全 文 ：Received 2 Jul. 2003 Accepted 2 Apr. 2004
Supported by the State Key Basic Research and Development Plan of China (G1999011602) and the Rockefeller Foundation (GN#97003).
* Author for correspondence. E-mail: .
** Author for correspondence. E-mail: .
http://www.chineseplantscience.com
Acta Botanica Sinica
植 物 学 报 2004, 46 (8): 973-981
Analysis of the Spatial and Temporal Expression of a MADS-Box Gene
M79 in Rice Using In Situ Reverse Transcription
MING Liang, GU Hong-Ya*, LIU Mei-Hua, PAN Nai-Sui, WEI Jun-Ming, CHEN Zhang-Liang, QU Li-Jia**
(Peking-Yale Joint Center for Plant Molecular Genetics and AgroBiotechnology, State Key Laboratory of Protein Engineering and
Plant Genetic Engineering, College of Life Sciences, Peking University, Beijing 100871, China)
Abstract: This study describes an innovative approach for detecting mRNA, called in situ reverse
transcription (ISRT), which marries in situ hybridization (ISH) with reverse transcription. Specific primers
were designed for mRNA-specific reverse transcription. The parameters and key steps of ISRT were
optimized. Based on our experience, with proper controls ISRT, can detect mRNA signals with high
sensitivity and high specificity. ISRT was used to detect the mRNA of a rice MADS-box gene, M79, which
belongs to the AGL2 subgroup. As seen with conventional ISH, M79 is expressed at all stages of rice
development, in both vegetative and reproductive processes. ISRT is relatively fast and easy-to-perform,
suggesting that this method will become a reliable, fundamental technique that can be used to localize
mRNA in situ.
Key words: M79 ; MADS-box; Oryza sativa ; in situ reverse transcription (ISRT); flower development
Many techniques have been developed to examine the
expression patterns of a target gene in various tissues or at
different developmental stages. Northern blot hybridiza-
tion is the conventional approach used to analyze mRNA
in specific tissues, but it cannot localize gene expression at
the subcellular level. To reveal the spatial distribution of
the mRNA of interest within cells, a powerful technique
called in situ hybridization (ISH) has been developed to
localize transcripts directly. The method is widely used to
localize mRNA temporally and spatially. In plants, ISH is
commonly used to perform hybridization on fixed plant tis-
sue sections with labeled probes. The key procedures in
ISH are all performed on slides and include the following
steps: fixing, dehydrating, and embedding tissue in paraf-
fin wax, sectioning, mounting sections on slides, dewaxing
sections, rehydrating, performing partial proteolysis, pre-
paring labeled probes, hybridizing blots, and detecting
hybridized signals (Jackson, 1991; McFadden, 1995). Based
on the basic principles of ISH, we integrated some new
procedures and developed a novel approach we call in situ
reverse transcription (ISRT). As an alternative to
hybridization, reverse transcription is a sensitive and effi-
cient way to incorporate labeled nucleotides into cDNA
generated from target mRNA template, which can be de-
tected by immunohistochemical methods. ISRT produces
a signal as good as that of ISH. Moreover, several treat-
ment steps are omitted, so that the overall time for ISRT is
reduced, simplifying the technique.
A known Arabidopsis MADS-box gene, AGL2, together
with the rice MADS-box gene M79 was used as target gene
to compare ISRT with ISH. The MADS-box, a 56-amino-
acid sequence motif shared by different floral organ iden-
tity genes, is an acronym for the four proteins in the family:
MCM1, AGAMOUS, DEFICIENS, and SRF (Schwarz-
Sommer et al., 1990). MADS-box genes have been found
in diverse eukaryotes, from fungi to plants to animals
(Yanofsky et al., 1990; Dolan and Fields, 1991; Johansen
and Prywes, 1995). They function as transcription factors
involved in a wide range of biological pathways (Shore and
Sharrocks, 1995; Theißen et al., 1996). For instance, sev-
eral MADS-box genes play key roles in organ development
in plants, especially in flower development; these are widely
known as floral meristem identity genes and organ identity
genes (Angenent et al., 1992; Huijser et al., 1992; Mandel
et al., 1992; Schwarz-Sommer et al., 1992; Angenent et al.,
1994; Goldberg et al., 1994; Goto and Meyerowitz, 1994;
Rigola et al., 1998). In addition, several MADS-box genes
are also involved in vegetative growth, such as shoot
(Huang et al., 1995) and root (Zhang and Forde, 1998)
development, embryogenesis (Heck et al., 1995), and seed
germination (Sung and An, 1997). The functions of plant
MADS-box proteins are not limited to flower development,
but include various development processes (Rounsley
et al., 1995). Phylogenetic analyses have classified plant
Acta Botanica Sinica 植物学报 Vol.46 No.8 2004974
MADS-box genes into several distinct subfamilies (Theißen
et al., 1996).
Many MADS-box genes have been cloned from
monocots, including maize (Mena et al., 1995), rice (Kang
et al., 1997), sorghum (Greco et al., 1997), and orchid (Lu
et al., 1993). Previously, we cloned a new MADS-box gene,
M79, from rice, which was orthologous to plant AGL2-like
genes. Transgenic tobacco plants with sense M79 showed
an early flowering phenotype, implying that M79 is in-
volved in controlling the flowering time. Moreover, M79
may also contribute to the control of the branching pro-
cess to form more flower buds (Qu et al., 2001). For an in-
depth study of the function of this transcription factor in
rice development, we need a precise description of M79
expression. In this paper, we report the expression pattern
analysis of M79 in rice tissues at different developmental
stages using in situ reverse transcription (ISRT)
hybridization, and compare the technique with ISH.
1 Materials and Methods
1.1 Plant material
Rice (Oryza sativa L. ssp. japonica var. Zhonghua 8)
plants at different developmental stages and Arabidopsis
flowers were collected from plants grown in a greenhouse
(College of Life Sciences, Peking University) under normal
conditions.
1.2 Tissue preparation and sections
The fixation step is critical for preserving the morphol-
ogy of tissue samples and determining the overall results
of in situ detection. Successful outcomes have been
achieved using rice tissues (flowers, roots, shoots, embryos,
and kernels) and Arabidopsis flowers fixed in FAA (4.5%
formaldehyde, 50% ethanol, 3% acetic acid) and then em-
bedded in paraffin. Embedded tissues were sliced into 10-
mm sections, which were then attached to poly-lysine-
coated glass slides and dried at 45 ℃ overnight.
1.3 In situ hybridization
We used conventional in situ hybridization in parallel
with the novel in situ reverse transcription to detect the
M79 expression pattern in order to compare the two
techniques. To produce the probe for ISH, a fragment from
the 3-terminus of M79 cDNA was amplified by PCR using
the primers 5-CTGAAGCAAATAGATGCC-3 (sense
primer) and 5-CAAGTGGATGGAAGAACC-3 (Primer Ⅰ),
and cloned into T-vector (Promega, USA). The DNA se-
quence encoding the MADS-box domain was not included
to avoid cross-hybridization with other MADS-box genes.
To make digoxigenin (DIG)-labeled antisense and sense
RNA probes, the plasmid harboring the M79 cDNA
sequence was linearized and used as the template for RNA
synthesis using an in vitro transcription kit (Roche,
Sweden). The resulting probes were then hydrolyzed to an
average length of 100 nucleotides. Sections of rice tissues
were treated with 1 mg/mL protease K for 30 min at 37 ℃
and hybridized with the probes in hybridization buffer (50%
formamide, 300 mmol/L NaCl, 10 mmol/L Tris-HCl, pH 7.5, 1
mmol/L EDTA, 5% dextran sulfate, 1% blocking reagent,
150 mg/mL tRNA, 200-800 mg/mL probe) at 43 ℃ overnight.
The samples were incubated in BSA washing solution (150
mmol/L NaCl, 100 mmol/L Tris-HCl, pH 7.5, 1% BSA, 0.3%
Trition-X-100) for 3 h. Anti-digoxigenin antibody coupled
with alkaline phosphatase was then added and incubated
for 2 h at room temperature. After washing once with BSA
washing buffer and TMN-50 for 2 min, NBT/BCIP enzy-
matic detecting solution, which is the substrate of alkaline
phosphatase in TMN-50 (100 mmol/L Tris-HCl, pH 9.5, 100
mmol/L NaCl, 50 mmol/L MgCl2, 330 mg/mL NBT, 170
mg/mL BCIP), was used to visualize the hybridization signals
according to described methods (Flanagan and Ma, 1994).
1.4 In situ reverse transcription
We used an intracellular reverse transcription step to
synthesize cDNA using labeled nucleotides, so that the
location of the target mRNA could be detected without
hybridizing steps. In order to optimize reverse transcription,
the antisense primers should contain 18 to 22 bp, be posi-
tioned in the 3-terminal region of the target mRNA, and
contain at least one GC-type dinucleotide at the 3 end to
facilitate complementary-strand formation and avoid sec-
ondary structures. Because the detailed spatial and tem-
poral expression pattern of Arabidopsis AGL2 is known
(Flanagan and Ma, 1994), we can verify the novel approach
using ISRT to detect AGL2 in floral sections by comparing
it with reported outcomes. To amplify the 3-terminal region
of Arabidopsis AGL2, the primer 5-CCATCCTGGAATGTA-
ACC-3 was designed, and in situ reverse transcription was
performed on sections of Arabidopsis flowers. Sections
were treated with 10 mg/mL protease K for 30 min at 37 ℃
for permeabilization to allow the RT reagents access to in-
tracellular mRNA. When digestion was complete, the pro-
teolysis was stopped by washing the slides with fresh PBS
three times for 5 min, and the slides were rehydrated through
an ethanol series. Reverse transcription reaction was then
performed using SuperScriptⅡ reverse transcriptase
(Gibco-BRL, USA) on slides in about 100 mL of reaction
buffer (50 mmol/L Tris-HCl, pH 8.3, 75 mmol/L KCl, 3 mmol/
L MgCl2, 10 mmol/L DTT, 1 mmol/L primer, 225 mmol/L
dNTPs, 10 mmol/L DIG-dUTP) per section at 43 ℃ for 2 h.
The reaction buffer was removed, and the samples were
MING Liang et al.: Analysis MING Liangof the Spatial and Temporal Expression of a MADS-Box Gene M79 in Rice Using In
Situ Reverse Transcriptiona 975
washed with PBS at 43 ℃ and incubated in BSA washing
solution (150 mmol/L NaCl, 100 mmol/L Tris-HCl, pH 7.5,
1% BSA, 0.3% Trition-X-100) at room temperature for 3 h.
Then, the anti-digoxigenin antibody (dilution of 1:500) was
added to the sections, and the sections were incubated for
2 h at room temperature. The immunological detection pro-
tocol was the same as described above.
To detect the expression of M79, two primers, the Primer
I and 5-GTCCAAATCCATCTCTCC-3 (Primer Ⅱ), were de-
signed to amplify the 3-end sequence of M79 cDNA. As a
control, reverse transcription using the sense primer was
performed.
1.5 Probe verification using RT-PCR
Total RNA from rice flowers collected at the meiosis
stage was prepared using an RNeasy Plant Mini Kit (Qiagen,
Germany). For RT-PCR, first-strand cDNA was synthesized
using SuperScript Ⅱ (Gibco-BRL, USA) from 3 mg of total
RNA. The two primers, the sense primer and the Primer Ⅰ,
were used to amplify the 3 region of M79 from the synthe-
sized cDNA by PCR. PCR analysis was performed in 100 mL
of PCR buffer (20 mmol/L Tris-HCl, pH 8.4, 50 mmol/L KCl,
1.5 mmol/L MgCl2) containing 20 pmol of the 5 and 3 primers,
200 mmol of each deoxyribonucleotide triphosphate, cDNA
synthesized from 300 ng of total RNA, and 2 U Taq DNA
polymerase. Amplification began with a 2-min Taq activa-
tion step at 94 ℃, followed by 35 cycles of 30 s at 94 ℃, 30
s at 56 ℃, and 30 s at 72 ℃. The product was analyzed on
a 2% agarose gel to reveal the amplified fragment.
2 Results
2.1 Application of in situ reverse transcription
Several approaches have been used to detect gene tran-
scripts in situ. Of these, in situ hybridization is widely
used to study gene expression patterns. However, this
technique requires cloning the gene of interest, which is, in
turn, used as template for in vitro transcription to synthe-
size the probe. This multiple-step method is time consuming,
especially when a set of genes is to be investigated. If
those genes contain many conserved regions or belong to
a closely related gene family, it may be problematic to ob-
tain gene-specific probes. To avoid these troubles, we used
an innovative method, called ISRT, and compared it with
standard ISH, to examine the expression pattern of a rice
MADS-box gene, M79. The basic principle of ISRT is quite
similar to that of ISH, except for the way the labeled nucle-
otides are incorporated into samples. The ISRT technique
uses reverse transcription to incorporate the labeled nucle-
otides into a synthesized DNA chain, whereas ISH visual-
izes the transcript by hybridizing it with a labeled RNA
probe. To exclude false-positive and false-negative results,
we performed ISRT using two antisense primers correspond-
ing to two different regions in the 3 non-conserved termi-
nus and one sense primer corresponding to the 5 region of
M79 cDNA as controls. We also performed ISRT on samples
pre-treated with RNase. In addition to ISRT, we performed
ISH to detect M79 expression pattern in parallel.
To determine whether the primers used for in situ hy-
bridization were specific to the corresponding mRNA, the
sense and antisense primers were used in RT-PCR. As a
control, ISRT was performed on sections of Arabidopsis
flowers to examine the Arabidopsis AGL2 gene, whose ex-
pression pattern is known.
The comparison of the results of ISRT using two differ-
ent antisense primers at the same stage of rice flower devel-
opment revealed transcription signals in the same regions
(Fig.1A, B). Therefore, primers designed for different posi-
tions in the 3-terminal region of the mRNA would not pro-
duce different results, as long as the primer is specific for
the target gene. No significant signal beyond background
was detected in the sample pre-treated with RNase or when
ISRT was performed using the M79 sense primer (Fig.1C,
D), suggesting that ISRT has sensitivity and specificity
similar to that of standard in situ hybridization. Single RT-
PCR products of the correct sizes were obtained and
Figs.1-3. 1. M79 expression pattern using in situ reverse transcription. Reverse transcription was performed on cross-sections using
primers Ⅰ (A) and Ⅱ (B) at the meiosis stage of pollen mother cells. Similar results were obtained using different primers, showing
transcripts in microspore mother cells, the tapetum, vascular bundles of anther septum palea, lemma, and sterile lemma. Two controls
were not included, in which M79 was not expressed above background levels. a, anther; L, lemma; P, palea; SL, sterile lemma. 2. Detection
of AGL2 mRNA during developmental stage 7 of the Arabidopsis flower. A longitudinal section of the flower was treated using in situ
reverse transcription (ISRT). The result was consistent with the published expression pattern obtained using in situ hybridization (ISH)
(Flanagan and Ma, 1994). PP, pistil primordium; Se, sepal; SP, spikelet. 3. The distribution of M79 transcripts in rice flowers at different
stages. Each print shows sections at the same stage subjected to in situ reverse transcription (ISRT). A-D, F are longitudinal sections.
E is a cross-section. A. The first bract primordium stage. B. Spikelet meristem stage. C. Stamen and pistil primordium formation stage.
D. Longitudinal sections of an anther at the pollen mother cell formation stage. E. Cross-sections of an anther at the pollen mother cell
meiosis stage. F. Longitudinal sections at the mature pollen stage; No signals were detected in vacuolated pollen and the transcripts in the
tapetum and vascular bundles of the anther septum were faint. BP, bract primordium; PMC, pollen mother cell; T, tapetum. Other
abbrevations are the same as in Figs.1, 2.
→
Acta Botanica Sinica 植物学报 Vol.46 No.8 2004976
sequenced (data not shown), showing that sense and
antisense primers synthesized specific M79 DNA
fragments.
The spatial expression pattern of AGL2 in Arabidopsis
flower has been well established using ISH (Flanagan and
Ma, 1994). We detected the expression of AGL2 during
stage 7 using ISRT hybridization. The expression of AGL2
in the stamen and pistil primordia remained high, while it
MING Liang et al.: Analysis MING Liangof the Spatial and Temporal Expression of a MADS-Box Gene M79 in Rice Using In
Situ Reverse Transcriptiona 977
was hardly detectable in sepals during the period when the
stamen primordia developed into stamens with a stalked
base (Fig.2). This is in agreement with published patterns
(Flanagan and Ma, 1994).
2.2 Expression of M79 during rice flower development
Because it is a monocot, rice has a flower structure dis-
tinct from that of Arabidopsis, a model dicot plant widely
used in studies of flower development. The Arabidopsis
flower consists of four whorls of floral organs: four sepals,
four petals, six stamens, and two fused carpels. By contrast,
rice lacks the two outer whorls, sepals, and petals, which
are replaced by a pair of lodicules. In addition to the
lodicules, the rice floral organ consists of rudimentary
glumes, the lemma, the palea, six stamens, and the pistil.
The differentiation and development of the rice flower be-
gins with the differentiation of the first bract primordium, a
conspicuous change from the vegetative meristem to the
reproductive meristem. The floral meristem is produced
when the spikelet apical meristem emerges; then, the sta-
mens and pistils appear. In the stamen, pollen mother cells
are formed and begin to undergo meiosis, leading to the
formation of microspore tetrads. After formation of the
pollen wall and the germination apertures, the pollen is
mature (Ting et al., 1959).
M79 expression was clearly detected from emergence
of the bract primordium until pollen ripening. In the early
stages of rice flower development and differentiation, M79
transcripts were detectable in the inflorescence meristem,
specific to the regions of the bract primordium (Fig.3A),
and then in the rice spikelet primordia (Fig.3B). When the
floral organ primordia emerged, M79 appeared to localize
predominately in floral buds, such as the organ primordia
of the lodicules, lemmas, paleas, developing stamen, and
pistil primordia (Fig.3C). This expression pattern was main-
tained while the stamens differentiated into anthers and
filaments and the ovary and style developed into the young
pistil. There were obvious transcription signals in the
nucleus of pollen mother cells and the tapetum shortly af-
ter their emergence within the anther (Fig.3D). M79 expres-
sion was still abundant throughout the pollen mother cell
stage of meiosis (Fig.3E). The M79 gene was strongly
expressed in microspore mother cells, microspores,
filaments, tapetum, and the vascular bundles of the anther
septum. Weaker, but significant, transcript signals were
also detected in the junction area of the palea and lemma,
where the vascular bundles are located (Fig.1). As devel-
opment proceeded, M79 expression in the anther decreased
rapidly, primarily in the pollen. During the pollen plenum
and final ripening stages, the level of M79 expression in
the tapetum, filaments, and vascular bundles of the anther
septum declined gradually (Fig.3F). Unlike in the stamens,
the expression of M79 remained constitutively high and
appeared uniform throughout carpel development. To test
the reliability of ISRT, conventional ISH was performed in
parallel and similar reproducible outcomes were obtained
(data not shown).
2.3 Expression of M79 during embryo development and
seed germination
M79 is expressed at a high level in the carpels of mature
rice flowers and remains detectable in embryo sacs after
anthesis and fertilization. The rice embryo consists of two
primary organ systems: the axis (including the radicle,
hypocotyl, and embryo bud) and the cotyledon (Goldberg
et al., 1994). The axis contains the shoot and root mer-
istems and gives rise to the mature rice plant after seed
germination. By contrast, the scutellum is a terminally dif-
ferentiated organ rich in storage materials that are used for
seed germination before photosynthesis begins to function.
During embryonic development, M79 expression was de-
tected in all embryo organs and remained largely unchanged
afterwards (Fig.4A). When the seed started to germinate,
large accumulations of M79 mRNA were found in the
radicle, coleorhiza, and coleoptile, while a weak signal was
detected in the scutellum. During early seed germination,
M79 was expressed at a high level, and then declined gradu-
ally and became constrained within the apical meristem of
the radicle and embryo bud (Fig.4B). This expression pat-
tern persisted after the rice plant matured. In adult rice
plants, a signal was detected in the root cap, root meristem,
cell pericycle (Fig.5B), and shoot apical meristem (Fig.5A).
However, no M79 transcription was found in other vegeta-
tive tissues of the root or shoot (data not shown).
3 Discussion
In order to detect the pattern of M79 expression in rice,
we successfully applied an innovative in situ technique,
ISRT, in combination with ISH. The procedures used in
ISRT are similar to those of ISH, except permeabilization
and hybridization. In order to optimize the conditions for
intracellular reverse transcription in ISRT, the concentra-
tion of protease K, which facilitates the penetration of re-
agents into the cells, is about ten times that used in ISH.
Because hybridization is not involved, ISRT omits many
cumbersome steps, such as synthesizing and hydrolyzing
the riboprobe, post-hybridization RNase treatment, and the
stringent washing crucial for reducing the signal-to-noise
ratio. The synthesized cDNA forms hybrid double-strand
molecules with the target mRNA, preventing the diffusion
Acta Botanica Sinica 植物学报 Vol.46 No.8 2004978
of the signal into cells in which the gene was not originally
expressed and efficiently reducing the final signal-to-noise
ratio. The hybridization probe used in the conventional
ISH protocol can cross-hybridize with homologous gene
transcripts via conserved domains, making it difficult to
detect the expression of a specific gene member in a
multigene family or to distinguish the expression pattern of
a specific mRNA in a mutant from that in a wild-type plant.
With ISRT, because the primers are designed for a specific
region of a particular gene, it is easier to distinguish the
expression pattern of one gene from another. This tech-
nique proved to be an easy-to-perform, highly effective
approach for detecting specific RNA sequences in situ
within a reasonable period compared with the standard ISH
method. The technique might have a wide range of
applications, especially in examining the expression of
genes exhibiting high homology, such as differentially
spliced products encoded by the same gene.
Comparison of the ISRT data with parallel ISH data
showed a uniform gene expression pattern. However, we
observed a faint signal of M79 expression in vascular
bundles in the palea and lemma before the vacuolated
pollen stage. This is not consistent with the RNA blotting
analysis of OsMADS7 and in situ studies of OsMADS45
by other independent groups (Kang et al., 1997; Greco
et al., 1997), which did not detect expression in this area.
Because both ISRT (Fig.1) and conventional ISH (data not
shown) reproduced this expression pattern and because
the signal was greatly weakened at the final stage of ma-
ture pollen (Fig.3F), we concluded that M79 is indeed
Figs.4-5. 4. Distribution of M79 mRNA during embryogenesis and seed germination detected using in situ reverse transcription (ISRT).
A. Longitudinal section of an embryo. B. Longitudinal sections of a germinating seed. EB, embryo bud; R, radicle. 5. Expression of M79
in the shoot and root apical meristem. A. A longitudinal section of the shoot apical meristem treated using ISRT. B. A longitudinal section
of the primary root at the tip. RC, root cap.
MING Liang et al.: Analysis MING Liangof the Spatial and Temporal Expression of a MADS-Box Gene M79 in Rice Using In
Situ Reverse Transcriptiona 979
expressed in vascular bundles in the palea and lemma be-
fore the vacuolated pollen stage. The difference from the
previously reports may arise from the selection of a differ-
ent probe sequence and the different systems used to de-
tect the hybridized probe, which might alter the sensitivity.
M79 mRNA was expressed during all stages of rice plant
differentiation and development, in both vegetative and
reproductive tissues. Therefore, the function of M79 is not
flower-specific; rather, it might be involved in a diverse
range of biological processes. The absence of detectable
signals in vegetative tissues in our previous report and the
results of independent groups using Northern analysis seem
inconsistent with the in situ experiments reported here,
which showed M79 expression in the shoot and root apical
meristem (Greco et al., 1997; Qu et al., 2001). This discrep-
ancy might be explained by the observation that the M79
transcripts were localized to the apical meristem, which is
merely a small region of the shoot and root. While North-
ern blot demonstrates the relative abundance of a specific
gene in homogenized tissues, our in situ technique distin-
guishes the highest levels of target mRNA in one cell type
compared to the surrounding tissues; thus our in situ tech-
nique might reveal a small area of tissue expressing the
target gene at a level that is too weak to be distinguished
from background by Northern analysis, producing a nega-
tive result (Poulsom et al., 1998).
Previously, we reported that M79 cDNA had high nucle-
otide and deduced amino acid sequence homologies with
the AGL-2-like gene family. AGL-2-like genes have a great
diversity of expression patterns, especially in monocots.
The functions of the genes in that subfamily remain largely
unclear. The expression pattern of M79 is similar to that of
AGL2, which is expressed abundantly in the primordia of
all four flower organs, as well as in the three inner whorls,
developing ovules and embryo, and seed coats (Flanagan
and Ma, 1994; Pelaz et al., 2000). Unlike AGL2, which is not
expressed until the emergence of the floral meristem, M79
is produced in the florescence meristem and is expressed
during all developmental stages of the rice flower. Therefore,
M79 might have a higher regulatory position than AGL2 in
flower development. Combined with the fact that ectopic
M79 expression results in early flowering (Qu et al., 2001),
this suggests that M79 is involved in flowering initiation.
Because M79 expression extended to a late-development
stage, it is probably not only required for the transition
from vegetative to reproductive shoot apical meristem, but
is also involved in the establishment of the floral meristem
and floral organs.
M79 expression in embryo and seed development was
not unexpected because many MADS-box genes are ex-
pressed in the ovules, embryo, seed, or fruit, such as tran-
scripts of AGL2 (Flanagan and Ma, 1994), AGL3 (Huang et
al., 1995), AGL15 (Heck et al., 1995), and AP3 (Jack et al.,
1992) from Arabidopsis; MdMADS1 and MdMADS3-11 from
apple (Sung and An, 1997; Yao et al., 1999; Sung et al.,
2000); and PrMADS1-3 from Monterey pine (Mouradov et
al., 1998). This also was consistent with published results
that OsMADS45 is strongly expressed in developing ker-
nels (Greco et al., 1997). Embryogenesis is a successive
process that follows flowering, so M79 may function in
embryo development associated with flower development.
The constitutive expression pattern of M79 transcripts
throughout embryo development until embryo maturity
suggests that M79 expression is important in this process.
The pattern of M79 expression in rice flowers implies
that M79 protein regulates many of the processes of flower
development, facilitating the transition from vegetative to
reproductive meristem and regulating the formation of the
floral meristem and organ. M79 is expressed in the shoot
and root apical meristems of adult rice during embryo and
seed development; the results of a previous study showed
that many new flower buds emerged at the apical lobes of
main stems and many new branches formed from leaf axils
in a majority of transgenic tobacco plants (Qu et al., 2001).
Taken together, these findings suggest that M79 expres-
sion in the shoot apical meristem is involved in controlling
branching during the vegetative stage of rice.
We successfully applied the ISRT method to reveal the
novel expression pattern of M79, which was consistent
with the ISH results. In summary, our results suggest that
M79, as a transcription regulator, is involved in the mainte-
nance of the meristem, initiation of organogenesis, regula-
tion of branching, and timing of floral morphogenesis.
Acknowledgements: The authors thank Prof. BAI Shu-
Nong, Ms. BAI Su-Zhen (College of Life Sciences, Peking
University), Dr. GUO Hong-Wei (Molecular, Cellular and
Developmental Biology Department, University of
California, Los Angeles), and Prof. DENG Xing-Wang
(Molecular, Cellular and Developmental Biology
Department, Yale University) for their valuable comments
on both the ISRT technique and the manuscript, and Ms.
LIANG Xi-Hui for help with the figures.
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