Download Floral organ identity genes in the orchid

The Plant Journal (2006) 46, 54–68
doi: 10.1111/j.1365-313X.2006.02669.x
Floral organ identity genes in the orchid Dendrobium
crumenatum
Yifeng Xu1,2, Lai Lai Teo1,2, Jing Zhou1,2, Prakash P. Kumar1,2,* and Hao Yu1,2,*
Department of Biological Sciences, Faculty of Science, National University of Singapore, 10 Science Drive 4, 117543 Singapore,
and
2
Temasek Life Sciences Laboratory, 1 Research Link, National University of Singapore, 117604, Singapore
1
Received 9 November 2005; revised 29 November 2005; accepted 29 November 2005.
*
For correspondence (fax þ65 677 92 486; e-mail [email protected] or [email protected]).
Summary
Orchids are members of Orchidaceae, one of the largest families in the flowering plants. Among the
angiosperms, orchids are unique in their floral patterning, particularly in floral structures and organ identity.
The ABCDE model was proposed as a general model to explain flower development in diverse plant groups,
however the extent to which this model is applicable to orchids is still unknown. To investigate the regulatory
mechanisms underlying orchid flower development, we isolated candidates for A, B, C, D and E function genes
from Dendrobium crumenatum. These include AP2-, PI/GLO-, AP3/DEF-, AG- and SEP-like genes. The
expression profiles of these genes exhibited different patterns from their Arabidopsis orthologs in floral
patterning. Functional studies showed that DcOPI and DcOAG1 could replace the functions of PI and AG in
Arabidopsis, respectively. By using chimeric repressor silencing technology, DcOAP3A was found to be
another putative B function gene. Yeast two-hybrid analysis demonstrated that DcOAP3A/B and DcOPI could
form heterodimers. These heterodimers could further interact with DcOSEP to form higher protein complexes,
similar to their orthologs in eudicots. Our findings suggested that there is partial conservation in the B and C
function genes between Arabidopsis and orchid. However, gene duplication might have led to the divergence
in gene expression and regulation, possibly followed by functional divergence, resulting in the unique floral
ontogeny in orchids.
Keywords: ABCDE model, flower development, functional divergence, gene duplication, orchid, organ
identity.
Introduction
Molecular and genetic regulation of flower development has
been extensively investigated in the last two decades (Jack,
2004; Theissen, 2001). As a result, the classical ABC model
has been proposed to explain the specification of floral organ identity in Arabidopsis and Antirrhinum (Coen and
Meyerowitz, 1991; Weigel and Meyerowitz, 1994). Based on
this model, three classes of functions, A, B and C with each
consisting of one or more genes, determine various floral
organ identities. Function A alone controls the development
of sepals in the first whorl of a flower. Functions A and B
specify petal development in the second whorl, while B and
C functions control stamen development in the third whorl.
Function C alone determines carpel development in the
fourth whorl. Functions A and C are considered mutual
antagonists and the expression of one represses the other
54
(Bowman et al., 1991; Drews et al., 1991; Gustafson-Brown
et al., 1994). The model has been used to explain floral organ
development in a wide range of plant species (Ambrose
et al., 2000; Kang et al., 1998; Kramer et al., 1998; Kyozuka
and Shimamoto, 2002; Mena et al., 1996; Whipple et al.,
2004). Recently, two classes of new functions, namely D and
E, were added, resulting in the revised ABCDE model (Pelaz
et al., 2000, 2001; Pinyopich et al., 2003). In the revised
model, function D determines ovule development, while
function E is required for development of all the floral organs
(Ditta et al., 2004; Pelaz et al., 2000, 2001; Pinyopich et al.,
2003).
With the exception of the function A gene APETALA2
(AP2), all known homeotic genes in the ABCDE model
belong to the MADS-box family of transcription factors. In
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Flower development in orchids 55
plants, MADS-box genes are important regulators controlling developmental programs from vegetative growth,
flowering process, to seed development (Theissen, 2001).
To date, five types (Ma, Mb, Mc, Md and MIKC) of MADS-box
genes have been identified in Arabidopsis (Parenicova et al.,
2003; Riechmann and Meyerowitz, 1997). Studies on the
plant MADS-box genes have been mainly focused on the
MIKC type, whose members have a similar modular structure. This includes a highly conserved domain of approximately 56 amino acids called MADS-box (M), a weakly
conserved intervening region (I), a moderately conserved K
domain (K) and a variable carboxyl-terminal (C; Kaufmann
et al., 2005). MIKC-type MADS-box transcription factors are
specific to plants and are essential for plant growth and
development (Becker and Theissen, 2003).
AP2, on the other hand, encodes a transcription factor with
two continuous AP2 domains, which are specific to plants.
AP2 confers function A in Arabidopsis and represses AG in
floral whorls 1 and 2 (Drews et al., 1991; Jofuku et al., 1994).
Although the transcripts of AP2 can be detected in both
reproductive and vegetative tissues (Drews et al., 1991), the
function of AP2 is restricted to whorls 1 and 2, which is caused
by miRNA-mediated translational regulation (Chen, 2004).
The above-mentioned mechanisms of floral patterning
may be applicable to diverse plant species, while unique
flower structures in some species may indicate the divergence of the genetic mechanism involved (Irish and Litt,
2005; Zahn et al., 2005). Some monocot species possess
distinct floral structures. These include the grass family (e.g.
rice and maize) and the orchid family. Floral development has
been extensively studied in two typical monocotyledonous
crop species, namely, rice and maize. The B and C function
genes are conserved in the grass family and Arabidopsis
(Ambrose et al., 2000; Kyozuka and Shimamoto, 2002; Schmidt et al., 1993; Whipple et al., 2004). However, floral
patterning in grasses may not be comparable to those from
other flowering plants, as they usually possess highly
reduced floral organs. Orchids belong to Orchidaceae, one
of the largest families of flowering plants. As a group of
important ornamental plants with great diversity and specialization in floral morphology, orchids are one of the
desirable monocot species for studying the genetic basis of
flower development. Although there have been many reports
on the physiological and horticultural aspects of orchids,
only a limited number of reports on molecular genetic
analysis of orchid flower development are available (Hsu
and Yang, 2002; Tsai et al., 2004; Yu and Goh, 2000, 2001).
In orchids, the only functional study of a putative floral
organ identity gene OMADS3 (a paeloAP3 gene from Oncidium) indicated that it may be an A function gene involved in
flower formation and floral initiation, despite its sequence
similarity to the known B function genes (Hsu and Yang,
2002). Recently, four APETALA3 (AP3)/DEFIENCE (DEF)-like
MADS box genes have been isolated from the Phalaenopsis
orchid (Tsai et al., 2004). These genes demonstrated distinct
expression profiles in wild-type orchids and the peloric
mutants with lip-like petals, indicating that the regulation of
AP3/DEF-like genes in orchids may be more complicated
than those in eudicots and grass species, where only one
AP3/DEF-like gene is usually found in each species (Ambrose
et al., 2000; Jack et al., 1992; Moon et al., 1999). Thus, it is so
far unknown if the counterparts of the floral organ identity
genes are functional in the highly specialized structures of
orchid flowers.
In our efforts to investigate the unique features in orchid
flowers, such as the modifications of perianth organs and
the fusion of reproductive organs, we systematically isolated and characterized a group of putative floral organ
identity genes from the orchid Dendrobium crumenatum.
Investigation of their expression profiles in the orchid and
functional characterization of these genes in Arabidopsis
revealed that DcOAP3A/B, DcOPI and DcOAG1 are B and C
function genes, respectively. Our studies suggested that
gene duplication followed by the divergence in expression
patterns or regulatory mechanisms may be responsible for
the diverse floral morphology in orchids.
Results
Floral morphology in D. crumenatum
Among the numerous orchid species and hybrids, floral
characteristics of Dendrobium species are comparatively
constant throughout the genus despite their variations at
vegetative stages. D. crumenatum has typical orchid flowers
that develop from young floral buds at the nodes of an
inflorescence axis (Figure 1a,b). Like most of the angiosperms, D. crumenatum flowers are made up of four whorls,
namely sepals, petals, anthers and pistils (Figure 1c). However, there are several unique features in orchids. First, the
first whorl of orchid flowers is made up of one dorsal and
two lateral petaloid sepals (Figure 1c). Even though the
petals in the second whorl are usually broader than sepals,
the general appearance of the two lateral petals is very
similar to the petaloid sepals. Secondly, the median petal is
modified into a lip with different size, shape and color from
other petals, which generally serves as a ‘landing platform’
for insect pollinators (Figure 1c). Lastly, unlike the configuration of separate male and female organs in most other
flowers, the stigmatic region (part of female organs) and the
anther (male organs) of orchid flowers are fused into a finger-like structure located at the tip of a column (Figure 1c,d).
The anther under the anther cap contains clusters of pollen
grains called pollinia. Closer to the base of the anther is the
shallow and sticky stigma (Figure 1d). This region is connected to the inferior ovary located at the flower stalk. After
successful pollination, the ovary develops into a capsule
containing numerous seeds.
ª 2006 National University of Singapore, The Plant Journal, (2006), 46, 54–68
56 Yifeng Xu et al.
Figure 1. Flowers and floral buds of Dendrobium crumenatum and expression patterns of orchid floral organ identity genes.
(a) Developing young flower bud (Yb) at the node on an inflorescence.
(b) Dormant flower bud (Db) that will develop into a flower upon receiving
environmental cues, such as low temperature.
(c) Top and back view of open flowers. In the top view, sepals in the first whorl
and the lateral petals of the second whorl have similar color, shape and size.
The median petal has been modified into a lip that has a distinct appearance
from two lateral petals. In the back view, the wide base of two lateral sepals
and the lip join at the column foot.
(d) Close-up view of the fusion structure of stigma and anther shown in (c).
Pollen grains packaged into four pollinia are covered by the anther cap, which
is located above the stigma at the tip of the column.
(e) Expression patterns of orchid floral organ identity genes in floral organs
and vegetative tissues. Total RNA was extracted from leaves, dormant flower
buds and floral organs of newly open flowers. Columns in this experiment
excluded anthers and their caps. Ac, anther cap; An, anther; Co, column; Cf,
column foot; Ds, dorsal sepal; Pe, petal; Po, pollinia; Le, leaf; Li, lip; Ls, lateral
sepal; St, stigma. Scale bar ¼ 1 mm in (d).
Identification of floral organ identity genes from
D. crumenatum
To identify genes involved in the development of orchid
flowers, we isolated candidates for A, B, C, D and E function
genes from D. crumenatum. A group of MADS-box genes
and one AP2-like gene were isolated from dormant flower
buds or newly open flowers using PCR amplification of the
cDNAs with degenerate primers as described in Experimental procedures. The rapid amplification of cDNA ends
(RACE) method was employed to further isolate full-length
cDNAs of AP2, AP3, PISTILLATA (PI), AGAMOUS (AG)
and SEPALLATA (SEP) orthologs. These orthologs were
named DcOAP2 (D. crumenatum AP2-like gene), DcOAP3A,
DcOAP3B, DcOPI, DcOAG1, DcOAG2 and DcOSEP1 (GenBank accession numbers DQ119837, DQ119838, DQ119839,
DQ119840, DQ119841, DQ119842 and DQ119843, respectively).
DcOAP2 cDNA is 1711 bp long with a 1338-bp coding
region (Figure S1). Similar to the known AP2-like genes, the
predicted amino acid sequence of DcOAP2 contains two AP2
domains, which bear a high similarity with other AP2-like
proteins (Figure 2a; Okamuro et al., 1997). This suggests
that DcOAP2 is an AP2-like transcription factor, which may
play a role in orchid flower development.
All of the predicted protein sequences of MADS box genes
cloned contain the conserved MIK region and a divergent Cterminal region. DcOPI possesses a highly conserved PI
motif at the C-terminal region, which is characteristic of PI
lineage and shared by other members in this lineage from
both dicots and monocots (Figure 2b; Kramer et al., 1998). In
contrast, AP3/DEF-like genes are separated into euAP3, TM6
and paleoAP3 lineages (Kramer et al., 1998). Proteins of
euAP3 lineage in core eudicot species possess a euAP3 motif
and a PI-derived motif, while members of the paeloAP3
lineage in basal eudicots, magnoliids and monocots, including DcOAP3A and DcOAP3B of D. crumenatum, contain a
paleoAP3 motif and a distinct PI-derived motif (Figure 2c;
Kramer et al., 1998; Moon et al., 1999; Tzeng and Yang,
2001). Sequence analysis revealed that both DcOAG1 and
DcOAG2 belong to AG lineage (Figure 2d). DcOAG1 encodes
a protein containing an N-terminal region upstream of the
MADS-box domain. In addition, it contains the eighth intron
(Figure 2e), which is present in the members of the function
C lineage, while most of the genes reported in the D lineage
from core eudicots and grass species lack this intron
(Kramer et al., 2004).
Phylogenetic analysis further confirmed that these orchid
MADS box genes belong to plaeoAP3, PI, SEP and AG
lineages (Figures 3 and 4). DcOAP3A and DcOAP3B were
found to be members of the paleoAP3 lineage (Figure 3),
which was consistent with the sequence alignment shown in
Figure 2(c). The paleoAP3 genes from the orchids Phalaenopsis, Oncidium and Dendrobium were subdivided into two
subclusters. DcOAP3A belongs to the subcluster containing
OMADS3, PeMADS2 and PeMADS5. The transcripts of
PeMADS2 and PeMADS5 were found in all floral organs
except the pollinia (Tsai et al., 2004). PeMADS2 might be
required for sepal development, while PeMADS5 might be
ª 2006 National University of Singapore, The Plant Journal, (2006), 46, 54–68
Flower development in orchids 57
Figure 2. Sequence analyses of orchid floral organ identity genes.
(a) Alignment of the predicted amino acid sequences of the two AP2 domains (AP2R1 and AP2R2) and the linker region of DcOAP2 with those of the homologs from
other plant species. Identical or similar residues are shaded black or gray. Dashes indicate gaps. Protein names are indicated at the left.
(b, c) Alignment of the predicted amino acid sequences of C-terminal regions of DcOPI, DcOAP3A and DcOAP3B with those of the orthologs from other plant species.
DcOPI contains the PI motif that is conserved in all PI/GLO orthologs. DcOAP3A and DcOAP3B contain the PI-derived and paleoAP3 motifs, which are only conserved
in monocots and basal angiosperms, but are different from the PI-derived and euAP3 motifs of euAP3 lineage from eudicots. OMADS3 of Oncidium and PeMADS5 of
Phalaenopsis lack PI-derived and paeloAP3 motifs.
(d) Alignment of the predicted amino acid sequences of DcOAG1, DcOAG2 with AG. (e) The presence of intron 8 in the genomic sequence of DcOAG1. A 94-bp intron
8 is located in the last codon (arrow).
involved in petal and stamen development. OMADS3 was
ubiquitously expressed and could confer function A in
Arabidopsis (Hsu and Yang, 2002). DcOAP3B was clustered
with PeMADS3, which was detectable in the lip and column
with unknown functions (Tsai et al., 2004). DcOPI was within
the PI/GLOBOSA (GLO) cluster (Figure 3). In this cluster,
DcOPI and other orthologs from monocots formed one
subcluster, which was separated from the members of
eudicots. Compared with the PI/GLO and AP3/DEF lineages,
the members of the SEP lineage were more diversified in
protein sequences. DcOSEP1 (Figure S2) was clustered with
other SEP-like genes from orchids, including OM1 and
DOMADS1 (Figure 3).
In the AG subfamily, phylogenetic analyses of 96 previously released AG-like sequences and two orchid AG-like
sequences obtained in our study revealed the division of the
ª 2006 National University of Singapore, The Plant Journal, (2006), 46, 54–68
58 Yifeng Xu et al.
of Dendrobium demonstrated different expression patterns
from their counterparts in angiosperms, where the
expression is usually restricted to whorls 2 and 3 of floral
organs. DcOPI was expressed equally in all floral organs,
including sepals, petals, lips, male organs (anther caps and
pollinia) and female organs (lower part of column with
ovary; Figures 1e and 5a–f). The two orchid AP3/DEF
orthologs showed distinct expression patterns from each
other. The transcripts of DcOAP3A were present in all the
floral organs and vegetative tissues (Figures 1e and 5g–i),
whereas DcOAP3B was only strongly expressed in the
second whorl of petals and lips and in the reproductive
organs including pollinia and columns (Figure 1e). Thus,
the expression patterns of function B genes in Dendrobium
showed two features. First, DcOPI and DcOAP3A were
clearly detectable in the first whorl of sepals. Secondly, all
the putative function B genes, including DcOPI, DcOAP3A
and DcOAP3B, were expressed at various levels in the
lower part of the column (ovary), which was equivalent to
the fourth whorl (carpels) in Arabidopsis. In contrast to its
orthologs in most of the angiosperms (Benedito et al.,
2004; Kang et al., 1998; Kater et al., 1998; Kim et al., 2005;
Pnueli et al., 1994), the putative function C gene DcOAG1
was expressed in all floral organs (Figures 1e and 5j–l).
However, the function D gene DcOAG2 was expressed
mainly in the lower part of columns (ovary) and in the
envelop cells of pollinia (Figures 1e and 5m,n).
Analysis of putative B function genes of D. crumenatum
Figure 3. Phylogenetic analysis of B and E class genes.
Numbers on the major branches indicate bootstrap values (>50%) for 10 000
replicates, and the values below 50 are omitted. Gene names shown in
rectangles were identified in this study.
angiosperm sequences into two major clades, termed the C
and D lineages (Kramer et al., 2004; Figure 4). DcOAG1 was
clustered in the C lineage, while DcOAG2 was in the D
lineage.
Expression patterns of orchid floral organ identity genes
In order to investigate the function of orchid floral organ
identity genes, their expression patterns in vegetative and
reproductive organs were examined by reverse transcription-PCR and in situ hybridization. With the exception of
the function A ortholog DcOAP2 and the function B ortholog DcOAP3A, other floral organ identity genes were
expressed specifically in Dendrobium flowers (Figure 1e).
Transcripts of DcOAP2 and the function E ortholog DcOSEP1 were detected in all the floral organs (Figure 1e), as
with their orthologs in Arabidopsis (Drews et al., 1991;
Pelaz et al., 2000). However, the putative function B genes
To study the biological function of DcOPI, we overexpressed
the DcOPI cDNA under the control of the constitutive CaMV
35S promoter in Arabidopsis. Among 33 independent lines
of kanamycin-resistant transgenic plants, 10 lines showed
partial transformation of sepals into petaloid structures in
the first whorl (Figure 6a,b), which resembled the homeotic
transformation in the flowers of 35S::PI lines (Krizek and
Meyerowitz, 1996). SEM analysis further showed that the
adaxial surface of the base and margin of the petaloid sepals
in 35S::DcOPI contained regular rounded cells, which was
characteristic of the adaxial surface of wild-type petals
(Figure 6e,g,h), while irregular rectangular cells interspersed
with stomata were arranged in the upper center region,
which was characteristic of the adaxial surface of wild-type
sepals (Figure 6e,i,j). Similarly, the abaxial surface of the
petaloid sepals also showed the mixed features of sepals
and petals (Figure 6f,k–n).
A complementation study was further performed to
examine if DcOPI could provide the same activity as
endogenous PI in Arabidopsis flower development. In
Arabidopsis loss-of-function mutant pi-1, flowers consist of
two outer whorls of sepals and an abnormally large gynoecium in the center (Figure 6c; Bowman et al., 1991; Hill and
Lord, 1989). Similar to 35S::PI (Krizek and Meyerowitz, 1996),
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Flower development in orchids 59
Figure 4. Phylogenetic analysis of AG-like genes.
Numbers on the major branches indicate bootstrap values (>50%) for 10 000 replicates, and the values below 50 are omitted. Gene names shown in rectangles were
identified in this study.
35S::DcOPI could largely, but not fully, rescue pi-1 (Figure 6d). 35S::DcOPI pi-1 flowers developed petaloid sepals
in the first whorl, petals in the second whorl, stamens,
carpeloid stamens or pistils in the third whorl, and normal
gynoecium in the fourth whorl (Figure 6d). Compared with
the full rescue of petals in the second whorl, the rescue of
organ defects in the third whorl was weaker, because
many carpeloid organs remained. These patterns mimicked
ª 2006 National University of Singapore, The Plant Journal, (2006), 46, 54–68
60 Yifeng Xu et al.
Figure 5. In situ localization of DcOPI, DcOAP3A, DcOAG1 and DcOAG2 in orchid floral buds.
(a, b, c) DCOPI antisense probe.
(d, e, f) DCOPI sense probe.
(g, h, i) DcOAP3A antisense probe.
(j, k, l) DcOAG1 antisense probe.
(m, n) DcOAG2 antisense probe.
Ac, anther cap; Po, pollinium; Pe, petal; Se, sepal;
Pl, placenta; Li, lip; Co, column. Scale
bar ¼ 1 mm in (a–n).
overexpression of PI in pi-1, suggesting that DcOPI was able
to substitute PI in Arabidopsis.
As suggested by sequence analysis, DcOAP3A and
DcOAP3B belonged to paleoAP3 lineage and had high
sequence similarity in the predicted protein sequences. To
address if orchid B-function genes contribute to the identity
of petaloid sepals, DcOAP3A was further characterized,
because only DcOAP3A was detectable in the first whorl of
orchid flowers. However, the phenotype of all 43 transgenic
Arabidopsis lines overexpressing DcOAP3A was indistinguishable from that of wild-type plants (data not shown). In
Arabidopsis, flowers of function B mutant ap3-3 contained
sepals in the outer two whorls and carpels in the center
(Figure 7a; Jack et al., 1992). Previous reports have suggested that the paeloAP3 motif was able to promote stamen
identity in the fourth whorl and partially rescue the third
whorl into stamen-like organs in ap3-3 (Lamb and Irish,
2003). Therefore, we further examined if 35S::DcOAP3A was
able to rescue ap3-3. Consistent with void effects in wildtype plants, 35S::DcOAP3A in ap3-3 also showed indistinguishable phenotypes from ap3-3 (data not shown). To test if
DcOAP3A could function in the presence of its potential
partner DcOPI, we produced double transgenic plants
35S::DcOAP3A 35S::DcOPI and also introduced both transgenes into ap3-3. The double transgenic plants showed the
same phenotypes as 35S::DcOPI (Figure 6b), and double
transgenes could not rescue floral defects in ap3-3 (data not
shown). Taken together, these results demonstrated that
DcOAP3A cannot function as AP3 in Arabidopsis.
In order to further clarify the roles of DcOPI and DcOAP3A,
we applied a chimeric repressor silencing technology by
using the SRDX domain, a super repression motif derived
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Flower development in orchids 61
Figure 6. Rescue of pi-1 by DcOPI.
(a) A wild-type Arabidopsis flower with four whorls of floral organs.
(b) A 35S::DcOPI flower showing the petaloid sepals (arrows).
(c) A pi-1 flower. Petals are transformed into sepal-like structures; stamens are absent, with an enlarged gynoecium.
(d) Rescue of pi-1 by 35S::DcOPI. Sepals are transformed into petaloid structures in the first whorl (arrowheads); petals are rescued in the second whorl; organs in the
third whorl are transformed into normal stamens (St), carpeloid stamen (Cs) or pistils (Pi).
(e) Adaxial surface of a petaloid sepal in 35S::DcOPI. Two different types of cells are present. Rounded cells are arranged regularly in the middle and lower parts of
the panel, while rectangular cells are present irregularly in the upper left corner of the panel.
(f) Abaxial surface of a petaloid sepal in 35S::DcOPI. Ovoid cells are regularly arranged in the lower left of the panel, while irregular rectangular cells are arranged in
the upper right corner.
(g) Close-up view of the rounded cells shown in (e).
(h) Close-up view of adaxial surface of a wild-type petal.
(i) Close-up view of the rectangular cells shown in (e). Stomata (arrow) are present among cells.
(j) Close-up view of adaxial surface of a wild-type sepal with stomata (arrows).
(k) Close-up view of the ovoid cells shown in (f).
(l) Close-up view of abaxial surface of a wild-type petal.
(m) Close-up view of the irregular rectangular cells shown in (f).
(n) Close-up view of abaxial surface of a wild-type sepal. Scale bar ¼ 50 lm in (e, f), 10 lm in (g–n).
from the EAR-motif of the repression domain of tobacco
ETHYLENE-RESPONSIVE ELEMENT-BINDING FACTOR 3
(RD) and SUPERMAN (SUPRD) (Hiratsu et al., 2003). When
it is fused translationally with the genes of interest, it can
efficiently convert transcription activators into dominant
repressors, which suppress the expression of target gene(s)
ª 2006 National University of Singapore, The Plant Journal, (2006), 46, 54–68
62 Yifeng Xu et al.
Figure 7. Overexpression of DcOPI-SRDX, DcOAP3A-SRDX and DcOAG1 in
Arabidopsis.
(a) An ap3-3 flower. The second and third whorl organs have been
transformed into sepal- and pistil-like structures, respectively.
(b) A flower of 35S::DcOPI-SRDX transgenic Arabidopsis with weak phenotype. The organs in the second whorl have been transformed into sepal-like
structures (arrows) and the third whorl stamens are absent.
(c) A flower of 35S::DcOPI-SRDX transgenic Arabidopsis with severe phenotype. The second whorl cannot be clearly identified and the third whorl organs
have stigmatic regions bearing ovule-like structures (arrows).
(d) A flower of 35S::DcOAP3A-SRDX transgenic Arabidopsis with weak
phenotype. The organs at two outer whorls have stigmatic regions (arrows)
without any ovule-like structures.
(e) Close-up view of the stigmatic regions (arrows) shown in (d).
(f) A flower of 35S::DcOAP3A-SRDX transgenic Arabidopsis with severe
phenotype. The second whorl cannot be clearly identified and the third whorl
has stigmatic regions (arrows) and ovule-like structures (arrowheads).
(g) Close-up view of the stigmatic region (arrow) and the ovule-like structure
(arrowhead) shown in (f).
(h) Adaxial surface of the sepal-like petal shown in (b).
(i) Adaxial surface of the third whorl of pistil-like structure shown in (c).
(j) Early flowering phenotype of 35S::DcOAG1 with curled rosette leaves. The
transgenic plant flowers at the four-leaf stage.
(k) A determinate inflorescence of 35S::DcOAG1.
(l) A flower of 35S::DcOAG1. Transformation of the petal into stamen is
apparent as indicated by the presence of eight stamens (1–8). The sepals are
carpeloid as hallmarked by the presence of the stigmatic papillae (arrows) at
the tips. A stamen (2) transformed from petals in the second whorl resembles
a true stamen in the third whorl (1). Scale bar ¼ 500 lm in (d, f, k), 100 lm in
(e, g, l), 50 lm in (h, i).
and generate phenotypes similar to those caused by loss-offunction of the transcriptional activators. The investigation
of different classes of transcription factors by the chimeric
repressor silencing technology has confirmed that the
generated loss-of-function phenotypes are specific to the
transcription factors in question (Hiratsu et al., 2003; Markel
et al., 2002). A total of 35 transgenic Arabidopsis lines
overexpressing DcOPI-SRDX were obtained. Nine of these
transgenic lines showed various degrees of phenotypes like
the function B null mutants. In flowers of these transformants, the second and third whorl organs turned into sepaland pistil-like structures, respectively (Figure 7b,c). SEM
showed that the sepal-like organs in the second whorl of
35S::DcOPI-SRDX had irregular rectangular epidermal cells
on the adaxial surface, which was characteristic of wild-type
sepals (Figure 7h), while the pistil-like organs in the third
whorl had well arranged rectangular cells on the adaxial
surface, which was characteristic of wild-type gynoeciums
(Figure 7i). In several strong transgenic lines, ovule-like
structures were observed in the third whorl (Figure 7c).
These phenotypes resembled those of the function B mutant
ap3-3 (Figure 7a) and pi-1 (Figure 6c). It was surprising that
introduction of DcOAP3A-SRDX into Arabidopsis could also
cause similar phenotypes as function B mutants in transgenic flowers (25 out of 89 lines; Figure 7d–g), indicating that
DcOAP3A might regulate the same target genes as AP3 in
orchid flower development.
Several lines of evidence from Arabidopsis and Antirrhinum suggested that MADS-box proteins function by forming higher order complexes in flower development (Honma
and Goto, 2001; Theissen, 2001; Theissen and Saedler,
2001). In Arabidopsis and Antirrhinum, AP3 and PI, or DEF
and GLO were known to interact with each other to form
heterodimers, which in turn formed higher complexes with
SEP lineage proteins (Honma and Goto, 2001; Theissen,
2001; Theissen and Saedler, 2001). In monocot species, it
has been reported that TGGLO of tulip bound to DNA as a
homodimer (Kanno et al., 2003). To examine if orchid floral
organ identity regulators can form similar protein complexes, yeast two-hybrid analyses were performed using the
two-hybrid MATCHMAKER II system (Clontech, Palo Alto, CA,
USA). The coding regions of DcOPI, DcOAP3A, DcOAP3B
and DcOSEP1 were cloned into the GAL4 DNA activation
domain (AD) vector pACT2 and the GAL4 DNA-binding
domain (BD) vector pAS2-1. Yeast two-hybrid analyses
showed that, in contrast to their orthologs in tulip,
DcOAP3A/B and DcOPI formed heterodimers rather than
homodimers (Table 1). In addition, DcOAP3A and DcOAP3B
were not able to form dimers. Similar to their orthologs in
Arabidopsis and Antirrhinum, DcOAP3A/B and DcOPI alone
were not able to form heterodimers with DcOSEP1, but the
heterodimer of DcOAP3A or DcOAP3B and DcOPI could
interact with DcOSEP1 to form a higher order protein
complex (Table 1). Thus, the patterns of protein–protein
ª 2006 National University of Singapore, The Plant Journal, (2006), 46, 54–68
Flower development in orchids 63
Table 1 Yeast two-hybrid assays for the interactions of Dendrobium
crumenatum floral organ identity proteins
Activation domain
(AD) plasmid
Binding domain
(BD) plasmid
b-Galactosidase
activitya
pAD-DcOPI
pAD-DcOAP3A
pAD-DcOAP3B
pAD-DcOAP3A
pAD-DcOAP3B
pAD-DcOAP3A
pAD-DcOPI
pAD-DcOAP3B
pAD-DcOPI
pAD-DcOAP3A
pAD-DcOAP3B
pAD-DcOPI
pAD-DcOAP3A & pAD-DcOPI
pAD-DcOAP3B & pAD-DcOPI
pBD-DcOPI
pBD-DcOAP3A
pBD-DcOAP3B
pBD-DcOAP3B
pBD-DcOAP3A
pBD-DcOPI
pBD-DcOAP3A
pBD-DcOPI
pBD-DcOAP3B
pBD-DcOSEP1
pBD-DcOSEP1
pBD-DcOSEP1
pBD-DcOSEP1
pBD-DcOSEP1
NCb
NCb
NCb
NCb
NCb
53.12 5.37
57.25 3.20
55.06 5.50
49.65 6.40
NCb
NCb
NCb
43.06 4.50
39.76 6.57
a
b-Galactosidase activity unit ¼ 1000 · A420/[A600 · reaction time
(min) · volume of culture].
b
NC means no color change even after overnight incubation.
interactions between orchid B-function regulators were
comparable to those of their counterparts in Arabidopsis
and Antirrhinum.
Analysis of a putative function C gene of D. crumenatum
To elucidate the role of the putative function C gene
DcOAG1, we overexpressed it in Arabidopsis. Out of 35
independent transgenic lines obtained, 12 lines showed
early flowering with the phenotype of curly leaves (Figure 7j), and seven lines showed defects in the first and
second whorls of floral organs. The strong lines showed
typical ap2-like phenotypes with reduced height, small and
curled leaves, loss of inflorescence indeterminacy (Figure 7j,k), and homeotic transformation in the first and
second whorls of flowers (Figure 7l). In these flowers, the
sepals were modified into carpel-like structures with stigmatic papillae at the apex, and the petals were either absent or transformed into stamen-like structures (Figure 7l).
These results suggested that DcOAG1 had the same
function as AG in Arabidopsis.
Discussion
Although several MADS-box genes have been isolated from
orchids (Hsu and Yang, 2002; Tsai et al., 2004; Yu and Goh,
2000), molecular mechanisms underlying floral organ
development in this group of plants are unknown. In this
study, we isolated putative function A, B, C, D and E genes
from D. crumenatum orchid and carried out functional
characterization on the B and C function genes.
The B function, conferred collectively by the members of
the AP3/DEF and PI/GLO lineages, is necessary for the
formation of petals and stamens in flowers. Extensive
studies in the past decade have proven that AP3/DEF and
PI/GLO lineages are highly conserved in diverse plant
species. Gene duplication and divergence in these two
lineages have been known throughout the history of the
evolution of angiosperms (Kramer et al., 1998; Zahn et al.,
2005). While PI/GLO-like genes are highly conserved, AP3/
DEF-like genes have three distinct lineages, paleoAP3, TM6
and euAP3 (Kramer and Irish, 2000; Kramer et al., 1998). It
has been proposed that these genes may have evolved from
an ancestral gene possessing a paleoAP3 motif after two
major duplications (Kramer et al., 1998; Purugganan, 1997).
The first duplication generated the PI and paleoAP3 lineages. In the second duplication, the paleoAP3 lineage
remained in lower eudicots, magnolid dicots and monocots,
but further divided into the euAP3 and TM6 lineages in core
eudicots (Kramer et al., 1998).
We have isolated three putative B function genes, DcOPI,
DcOAP3A and DcOAP3B, from the orchid D. crumenatum.
DcOPI belongs to the PI lineage, while DcOAP3A and
DcOAP3B are typical genes in the paeloAP3 lineage. This
result indicates that gene duplication has at least taken
place in the AP3/DEF clade in the monocots. Overexpression of DcOPI in Arabidopsis wild-type plants and pi-1 null
mutants have shown that DcOPI could provide similar
regulatory effects as PI in flower development, indicating
that DcOPI is an ortholog of PI in orchids. In contrast,
overexpression of DcOAP3A in wild-type or ap3-3 plants
failed to cause significant phenotypic changes relevant to B
function genes. Compared with the PI lineage, the AP3/DEF
lineage is less conserved in protein sequences. DcOAP3A
and AP3 belong to paleoAP3 and euAP3 lineages, respectively. AP3 contains a distinct euAP3 motif in its C-terminal
region (Figure 2a) resulting from a frame-shift mutation in
one of the copies of a duplicated ancestral paeloAP3-type
gene (Vandenbussche et al., 2003). It has been shown that
the paeloAP3 and euAP3 motifs within the eudicot lineage
may have different functions (Lamb and Irish, 2003;
Vandenbussche et al., 2004). A chimeric construct for the
Arabidopsis AP3 gene, in which the C-terminal euAP3 motif
was replaced by a paeloAP3 motif, could partially rescue
stamen development in ap3-3 (Lamb and Irish, 2003). This
result indicates that the paeloAP3 motif can only promote
stamen but not petal development in core eudicots. However, another study has argued that the paeloAP3 genes
have a similar function to their euAP3 orthologs. Overexpression of maize Silky under the control of the AP3
promoter can partially rescue ap3-3 in Arabidopsis (Whipple et al., 2004). Our study suggests that the paeloAP3 gene
may have different functions from the euAP3 gene, demonstrating the functional diversity of the AP3/DEF-lineage
genes in different plant species.
We have further tested whether DcOPI and DcOAP3A
are function B orthologs in orchids by using a chimeric
ª 2006 National University of Singapore, The Plant Journal, (2006), 46, 54–68
64 Yifeng Xu et al.
repressor silencing technology. The transgenic plants
expressing DcOAP3A-SRDX and DcOPI-SRDX have shown
the dominant-negative phenotypes as those in B function
mutants. Because overexpression of DcOAP3A and DcOPI
did not produce any dominant-negative phenotypes, the
possibility that DcOAP3A-SRDX and DcOPI-SRDX cause the
gene-specific silencing of endogenous AP3 and PI is very
low. DcOAP3A-SRDX and DcOPI-SRDX may compete with
AP3 and PI for the same target genes, or substitute them in
critical regulatory protein complexes, thus indirectly causing
the phenotypes of B function mutants. In either case,
DcOAP3A and DcOPI show functional potentials similar to
AP3 and PI, respectively. It is interesting to note that, while
35S::DcOAP3A did not produce any phenotypes in Arabidopsis, 35S::DcOAP3A-SRDX induced a dominant loss-offunction phenotype. One possibility is that DcOAP3A is not
sufficient to activate downstream target genes of AP3, while
SRDX renders an activity to DcOAP3 to repress the same
targets of AP3.
One key feature of B function genes in most species is that
AP3/DEF-like proteins form obligate heterodimers with PI/
GLO-like proteins. In monocots, ZMM16 and SILKY1 of
maize (Whipple et al., 2004), as well as OsMADS16 and
OsMADS4 of rice (Moon et al., 1999), function as a heterodimer as indicated by in vitro DNA-binding and yeast twohybrid assays. However, it has been reported that in tulip
and lily, their PI/GLO-like proteins are able to form homodimers (Kanno et al., 2003). In our study, yeast two-hybrid
analyses have shown that AP3/DEF-like protein, DcOAP3A or
DcOAP3B, form heterodimers with DcOPI. All these function
B proteins are not able to form homodimers. Also, DcOAP3A
and DcOAP3B do not interact with each other. Furthermore,
the heterodimers formed by DcOPI and DcOAP3A/B can
form higher order complexes with DcOSEP1. These results
strongly suggest that the heterodimerization of B function
proteins is at least conserved among orchids, grasses and
eudicots.
The unique floral structures in orchids can be partially
explained by the conserved role of function B genes. RT-PCR
and in situ analyses have shown that both DcOPI and
DcOAP3A are expressed in all whorls of floral organs. These
expression patterns, together with the shown functions in
transgenic Arabidopsis, suggest that the expansion of
function B genes, represented by DcOPI and DcOAP3A, into
the first whorl may be responsible for the petaloid sepals in
orchids. Our results are consistent with the modified ABC
model (van Tunen et al., 1993). In Tulipa (Liliaceae), the
function B genes TGDEFA/B and TGGLO were also
expressed in whorls 1 and 2 (Kanno et al., 2003). On the
other hand, the mRNA expression of an AP3/DEF-like gene,
LMADS1 of Lilium longiflorum, was detectable strongly in
whorls 2 and 3, but weakly in whorls 1 and 4. In addition,
LMADS1 protein was only detectable in whorls 2 and 3,
suggesting that the outer two whorls of tepals in lily are
regulated by a different mechanism despite their similarity
in morphology (Tzeng and Yang, 2001). The above contradictory results may indicate the complexity in the role of
function B genes in the development of petaloid sepals.
In the second whorl of orchid flowers, multiple polymorphisms are widely observed in wild-type plants and naturally
occurring peloric mutants. The median petal of a wild-type
orchid flower is specialized as a lip, which may be differently
colored, shaped, or decorated with crests, tails, horns or
other accessories attractive to potential pollinators. In natural peloric orchid mutants, a specific type of organs such as
petals, lips or transformed sepals in the second whorl can be
transformed into another type without affecting the identities of other whorls of floral organs (Rudall and Bateman,
2002). This subtle regulation should require more regulatory
genes to be involved, which is consistent with the isolation
of at least two AP3/DEF orthologs from D. crumenatum.
Recently, four AP3/DEF-like MADS-box genes were also
identified in Phalaenopsis orchid (Tsai et al., 2004). Comparison of their expression patterns between wild-type
plants and peloric mutants has shown the functional
specialization of these paralogous genes in perianth whorls.
The above observations suggest that the exquisite regulation of the second whorl in orchid flowers may need the
coordinated functions of several AP3/DEF-like genes.
There are several lines of evidence suggesting that two
AG-like genes from D. crumenatum may have different
biological functions. First, an N-terminal extension preceding the MADS domain found only in C-lineage members
is present in DcOAG1, but not in DcOAG2. Secondly, the
genomic sequence of DcOAG1 contains the intron 8, which
is absent in DcOAG2 and most other known function D
genes (Kramer et al., 2004; Zhang et al., 2004). Thirdly,
phylogenetic analyses show that DcOAG1 and DcOAG2
belong to function C and D genes in orchids, respectively
(Figure 4). Lastly, overexpression of DcOAG1 in Arabidopsis causes early flowering, curly leaves and, in particular,
the homeotic transformation of floral organs as observed in
overexpression of AG and AG orthologs in other plant
species. These results strongly support that DcOAG1 is the
orchid ortholog of AG, while DcOAG2 is a putative function
D gene according to its specific expression in the ovary
and its clustering with D-lineage genes by phylogenetic
analysis.
While the sequence analyses and functional studies have
suggested that DcOAG1 is a putative C function gene in
orchids, its expression is detectable in all the floral organs.
Recently, it has been reported that, in some basal angiosperm species, the expression of the AG orthologs was also
not restricted to the reproductive organs (Kim et al., 2005).
The Illicium AG ortholog is expressed strongly in the inner
tepals, while the Persea AG orthologs are detectable in all
floral organs. The broader expression of AG orthologs in
basal angiosperms Illicium and Persea as well as the
ª 2006 National University of Singapore, The Plant Journal, (2006), 46, 54–68
Flower development in orchids 65
monocot orchids indicates that the expression of AG orthologs or the regulatory mechanisms involved can be independently derived. In orchids, the concurrence of the
expression of DcOAP2, a putative AP2 ortholog and DcOAG1
in all floral organs implies that the mechanism underlying
the regulation of AG orthologs may be different from that in
Arabidopsis, as observed in Antirrhinum and petunia (Keck
et al., 2003; Maes et al., 2001).
The column of an orchid flower is thought to be the fused
structure of male and female organs. The finding of the coexpression of orchid B, C and D function genes in the column
(Figure 4), provides molecular evidence to support the
chimeric nature of the column. Because the expression of
B function genes in female organs has been reported in
many angiosperm lineages (Zahn et al., 2005), the unique
structure of orchid reproductive organs may indicate a
hitherto unidentified function of B function genes in flower
development. For example, it will be interesting to explore if
expression of function B genes in the column causes the
fusion of male and female organs in orchids.
In conclusion, our study demonstrates that the molecular
mechanisms underlying flower development are partly
conserved between Arabidopsis and orchid despite extensive differences in their floral morphology. During floral
evolution, gene duplication events as revealed in this study
may have given rise to a diversity of gene functions or
expression domains, thus leading to the specification of
unique floral structures in orchids. Further functional analyses of orchid floral organ identity genes are necessary to
achieve a better understanding of the evolution of the
unique structures in orchid flowers and to provide a
mechanistic basis for genetic engineering of orchids.
Experimental procedures
Plant materials and growth conditions
Dendrobium crumenatum plants used in this study were grown in
the garden of the Department of Biological Sciences, National
University of Singapore, Singapore. Arabidopsis mutants of pi-1
and ap3-3 as well as transgenic plants were grown in growth
chambers (22C, 16 h of light/8 h of darkness).
Molecular cloning of MADS-box genes
Total RNA from different floral tissues and vegetative parts of
D. crumenatum was isolated using the RNeasy Plant Mini Kit (Qiagen, Hilden, Germany). To isolate the putative AP3/DEF and PI/GLO
orthologs, fragments around 850 bp were amplified by one
degenerate primer dMADS1 and 3¢-RACE with SMART RACE cDNA
Amplification Kit (Clontech). Comparison with sequences in the
database revealed two fragments corresponding to AP3/DEF and PI/
GLO homologs. To get the complete cDNAs, 5¢-RACE was performed with the gene-specific primers 5RACEOAP3A and 5RACEOPI. To isolate more AP3/DEF-like genes, a degenerate primer
DODEF1 corresponding to the motif specifically conserved among
AP3/DEF-lineage members was designed. Using this degenerate
primer, 3¢-RACE PCR was performed to obtain another AP3/DEF
gene fragment with the partial K domain, C region and 3¢-UTR. The
upstream sequence of this cDNA was isolated by 5¢-RACE PCR with
the gene-specific primer 5RACEOAP3B. During isolation of the full
length of DcOAP3B, a cDNA fragment containing the 3¢-terminal
region of DcOAP2 was obtained, 5¢-RACE was performed to isolate
the upstream cDNA fragment with the gene-specific primer
5RACEOAP2. Partial cDNAs of AG orthologs were isolated by RTPCR using two degenerate primers dgAGF and dgAGR, which correspond to the conserved sequence of MADS-box regions. This led
to the isolation of two fragments corresponding to DcOAG1 and
DcOAG2. To obtain the full cDNA sequences, 5¢- and 3¢-RACEs were
performed respectively with the internal gene-specific primers:
5RACEOAG1, 3RACEOAG1, 5RACEOAG2 and 3RACEOAG1. The
partial genomic sequences of DcOAG1 and DcOAG2 were isolated
using primers GDOAG1-F1, GDOAG1-R1, GDOAG2-F1 and GDOAG2-R1, which were designed near the last codon of their protein
sequences to examine the existence of intron 8. A partial cDNA
fragment of DcOSEP1 was amplified by degenerate primers
dgSEP1F and dgSEP1R based on the conserved sequence of the
AP1/AGL9-lineage genes. 5¢- and 3¢-RACEs were further performed
to obtain upstream and downstream fragments with the internal
gene-specific primers 5RACEOSEP1 and 3RACEOSEP1. All the
primers used for cloning are listed in Table S3.
Sequence analysis
Sequencing was performed using the Big Dye Terminator Cycle
Sequencing Ready Reaction Kit (Applied Biosystems, Foster City,
CA, USA). Sequence data were compared with the relevant sequences retrieved from the National Center for Biotechnology
Information (http://www.ncbi.nlm.nih.gov) database.
Phylogenetic analyses for B and E class genes were carried out
using the program PAUP version 4.0b4 (Thompson et al., 1997).
DEFICIENS-AGAMOUS-LIKE2 (DAL2) (X79280) of Picea abies was
chosen as an outgroup. The sequence used for phylogenetic
analysis included the MADS-box, I-region and K-box of MADS-box
genes. These sequences were aligned using CLUSTALW and then
used for neighbor-joining and bootstrapping analyses.
Phylogenetic analyses for AG-like genes were preformed as the
published protocols (Kramer et al., 2004) with some modifications.
The amino acid sequences of putative orchid AG-like genes
DcOAG1 and DcOAG2 and 96 previously published AG-like genes
from angiosperms and gymnosperms were aligned by MUSCLE
(Edgar, 2004) and manually adjusted. Their nucleotide sequences
were aligned through the protal2dna server (http://bioweb.pasteur.fr/seqanal/interfaces/protal2dna.html), and this alignment was
further adjusted based on the alignment of amino acid sequences.
These aligned nucleotide sequences were used for phylogenetic
analysis by using the NEIGHBOR-JOINING program in the PHYLOGENY
INFERENCE PACKAGE, version 3.65 (PHYLIP 3.65; Department of
Genome Sciences, University of Washington, Seattle, WA, USA).
The statistical significance was tested by bootstrap analysis for
10 000 replicates.
RT-PCR analysis and in situ hybridization
Total RNA extracted from different tissues served as the template
for first strand cDNA synthesis using the SuperScript First Strand
Synthesis System for RT-PCR (Invitrogen, Carlsbad, CA, USA).
Orchid polyubiqutin (DcOUPQ1) was amplified as a quantitative
ª 2006 National University of Singapore, The Plant Journal, (2006), 46, 54–68
66 Yifeng Xu et al.
control for RT-PCR. RT-PCR was preformed using gene-specific
primers listed in Table S4.
In situ hybridization was preformed according to Long and Barton
(1998) with minor modifications. In brief, the probe fragments were
amplified by the gene-specific primers and cloned into pGEM-T
Easy vectors (Promega, Madison, WI, USA). The single-stranded
antisense and sense RNA probes were transcribed in vitro with T7 or
SP6 polymerases using the DIG RNA Labeling Kit (Roche Diagnostic, Mannheim, Germany).
Construction of binary vectors
The constructs used for overexpression of DcOPI and DcOAP3A
were prepared from pBI121 binary vector (Clontech). The open
reading frames (ORFs) of cDNAs were amplified using the proofreading Vent DNA polymerase (New England Biolabs, Beverly, MA,
USA) with primers incorporating relevant restriction sites. The
primers used in PCR were OPI-F(BamHI), OPI-R(XbaI), OAP3AF(BamHI) and OAP3A-R(XbaI). The PCR products were digested with
XbaI and BamHI, and cloned into the pBI121 binary vector, where
the b-glucuronidase (GUS) gene has been removed. For the preparation of 35S::DcOAG1 construct, the primers OAG1-F(XhoI) and
OAG1-R(BamHI) were used to amplify the DcOAG1 coding region.
The PCR products were digested with XhoI and BamHI, and inserted
into the corresponding sites of a pGreen 0229-35S binary vector (Yu
et al., 2004). For chimeric repressor constructs of DcOAP3A and
DcOPI, a binary vector pGreen-SRDX with SRDX motif, a derived
motif of SUPERMAN, was first prepared. SRDX was amplified from
Arabidopsis genomic DNA using the primers SUPRD1(BamHI) and
SUPRD2(XbaI). Because the resulting DNA fragment contained one
internal XbaI restriction site, partial digestion with XbaI was preformed after complete digestion with BamHI. The resulting DNA
fragment with the expected size of 89 bp was inserted into BamHI
and XbaI sites of pGreen 0229-35S binary vector. cDNA fragments of
DcOPI and DcOAP3 were amplified by primers OPI-Fus1(XhoI), OPIFus2(BamHI), OAP3A-Fus1(XhoI) and OAP3A-Fus2 (BamHI). After
restriction digestion, the resulting fragments were inserted into
XhoI and BamHI of pGreen-SRDX to be translationally fused with
SRDX domain at the C terminus. Primers used for construction of
the above vectors are listed in Table S3.
Arabidopsis transformation
Agrobacterium tumefaciens–mediated plant transformation was
performed by the floral dipping method (Clough and Bent, 1998).
Seeds of 35S::DcOAP3A and 35S::DcOPI transgenic plants were
selected on MS medium supplemented with kanamycin, and placed
at 23C in a growth room under long-day conditions (16 h light).
Seeds of 35S::DcOAG2, 35S::DcOAP3-SRDX and 35S::DcOPI-SRDX
were sown directly on soil and screened by Basta selection.
Yeast two-hybrid analysis of protein–protein interaction
Two-hybrid analyses were preformed by the MATCHMAKER II system
according to the manufacturer’s recommendation. Full-length
cDNAs of DcOAP3A, DcOAP3B, DcOPI and DcOSEP1 were cloned
into pACT2 and pAS2-1 vector. Yeast strain Y190 was used.
Selection for interaction was preformed on medium lacking His
and Leu, and supplemented with 10 mM 3-amino-1,2,4-triazole
(Sigma-Aldrich, St. Louis, MO, USA). The interaction activity was
quantified by b-galactosidase assay in liquid culture using
o-nitrophenyl-b-D-galactoside (Sigma-Aldrich) as the substrate.
Scanning electron microscopy
Freshly harvested plant materials were flash-frozen in liquid nitrogen. Examination of specimens was carried out with the (JSM5600LV) electron microscope at 10 kV.
Acknowledgements
We thank Qingwen Lin for SEM analysis, Lesheng Kong for phylogenetic analysis, Mohan K. Balasubramanian for pACT2 and pAS2-1
vectors, and Toshiro Ito for critically reading the manuscript. This
work was supported by Research Grants R-154-000-232-101 and R154-000-125-112 and PhD scholarships (to YX, LLT and JZ) from the
National University of Singapore, and the intramural research funds
from Temasek Life Sciences Laboratory.
Supplementary Material
The following supplementary material is available for this article
online:
Figure S1. cDNA and predicted protein sequence of DcOAP2.
Figure S2. cDNA and predicted protein sequence of DcOSEP1.
Table S3 Primers used for gene cloning and construction of binary
vectors
Table S4 Primers used for RT-PCR and generating probes for in situ
hybridization
This material is available as part of the online article from http://
www.blackwell-synergy.com
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