Brassinosteroid actions in plants
Journal of Experimental Botany
Brassinosteroid actions in plants
Jianming Li 0
Joanne Chory 0
0 Howard Hughes Medical Institute, Plant Biology Laboratory, The Salk Institute for Biological Studies , 10010 N. Torrey Pines Road, La Jolla, CA 92037 , USA
Recent studies on dwarf mutants of Arabidopsis, tomato and pea have provided convincing evidence that brassinosteroids are a unique class of plant hormones that are essential for normal plant growth. Detailed metabolic analyses of these mutants, coupled with thorough molecular and biochemical studies of their corresponding genes and gene products, are essential for a better understanding of brassinosteroid biosynthesis and its regulation. Molecular and genetic approaches have been undertaken to dissect the brassinosteroid signalling pathway, leading to the identification of a putative brassinosteroid receptor and a few brassinosteroid-response genes. Further studies should expand our knowledge on how brassinosteroids are perceived and transduced to regulate plant development.
Brassinosteroid; biosynthesis; signal transduction; dwarf; receptor kinase
Steroid hormones are crucial for embryonic development
and adult homeostasis in animals ( Evans, 1988 ).
Similarly, the insect steroid hormone, ecdysone, triggers
a key regulatory cascade of gene expression controlling
many developmental processes such as moulting,
metamorphosis and reproduction ( Thummel, 1995). In
plants, many steroids have been identified, but only one
class of steroids, called collectively brassinosteroids, have
wide distribution throughout the plant kingdom and
unique growth-promoting activity when applied
exogenously (Mandava, 1988; Fujioka and Sakurai, 1997).
Physiological studies done in several laboratories have
demonstrated that brassinosteroids can induce a broad
spectrum of cellular responses such as stem elongation,
pollen tube growth, leaf bending and epinasty, root
inhibition, induction of ethylene biosynthesis,
protonpump activation, xylem diVerentiation, and regulation of
gene expression (Mandava, 1988; Clouse and Sasse,
1998 ). In addition, useful agricultural applications have
been found such as increasing yield and improving stress
resistance of several major crop plants (Cutler et al.,
1991 ). Despite this extensive research, a definitive proof
that these steroids are essential for normal plant growth
has been lacking.
A clear role for these steroids in plant growth and
development came from an entirely diVerent field of
studies: genetic analysis of plant photomorphogenesis. In
plants, immediate post-germinative growth usually occurs
beneath the soil surface and newly germinated seedlings
need to bring their leaves into full sunlight so that they
can start photosynthesis to provide the essential raw
materials needed for long-term growth. An etiolated
seedling, with an apical hook, closed cotyledons, and a
rapidly elongating hypocotyl, can reach the soil surface
quickly without damaging its young leaves. Once the
seedling is in full sunlight, its growth pattern changes:
the cotyledons expand and new leaves develop,
accompanied by the induction of many genes involved in
photosynthetic function, while the hypocotyl, whose elongation
is dramatically inhibited, becomes thicker and acts as a
physical support to hold newly developed leaves. Such a
developmental switch from one suited to an underground
environment to one suited to full sunlight is often referred
to as photomorphogenesis, or de-etiolation (Cosgrove,
In order to understand the molecular mechanisms
regulating this process, a genetic screen was initiated to
identify Arabidopsis mutants that display many
characteristics of light-grown plants when grown in the dark
(Chory et al., 1989). So far, mutants of over 18 loci have
been isolated and characterized ( Wei and Deng, 1996).
Among these mutants are recessive alleles of a gene
called DET2. Loss of-function mutations in DET2 have
pleiotropic eVects on Arabidopsis development (Chory
et al., 1991 ). In the dark, det2 mutants are short, have
thick hypocotyls, accumulate anthocyanins, have open,
expanded cotyledons, develop primary leaf buds, and
inappropriately express light-regulated genes. In the light,
det2 mutants are dark-green dwarfs, have reduced apical
dominance and male fertility, display altered
photoperiodic responses, show delayed chloroplast and leaf
senescence, and respond improperly to fluctuations in
their light environment. Such phenotypic diVerences
between det2 mutants and wild-type Arabidopsis plants
indicate that DET2 plays an important role throughout
DET2 encodes a protein that exhibits significant sequence
identity with mammalian steroid 5a-reductases that are
involved in animal steroid metabolism (Li et al., 1996 ).
In addition, a conservative substitution of a glutamate
(Glu204), which is absolutely conserved among all
mammalian steroid 5a-reductases and is essential for the
activity of a type 2 human 5a-reductase ( Russell and
Wilson, 1994 ), completely abolishes the in vivo activity
of DET2 protein, strongly suggesting that DET2 might
be involved in the biosynthesis of plant steroids such as
brassinosteroids. Consistent with this hypothesis,
brassinosteroid treatment rescued the growth defects of both
dark and light-grown det2 mutants (Li et al., 1996;
Fujioka et al., 1997 ). When expressed in mammalian
cells, DET2 protein was able to 5a-reduce several animal
steroids including testosterone and progesterone.
Moreover, the expression of human genes encoding
steroid 5a-reductases complemented a det2 mutation,
demonstrating that DET2 is a functional homologue of
mammalian steroid 5a-reductases ( Li et al., 1997). det2
mutants accumulated only 8–15% of the wild-type level
of campestanol and less than 10% of the wild-type levels
of other brassinosteroids and failed to convert 2H-labelled
campesterol to 2H-labelled campestanol, showing that
det2 is a brassinosteroid-deficient mutant. A further proof
came from rescue experiments in which all intermediates
of the brassinosteroid biosynthetic pathway after the
DET2 reaction were capable of rescuing det2 mutant
phenotypes. Taken together, these results establish that
DET2 is a brassinosteroid biosynthetic enzyme catalysing
an early step in the brassinosteroid biosynthetic pathway
( Fig. 1; Fujioka et al., 1997 ).
In addition to the DET2 gene, there are several putative
brassinosteroid biosynthetic genes that have been cloned
recently in Arabidopsis. Both CPD and DWF4 encode
cytochrome P450 proteins that share sequence similarities
with mammalian steroid hydroxylases and feeding
experiments with various brassinosteroids suggest that they act
at the C-23 and C-22 hydroxylation steps, respectively,
in the biosynthetic pathway ( Fig. 1; Szekeres et al., 1996;
Choe et al., 1998a). Both CBB3 ( Kauschmann et al.,
1996 ) and DWF3 (Choe et al., 1998b) are allelic to CPD.
The Arabidopsis DWF1/DIM1/CBB1 protein ( Feldmann
et al., 1989; Takahashi et al., 1995; Kauschmann et al.,
1996 ) has strong homology with several unknown
proteins of pea, human, and C. elegans (J. Li and
J. Chory, unpublished data), and is suggested to be an
FAD-dependent oxidoreductase (Mushegian and Koonin,
1995 ). Quantitative analysis of endogenous
brassinosteroid levels in dim1 mutants suggests that the
dim1 mutation might block the conversion of
24-methylenecholesterol to campesterol ( Fig. 1; Clouse
and Sasse, 1998). Mutations in each of these genes result
in phenotypes that are very similar to those of det2
mutants and these mutants can also be rescued by
brassinosteroid treatment. Given the fact that mutations in
each of these genes aVect many aspects of light-regulated
Arabidopsis development, the recent molecular genetic
analyses of these dwarf mutants have provided convincing
evidence that brassinosteroids are a unique class of plant
hormones that are essential for normal plant growth and
The identification of additional
brassinosteroiddeficient dwarf mutants in both Arabidopsis and other
plant species further confirms the importance of these
steroids in plant development. Three novel Arabidopsis
dwarf mutants including dwf5, dwf7 and dwf8 were
recently found to be defective in brassinosteroid
biosynthesis (Choe et al., 1998b). Results from feeding
experiments and analyses of endogenous brassinosteroid
levels have indicated that both dwf5 and dwf7 mutants
are defective in the formation of 24-methylenecholesterol,
a precursor to campesterol, while the dwf8 mutation is
suggested to block a late step after 3-dehydroteasterone
in the brassinolide biosynthetic pathway ( Fig. 1; Choe
et al., 1998b). The lkb mutant of pea is a
brassinosteroiddeficient dwarf that shows normalization of internode
elongation upon application of a range of brassinosteroids
(Nomura et al., 1997 ) and a recent study suggested that
the lkb mutation, like the Arabidopsis dim1 mutation,
blocks the conversion of 24-methylenecholesterol to
campesterol ( Fig. 1; Clouse and Sasse, 1998). Another pea
mutant, lk, is believed to be defective in the conversion
of campesterol to campestanol and the corresponding
gene LK might encode a pea homologue of the Arabidopsis
DET2 ( Fig. 1; Yokota et al., 1997 ). The DWARF gene of
tomato was recently cloned by transposon tagging and
shown to encode a cytochrome P450 with 38% sequence
identity to the Arabidopsis CPD (Bishop et al., 1996).
Preliminary feeding experiments suggest that the tomato
DWARF is also a brassinosteroid biosynthetic enzyme,
acting later in the biosynthetic pathway than CPD (Clouse
and Sasse, 1998). Another tomato mutant, dpy, was
found to be deficient in C-23 hydroxylation of both
cathasterone and 6-deoxocathasterone, implying that
DPY is the tomato homologue of CPD ( Fig. 1; Clouse
and Sasse, 1998).
The brassinosteroid-deficient mutants and the cloned
brassinosteroid-biosynthetic genes provide an important
tool for investigating not only the physiological functions
but also the biosynthetic pathways of brassinosteroids.
The detailed metabolic analyses of det2 mutants coupled
with the biochemical characterization of DET2 protein
aided the identification of a 3-oxo, D4–5 steroid,
3-dehydro-D4–5-campesterol, as a direct substrate of the
DET2 reductase ( Fig. 1; Li et al., 1997; Fujioka et al.,
1997). Such a study also concluded that Arabidopsis
must have a plant orthologue of the mammalian
3b-hydroxysteroid dehydrogenase/D5–6–D4 –5
isomerases to catalyse the conversion of campesterol to
3-dehydro-D4–5-campesterol and the conversion of
3-dehydrocampestanol to campestanol ( Fig. 1). Indeed,
recent database searches indicate that the Arabidopsis
genome contains at least two genes for such an enzyme
(J Li and J Chory, unpublished results). Quantitative
analysis of campestanol levels in three presumed null
alleles of det2 has suggested that there is a second steroid
5a-reductase in Arabidopsis that plays a minor role in
brassinosteroid biosynthesis ( Fujioka et al., 1997), which
might explain the relatively weak phenotypes of det2
mutants compared to cpd/cbb3/dwf3 mutants. Further
molecular and genetic studies should aid in the
identification of this second reductase and the elucidation of its
physiological function during plant development.
The existence of various brassinosteroid-deficient
mutants allowed the testing of the possibility that brassinolide
is not synthesized via a simple linear biosynthetic
pathway. Recently, two pathways, the so-called early C-6
oxidation and late C-6 oxidation pathways, were proposed
for the biosynthesis of brassinolide ( Fig. 1; Fujioka and
Sakurai, 1997 ). Feeding experiments with intermediates
of both pathways provided strong genetic evidence that
both pathways are operating in Arabidopsis ( Fujioka
et al., 1997; Choe et al., 1998a). A recent study with dwf4
mutants suggests that 22-hydroxyl-campesterol might be
a starting point for a new subpathway since this
compound is able to rescue dwf4 mutations (Choe et al.,
1998a). Similarly, 6a-hydroxycampesterol could also be
a starting point for a diVerent subpathway whose
intermediates act as ‘bridging molecules’ between the early
and late C-6 oxidation pathways (Choe et al., 1998a).
Identification of new brassinosteroid biosynthetic
intermediates in vivo are needed to confirm whether these
hypothesized subpathways exist in plants. One simple
explanation for plants having multiple pathways of
brassinosteroid biosynthesis is that these subpathways might
be diVerentially regulated by various environmental or
developmental signals. Feeding experiments using det2
and dwf4 mutants have consistently shown that
brassinosteroids in the late C-6 oxidation pathway are more
eVective in rescuing light phenotypes whereas the
brassinosteroids in the early C-6 oxidation pathways show
stronger activity in promoting hypocotyl elongation of
dark-grown seedlings ( Fujioka et al., 1997; Choe et al.,
1998a). Further measurements of endogenous
brassinosteroid intermediates of diVerent subpathways and detailed
analysis of expression patterns of key brassinosteroid
biosynthetic genes under diVerent environmental
conditions or at diVerent developmental stages should help to
understand the regulation of brassinosteroid biosynthesis.
Metabolic studies with wild-type Arabidopsis and det2
seedlings indicate that the reaction catalysed by DET2 is
a major rate-limiting step in the brassinosteroid
biosynthetic pathway (S Fujioka, personal communication).
Consistent with such an observation, overexpression of
DET2 in Arabidopsis results in transgenic plants that are
significantly larger than wild-type plants in the light, while
underexpression of DET2 using an antisense DET2
construct leads to an allelic series of plants ranging from
det2-like dwarfs to wildtype in stature (Chory and Li,
1997 ). These results support the early claims that
brassinosteroid applications could increase crop productivity.
Likewise, the 22-hydroxylation reaction has also been
hypothesized to be a rate-limiting step ( Fujioka et al.,
1995 ). Therefore, DWF4 might be another limiting
activity in the biosynthesis of brassinolide (Choe et al., 1998a).
Biochemical, metabolic and transgenic studies are needed
to confirm whether DWF4 does catalyse a rate-limiting
reaction in the brassinosteroid biosynthetic pathway. If
so, it will be interesting to see whether overexpression of
both DET2 and DWF4 genes will have a great eVect on
the biomass of transgenic plants.
Analysing the expression of the cloned brassinosteroid
biosynthetic genes can provide hints to where
brassinosteroids are synthesized and how brassinosteroid
biosynthesis is regulated. DET2 appears to be constitutively
and ubiquitously expressed throughout Arabidopsis
development and in response to diVerent light conditions (Li
and Chory, 1997; D Friedrichsen and J Chory,
unpublished observations). In contrast, the expression of CPD
is confined to cotyledon and leaf primordia in dark-grown
seedlings and the adaxial parenchyma of expanding leaves
in light-grown plants. CPD expression is not detected in
either actively elongating cells of hypocotyls and roots,
or pollen and seeds where the highest brassinolide levels
have been detected (Mathur et al., 1998 ). Such an
expression pattern of CPD might suggest that brassinosteroids
are synthesized in both cotyledons and leaves and are
then transported to the organs that require high levels of
brassinosteroids for normal growth. Alternatively, there
might be enzymatic machineries that reduce the levels of
active brassinosteroids to relieve a feedback inhibition on
CPD expression in either cotyledons or expanding leaves.
Several diVerent enzyme reactions have been implicated
in brassinosteroid inactivation such as hydroxylation,
glycosylation, acylation, and side-chain degradation
( Fujioka and Sakurai, 1997; Adam et al., 1996). Both
the transport and metabolism of 14C-labelled
24-epibrassinolide have been studied in cucumber and
wheat seedlings ( Fujioka and Sakurai, 1997). When
applied to roots, 24-epibrassinolide is readily taken up
and transported to leaves; in contrast, when applied to
leaves, its transport is quite slow. It was later shown that
this compound is quickly metabolized in leaves, but
not in hypocotyls and roots. As such, these results are
consistent with the second hypothesis.
Brassinosteroid signal transduction
Having established the essential role of brassinosteroids
in plant growth and development, the most interesting
question to ask is how plants recognize these steroids and
transduce their signals to regulate a wide spectrum of
developmental processes. To understand the molecular
mechanism by which brassinosteroids regulate plant
development, it is necessary to identify components of
the brassinosteroid response pathway, including
receptor(s), intermediates, and targets. Currently, both
molecular and genetic approaches have been taken to investigate
the molecular mechanisms of brassinosteroid signalling.
The molecular approach is to clone
brassinosteroidregulated genes, understand their functions, map their
corresponding brassinosteroid-responsive cis-acting
elements and identify trans-acting regulatory factors
mediating brassinosteroid-regulated gene expression. It has been
demonstrated that brassinosteroid-induced responses
require de novo protein synthesis ( Mandava, 1988 ) and
brassinosteroid-treatment induces synthesis of both
mRNAs and proteins (Clouse, 1996). Using a diVerential
hybridization method, a br assinosteroid up-regulated
(BRU1) gene was identified from elongating soybean
epicotyls ( Zurek and Clouse, 1994 ). BRU1 encodes a
protein that showed significant homology to various
xyloglucan endotransglycosylases ( XETs) which have
been implicated in cell wall loosening during cell
elongation (Clouse, 1996). A later study proved that
BRU1 is indeed a functional XET (Oh et al., 1998 ). The
expression level of BRU1 correlates with the extent of
brassinosteroid-promoted stem elongation and the
accumulation of the BRU1 transcript parallels the
brassinosteroid-mediated increases in plastic extensibility of the
cell wall ( Zurek and Clouse, 1994; Zurek et al., 1994 ).
Moreover, a linear relationship has been observed
between brassinosteroid concentrations and extractable
XET activities in brassinosteroid treated soybean
epicotyls (Oh et al, 1998), strongly suggesting an involvement
of BRU1 in brassinosteroid-stimulated stem elongation.
Surprisingly, brassinosteroid regulates BRU1 expression
at a post-transcriptional rather than a transcriptional
level ( Zurek and Clouse, 1994 ). A
brassinosteroidregulated XET has also been identified in Arabidopsis
( Xu et al., 1995 ). The TCH4 gene encodes an XET whose
expression is increased within 30 min of brassinosteroid
treatment, with a maximum at 2 h. In addition to
brassinosteroids, this gene is also regulated by touch, darkness,
temperature shock and auxin. In contrast to soybean
BRU1, the brassinosteroid-regulated TCH4 expression
occurs at the transcriptional level ( Xu et al., 1995). The
brassinosteroid-responsive element has been mapped to a
100 bp fragment of the TCH4 promoter (Clouse and
Sasse, 1998 ) and linker-scanning mutagenesis is underway
to identify the specific brassinosteroid response element(s)
for this gene.
A recent study has shown that expression of CPD is
specifically down-regulated by brassinolide although its
transcription is not aVected by other plant hormones
including auxin, ethylene, gibberellin, cytokinin, jasmonic
acid, and salicylic acid (Mathur et al., 1998). Such a
brassinosteroid-induced repression of CPD transcription
is sensitive to the protein synthesis inhibitor
cycloheximide, indicating a requirement for de novo synthesis
of a negative transcriptional regulator (Mathur et al.,
1998 ). Mapping of brassinosteroid-response element(s) in
the CPD promoter is necessary to find transcriptional
factors regulating CPD gene expression.
The genetic approach to the identification of
components of the brassinosteroid signalling pathway is the
isolation and characterization of Arabidopsis mutants
deficient in brassinosteroid responses. Screens for
hormone-insensitive mutants in Arabidopsis have proven
to be fruitful in the study of plant hormone action. As
examples, screens for mutants that are insensitive to high
levels of exogenously applied auxins, ethylene,
gibberellins, and abscisic acid have led to the identification of
loci involved in signalling from these plant hormones.
Among the ethylene-insensitive loci are ETR1 and ETR2,
which encode ethylene receptors (Chang et al., 1993;
Sakai et al., 1998 ).
An Arabidopsis brassinosteroid-i nsensitive mutant
(bri1 ) was originally identified by a root-growth inhibition
assay (Clouse et al., 1996). bri1 seedlings display similar
pleiotropic phenotypes to brassinosteroid-deficient
mutants, yet can not be rescued by brassinosteroid
treatment. The bri1 mutants showed marked insensitivity only
to brassinosteroids but retain complete sensitivities to
auxins, cytokinins, gibberellins, abscisic acid, and
ethylene. A second brassinosteroid insensitive mutant
cbb2, has been isolated independently in a genetic screen
for cabbage-like Arabidopsis mutants ( Kauschmann et al.,
1996 ) and was later found to be allelic to bri1.
Brassinosteroid insensitivity in the cbb2 mutant has also
been observed at the molecular level. The
brassinosteroidinduced expression of two XET genes, TCH4 and meri5,
is missing in the mutant, although GA-induced meri5
expression is still observed ( Kauschmann et al., 1996 ). A
diVerent genetic screen, designed specifically for
brassinosteroid-response mutants with brassinosteroid-deficient
phenotypes, has also been performed. The screen involved
two steps: a primary screen for det2 or cpd-like dwarf
mutants followed by a secondary screen for dwarfism
that could not be rescued by brassinosteroid treatment.
This screen yielded 18 bin (b rassinosteroid insensitive)
mutants that were later found to be new alleles of the
BRI1/CBB2 locus. Recently, three dwf2 alleles were found
to be allelic to bri1 (Clouse and Sasse, 1998) and two
new bri1 alleles were identified from a collection of dwarf
mutants at the Arabidopsis Biological Resource Center
( The Ohio State University, Ohio, USA) and a private
collection of T-DNA insertion lines (J. Li and J. Chory,
unpublished data), thus bringing the total number of bri1
alleles to 25. It is somewhat surprising that all
brassinosteroid-insensitive mutants isolated in four diVerent genetic
screens are all alleles of a single gene. This suggests that
BRI1 is the only unique and specific component in the
brassinosteroid signalling pathway and other components
of the pathway are either redundant (mutants would have
no obvious phenotype) or shared with other signalling
cascades (mutants would be lethal ).
Brassinosteroidinsensitive mutants have also been identified in both pea
(lka, Nomura et al., 1997) and tomato (cu-3, Clouse and
Sasse, 1998 ) ( Fig. 1).
The almost identical phenotypes between bri1 and cpd,
the most severe brassinosteroid-deficient mutant isolated
to date, strongly suggests that BRI1 encodes an early
component of the brassinosteroid response pathway, most
likely a brassinosteroid receptor. Recently, the Arabidopsis
BRI1 gene was cloned by chromosome walking (Li and
Chory, 1997) and found to encode a protein that has
significant sequence identity to a family of plant
leucinerich-repeat (LRR) receptor-like kinases including Xa21
of rice (Song et al., 1995 ), ERECTA and CLV1 of
Arabidopsis ( Torii et al., 1996; Clark et al., 1997). The
predicted BRI1 protein contains several distinct domains:
a signal peptide, a putative leucine-zipper motif, 25
leucine-rich-repeats, a 70 amino-acid island buried
between the 21st and 22nd LRR, and a cytoplasmic
kinase domain that has serine/threnine kinase activity
when expressed in E. coli or animal cells (J Li, C
Joazeiro and J Chory, unpublished data).
The finding that BRI1 is a LRR-containing receptor
kinase is quite a surprise. First, while a transmembrane
receptor kinase is a central theme in many signal
transduction events ( Ullrich and Schlessinger, 1990), no such
receptor kinase has ever been found to be involved in a
steroid signal transduction pathway. All known steroid
receptors belong to a superfamily of nuclear receptor
proteins that are ligand-dependent transcription factors
that regulate gene expression ( Beato et al., 1995);
however, cell surface steroid receptors are known to be present
in animal cells mediating non-genomic eVects of steroid
hormones (McEwen, 1991 ) and protein tyrosine
phosphorylation has been implicated in such a
membraneinitiated steroid signalling pathway ( Tesarik et al., 1993;
Mendoza et al., 1995).
Second, while many LRR-containing proteins are
involved in signal transduction pathways, LRRs are
believed to mediate protein–protein interactions and, in
many cases, provide binding sites for protein ligands
( Kobe and Deisenhofer, 1994 ). So far, no LRR has ever
been found to interact with small compounds.
Interestingly enough, the predicted BRI1 protein contains
a unique 70 amino-acid island buried between the 21st
and 22nd LRRs. The importance of this island in BRI1’s
function is self-evident by sequencing two bri1 alleles,
both of which have a missense mutation in this domain
(Li and Chory, 1997; J. Li and J. Chory, unpublished
results). In contrast, of 19 bri1 alleles that have been
sequenced, no allele contains a mutation in the LRRs
themselves. It is worthwhile to note that mutations in
LRRs have been identified in both erecta and clv1 alleles
( Torri et al., 1996; Clark et al., 1997). Careful biochemical
studies are needed to determine whether BRI1 is a
brassinosteroid receptor, and if so, whether the island is
directly involved in binding brassinosteroids.
BRI1’s kinase domain must be essential for the BRI1’s
function. Of the 19 bri1 alleles sequenced to date, 15 have
mutations in this domain (Li and Chory, 1997; J. Li and
J. Chory, unpublished results). This domain must interact
with other downstream signalling components to
transduce the steroid signal. One protein that might interact
with BRI1’s kinase domain is a type 2C phosphatase
( KAPP) which has been implicated to be involved in
both RLK5 and CLV1-mediated signalling pathways
(Stone et al., 1994; Williams et al., 1997). Indeed, a fusion
protein between glutathione S-transferase (GST ) and
BRI1’s kinase domain interacts with this phosphatase in
vitro (J. Li and J. Chory, unpublished results). Transgenic
experiments are underway to find the physiological
relevance of such an interaction.
Conclusion and perspectives
To understand completely how brassinosteroids act in
plants, more components of the brassinosteroid response
pathways need to be isolated and characterized. It might
be possible to identify proteins involved in brassinosteroid
signal transduction that directly interact with BRI1’s
kinase domain by a yeast two-hybrid screening or
interactive cloning strategy. Although four diVerent genetic
screenings have resulted in the identification of only one
brassinosteroid signalling component, BRI1, at least two
other genetic screens can be used to identify new
components involved in brassinosteroid signal transduction. One
is a transgene-based screening using a well-characterized
brassinosteroid-regulated promoter fused to a reporter
gene. Such a genetic screen has been used successfully to
identify mutants aVecting phytochrome responses and
chloroplast-nuclear communication (Li et al., 1994, 1995;
Susek et al., 1993 ). The other screen is to identify mutants
that suppress or enhance either brassinosteroid-deficient
or brassinosteroid-response mutations. Results from these
studies will certainly expand our knowledge of how
steroids regulate plant development. A better
understanding of the brassinosteroid signal transduction pathway
will make it possible to manipulate plant growth
genetically in order to improve crop productivity.
Work on brassinosteroids in our laboratory is supported by the
United States Department of Agriculture. JC is an investigator
of the Howard Hughes Medical Institute and JL is an American
Cancer Society postdoctoral fellow.
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