (approx. 5% of the total proteome),
with the E3 ubiquitin ligase class, with more than 1000 genes,
being the largest family (for information visit http://bioinformatics.cau.edu.cn/plantsUPS/ and http://plantsubq.genomics.purdue.edu/ and see reviews by Vierstra, 2009; Chen and
Hellmann, 2013). These E3 enzymes recognize the target
protein and provide system specificity. Classification of the
different E3 ligases is based on the presence of a homology
to E6-AP C terminus (HECT) domain or a really interesting new gene (RING)/U-box domain. With regard to this
last group, there are RING-E3 members that contain the E2
ubiquitin-binding site and the target recognition domain in
a unique peptide while other RING-E3 constituents form
multisubunit complexes: Skp/cullin/F-box-containing (SCF);
bric-a-brac, tramtrack and broad complex/pox virus and zinc
finger (CUL3-BTB/POZ); UV-damaged DNA binding protein 1 (CUL4-DDB1) and the anaphase-promoting complex/
cyclosome (APC/C). In the case of plants, the number of E3
ligases is much higher than in other eukaryotes (yeast and
mammals for example), also suggesting a broader number of
targets that might be regulated by degradation.
In eukaryotes, cell division is an important process that
is under strict regulatory systems to ensure the successful
production of descendant cells. The cell cycle is controlled
through a number of different regulatory mechanisms, with
phosphorylation and ubiquitin-dependent degradation
of key regulatory proteins being the most relevant among
them (Hershko, 2005; Nakayama and Nakayama, 2006;
Genschik and Criqui, 2007; Johnson, 2009). The majority of
the cell-cycle-related processes are carried out primarily by
two families of E3 ubiquitin ligases: SCF and APC/C complexes (Skaar and Pagano 2009; Teixeira and Reed, 2013),
but recently RING-E3 activities involved in the proteolysis
of cell proliferation regulators have been reported in plants
(Ren et al., 2008; Liu et al., 2008; Strzalka et al., 2013) and in
mammals (Lee and Gu, 2009).
The APC/C complex is the largest E3 ubiquitin ligase
described so far and consists of the assembly of 11 subunits
in vertebrates (Fig. 1A) (Peters, 2002) and 13 in yeast (Yoon
et al., 2002). The APC/C core is formed by APC2, a distant
CULLIN-like protein, and APC11, which is similar to the
RING-finger subunit. These two proteins build up a basic
and non-specific ubiquitin ligase (Gmachl et al., 2000). Other
APC/C subunits assemble onto this minimal module in order
to confer specificity to the E3 ligase. The substrate recognition and poly-ubiquitination requires the additional function
of APC10/DOC1 (Carroll et al., 2005). This selective target
recognition is activated during the cell cycle by co-activators
such as CDC20 (cell division cycle protein 20) and Cdh1 (cadherin 1) (Vodermaier et al., 2003). In addition, the APC/C
core incorporates other essential subunits (APC3/CDC27;
APC8/CDC23, and APC6/CDC16) that contain a tetratricopeptide repeat, a domain motif involved in protein–protein
interactions. In plants, all the APC/C components have been
identified (Capron et al., 2003; Genschik and Criqui, 2007).
The SCF is a multisubunit complex formed by the structural protein CULLIN1, which interacts at the C terminus
with RING-box 1 (RBX), which recruits the cognate E2,
and at the N terminus with ASK1 (SKP1 homologous in
Auxin, ubiquitin and the cell cycle | 2619
A
APC/C
SCF
CAND1
COP/CSN
Ub
TARGET
AXR1
RUB
E2
CULLIN1
CDH1/CDC20
APC11
ASK
APC2
RBX
F-BOX
E2
TARGET
B
SCFSKP2B SCFFBL17 RHF1a
E3 ligases SCFCLINK SCFSKP2A RKP
RHF1b
targets
RBR
E2FC KRP1
E3 ligases
CYCA
CYCB
KRP3-7 KRP6
G1
targets
APCCDH1
APCCDC20
S
G2
M
E2FB
CYCD
PCNA
CDT1a
CDC6
SECURIN-like?
?
?
?
APC?
Ub
APCCCS52
CYCB
CYCA
Endo
cycle
G1
Fig. 1. (A) Schematic representation of the APC/C and SCF complexes. Both contain a structural core that is composed of CULLIN1
and APC2 and a RING-finger protein (APC11 in APC/C and RBX1 in SCF). These RING-finger proteins recruit the E2 conjugating
enzyme, which loads the ubiquitin (Ub) moiety onto the target. The target proteins are specifically recognized by the F-box (SCF) or the
CDH1/CDC20 or APC10 (APC/C). (B) Plant key cell-cycle regulators regulated by UPS-dependent proteolysis by the action of SCF and/
or APC/C complexes at specific cell-cycle phases: G1, S, G2, and M, or during DNA endoreplication. SCF activities are predominant
from late G1 to S phase, whereas APC/C ubiquitinates targets basically during mitosis and early G1.A question mark indicates that the
E3 ligase is unknown. Grey names and arrows show unproven targets and E3.
plants) (Fig. 1A). The F-box, which is anchored to the complex through the interaction with ASK1, recruits and positions the target proteins in close proximity to E2 for ubiquitin
uploading (Cardozo and Pagano, 2004; Skaar et al., 2013;
Suryadinata et al., 2013). SCF activity is regulated at different levels. Several modes of regulation are exerted on the
CULLIN1 subunit. Post-translational modification of the
CULLIN1 subunit with neural precursor cell expressed, developmentally downregulated 8/related to ubiquitin 1 (NEDD8/
RUB1) regulates the ability of SCF complexes to ubiquitinate
their targets. In a genetic screening based on auxin resistance,
several mutants were isolated and two of them (axr1 and tir1)
were proven to encode components or regulators of the SCF
complex. Auxin Resistant 1 (AXR1) is part of the NEDD8/
RUB1 E1-activating enzyme that modifies CULLIN1 and
regulates the SCFTIR1 activity (Leyser et al., 1993; del Pozo
et al., 1998; Gray et al., 1999; del Pozo et al. 2002a). Lack of
this modification compromised the TIR1/AFBs-dependent
degradation of several auxin/indole-3-acetic acid (Aux/IAA)
proteins (Gray et al., 2001; Dharmasiri et al., 2005), but also
affected, to a lesser extent, other hormonal pathways, as axr1
mutant were partially resistant to jasmonic acid or blocked
DELLA degradation in response to gibberellins (Tiryaki and
Staswick, 2002; Weiss and Ori, 2007). This RUB1 modification is also needed for the proper degradation of cell-cycle regulators such as the cell-cycle repressors E2-promoter binding
2620 | del Pozo and Manzano
factor C (E2FC) and E2F dimerization partner B (DPB) (del
Pozo et al., 2002b). An additional regulatory system of the
SCF is exerted by the COP9/SIGNALOSOME (CSN). This
macro-complex removes the conjugated NEDD8/RUB1 from
CULLIN1 (CUL1) (Schwechheimer et al., 2001). A correct
balance in this ‘cycling’ process of NEDD8/RUB1 conjugation/deconjugation is absolutely required for the proper activity of the SCF complexes. Mutations that affect CSN activity
lead to growth arrest and constitutive photomorphogenesis
development. Recently, an interesting study revealed that
CSN activity is also needed for cell-cycle progression. Loss of
CSN activity leads to delays in the G2-phase transition due to
an intensification of the DNA damage checkpoint (Dohmann
et al., 2008). Moreover, SCF activity is negatively regulated
by the CULLIN1-associated (CAND1) protein, which interacts with both the C and N termini of CUL1, blocking the
RUB1 attachment site and affecting the turnover of active
SCF complexes (Dreher and Callis, 2007). Identification of
loss of function of CAND1 in plants has raised several questions, because, in contrast to what might be expected, cand1
mutants show increased stability of Aux/IAA7 and repressor of ga1-3 (RGA), two targets of the SCFTIR1 and SCFSLY1,
respectively (Chuang et al., 2004; Feng et al., 2004), suggesting a positive role of CAND1 in SCF activity. This contrary
data could be explaining by a ‘cycling model’ where full SCF
assembly and disassembly, coupled with NEDD8/RUB1
modification and deconjugation, is necessary for an accurate
activity of the SCF and proper target degradation (Zhang
et al., 2008).
UPS-dependent degradation and the
cell cycle
Control of cell division in eukaryotes is ensured through a
number of regulatory mechanisms, including specific phasetime gene transcription and post-translational modification of key regulatory proteins, including phosphorylation,
SUMOylation (SUMO: small ubiquitin modifier) and ubiquitin-dependent degradation (Hershko, 2005; Nakayama and
Nakayama, 2006; Johnson, 2009). One of the most secure
systems to control protein activity is selective degradation
through the UPS, as it is an irreversible and highly precise
mechanism that will ensure the complete loss of function
of a given protein. Plant cell-cycle core proteins are also
largely regulated by UPS-dependent proteolysis, although
our knowledge, compared with mammals, is limited (Fig. 1B,
Fig. 2).
Events during cell division are tightly regulated. When
cells commit to divide, the first check point that they have
to overcome is the G1-to-S-phase transition, which is strictly
governed by different molecular mechanisms involving phosphorylation and degradation of key regulators. This check
point is largely controlled by the retinoblastoma (Rb) protein,
cyclin-dependent kinase (CDK) activities, and E2F transcription factors (Adams, 2001). Rb, the first tumor suppressor
gene identified in animals, is a negative cell-cycle regulator
that blocks the function of the E2F-associated transcription
Mammals
RBX
Arabidopsis
Skp2
SKP2A
RBX
Skp1
ASK1
CUL1
CUL1
Skp2
P27
P57
P21
P130
Cyc A
Cyc D
Cyc E
CDK9
MYC
E2F1
ORC1
CDT1
B-MYB
RAG2
SMAD4
FOXO1
UBP43
Vira E7
SKP2A
SKP2B
KRP1
E2FC / DPB
Known targets
Fig. 2. SCFSkp2 is a highly versatile E3 ubiquitin ligase. Human
Skp2 targets for ubiquitination/degradation a large number of
cell-cycle and non-cell cycle proteins (Nakayama and Nakayama,
2006). At present, in Arabidopsis, the only known targets of the
SCFSKP2A and SCFSKP2B complexes are E2FC-DPB and KRP1.
Nevertheless, it is possible that other cell-cycle and non-cell cycle
proteins might be also recruited by SKP2A or SKP2B.
factors, which are needed to progress through the G1/S transition. Rb is regulated mainly by CDK-dependent phosphorylation but also by a proteasome-mediated degradation
(Sdek et al., 2005). Plants have a unique Rb-related (RBR)
coding gene which is also regulated by CDK phosphorylation (Boniotti and Gutierrez, 2001). However, whether RBR
is regulated by a proteasome-dependent proteolysis is still
poorly known. There is some evidence that Arabidopsis RBR
is degraded by the proteasome in cultured cells arrested by
sucrose starvation (Hirano et al., 2011). Under sucrose deprivation, three E2F transcription factors (E2FA, -B, and -C),
which interact with RBR, are also regulated by proteasomedependent proteolysis. However, whether this proteolysis
Auxin, ubiquitin and the cell cycle | 2621
is relevant for cell-cycle control or is just a consequence of
energy deprivation needs to be analysed. To date, it has only
been shown that RBR is targeted for degradation by a virusencoded F-box named CLINK (cell-cycle link), which interacts with ASK1 and RBR. It has been proposed that RBR
is destroyed via a SCFCLINK complex to enable cell progression and virus DNA replication together with amplification (Lageix et al., 2007). Nonetheless, further experiments
need to be performed until this possibility is fully corroborated. However, it is remarkable that a similar viral cell-cycle
machinery appropriation mechanism has been described in
mammals, where the virus oncoprotein E7 targets the Rb protein for degradation (Ying and Xiao, 2006; Oh et al., 2010).
E2F transcription factors function in cell-cycle control and
are intimately regulated by RBR. These factors are capable
of establishing transcriptional regulatory loops to ensure,
in coordination with the UPS-dependent proteolysis, a balanced level of proteins needed in each phase of the cell cycle.
In plants, six different E2F genes that encode transcription
factors with activating or repressing activities have been
identified, and some are regulated by the UPS. E2FC interacts with DPB to form heterodimers in vitro and in vivo, and
these have been found to function as cell-cycle repressors of
S-phase genes (del Pozo et al., 2002b, 2006). Both E2FC and
DPB are regulated by UPS-dependent proteolysis. In this
case, the E3 involved in such degradation is the SCFSKP2A
complex, which forms part of the G1/S checkpoint in cellcycle progression (del Pozo et al., 2002b, 2006). Arabidopsis
SKP2A, and its closet homologous SKP2B, were identified
by sequence homology to human Skp2, an F-box protein
that regulates the stability of a large number of cell-cycle
and non-cell-cycle regulators (Nakayama and Nakayama,
2006; Frescas and Pagano, 2008; Bassermann et al., 2013)
(Fig. 2). Despite the fact that these two proteins share more
than 80% sequence identity, they play entirely different roles
during the plant cell cycle. SKP2A is a positive regulator of
cell division, while SKP2B functions as a negative regulator
(Jurado et al., 2008; Manzano et al., 2012). Loss of function
of SKP2A led to lower cell proliferation rates in root meristems, while skp2b mutants showed higher cell division rates
in the root meristem as well as a premature division of lateral
root founder cells, suggesting that SKP2B degrades a cellcycle activator in these cells (Jurado et al., 2008; Manzano
et al., 2012). SKP2A preferentially recognizes the CDKA/
Cyclin D (CYCD)-dependent phosphorylated E2FC isoform,
linking CDK activity to UPS-dependent proteolysis in order
to activate cell division (del Pozo et al., 2002b). In the case
of DPB, it seems that its degradation is also dependent on
phosphorylation, but whether a CDK activity participates
in such phosphorylation is unknown (del Pozo et al., 2006).
Interestingly, E2FC degradation is attenuated in dark-grown
seedlings but is triggered during light exposure, suggesting
a connection between cell division and photomorphogenesis mediated through the function of E2F factors (del Pozo
et al., 2002b). In this regard, it was shown that another E2F
factor, E2FB, is regulated by proteolysis and its turnover is
regulated by light, through the action of CSN5 and COP1
(Lopez-Juez et al., 2008), and by the plant hormone auxin
(Magyar et al., 2005). During skotomorphogenesis, growth
is based mainly on cell elongation, with cell division being
attenuated. As E2FC encompasses a repressor activity, it has
been speculated that E2FC degradation in response to light
is a requirement to allow meristem proliferation activation,
probably through the positive cell-cycle regulation of E2FB
and/or E2FA. Likewise, transcriptional analyses have shown
that E2FC regulates a large number of genes involved in light
signalling, supporting a direct link between light-dependent
development and E2FC (de Jager et al., 2009). Arabidopsis
DPB proteolysis is governed by SCFSKP2A, although not by
SCFSKP2B, demonstrating the high degree of recognition
specificity despite the high homology between these two
F-box proteins (del Pozo et al., 2006; Ren et al., 2008). In
addition, E2FC/DPB function is also important for DNA
endoreduplication control. Variations in E2FC/DPB levels by overexpressing these transcription factors, by E2FC
RNA interference, or by overexpressing the F-box SKP2A,
which also reduces the E2FC amount, alter ploidy levels (del
Pozo et al., 2006). All these data suggest that a high levels of
E2FC/DPB blocks cell proliferation and switches them into
the endoreduplication programme, while, on the other hand,
a low E2FC level favours cell division.
Protein phosphorylation by CDK plays a pivotal role in
cell-cycle regulation. To be fully active, CDK needs the collaborative interaction with cyclin-type regulatory components. On the other hand, CDK activity can be inhibited
by cyclin kinase inhibitors (CKIs). Plants possess two types
of CKI-related families: KRP/ICK and SIM/SMR. Several
studies have shown that KRP proteins function as inhibitors of cell proliferation. Gain of function of KRP activities entails severe plant developmental alterations, many of
which are associated with defects in cell division (Wang et al.,
2000; De Veylder et al., 2001; Zhou et al., 2003; Bemis and
Torii, 2007; Ren et al., 2008). In a similar manner to other
eukaryotes, many of these plant KRP proteins are regulated by ubiquitin-dependent degradation during the G1/S
transition. Arabidopsis KRP1, which binds to the CDKA;1/
CYCD2;1 complex to inhibit its function, negatively regulates
the G1/S phase transition. Ectopic overexpression of KRP1
severely compromises plant development and blocks the initial auxin-mediated pericycle cell division in founder cells in
roots (Ren et al., 2008). KRP1 is targeted for degradation by
the SCFSKP2B complex (but not by SCFSKP2A) and a RING-E3
enzyme called RKP [homologous to the human Kip1 ubiquitination promoting complex (Kpc1)] (Ren et al., 2008).
This regulation is similar to that found for the p27 protein,
a mammalian CDK inhibitor, which is ubiquitinated by two
E3 ligases, SCFSkp2 and the RING-E3 Kpc1 (Carrano et al.,
1999; Kotoshiba et al., 2005). In Arabidopsis, the RKP gene
is needed for cell division during callus formation (Lai et al.,
2009) and for beet severe curly top virus virus replication,
again linking cell division control to virus infection, as was
shown for RBR (Lai et al., 2009). In maize and Arabidopsis,
UPS-dependent proteolysis of KRP2 is CDK phosphorylation dependent and requires the N-terminal region of KRP2
(Coelho et al., 2005; Verkest et al., 2005). Interestingly,
upon beet severe curly top virus infection, KRP2 levels also
2622 | del Pozo and Manzano
decrease, suggesting either that RKP ligase targets KRP2 for
degradation or, alternatively, that the virus machinery appropriates another E3 ligase activity to promote the proteolysis
of KRP2. In addition to these examples, KRP6 and KRP7
are also degraded through the UPS. The E3 responsible for
their ubiquitination corresponds to the SCFFBL17 complex.
The F-box and leucine-rich repeat 17 (FBL17) seems to be
transiently assembled into a functional SCF during the male
germ-line formation and thereafter is destabilized before germ
cells divide (Kim et al., 2008). Interestingly, loss of function
of FBL17 somehow resembles the cdka;1 mutant phenotype,
whose unique-formed male germ cell is able to fertilize the
egg cell to generate an embryo, yet subsequently fails to fully
develop a functional endosperm, with a resulting abortion of
the embryo (Iwakawa et al., 2006; Nowack et al., 2006; Kim
et al., 2008). Recently, it has been reported that the energy
kinase sensor AtSnRK1 phosphorylates both KRP6 and
KRP7 (Guérinier et al., 2013). In this work, several evidences
suggest that this phosphorylation inactivates KRP6 function
likely by triggering its proteolysis, linking energy status with
cell division. SCFFBL17 complex has emerged as an important
cell-cycle regulator. By using transient expression assays, it
was found that FBL17 targets preferably KRP3, KRP4,
KRP5, and KRP7 for degradation (moderately for KRP6),
whereas KRP1 and KRP2 stability seems to be not affected
(Zhao et al., 2012). Furthermore, RHF1a and RHF2a, two
RING-finger E3 ligases, are essential for mitotic cell-cycle
progression during Arabidopsis gametogenesis. The defects
in male and female gametophytes found in the double rhf1a/
rhf2a mutant correlate with the KRP6 accumulation during
gametogenesis, suggesting that these two RING-finger E3
ligases target KRP6 for degradation (Liu et al., 2008).
In a similar manner to other eukaryotes, several components of the plant DNA replication machinery are also regulated by the UPS. CDC6, a protein that binds to the DNA
origin recognition complex in the G1 phase, confers on
proliferative cells the capacity to initiate DNA replication.
Arabidopsis CDC6 is degraded in a proteasome-dependent
manner in extracts of proliferating cells; nevertheless, its
turnover in contrast is reduced in extracts derived from darkgrown hypocotyls, cells that are endoreplicating rather than
dividing (Castellano et al., 2001). In this case, unfortunately,
the E3 responsible for CDC6 degradation remains unknown.
Another S-phase protein is chromatin licensing and DNA
replication factor 1 (CDT1), which is a chromatin licensing
and DNA replication factor. In Arabidopsis, two genes that
encode for CDT1-like proteins were identified. CDT1a stability is also regulated by the UPS in a CDK phosphorylationdependent manner, although the E3 ligase involved remains
unknown (Castellano et al., 2004). In other eukaryotes,
CDT1 stability seems to be regulated by two different types
of E3: by a RING-Cul4 based E3 ligase (Hu et al., 2004) or
by the SCFSkp2 complex (Li et al., 2003). Whether Arabidopsis
SKP2A or SKP2B is able to target CDT1 for proteolysis is
still unknown, and it would be interesting to explore this
possibility. Another important protein during DNA replication comprises the proliferating cell nuclear antigen
(PCNA), which is a key component of the DNA replication
machinery. In eukaryotes, this protein is subjected to a complex post-translational control, involving mono- and polyubiquitination, phosphorylation, and SUMOylation (Lee
and Myung, 2008; Zhao and Ulrich, 2010; Lo et al., 2012;
Gali et al., 2012). Arabidopsis PCNA is equally modified by
SUMO (Strzalka et al., 2012) and by ubiquitin by the RAD5a
E3 ligase (Strzalka et al., 2013). These authors showed that
the E2 AtUBC2, a plant homologue of yeast RAD6, without the participation of an E3, can ubiquitinate Arabidopsis
PCNA, suggesting a possible role in DNA repair. In addition,
AtUBC25 collaborates with the E2 AtRAD5a and the accessory protein AtUEV1A to poly-ubiquitinate PCNA (Strzalka
et al., 2013). Despite these abundant molecular data, whether
the plant PCNA is degraded by the UPS has still not been
elucidated.
The mitotic phase, which is preceded of the G2 checkpoint control to ensure that all is correct before entering the
M phase, is the process by which a eukaryotic cell separates
the chromosomes into two identical sets. Mitosis is a complex
and highly regulated phase. One of the components of the
M-phase control is the APC/C ubiquitin ligase, which promotes degradation of structural proteins associated with the
chromosomal kinetochore and mitotic cyclins (Primorac and
Musacchio, 2013). In Arabidopsis plants, the loss of APC2
function compromises female mega-gametogenesis, although
the apc2 mutant does not arrest in metaphase and accumulates high levels of cyclin, indicating that a functional apc2
allele is needed for cyclin degradation (Capron et al., 2003).
One of the two CDC27 homologues, HOBBIT/CDC27B, is
expressed mainly in the G2/M phase and seems to be required
for the correct balance between cell division and differentiation (Willemsen et al., 1998; Blilou et al., 2002). In addition,
six CDC20 and three CDH1 genes have been identified, alluding to the diversity and complexity of the APC function in
plants. It is remarkable how reduction of the APC activity
affects several developmental plant processes in addition to
cell division, including vascular tissue development, leaves,
and inflorescence architecture, as well as DNA endoreduplication (Marrocco et al., 2009). These authors also showed
that the APC/C activity remains in fully differentiated cells,
a characteristic shared by mammals, whose APC/C-encoding
genes are expressed in differentiated cells (Kim and Bonni,
2007).
In mammals, mitosis entry has been well characterized
by an increment of mitotic kinase activities. Later, the transit from metaphase to anaphase requires inactivation of the
spindle checkpoint, which involves Securin degradation by
the APC/C complex. This degradation allows the proteolytic
activity of Separase to cleave the Cohesin complex, which
holds sister chromatids together (Uhlmann et al., 2000).
Additionally, the APC/C also targets CYCB1for degradation,
reducing the level of CDK activity below a required threshold to activate cytokinesis and the exit out of mitosis (de
Gramont and Cohen-Fix, 2005). In plants, no sequence-based
SECURIN homologues have yet been identified, probably
due to sequence divergence; nonetheless, indirect studies have
suggested that functional SECURIN-like proteins might exist
(Genschik et al., 1998). Even so, APC-dependent degradation
Auxin, ubiquitin and the cell cycle | 2623
of the plant mitotic cyclins is needed for a correct cytokinesis
mediated via the mitotic microtubule reorganization to the
phragmoplast (Weingartner et al., 2004).
Alternatively to the mitotic cycle, cells can undertake a
differentiation-coupled DNA endoreduplication programme,
which in many cases has been associated with developmentally regulated processes such as endosperm formation, cell
elongation of roots or the hypocotyl, and inclusive trichome
growth (Kondorosi et al., 2000; Larkins et al., 2001). The
mechanism by which several DNA replication cycles are carried out is broadly similar among evolutionarily distant organisms. Although the majority of the knowledge gained as to
the APC/C function has been made in Arabidopsis, it was the
model legume Medicago that allowed the first identification
of a CDH1 homologue, namely, CCS52 (Cebolla et al., 1999).
CCS52 regulates the switch between a mitotic status and the
endoreduplication programme (Vinardell et al., 2003). The
key steps required for initiation of the endoreplication are the
upregulation of both CKIs, which inhibits CYC/CDK activity, and CCS52A, an APC/C activator that favours mitotic
cyclin proteolysis. The APC/C (CCS52A) recruits CYCA2;3,
CYCB1;1, CYCB1;2, and CYCA3;1 for ubiquitination and
subsequent degradation (Boudolf et al., 2009; Kasili et al.,
2010; Mathieu-Rivet et al., 2010). The CYCA2;3-CDK1;1
complex has been pointed out as a key regulatory factor
of ploidy levels. A semidominant cyca2;3 mutation induces
DNA endoreplication, while overexpression of CYCA2;3
leads to inhibition of the endocycle (Imai et al., 2006). With
this data in mind, it is clear that degradation of CYCA2;3 via
the APC is a vital mechanism to control the endoreplication
of DNA. The role of APC in plants with regard to endoreduplication and degradation of mitotic cyclins has been thoroughly reviewed (Genschik and Criqui, 2007; Xu et al., 2010).
One of the most important questions is to understand how
cells decide to exit cell division and undertake the differentiation-coupled DNA endoreduplication programme. For many
years, it was clear that plant hormonal signalling was essential
in cell division control. Recent data also show that auxin and
cytokinin cross-talk regulates the transition from the mitotic
programme to the endocycle. A reduction in auxin biosynthesis, transport, or TIR1-dependent signalling entails a premature transition to the endocycle programme (Ishida et al.,
2010). These authors proposed that auxin maxima are needed
to maintain cell division, while low auxin signalling entails
cell-division cessation, DNA endoreplication, and cell differentiation. Interestingly, overexpression of CYCA2;3 partially
rescued the premature entry into the endocycle and cell differentiation promoted by low auxin signalling, suggesting that
CYCA2;3 is regulated by auxin to a certain level. Recently, it
has been shown that cytokinin signalling through the action
of ARABIDOPSIS RESPONSE REGULATOR 2 (ARR2)
directly induces the expression of CCS52A1 in Arabidopsis
roots. This increase in CCS52A1, along with the cytokinin
induction of SHY2/IAA3 to repress auxin response, blocks
cell division and induces DNA endoreduplication in the root
differentiation zone (Takahashi et al., 2013).
Massive proteomic approaches have been carried out to
identify ubiquitinated proteins in plants, but we are still far
from knowing the identity of all the different targets of the
ubiquitin pathway in plants (Maor et al., 2007; Manzano
et al., 2008; Kim et al., 2013). In these studies, four cellcycle core proteins (CDKB1;1, CYCB3;1, CDKA;1, and
CYCD3;3) were identified as putative targets, although
the E3 ligase responsible for their ubiquitination remains
unknown. Interestingly, Arabidopsis Cyclin D3;1, which
is similar to the mammalian D-type cyclins, was reported
to be regulated by an SCF-E3 ligase type for proteolysis
(Lechner et al., 2002; Liu et al., 2008). In these works, no
further cell-cycle proteins known to be degraded through
the UPS were identified. This is probably due to their low
in planta protein level, even more so considering that their
expression could be restricted to dividing cells, or else due
to their fast turnover. Therefore, stability analyses of individual cell-cycle proteins will be required in the future to
study the amplitude of UPS-dependent proteolysis in the
control of the plant cell cycle and identification of the E3
ubiquitin ligases involved.
Auxin signalling and the cell cycle
The plant hormone auxin (IAA) is a central regulator of
essentially every aspect of plant growth and development.
Auxin activity has been implicated in a large and diverse
range of processes such as cell division and elongation,
embryogenesis, phloem and xylem differentiation, lateral
root initiation, tropic responses in response to gravity and
light, growth arrest of lateral buds, leaf senescence, fruit ripening, flowering and flower organ growth, ethylene biosynthesis, nutrient starvation responses and nutrient transport,
and in the establishment of cellular patterning in both the
shoot and root meristems as well as in the embryo (Davies,
1995; Salisbury, 1992; Mockaitis and Estelle, 2008; PerezTorres et al., 2008; Möller and Weijers, 2009; Mironova
et al., 2010; Vernoux et al., 2010; Zhao, 2010; Raven, 2012;
Shen et al., 2012; Sauer et al., 2013). In addition, the positive
effect of auxin on cell division is well known. It was proposed
that ARF2 is a negative regulator of cell division by repressing the expression of CYCD3;1 and AINTEGUMENTA, a
transcription factor that seems to be necessary and sufficient
to control cell number and growth (Schruff et al., 2006).
However, the mechanisms underlying this regulation remain
unknown. The cross-talk between auxin and cytokinin is
essential to regulate cell division (Moubayidin et al., 2009;
Su et al., 2011). These two hormones show opposite effects
concerning cell division-associated processes, such as in the
case of lateral root primordia development. A considerable
body of evidence indicates that auxin signalling plays a critical role in the lateral root formation founder cell specification
and division and in the promotion of lateral root primordia
(De Rybel et al., 2010; Overvoorde et al., 2010). In contrast,
cytokinin seems to arrest lateral root primordia development
by perturbing the formation of a correct auxin gradient in
primordia (Laplaze et al., 2007). Recently, Takahashi et al.
(2013) showed that cytokinin induces ARR regulators to
activate cytokinin-dependent gene transcription and SHY2/
2624 | del Pozo and Manzano
IAA3 to repress auxin signalling, leading to cell-cycle arrest
and subsequent cell differentiation in the root transition zone.
In the last decade, several genetic studies have shed light on
auxin signal transduction. Likewise, molecular studies have
recently showed that auxin is perceived by a co-receptor that
involves an F-box protein and its Aux/IAA targets (reviewed
by Mockaitis and Estelle, 2008; Hua and Vierstra, 2011; Peer,
2013). However, there is a set of auxin responses that take
place almost immediately, occurring within a few minutes
after hormone exposure and that are unlikely to be controlled
by a specific transcriptional network (Badescu and Napier,
2006). Physiological and molecular studies have indicated that
the AUXIN-BINDING PROTEIN1 (ABP1) regulates part of
these fast responses, which are essential for plant cell division
and development (Sauer and Kleine-Vehn, 2011). Although
the molecular mechanisms are unknown, there is a clear link
between ABP1 function and cell cycle. Plants with reduced levels of ABP1 show a decrease in CYCD3;1 and CYCD6 levels
and an increase in RBR, leading to cell-cycle cessation (Braun
et al., 2008; Tromas et al., 2009; Borghi et al., 2010), suggesting a role of ABP1 in the RBR–CYC pathway. In addition to
this fast responses, auxin alters the expression level of a large
number of genes includin many and diverse functional categories (Goda et al. 2004; Overvoorde et al. 2005; Nemhauser
et al. 2006; Bargmann et al., 2013; Lewis et al., 2013). The
auxin transcriptional response is essentially regulated by
large families of transcription factors: the Aux/IAA family,
which contains 29 members and which have been described
as transcriptional repressors, plus the auxin-response factor (ARF) family encompassing 23 members, which act as
transcriptional activators, although some have also been
reported to act as repressors (Pierre-Jerome et al., 2013). Aux/
IAA factors lack the ability to bind DNA, but they have the
capability to heterodimerize with ARFs, through domains
III and IV, forming diverse Aux/IAA–ARF modules that
cooperatively regulate the transcription of auxin-regulated
genes during organ formation and plant development (Goh
et al., 2012; Peer, 2013; Pierre-Jerome et al., 2013) (Fig. 3).
Several studies have shown that Aux/IAA proteins are highly
unstable and are degraded in an ubiquitin-dependent manner
(Worley et al., 2000; Ramos et al., 2001; Dos Santos et al.,
2009; Chapman and Estelle, 2009; Pierre-Jerome et al., 2013).
TIR1 and TIR1-related genes [Auxin signalling F-box (AFBs)]
encode F-box proteins, which hold the ability to recruit Aux/
IAA repressors (Gray et al., 2001; Dharmasiri et al., 2005).
In the current model, TIR1/AFBs and Aux/IAA form a
co-receptor system, in which auxin favours the interaction
between the F-box protein and the Aux/IAA repressor, acting
as a ‘molecular glue’ (Tan et al., 2007). Once the co-receptor
is assembled, Aux/IAA proteins are initially ubiquitinated
and subsequently degraded via the proteasome 26S, releasing ARF transcription factor from the repression executed by
the Aux/IAA proteins (Fig. 3). Finally, ARFs activate auxinresponse genes through the auxin-response element (AuxRE)
located in the promoter region of these genes. Interestingly,
in Arabidopsis, the auxin-binding and Aux/IAAs interaction
sites are conserved among all six TIR1/AFBs. However, not
all TIR1/AFBs function as an activator of auxin signalling.
TIR1 and AFB2 proteins are undoubtedly positive regulators
of the auxin response, while AFB4 seems to be a negative
effector (Greenham et al., 2011). In principle, these six TIR1/
AFBs might recognize any of the 29 members of the Aux/
IAA proteins, forming several types of auxin co-receptor
combinations that might respond to different auxin concentrations and/or cell types. To provide answers to this complex
combinatorial problem an elegant heterologous interaction
system based on a yeast two-hybrid system and different
auxin concentrations in medium was designed (CalderonVillalobos et al., 2012). These authors obtained evidence that
even Aux/IAA proteins sharing high homology in the interaction domain (DII) interact in a different manner with TIR1/
AFBs, thus indicating the high complexity of this signalling
network, which might explain, at least in part, the different
responses experienced in response to wide-ranging auxin
concentrations and the large variety of processes regulated
by auxin.
Although the function of auxin in cell-cycle control is clear,
it has been widely discussed that the TIR1/AFBs–Aux/IAA
system per se does not explain this extensive role of auxin in
cell division. Several core cell-cycle genes contain the AuxRE
elements (forward: TGTCnC or reverse: GnGACA) in their
promoter regions, suggesting that they could act as direct
targets of the ARF–Aux/IAA regulatory systems (Fig. 4).
However, a large number of these AuxRE-containing cellcycle genes are not induced by auxin treatment, as has been
shown in several transcriptomic analyses (Vanneste et al.,
2005; De Smet et al., 2008). In contrast, there are genes that
do not contain AuxRE but are induced by auxin early on,
such as E2FA, DEL3, CYCB2;2, and CYCA2,2 (Fig. 4).
Although it is evident that auxin is directly linked to cellcycle regulation, the details of this connection at the molecular level remain obscure. It has been proposed that further
pathways, independent of Aux/IAA–ARF, operate to regulate cell division. Recently, another F-box protein, SKP2A,
was reported to bind to auxin and target E2F/DPB repressors
for degradation. With this fact in mind, it could be possible
that some cell-cycle core genes induced by auxin, which do
not contain AuxRE but instead E2F sites in their promoters, might respond to this hormone by degradation of E2F
repressors rather than by ARF activation. However, further
studies will be needed to prove this possibility. In addition,
there are cell-cycle genes that contain AuxRE and E2F sites.
Although it is still speculative, it is reasonable to think that, in
order to be induced by auxin, both Aux/IAA and E2F repressors have to be degraded simultaneously by SCFTIR1/AFBs
and SCFSKP2A, allowing ARFs and positive E2F factors to
activate the transcription of genes needed for cell division.
Afterwards, E2F repressors will again occupy these E2F sites
to limit the re-entry into a new cell cycle, accordingly preventing a non-programmed cell division (Fig. 3).
Several lines of evidence, such as the increasing sensitivity
to auxin in SKP2A-overexpressing roots, the auxin-resistant
growth of skp2a roots or the direct positive effect of auxin
in promoting SKP2A degradation in a cell-free system, have
hinted at a connection between this phytohormone and
SCFSKP2A (Jurado et al., 2008). Recently, it was shown that
Auxin, ubiquitin and the cell cycle | 2625
Fig. 3. Model of action of TIR1 and SKP2A in response to high auxin levels during the cell cycle. When the auxin concentration is low,
Aux/IAA and E2FC/DPB repress the auxin response as well as cell-cycle genes. Many core cell-cycle genes contain AuxRE and E2F
sites in their promoters (Fig. 4). When the auxin level increases, Aux/IAA and E2FC/DPB repressors are degraded by the action of TIR1/
AFBs and SKP2A, respectively. These sites are occupied by the ARFs and E2F/DP-positive dimers (E2F+/DP+: E2FA/DPA or E2FB/
DPA) to activate transcription of cell-cycle genes that are coupled to the auxin response. After the cell cycle has initiated the progression
through S phase, the SKP2A F-box is degraded to prevent further G1/S-phase transition signalling, allowing progression into the S/G2
phase. After SKP2A is degraded, E2FC/DPB reoccupies the E2F sites to prevent a premature entry or re-entry into S phase.
SKP2A binds to auxin (Jurado et al., 2010). A structural
model for SKP2A was generated using the crystallization
data obtained from human Skp2 (Schulman et al., 2000).
The model revealed the high degree of similarity between
both proteins, showing a similar structure of leucine-rich
repeats, a protein structural motif that forms an α/β horseshoe fold involved in the recognition of targets and a disorganized tail at the C terminus that seems to function as a
cover of the leucine-rich repeat domains (Enkhbayar et al.,
2004; Jurado et al., 2010). In HsSkp2 protein, this C-terminal
tail displays specific functions, as a splicing variant, Skp2B,
recognizes different target proteins through the alternative
tail (Germain, 2011). By overlapping the SKP2A model
onto the TIR1 crystal structure containing the IAA molecule, Jurado et al. (2010) were able to identify the putative
auxin-binding site, which was corroborated by mutations in
critical amino acids in this domain. These mutants were no
longer able to bind auxin. In vivo analyses showed that this
binding site is also needed for E2FC and DPB degradation
(Jurado et al., 2010), and in vitro interaction assays showed
that the SKP2A–DPB interaction is enhanced by auxin in
a concentration-dependent manner, but whether this system
is mechanistically similar to the TIR1–Aux/IAA complex
currently continues to be unknown. Interestingly, SKP2A is
regulated by UPS-dependent proteolysis, and auxin triggers
its degradation in few minutes (Jurado et al., 2008, 2010).
Although similar to the Aux/IAA degradation by TIR1, it
has been shown that TIR1 is not the E3 ligase responsible
for targeting SKP2A (Jurado et al., 2010). It is possible that
the binding of auxin induces a conformational change in
SKP2A that allows ubiquitination by another E3 or, alternatively, exposes a specific motif of the SKP2A protein that
can be auto-ubiquitinated. Considering this mechanism,
it is tempting to speculate that auxin favours E2FC/DPB
2626 | del Pozo and Manzano
Protein
Name:
Genomic
Locus:
Auxin
AuxRE
E2F site induced in
site
roots
Auxin
induced
pericycle
cells
Protein
Name:
Genomic
Locus:
CDKB1;1 AT3G54180
DPA
AT5G02470
CDKB2;1 AT1G76540
E2FB
AT5G22220
CDKB2;2 AT1G20930
KRP1
AT2G23430
CDKC;1
AT5G10270
KRP2
AT3G50630
CDKC;2
AT5G64960
KRP4
AT2G32710
CDKD;2
AT1G66750
KRP6
AT3G19150
CYCA1;1 AT1G44110
KRP7
AT1G49620
CYCA1;2 AT1G77390
RBR
AT3G12280
CYCA2;1 AT5G25380
CDKA;1
AT3G48750
CYCA2;4 AT1G80370
CDKB1;2
AT2G38620
CYCA3;1 AT5G43080
CDKD;1
AT1G73690
CYCA3;2 AT1G47210
CDKD;3
AT1G18040
CYCA3;3 AT1G47220
CDKE;1
AT5G63610
CYCB1;1 AT4G37490
CDKF;1
AT4G28980
CYCB1;3 AT3G11520
CYCA2;2
AT5G11300
CYCB1;4 AT2G26760
CYCA2;3
AT1G15570
CYCB2;1 AT2G17620
CYCA3;4
AT1G47230
CYCB2;3 AT1G20610
CYCB1;2
AT5G06150
CYCB2;4 AT1G76310
CYCB2;2
AT4G35620
CYCB3;1 AT1G16330
CYCD2;1
AT2G22490
CYCD1;1 AT1G70210
CYCD4;1
AT5G65420
CYCD3;1 AT4G34160
CYCD5;1
AT4G37630
CYCD3;2 AT5G67260
CYCD7;1
AT5G02110
CYCD3;3 AT3G50070
CYCH;1
AT5G27620
CYCD4;2 AT5G10440
DEL3
AT3G01330
CYCD6;1 AT4G03270
DPB
AT5G03410
CKS1
AT2G27960
E2FA
AT2G36010
CKS2
AT2G27970
E2FC
AT1G47870
DEL1
AT3G48160
KRP3
AT5G48820
DEL2
AT5G14960
KRP5
AT3G24810
WEE1
AT1G02970
Auxin
AuxRE
E2F site induced in
site
roots
Auxin
induced
pericycle
cells
Fig. 4. Identified core cell-cycle genes of the Arabidopsis genome. Blue boxes indicate genes that show at least one auxin response
element (AuxRE: TGTCnC or GnGACA) within the genomic region located 1 kb upstream of the ATG codon. Yellow boxes indicate genes
that contain at least one minimum E2F consensus site motif (NNNSSCGS) within the genomic region located 1 kb upstream of the ATG
codon. Red boxes indicate genes that were induced in whole roots by auxin treatment (Vanneste et al., 2005). Green boxes indicate
genes that were induced in pericycle cells by auxin treatment (De Smet et al., 2008). In both cases, genes were considered to be auxin
induced when expression increased over 3-fold compared with untreated samples and the false discovery rate value was P < 0.025.
degradation and, sequentially, SKP2A degradation to prevent its overfunction and to impede the re-entry of the cell
into S phase, implying a highly sophisticated autoregulatory
system (Fig. 3).
This regulative fine-tuning of E3 ligase levels by the specific
UPS-dependent degradation has been described previously.
In yeast, SCFCdc4 regulates cell-cycle progression by degrading
diverse cell-cycle regulators. The F-box Cdc4 is a short-lived
protein that is targeted for proteolysis in a SCFCdc4-dependent
manner (Zhou and Howley, 1998). In mammals, Skp2 is also
targeted for degradation by APC/CCdh1 (Gao et al., 2009) and
by SCFSkp2 function during the G0/G1 transition, suggesting an autocatalytic mechanism (Wirbelauer et al., 2000). In
plants, the F-box COI1 has been shown to be regulated by
the UPS (Yan et al., 2013). Mechanically similar to TIR1,
COI1 targets the JAZ repressors in a jasmonate-dependent
manner (Chini et al., 2007; Katsir et al., 2008). In this case,
COI1 degradation is SCFCOI1 and jasmonate independent,
suggesting that another E3 ubiquitin ligase is responsible for
proteolysis (Yan et al., 2013). It is particularly intriguing that
Auxin, ubiquitin and the cell cycle | 2627
COI1 is degraded through the UPS in yeast cells, suggesting a
conserved degradation mechanism between plants and yeast.
It would be interesting to analyse whether this system is general for many plant F-box proteins or just specific to some
of them.
Overall, the data suggest that novel and specific mechanisms regulate the rapid assembly and disassembly of SCF
complexes by degrading the F-box protein, which is the
substrate recognition subunit. This mechanism allows the
cell to respond quickly and efficiently to different signals
on ‘demand’ and precisely controls the overfunction of the
E3 ligase activity. Several studies have provided evidences
that E3 ligases might act as receptors of different hormones
for signal transduction. Likewise, considering that plants
have more than 700 F-box proteins, it is highly likely, in the
near future, that there will be the discovery of novel small
molecules/hormones that will be shown to interact with E3
ligases to regulate different biological processes, including
cell division.
Acknowledgements
We would like to apologize to all those authors whose work
has not been cited directly in this review. We want to thank all
members of the laboratory for their work and for the hours
of discussion and support, and R.M. Fratini for the English
revision. This work was supported by grants from the Spanish
Government: BIO2008-00639, BIO2011-28184-C02-01 and
CDS2007-0057 to JCP.
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