Involvement of brassinosteroid signals in the floral-induction network of Arabidopsis
Key Laboratory of Arid and Pasture Agroecology of Ministry of Education, School of Life Sciences, Lanzhou University
Lanzhou, Gansu, 730000
The transition to flowering is known to be regulated by numerous interacting endogenous and environmental cues, of which brassinosteroids (BRs), a group of polyhydroxylated steroid phytohormones, appear to be linked to the regulation of flowering time. In Arabidopsis, BR biosynthetic det2 mutants exhibited delayed flowering time by at least 10 d compared with the wild type. The levels of endogenous BRs in det2 were below 10% of the wild type. The timing of flowering was also delayed in the BR biosynthetic dwf4 and cpd mutants and in the BR-insensitive bri1 mutants. Because brassinolide (BL) and different BL precursors were over-accumulated in BR biosynthetic mutants and BR-insensitive bri1 mutants, this showed that alterations in the endogenous BL content and the level of different BL precursors affect flowering time in Arabidopsis. The late-flowering phenotypes of bri1 also showed that components of the BR signal transduction pathway affect flowering time. So far, reports on a connection between BRs and flowering time are limited. This review summarizes recent advances regarding the action of BRs in the transition to flowering.
One of the most important events in the life cycle of
flowering plants is the transition from vegetative to
reproductive growth. This process is caused by floral
induction which is dependent on numerous interacting
endogenous and environmental cues (Boss et al., 2004). In
Arabidopsis, the results of extensive genetic studies of
lateflowering mutants revealed the existence of at least four
signalling pathways that co-ordinately promote flowering,
including environmental induction through photoperiod,
temperature, autonomous floral initiation, and regulation
by gibberellins (GAs) (Michaels, 2009). In addition,
endogenous ascorbic acid, ethylene, and some environmental
factors, such as ambient temperature and light quality, have
also been found to be critical in mediating flowering
(Achard et al., 2007; Lee et al., 2007; Wollenberg et al.,
2008; Kotchoni et al., 2009). Although major components
regulating flowering time have been identified in
Arabidopsis, other factors still await discovery.
Brassinosteroids (BRs) are a class of steroidal hormones
essential for plant growth and development, including
skotomorphogenesis, photomorphogenesis, xylem
formation, cell division, and cell elongation (Clouse, 1997; Clouse
and Sasse, 1998; Yang et al., 2005). Brassinolide (BL), the
most biologically active BR, was initially isolated from rape
pollen of (Brassica napus) and its structure was determined
in 1979 (Grove et al., 1979) and a variety of BRs have since
been identified from a broad range of plant species. These
molecules have been widely recognized as a major group of
plant growth regulators (Bajguz and Tretyn, 2003; Zullo
and Kohout, 2004). The typical phenotypes of BR-defective
or signalling mutants include dwarfism, small dark green
leaves, a compact rosette structure, delayed flowering and
senescence, and reduced male fertility (Clouse, 1997). In
addition, agricultural applications of BRs have been used as
BR treatments can increase yield and improve stress
resistance in several major crop plants (Symons et al., 2006;
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Wu et al., 2008). Although rapid advances in BR
biosynthesis and signal transduction, as well as their
interaction with other plant hormones have been reported (Arteca
and Arteca, 2008; Kim et al., 2008), there is limited
information about the functions of BRs in the flowering
transition. The involvement of BRs in regulating the floral
induction networks of Arabidopsis through the regulation of
critical flowering-time genes is described here.
Molecular pathways regulating flowering
induction in Arabidopsis
Photoperiod (day-length) signals were first described in
plants in the early 20th century and identified as one of the
most important environmental factors that affect the
transition to flowering (Hayama and Coupland, 2003) (Fig. 1).
Physiological and molecular genetic analyses have shown
that day-length signals are perceived by phytochromes,
cryptochromes, and ZEITLUPE (ZTL), which mediate the
light input to the downstream components including the
circadian clock (Somers et al., 1998; Martinez-Garcia et al.,
2000; Devlin et al., 2007). The circadian clock generates
a series of rhythmic output signals to regulate photoperiodic
flowering. This process is mainly mediated by GIGANTEA
(GI) (Rubio and Deng, 2007). GI-mediated photoperiodic
flowering is governed by the co-ordinated interaction of two
distinct genetic pathways. One is mediated via CONSTANS
(CO), which encodes a positive regulator for flowering and
the other is mediated via miR172 and its targets (Jung et al.,
2007). The CO-dependent pathway is mediated by the blue
light receptors FLAVIN BINDING, KELCH REPEAT,
and F-BOX 1 (FKF1) that interact with GI to cause
CYCLING OF DOF FACTOR 1 (CDF1) to release its
repression of CO, thus promoting the expression of
FLOWERING LOCUS T (FT) (Sawa et al., 2007). The
CO-independent miR172 pathway mediates light signals
from GI through miR172 to repress the expression of a set
of APETALA2 (AP2)-like genes including TARGET OF
EAT1 (TOE1), TOE2, and TOE3 and to promote the
expression of FT (Aukerman and Sakai, 2003; Jung et al.,
2007). FT and SUPPRESSOR OF OVEREXPRESSION
OF CONSTANS 1 (SOC1) act as floral integrators that are
repressed by FLOWERING LOCUS C (FLC) and SHORT
VEGETATIVE PHASE (SVP) (Searle et al., 2006). Finally,
FT is transported through the phloem to the shoot apex
where it interacts with the FD, a bZIP transcription
factor to activate the expression of APETALA1 (AP1),
FRUITFULL (FUL), and SEPALLATA3 (SEP3)
(TeperBamnolker and Samach, 2005; Wigge et al., 2005). FT and
FD also promote SOC1 expression at the shoot apical
meristem and then activate the expression of LEAFY (LFY)
to initiate flowering (Searle et al., 2006).
The vernalization pathway of Arabidopsis promotes
flowering in response to extended exposure to low
temperature (Fig. 1). Currently, two vernalization-related
pathways have been identified. One is FLC-dependent and the
other is FLC-independent (Alexandre and Hennig, 2008).
Fig. 1. Model of the floral-induction networks in Arabidopsis.
Arrows and perpendicular lines represent actions of promotion and
inhibition, respectively. Four molecular responses pathway
regulating floral induction have been identified in Arabidopsis:
photoperiod pathway, vernalization pathway, autonomous pathway, and
gibberellins pathway. Details and definitions are provided in the
The FLC-dependent pathway is thought to be the major
determinant of vernalization in Arabidopsis (Scho nrock
et al., 2006). FLC is a MADS-box transcription factor that
is expressed in both the vasculature of leaves and the shoot
apical meristem (SAM). FLC can interact with SVP to
inhibit flowering mainly through directly binding to specific
regulatory elements in the FT and SOC1 loci in Arabidopsis
(Searle et al., 2006; Li et al., 2008). The FLC-dependent
pathway promotes flowering in winter-annual accessions of
Arabidopsis through epigenetic silencing of FLC that is
mediated by histone modifications at the FLC locus
(Bastow et al., 2004). De Lucia et al. (2008) reported that
vernalization-induced epigenetic silencing of FLC involves
changed composition and dynamic redistribution of
Polycomb complexes at different stages of the vernalization
process. Vernalization-induced FLC repression is triggered
by association of the plant homeodomain (PHD) finger
protein VERNALIZATION 5 (VRN5) to a specific domain
in FLC intron 1, in a process that depends on the
coldinduced PHD protein VERNALIZATION INSENSITIVE
3 (VIN3). Subsequently, the VRN5 distribution changes,
which is accompanied by increased trimethylation of lysine
27 of histone 3 (H3K27me3) (De Lucia et al., 2008).
Maintenance of FLC repression requires H3K27me3; the
Polycomb Repressive Complex 2 (PRC2) component
VERNALIZATON 2 (VRN2) and the LIKE
HETERCHROMATIN PROTEIN 1 (LHP1)/TERMINAL FLOWER 2
(TFL2) are also important candidates (Mylne et al., 2006;
Sung et al., 2006; De Lucia et al., 2008). More recently, an
FLC-independent vernalization pathway has been
proposed. In this pathway, vernalization can directly promote
flowering through AGAMOUSLIKE19 (AGL19) and
AGAMOUS-LIKE 24 (AGL24) in Arabidopsis (Michaels
et al., 2003; Alexandre and Hennig, 2008). AGL24 and
AGL19 both act as activators of flowering in Arabidopsis.
AGL24 and SOC1 activate each others expression to
pomote flowering but AGL19 acts independently of SOC1
(Scho nrock et al., 2006; Liu et al., 2008). In the response to
vernalization, AGL24 and AGL19 expression is also
accompanied by chromatin modifications such as H3K27me3,
and they mostly act in the late stages of vernalization
(Alexandre and Hennig, 2008).
The autonomous pathway constitutes a heterogeneous
group of genes that includes FVE, FLOWERING LOCUS
D (FLD), LUMINIDEPENDENS (LD), FLOWERING
LOCUS K (FLK), FY, FCA, and FPA (Michaels and
Amasino, 2001). These genes regulate flowering
independently of environmental cues mainly by stably suppressing
FLC expression (Michaels and Amasino, 1999, 2001).
Mutations in any one of these genes result in elevated FLC
expression which can be epigenetically silenced by
vernalization (Michaels and Amasino, 1999; Sheldon et al., 1999,
2000). In addition, Veley and Michaels (2008) found that
fpa fve double mutants are late flowering in an flc-null
background, which showed that the autonomous pathway
also acts through FLC-independent responses (Fig. 1).
The acceleration of GAs in Arabidopsis flowering has
been confirmed (Blazquez et al., 1998). The GA pathway
has less influence on flowering time in long days than in
short days in Arabidopsis (Wilson et al., 1992; Chandler
et al., 2000). Gibberellins promote flowering in Arabidopsis
through the activation of SOC1, LFY, and FT in
inflorescence, floral meristems, and leaves, respectively (Lee
et al., 2000; Yu et al., 2004; Eriksson et al, 2006; Hisamatsu
and King, 2008) (Fig. 1).
Brassinosteroid biosynthetic pathway
In Arabidopsis, the main BR biosynthetic pathway has been
established via extensive stepwise metabolic studies (Divi
and Krishna, 2009). These studies showed that BL (C28
brassinosteroid) is synthesized from campesterol (CR), CR
is first converted to campestanol (CN), which is then
converted to castasterone (CS) through two parallel
branched biosynthetic pathways, the early and late C-6
oxidation pathways, and CS is finally converted to BL
(Noguchi et al., 2000). In the early stages of BL, the early
C-22 oxidation pathway is also the main BR synthesis
route. In this process, CR is initially converted to
22-OHCR, then gradually converted to 22-OH-4-en-3-one,
22-OH-3-one, and 6-deoxoCT, which is finally converted
to BL through the late C-6 oxidation pathways (Fujioka
et al., 2002). In addition, a shortcut in BR biosynthesis
has been proposed, which allows direct conversion of
early 22-hydroxylated intermediates to 6-deoxo3DT and
6-deoxoTY via C-23 hydroxylation (Ohnishi et al., 2006).
These cross-linked pathways together ensure the synthesis
of BL (Fig. 2).
In Arabidopsis, an analysis of BR-deficient mutants
revealed the existence of many enzymes that catalyse BL
biosynthesis. DEETIOLATED2 (DET2), a 5a-reductase,
catalyses the conversion of 4-en-3-one to 3-one and
22-OH4-en-3-one to 22-OH-3-one (Fujioka et al., 1997, 2002).
Some cytochrome P450 mono-oxygenases were found to
catalyse multiple oxidative conversions in BL biosynthesis.
The C-22 and C-23 hydroxylation reactions are mediated by
the P450s DWARF4 (DWF4) and CONSTITUTIVE
PHOTOMORPHOGENESIS AND DWARFISM (CPD),
respectively (Szekeres et al., 1996; Choe et al., 1998), and the
C-6 oxidation is catalysed by BR6ox (Shimada et al., 2001).
Two BR-biosynthetic P450s CYP90C1 and CYP90D1
preferentially convert 22-OH-4-en-3-one, 22-OH-3-one, and
3-epi-6-deoxoCT to 23-hydroxylated products (Ohnishi
et al., 2006). The typical phenotypes of mutants impaired in
BR biosynthesis include dwarfism, small dark green leaves,
a compact rosette structure, delayed flowering and
senescence, and reduced male fertility which can be rescued in the
wild-type when BRs are applied exogenously (Clouse,
Brassinosteroid signalling pathway
BRs are perceived by the cell surface receptor kinase complex
including BRASSINOSTEROID INSENSITIVE 1 (BRI1),
and BRI1 ASSOCIATED PROTEIN KINASE 1 (BAK1)
(Li et al., 2002; Nam and Li, 2002; Tang et al., 2010). BRI1
contains an extracellular domain, a single transmembrane
domain, and a cytoplasmic kinase domain that initiates
intracellular signal transduction. The extracellular domain of
BRI1 contains 24 leucine-rich repeats (LRRs) and an island
domain (ID) between LRR20 and LRR21, and its
cytoplasmic region can be subdivided into a juxtamembrane (JM)
domain, a Ser/Thr kinase domain and a C-terminal (CT)
domain (Vert et al., 2005). BRs bind directly to the ID
LRR21 domain to activate the BRI1 kinase. Activation of
BRI1 leads to disassociation with BRI1 KINASE
INHIBITOR 1 (BKI1) and its association with BAK1. The activated
BAK1 further transphosphorylates JM and CT domains in
BRI1, thereby increasing BRI1 kinase activity (Wang et al.,
2005, 2008). Activation of BRI1 leads to the
phosphorylation of Ser230 of BR-SIGNALLING KINASE 1 (BSKs),
which then interacts with and presumably activates BRI1
SUPPRESSOR 1 (BSU1) phosphatase (Tang et al., 2008;
Kim et al., 2009). BSU1 inactivates the GSK3-like kinase
BRINSENSITIVE 2 (BIN2) by dephosphorylating a conserved
phospho-tyrosine residue and inhibits phosphorylation of
BRASSINAZOLE RESISTANT 1 (BZR1) and
BRI1-EMSSUPRESSOR 1 (BES1)/BRASSINAZOLE RESISTANT 1
(BZR2) (Kim et al., 2009). BIN2 phosphorylates BZR1 and
BZR2/BES1 to inhibit their DNA binding activity and to
promote their cytoplasmic retention by the 14-3-3 proteins
(He et al., 2002; Bai et al., 2007). Inactivation of BIN2 allows
the accumulation of unphosphorylated BZR1 and BZR2/
BES1 in the nucleus, which, in turn, will regulate BR target
genes (Yin et al., 2002; He et al., 2005). BZR1 and BES1 can
bind directly to the promoter regions of the BR biosynthetic
genes, CPD and DWF4, and inhibit their expression, forming
a negative feedback loop (Tanaka et al., 2005) (Fig. 3)
Relationship between brassinosteroids and
In Arabidopsis, most BR-biosynthetic mutants and
BRinsensitive mutants exhibit a prolonged vegetative phase
and delayed flowering time. Under the same growth
conditions, det2 mutants produced a rosette of 1963 leaves
compared with 962 leaves in the wild type before flowering.
det2 mutants did not flower until 33 d post-germination
(delayed by at least 10 d compared with the wild type)
(Chory et al., 1991). The levels of endogenous BRs in det2
are below 10% of those of the wild type (Fujioka et al.,
1997), indicating that endogenous BR levels are closely
linked to the flowering time. dwf4, another BR biosynthetic
mutant, also possessed a prolonged vegetative phase and
produced twice the number of rosette leaves than the wild
type. The flowering time was delayed by 4 d in dwf4
mutants (Azpiroz et al., 1998). Shimada et al. (2003)
reported that the highest level of endogenous BRs and the
highest expression of the DWF4, BR6ox1, and BR6ox2
genes in Arabidopsis were observed in apical shoots. bri1
and cpd mutants exhibited a modest late-flowering
phenotype (bolting after ;13 leaves), but when flowering time was
measured as days to the start of bolting, cpd flowered later
than bri1 (41 d versus 33.8 d) (Li and Chory, 1997;
Domagalska et al., 2007). Although there are no precise
reports on the BL precursor levels in cpd and dwf4 mutants,
the BR-biosynthetic pathway has been shown to be blocked
in cpd and dwf4 mutants, which probably results in the
over-accumulation of different BL precursors (Szekeres
et al., 1996; Azpiroz et al., 1998). By contrast, it is clear
that bri1 mutants accumulated very high levels of
endogenous BRs, such as typhasterol (TY), CS, and BL (Noguchi
et al., 1999). These results indicated that alterations in the
endogenous BL content and different BL precursor levels
affect flowering time in Arabidopsis. Meanwhile, the
lateflowering phenotypes of bri1 also showed that components
of the BR signal transduction pathway affect flowering
CPD: CPD encodes a C-23 steroid hydroxylase CYP90A1,
which acts at one of the later steps of BR biosynthesis in
Arabidopsis (Szekeres et al., 1996). Bancos et al. (2006)
reported diurnal changes in CPD expression by analysis of
the CPD:LUC transgenic line and these changes were
clearly observed when the CPD:LUC seedlings were grown
under continuous white light (LL) and continuous dark
(DD), following alternation of 12 h of light and 12 h of
darkness (LD) for 7 d. Circadian oscillations in CPD
expression were observed in LL for at least 96 h. In DD,
circadian activity of the CPD promoter was also observed for
3 d. Furthermore, the diurnal rhythmicity of CPD is not
determined by feedback regulation because diurnal expression
of CPD is maintained in the bri1 mutant. It was also
indicated that endogenous BR content exhibits diurnal
changes, leading to BL accumulation in the middle of the
day. Bancos et al. (2006) speculated that the diurnal control
of transcription may involve the AAAATCT motif present in
both the CPD and CYP85A2 promoters because this
sequence is a potential binding site of CIRCADIAN CLOCK
ASSOCIATED 1 (CCA1), a core circadian clock component.
Hanano et al. (2006) reported that BR (homobrasinolide)
application shortened the period of the circadian rhythms of
CHLOROPHYLL A/B BINDING PROTEIN (CAB2),
COLD AND CIRCADIAN-REGULATED 2 (CCR2), and
CCA1 under both LL and DD. Moreover, cpd mutants
displayed a 3 h long-period of the CCR2 phenotype under
LL, since cpd mutants have impaired BR biosynthesis, all
these results indicated that BR can modulate the circadian
rhythms of these genes. They also reported that BRs play an
important role in promoting periodicity because periodicity of
CAB2 rhythms in DD was significantly lower after BR
treatment than without it.
The circadian clock is an endogenous mechanism that is
able to generate biological rhythms with a 24-h period to
regulate metabolism and physiology in plants (Harmer,
2009). CCA1 is a morning-expressed Myb-transcription
factors, and LATE ELONGATED HYPOCOTYL (LHY)
and the evening-phased pseudoresponse regulator TIMING
OF CAB EXPRESSION 1 (TOC1) constitute the core
feedback loop of the circadian oscillator (Salome and
McClung, 2004; McClung, 2006). In Arabidopsis, the
circadian system has dramatic effects on flowering time,
which it regulates through the CO and FT genes
(SuarezLopez et al., 2001; Yanovsky and Kay, 2002). Niwa et al.
(2007) suggested that CCA1/LHY act redundantly as
negative regulators of the photoperiodic flowering pathway,
but the TOC1 gene serves as an activator. In their model,
the core CCA1LHY/TOC1 coordinately modulate the
COFT photoperiodic flowerig pathway particularly
through the flowering-associated GI, CDF1, and FKF1.
Thus, based on the above findings, BR influences the
circadian clock, and the circadian clock regulates flowering
time, which might imply that BR regulates flowering time
through the circadian clock system (Fig. 4).
BRI1: In Arabidopsis, Domagalska et al. (2007) found that
BRs regulate flowering time by regulating expression of
FLC. FLC is a potent floral repressor, at least in part, by
binding directly to CArG boxes within the first intron of FT
and the promoter regions of SOC1 and FD (Searle et al.,
2006). LD and FCA are included in the autonomous
pathway. FLC transcript levels are higher and flowering is
later in autonomous mutants such as ld mutants than in
wild-type Arabidopsis grown under the same conditions
(Michaels and Amasino, 1999). Domagalska et al. (2007)
reported that bri1 delays flowering of the
autonomouspathway mutant ld, fca, and the dominant FRI line through
enhancing FLC expression. Moreover, the late-flowering
phenotype of bri1 ld, bri1 fca, and bri1 FRI can be
suppressed by vernalization or FLC-RNAi constructs. In
addition, bri1 the increased levels of histone H3 acetylation
at the FLC locus in the ld background. Histone acetylation
at the FLC genomic locus was found to be correlated with
actively transcribed FLC (He et al., 2003; Ausin et al.,
2004). These results showed that BR signalling promotes
flowering by enhancing the repression of the autonomous
pathway gene LD and FCA on FLC (Fig. 4). Further, they
found that the flowering time of double mutants bri1 gi and
ga1 bri1 was earlier than that of bri1 ld, bri1 fca, and bri1
FRI mutants. The gi mutant, impaired in the photoperiod
pathway, was the very late-flowering single mutant under
LL and was slightly later flowering under DD. ga1,
a gibberellin-deficient mutant, had a mild late-flowering
phenotype. These results showed that the BRI1 pathway
might not function via the photoperiod and the gibberellin
pathways. Meantime, bri1 mutants responded only partially
to vernalization and the expression of autonomous pathway
genes were not changed in bri1, which indicated that BR
signalling acts to repress the expression of FLC and that it
does so independently of vernalization and the autonomous
pathway (Domagalska et al., 2007). Thus, BR signalling
probably has previously unidentified independent genetic
function in the repression of FLC (Domagalska et al.,
2007). However, because single bri1 mutants only displayed
a weak late-flowering phenotype, BR probably has a role
assisting a different floral indution pathway in the
repression of FLC.
BES1: Evidence for another molecular interaction between
components of the BR signalling pathway and the flowering
pathway came from the research of Yu et al. (2008). The
study demonstrated that BES1 recruits two JmjN/C
domain-containing proteins, EARLY FLOWERING 6
(ELF6) and RELATIVE OF EARLY FLOWERING 6
(REF6), to regulate the expression of target gene (Fig. 4).
Previous genetic analyses showed that ELF6 and REF6
influenced flowering time (Noh et al., 2004; Clouse, 2008).
ELF6 is an upstream repressor in the photoperiodic
flowering pathway and elf6 causes early flowering. REF6 acts as
a repressor of FLC, leading to the accumulation of FLC
transcripts in ref6 mutants, which results in late flowering
(Noh et al., 2004). In addition to the flowering phenotype,
Yu et al. (2008) revealed that elf6 and ref6 mutants display
BR-response phenotypes, such as reduced cell elongation
and shorter leaf petioles compared with the wild type. The
ref6 mutation can also enhance the bri1-5 phenotype,
resulting in even shorter hypocotyls as well as darker green
and more curled leaves. Many BR-induced genes are
downregulated in ref6 and elf6 knockout mutants, which futher
revealed that ELF6 and REF6 can function as activators of
BR-regulated genes. In addition, ELF6 and REF6 are
recruited by BES1 to modulate gene expression through
histone modifications. The repression of FLC by REF6 is
associated with histone modifications in FLC chromatin
(Noh et al., 2004). Collectively, the interaction between
BES1 and ELF6/REF6 established another link between
floral induction and the BR pathway.
Recent studies demonstrated that all three rice SHORT
VEGETATIVE PHASE (SVP)-group genes OsMADS22,
OsMADS47, and OsMADS55 act as negative regulators of
BR responses (Duan et al., 2006; Lee et al., 2008). However,
the rice SVP-group genes seem not to be involved in
controlling flowering time. The heading dates were only
slightly delayed in the OsMADS22-OsMADS55 double and
OsMADS22-OsMADS47-OsMADS55 triple RNAi plants
(Lee et al., 2008). The complementation tests also showed
that the rice OsMADS22 and OsMADS47 are unable to
reverse the flowering-time phenotypes of two Arabidopsis
SVP-group gene mutants (Fornara et al., 2008).
Phylogenetic analysis showed that OsMADS47 is grouped with
Arabidopsis SHORT VEGETATIVE PHASE (SVP),
whereas OsMADS22 and OsMADS55 are clustered with
Arabidopsis AGAMOUS-LIKE24 (AGL24) (Lee et al.,
2003). SVP and AGL24 are two closely related Arabidopsis
MADS-box genes that control the floral transition. AGL24
acts as a dosage-dependent promoter of flowering (Michaels
et al., 2003). AGL24 and SOC1 regulate each other to
provide positive-feedback control of their expression at the
shoot apex to promote the flowering transition (Liu et al.,
2008). By contrast, SVP functions as a dosage-dependent
repressor of flowering (Hartmann et al., 2000). SVP
Plant flowering time is controlled by networks of signals.
Four major pathways regulating flowering transition have
been identified in Arabidopsis: photoperiod, vernalization,
autonomous, and gibberellin pathways. In addition, many
other factors are critical for flowering transition. BRs are
important plant hormones and the BR-deficient det2, cpd,
and dwf4 mutants and the BR-insensitive bri1 mutants all
showed late-flowering-time phenotypes (Chory et al., 1991;
Azpiroz et al., 1998; Domagalska et al., 2007). det2 was
particularly late flowering and was classified as a
lateflowering mutant (Chory et al., 1991). Because endogenous
BR production is blocked in det2, cpd, and dwf4 mutants,
alterations in the endogenous BR content affect flowering
time in Arabidopsis. The late-flowering phenotypes of bri1
also showed that components of the BR signal transduction
pathway affect flowering time. Recently, several lines of
molecular evidence showed that BR biosynthesis genes and
BR signal transduction genes interact with flowering-time
genes directly or indirectly. First, CPD expression exhibited
diurnal and circadian rhythmicity and endogenous BR
showed diurnal changes leading to BL accumulation in the
middle of the light phase (Bancos et al., 2006). BR
application shortened the period of the circadian rhythms
of CCR2, CAB2, and CCA1 under both LL and DD
(Hanano et al., 2006). These results indicated that
endogenous BR levels and the circadian clock rhythms influence
each other. In Arabidopsis, the circadian system influences
flowering time. Thus, endogenous BR levels might affect
flowering time indirectly via their influence on the circadian
system. Second, BRs regulate the timing of the floral
transition by regulating FLC expression. The late-flowering
phenotype of ld is enhanced in bri1 and cpd mutants of
Arabidopsis through enhancing FLC expression. Moreover,
bri1 increased levels of histone H3 acetylation at the FLC
locus in the ld background. Interestingly, these experimental
results indicate that BR signalling functions independently
of the four major flowering pathways in repressing FLC
expression (Domagalska et al., 2007). Thus, BR signalling
might represent a new avenue in the floral-regulating
network. Third, the recruitment of ELF6 and REF6 by
BES1 to regulate the target gene expression established
another link between floral induction and the BR pathway
We thank Professor Jia Li, Professor Tianshan Zha, and
two anonymous reviewers for their valuable comments on
the manuscript. This study has been supported by the
National Outstanding Youth Foundation of China
(30625008); the major project of cultivating new varieties of
Transgenic organisms (2009ZX08009-029B); the National
Basic Research Program of China (973 Program)
(2007CB108905); and the National High Technology
Research and Development Program (863 Program)
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