Download bearded-ear Encodes a MADS Box Transcription

The Plant Cell, Vol. 21: 2578–2590, September 2009, www.plantcell.org ã 2009 American Society of Plant Biologists
bearded-ear Encodes a MADS Box Transcription Factor
Critical for Maize Floral Development
W OA
Beth E. Thompson,a Linnea Bartling,a Clint Whipple,b Darren H. Hall,b Hajime Sakai,c Robert Schmidt,b
and Sarah Hakea,1
a Plant Gene Expression Center, U.S. Department of Agriculture–Agricultural Research Service and Plant and Microbial Biology
Department, University of California-Berkeley, Albany, California 94710
b Division of Biological Sciences, Section of Cell and Developmental Biology, University of California at San Diego, La Jolla,
California 92093
c Dupont Crop Genetics, Experimental Station E353, Wilmington, Delaware 19880
Although many genes that regulate floral development have been identified in Arabidopsis thaliana, relatively few are known
in the grasses. In normal maize (Zea mays), each spikelet produces an upper and lower floral meristem, which initiate floral
organs in a defined phyllotaxy before being consumed in the production of an ovule. The bearded-ear (bde) mutation affects
floral development differently in the upper and lower meristem. The upper floral meristem initiates extra floral organs that
are often mosaic or fused, while the lower floral meristem initiates additional floral meristems. We cloned bde by positional
cloning and found that it encodes zea agamous3 (zag3), a MADS box transcription factor in the conserved AGAMOUS-LIKE6
clade. Mutants in the maize homolog of AGAMOUS, zag1, have a subset of bde floral defects. bde zag1 double mutants have
a severe ear phenotype, not observed in either single mutant, in which floral meristems are converted to branch-like
meristems, indicating that bde and zag1 redundantly promote floral meristem identity. In addition, BDE and ZAG1 physically
interact. We propose a model in which BDE functions in at least three distinct complexes to regulate floral development in
the maize ear.
INTRODUCTION
Organogenesis in plants requires the ongoing activity of meristems. Meristems are groups of totipotent cells that give rise to
lateral organs, stems, and roots, while maintaining a population
of stem cells. Meristems can be indeterminate and give rise to an
unlimited number of primordia, or they can be determinate and
terminate in the production of primordia. Floral meristems (FMs)
are determinate meristems: they produce a defined number of
floral organs and terminate in the production of the ovule, which
is contained in the carpel whorl. A typical eudicot flower contains
four whorls of floral organs: sepals, petals, stamens, and carpels.
Grass flowers contain stamens and carpels but also contain
palea and lemma, organs unique to the grasses.
Floral development has been intensively studied in the model
plant Arabidopsis thaliana. The molecular regulation of floral
organ identity is described by the ABC model, which posits that
floral organ identity is determined by the combinatorial action of
the Class A, B, and C genes (Coen and Meyerowitz, 1991).
Briefly, Class A genes alone specify whorl 1 organs (sepals),
Class A and B genes together specify whorl 2 organs (petals),
Class B and C genes together specify whorl 3 organs (stamens),
1 Address
correspondence to [email protected].
The author responsible for distribution of materials integral to the
findings presented in this article in accordance with the policy described
in the Instructions for Authors (www.plantcell.org) is: Sarah Hake
([email protected]).
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www.plantcell.org/cgi/doi/10.1105/tpc.109.067751
and the Class C gene AGAMOUS (AG) specifies whorl 4 organs
(carpels); AG also promotes FM determinacy. The ABC model
has also been expanded to include Class D genes, which specify
ovules (Colombo et al., 1995; Dreni et al., 2007), and Class E, or
SEPALLATA genes, which function with the Class A, B, C, and D
genes to specify organ identity (Pelaz et al., 2000; Honma and
Goto, 2001; Ditta et al., 2004).
Much less is known about floral development in the grasses,
although some aspects of the ABCDE model are applicable. For
example, mutations in maize (Zea mays) and rice (Oryza sativa)
homologs of the Arabidopsis Class B gene APETALA3 (AP3)
result in homeotic conversions consistent with Class B function
(Ambrose et al., 2000; Nagasawa et al., 2003). Furthermore,
maize Class B homologs have similar biochemical activities as
their Arabidopsis counterparts (Whipple et al., 2004). The Class C
gene AG has been duplicated in the grasses and C function
subfunctionalized. In rice, the AG homologs MADS3 and
MADS58 regulate floral organ identity and FM determinacy,
respectively (Yamaguchi et al., 2006). Similarly, the maize AG
homolog zag1 is required for FM determinacy, suggesting that at
least some Class C function is conserved (Mena et al., 1996).
Mutants in maize Zea mays mads2, the other AG homolog, have
not been identified. In rice, leafy hull sterile1 (lhs1) mutants harbor
mutations in MADS1, a member of the SEP clade (Jeon et al.,
2000; Prasad et al., 2005). lhs1 mutants make extra palea/
lemma-like organs and also make an aberrant number of stamens and carpels, indicating that LHS1 functions in meristem
determinacy and organ fate. In maize, indeterminate floral apex1
(ifa1) is also required for FM determinacy, although the molecular
identity of ifa1 is unknown (Laudencia-Chingcuanco and Hake,
bde Controls Maize Floral Development
2002). Thus, the ABCDE model provides a framework for the
molecular regulation of floral development in grasses, but more
genes need to be identified and characterized to understand
how unique floral morphologies are specified in the grasses
(Thompson and Hake, 2009).
All MADS box genes required for floral development encode
MIKC-type MADS box transcription factors (Yanofsky et al., 1990;
Jack et al., 1992; Mandel et al., 1992; Goto and Meyerowitz,
1994; Jofuku et al., 1994). The MIKC class is named for its
characteristic structure, which includes the MADS box (M), the
intervening domain (I), the keratin-like domain (K), and C-terminal
domain (C) (reviewed in Kaufmann et al., 2005). The MADS box
binds DNA, the K-domain is involved in protein-protein interactions, the C terminus also contributes to protein–protein interactions, and some MADS box proteins harbor a transcriptional
activation domain (Fan et al., 1997; Honma and Goto, 2001).
The quartet model has been proposed to explain the molecular
underpinnings of the ABCDE model, which posits that four
different combinations of A, B, C, and E class transcription
factors determine floral organ identity in the four floral whorls
(Thiessen, 2001). Indeed, biochemical and genetic evidence
suggests that MADS box proteins form trimers and tetramers,
and these higher-order complexes regulate transcriptional programs required for floral organ identity (Egea-Cortines et al.,
1999; Honma and Goto, 2001). Thus, uncovering the protein
interaction network is critical to understand MADS box function
in floral development.
Here, we describe the cloning and characterization of the
maize floral mutant bearded-ear (bde). bde is critical for multiple
aspects of floral development, including FM determinacy, organ
development, and sex determination. bde encodes a MADS box
transcription factor belonging to the AGL6 clade. AGL6-like
genes are conserved among diverse angiosperms; however, the
role of AGL6-like genes in development has remained elusive
due to a lack of phenotypes in single loss-of-function mutants.
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Thus, characterization of bde in maize provides key insights into
the roles of AGL6-like genes.
RESULTS
bde Regulates Multiple Aspects of Floral Development
To understand the molecular regulation of floral development in
maize, we characterized the bde mutant phenotype in detail. bde
is defined by three alleles, bde-McClintock (bde-McC), bdeTR864, and bde-N868. bde-McC was obtained from Barbara
McClintock’s collection via the Maize Genetics Cooperative;
bde-TR864 was ethyl methanesulfonate (EMS) induced in the
A619 inbred background; bde-n868 was EMS induced and
found segregating in the et*N868A stock from the Maize Genetics Cooperative. All alleles are completely recessive. Both bdeTR864 and bde-N868 fail to complement bde-McC, indicating
these three alleles define a single locus. Phenotypic characterization focused on bde-McC, backcrossed three times to A619.
All mutant alleles exhibit qualitatively similar phenotypes with
variations in severity depending on allele and inbred background.
Maize has two types of inflorescences, the tassel and the ear,
which produce male and female florets, respectively. Spikelet
pair meristems (SPMs) arise on the flanks of the inflorescence
meristem (IM) and give rise to two spikelet meristems (SMs).
Each SM produces two bract leaves, called glumes, and two
FMs (Figure 1A). The SM initiates the lower floral meristem (LFM),
and then the SM either initiates the upper floral meristem (UFM)
or is itself converted to the UFM. The FM initiates a specific
number of floral organs in a defined phyllotaxy, including two
lodicules, three stamens, and three carpel primordia. Surrounding these floral organs are two bracts, called lemma and palea,
but it is not clear if these bracts are the product of the SM or
FM. Carpels abort in the tassel, and stamens arrest in the ear,
Figure 1. Maize Floral Development.
(A) Schematic of maize inflorescence development. The IM gives rise to the SPM, which initiates two SMs. Each SM initiates two FMs. Development in
the tassel and ear is similar, except in the tassel; the IM also initiates BMs.
(B) The tassel produces male florets. Cartoon of tassel floret (left) and spikelet pair (right). Tassel florets contain stamens but no pistils (known as silks).
The spikelet pair consists of two spikelets; each spikelet contains an upper floret and a lower floret.
(C) The ear produces female florets. Cartoon of ear floret (left) and spikelet pair (right). Ear florets contain a pistil, which is the product of two fused
carpels, but no stamens. The spikelet pair consists of two spikelets, each containing a single floret, due to the abortion of the lower floret. UF, upper
floret; LF, lower floret.
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resulting in unisexual flowers (Cheng et al., 1983). In the ear, the
LFM aborts, producing a single floret per spikelet (Figures 1B
and 1C).
bde mutants exhibit multiple defects in floral development but
not during earlier stages of inflorescence or vegetative development. The ear phenotype is striking and obvious in all alleles and
inbred backgrounds examined, whereas the severity of the tassel
phenotype is more variable. In the tassel, spikelets contain extra
florets, which produce extra floral organs and often silks (Figures
2A to 2F). In the ear, bde mutants make extra silks and are mostly
sterile (Figures 2G to 2L). We dissected individual spikelets and
found they contain multiple florets. In addition, the florets produce an excess of palea/lemma-like organs, and multiple silks
often emerge from a single ovule (Figures 2M to 2R). bde mutants
often exhibit protruding nucelli (Figure 2R), suggesting a defect in
ovule and/or integument development.
To confirm the defects we observed in the mature inflorescences, we examined earlier stages of development by scanning
electron microscopy. Early inflorescence development was indistinguishable from normal development, and no defects were
observed in branch meristems (BMs), SPMs, or SMs (Figures 3A
to 3D; data not shown). In both ears and tassels, SM initiated
LFM, although FMs were abnormal and misshapen compared
with those in normal siblings (Figures 3E to 3H). The UFM initiated
an excess of organ primordia in an aberrant phyllotaxy (Figures 3I
to 3L). In rare instances (<1% of spikelets), the UFM bifurcated
and formed two meristems (see Supplemental Figures 1A and 1B
online). The LFM initiated what appear to be ectopic meristems in
the axils of bract leaves (bracts were often removed during
dissections), which are likely the source of the extra florets
observed in the mature inflorescence (Figures 3N and 3P). To
confirm these were meristems and not aberrant floral organs, we
examined the expression of the meristem marker knotted1 (kn1)
(Jackson et al., 1994) in bde mutants. Indeed, multiple loci of kn1
expression were apparent in bde mutants in the region corresponding to the LFM, indicating these primordia are meristems
(see Supplemental Figure 1C online). Since FMs are terminal
meristems and never initiate other meristems, this result suggests that FM identity has been lost in the LFM in bde mutants.
Thus, bde mutants affect the UFM and LFM differentially.
To characterize the bde tassel defects in more detail, we
counted floral organs in the upper floret (Figure 4A). In normal
florets, lemma and palea are easily distinguishable based on
position and vasculature. However, in bde florets, lemma and
palea were indistinguishable and therefore were scored together. bde mutants made excess palea/lemma and often an
atypical number of stamens and lodicules. Normal tassel florets
Figure 2. bde Mutant Phenotype
(A) to (C) Mature tassels.
(D) to (F) Dissected tassel florets; glumes have been removed. Arrows
indicate stamens. Arrowheads indicate silks.
(G) to (I) Mature ears. bde-McC (H) and bde-TR864 (I) ears make extra
silks compared with normal (G).
(J) to (L) Cross section of ears. Normal spikelets (J) have a single silk,
while bde-McC (K) and bde-TR864 spikelets (L) produce multiple silks.
(M) to (O) Dissected spikelets. A normal spikelet contains a single floret
(M), while bde-McC (N) and bde-TR864 (O) spikelets contain multiple
florets.
(P) to (R) A normal ovule is contained within a single silk (P), while bdeMcC (Q) and bde-TR864 (R) ovules are surrounded by multiple silks.
bde-TR864 also exhibits a protruding nucellus.
Bars = 2 mm.
bde Controls Maize Floral Development
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Figure 3. Early bde-McC Development in Ear and Tassel.
(A) to (D) SMs.
(E) to (H) The SM first initiates the LFM and then itself converts to the UFM in both wild-type and bde ears and tassels. Meristems are abnormal and
misshapen in bde mutants ([F] and [H]).
(I) and (K) In normal florets, the UFM develops first and initiates a palea, three stamens, and three carpels.
(J) and (L) In bde mutants, the UFM develops first and initiates ectopic organ primordia in a disorganized phyllotaxy.
(M) A normal tassel spikelet consists of an upper and lower floret; no ectopic meristems are initiated.
(N) A bde tassel spikelet consists of abnormal upper and lower florets; the lower floret initiates an ectopic meristem (arrow).
(O) A normal ear spikelet contains an upper floret; the lower floret does not develop.
(P) A bde ear spikelet contains an abnormal upper floret; the lower floret initiates an ectopic meristem (arrow).
Bars = 100 mM. st, stamen; pal, palea; car, carpel; si, silk.
do not produce silks due to carpel abortion. bde upper florets did
not produce silks most of the time, in contrast with bde lower
florets, which often produced silks. In addition, floral organs in
the upper floret exhibited developmental defects, and organs
were often mosaic or fused (Figures 4B to 4D). Thus, bde is
required to specify proper floral organ number and development
in the upper floret.
The scanning electron microscopy and kn1 expression data
indicate that the extra florets were the product of axillary meristems initiated from the lower floret. When we dissected spikelets, the orientation of the extra florets also suggested they arose
from the lower floret. We counted florets that appeared to be
derived from the UFM and LFM. The UFM produced a single
floret, while the LFM usually produced multiple florets (Figure
4E). Florets derived from the LFM did not produce a normal
complement of floral organs and often contained only a couple of
lemma/palea-like organs (Figure 4F).
bde Encodes a MADS Box Transcription Factor
To identify the gene responsible for the bde mutant phenotype,
we pursued a map-based cloning approach. bde was crossed to
the B73 and Mo17 inbred lines and self-pollinated to generate F2
mapping populations. We established linkage to bnlg118 on
chromosome 5, bin 7 by bulked segregant analysis. We further
defined the interval between the markers umc1752 (34/2066
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Figure 4. Quantification of Tassel Defects in bde Mutants.
(A) Quantification of floral organs in the upper floret of normal and bdeMcC tassels. Palea and lemma were scored together because they are
indistinguishable in bde.
(B) to (D) Examples of floral organ defects in the upper floret of bde
tassels.
(B) Floret containing a partially transformed lodicule (arrowhead) and four
stamens.
(C) A partially transformed stamen (arrowhead).
(D) A stamen fused to a palea-like organ (arrowhead).
(E) Quantification of florets originating from the UFM and LFM in normal
and bde tassels. The UFM produces a single floret in both normal and
bde tassels (left). The LFM produces a single floret in normal tassels but
multiple florets in bde (right).
(F) Three florets (white arrowheads) produced from the lower floret in a
bde tassel. Bar = 2 mm.
chromosomes; 1.65 centimorgans) and umc1524 (7/2066; 0.34
centimorgans) (Figure 5A). We identified a syntenous region in
rice on the long arm of chromosome 2 and designed single
nucleotide polymorphism markers using maize ESTs corresponding to rice genes in the candidate region. Eventually, we
narrowed the interval to a region that contained six genes in rice,
four of which could be amplified from maize BACs present in the
region (see Supplemental Figure 2A online).
Of these four genes, the MADS box gene zag3 was a particularly strong candidate to underlie the bde mutant phenotype
given the role of MADS box genes in floral development and
patterning. Furthermore, zag3 is specifically expressed in the
inflorescence (Mena et al., 1995). We analyzed the zag3 genomic
sequence in the three bde alleles and identified lesions in each
allele (Figure 5B). bde-McC contains a single nucleotide insertion
in the fourth exon of zag3, resulting in a frame shift that introduces 36 novel amino acids and a premature stop codon. bdeTR864 contains a large deletion that removes all zag3 coding
sequences after the first exon. DNA gel blot analysis indicated a
polymorphism between bde-TR864 and the A619 progenitor
inbred (see Supplemental Figure 2B online), consistent with a
deletion. To ask if other genes were also removed by the deletion,
we identified syntenous genes in Sorghum bicolor, whose genome is fully sequenced (www.phytozome.net). We were able to
amplify zag3-flanking genes from bde-TR864 homozygotes,
indicating that only zag3 is likely to be disrupted in the bdeTR864 allele. The exact breakpoints of the deletion are unknown.
bde-N868 contains an insertion/deletion in the 59 region, 825 bp
upstream of the start codon, that deletes 39 bp and inserts ;150
nucleotides. A BLAST search with the inserted nucleotides yields
no significant hits. bde-N868 also contains a missense mutation
at position 180, a nonconserved residue in the C-terminal region
of zag3. Together, these data indicate that the bde mutant
phenotype is caused by mutations in the MADS box gene zag3.
Per convention, we refer to the zag3 gene as bde.
To determine the effect of these mutations on bde gene
expression, we compared bde mRNA levels in mutant alleles
and the A619 inbred using quantitative RT-PCR (Figure 5C). bde
mRNA levels are slightly reduced in bde-McC, consistent with
decreased RNA stability due to the introduction of a premature
stop codon and nonsense-mediated decay. As expected, bde
mRNA is not detectable in bde-TR864; the primers flank the
sequence deleted in this allele. Finally, the amount of bde mRNA
is greatly reduced in bde-N868, suggesting that the insertion/
deletion in the 59 flanking sequence affects transcriptional efficiency.
bde Expression
Previous work showed that bde was expressed in the tassel and
ear but was not detectable in vegetative tissues (Mena et al.,
1995), consistent with where we observe the bde mutant’s
defects. To more carefully investigate bde expression in normal
plants, we performed RNA in situ hybridization analysis. bde was
first detected in the UFM and LFM in both the tassel and the ear
(Figures 6A and 6B) but not in initiating organ primordia (Figures
6C and 6D). In the tassel, bde was also detected in lodicules and
mature palea (Figure 6E). In the ear, bde mRNA was present in
bde Controls Maize Floral Development
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Figure 5. bde Encodes the MADS Box Transcription Factor ZAG3.
(A) Schematic of genetic location of bde. bde showed no recombination with the MADS box gene zag3. cM, centimorgans
(B) Schematic of zag3 genomic region and bde lesions.
(C) Quantitative RT-PCR showing expression of bde mRNA in bde mutant alleles compared with the A619 inbred. bde levels are normalized against
gapdh levels. Error bars represent 1 SD.
carpel primordia but was downregulated later in carpel development (Figures 6F and 6G). In addition, bde was strongly
expressed in the inner integument and the region underlying the
ovule primordium (Figure 6G). The integument expression suggests a role for bde in integument development and might
explain the protruding nucellar phenotype. bde was not detected
in earlier stages of inflorescence development in the IM, SPM, or
SM or with a sense probe.
LFM Identity in bde Mutants
The identity of the LFM is unclear in bde mutants given that FMs
normally do not initiate meristems. One possibility is that the LFM
is converted to a SM, which normally initiates FM. To determine if
the LFM has SM identity in bde mutants, we examined expression of the SM marker branched silkless1 (bd1). bd1 is transiently
expressed in a semicircular pattern at the base of the SM (Chuck
et al., 2002). In bde-McC, bd1 was not detectable in the LFM at
any stage of floral development, suggesting that the LFM does
not have SM identity (see Supplemental Figure 3 online). To ask if
the LFM retains FM identity, we examined expression of bde,
which is only expressed in developing florets (Figure 6). We used
the bde-McC allele, in which bde mRNA is reduced but still
present (Figure 5C). bde was expressed in the UFM and LFM of
bde-McC, and in some spikelets, the LFM contained additional
foci of bde expression (see Supplemental Figure 3 online). These
foci likely correspond to the axillary meristems observed by
SEM. Taken together, these results indicate that the LFM initiates
additional FM yet retains at least some FM identity in bde
mutants.
bde and zag1 Function Together to Regulate
FM Determinacy
bde likely functions in a pathway with additional genes to
regulate floral development. One candidate gene is zag1, mutants in which make extra carpels in the ear, similar to the upper
floret of bde, and is expressed in an overlapping domain
(Schmidt et al., 1993; Mena et al., 1996; this work). We constructed bde zag1 double mutants to investigate their genetic
interaction. bde zag1 double mutants had a novel ear phenotype
(Figures 7A to 7E), not observed in either single mutant (Figures
7F and 7G), while bde zag1 tassels resembled bde tassels. In
place of normal ear florets were branch-like structures that
initiated ectopic meristems in the axils of bracts. Occasionally,
radialized organs that resembled silks were observed in the
mature ear (Figure 7C); however, no other obvious floral organs
were observed. In most plants, initiation of ectopic meristems
was random. However, in one plant, the branch structure
appeared to be a complete conversion to an inflorescence
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Figure 6. bde Expression in Developing Inflorescences.
In situ hybridization showing bde expression in the UFM and LFM of
tassels (A) and ears (B) (longitudinal sections). Transverse sections
showing lack of bde expression in initiating organ primordia in tassels (C)
and ears (D). Later, bde is expressed in lodicules (black arrowheads) and
palea (white arrowheads) of tassels (E). A single palea is present,
although in the section there appear to be three. In ears, bde is
expressed in carpel primordia (F), the inner integument, and tissue
underlying the ovule (G). st, stamen; lod, lodicule; UF, upper floret; car,
carpel; si, silk; int, integument. Bars = 100 mm.
structure, as branches initiated spikelet pairs in a distichous
phyllotaxy (Figures 7D and 7E).
To investigate the origin of these branch structures, we examined early development of bde zag1 double mutants by
scanning electron microscopy. Early stages of inflorescence
development appeared normal in double mutants, and the IM,
SPM, and SM were indistinguishable from those of normal plants
(Figure 7H; data not shown). bde zag1 double mutants initiated a
meristem in the proper position for the LFM (Figure 7I); however,
neither the UFM or the LFM initiated floral organs and instead
continued to initiate ectopic meristems (Figures 7J to 7M).
Sometimes structures that resembled a carpel ridge (Figure 7L,
white arrowhead) were present, suggesting that the radial organs
observed in the mature ear were silks.
MADS box proteins form dimers and higher-order complexes
to regulate the expression of downstream target genes (Fan
et al., 1997; Honma and Goto, 2001). Both bde and zag1 mutants
make extra carpels, raising the possibility that BDE and ZAG1
function in the same complex to regulate floral organ number.
Furthermore, ZAG1 and BDE homologs interact in Arabidopsis
and petunia (Petunia hybrida) (Fan et al., 1997; de Folter et al.,
2005; Rijpkema et al., 2009). To establish if BDE and ZAG1 also
physically interact, we first used the yeast two-hybrid system
(Figure 8A). We tested if full-length BDE fused to the GAL4 DNA
binding domain (BD-BDE) activated lacZ reporter gene expression and found that BD-BDE possesses transcriptional activity in
the absence of the GAL4 activation domain. In other MADS box
proteins, the activation domain resides in the C terminus (Honma
and Goto, 2001). Therefore, we deleted the C terminus of BDE
(amino acids 164 to 256) and found that the truncated protein
(BD-BDEDC) no longer autoactivated in the yeast two-hybrid
system. We also examined if BD-BDE autoactivated in a more
stringent assay, which assays growth in the absence of histidine
and adenine, indicating activation of reporter genes. BD-BDE did
not autoactivate in this assay; however, yeast grew on restrictive
media when both BD-BDE and AD-ZAG1 were present, indicating that BDE and ZAG1 interact. To eliminate the possibility that
observed growth was due to some transcriptional activation by
BDE alone, we also tested if BD-BDEDC, which did not autoactive in the b-gal assay, interacted with AD-ZAG1. Indeed BDBDEDC interacted with AD-ZAG1. The interaction was also
observed with ZAG1 fused to the DNA binding domain (BDZAG1) and BDE fused to the activation domain (AD-BDE). BDE
does not interact with itself, indicating that the BDE–ZAG1
interaction is specific.
To confirm the interaction we observed in the yeast-two hybrid
system, we used bimolecular fluorescent complementation
(BiFC) (Citovsky et al., 2006). We fused full-length ZAG1 to the
N-terminal portion of yellow fluorescent protein (ZAG1-nYFP)
and full-length BDE to the C-terminal portion of YFP (ZAG3cYFP) and cobombarded these constructs into epidermal onion
cells, along with a plastid red fluorescent protein (RFP) marker as
an internal control. We observed nuclear YFP expression in 50%
of RFP-expressing cells (n = 290 cells), indicating that BDE and
ZAG1 also interact in planta and are localized to the nucleus,
consistent with their function as transcription factors (Figure 8B).
We observed no YFP expression when ZAG1-nYFP or BDEcYFP were cobombarded with the cYFP or nYFP, respectively
bde Controls Maize Floral Development
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(n = 193 cells for ZAG1-nYFP + cYFP and n = 175 cells for BDEcYFP + nYFP). ZAG3-nYFP cobombarded with cYFP resulted in
nonspecific nuclear YFP expression; thus, we were unable to
conduct BiFC experiments in the opposite orientation or ask if
BDE interacts with itself in this assay. Based on the BiFC, yeast
two-hybrid analysis, bde zag1 genetic interaction data, and the
observation that the BDE and ZAG1 homologs interact in other
species (Fan et al., 1997; de Folter et al., 2005; Rijpkema et al.,
2009), we conclude that BDE and ZAG1 are likely to physically
interact in vivo.
DISCUSSION
Here, we describe the characterization and cloning of the maize
floral mutant bde. bde encodes a member of the AGL6 clade of
the MADS box family. bde is expressed in FMs and a subset of
floral organs. The phenotype reveals a role for bde in multiple
stages of floral determinacy and floral organ fate. Given that
AGL6-like genes are conserved among diverse angiosperms and
that no other single loss-of-function mutants have yet been
described, our analysis provides key insights into the roles of this
group of MADS box regulators.
bde Mutants Affect the Upper and Lower FM Differently
Figure 7. bde zag1 Double Mutants in the Ear.
(A) bde zag1 double mutants are synergistic, failing to make florets in
the ear.
(B) bde zag1 spikelet.
(C) Dissected spikelet in (B) contains no florets, but a branch-like
structure with ectopic meristems.
(D) bde zag1 spikelet, which produces branches.
(E) Dissected spikelet in (D) contains a branch, with SM in a distichous
phylotaxy.
(F) and (G) bde (F) and zag1 (G) single mutant spikelets are shown for
comparison.
(H) to (M) Scanning electron micrographs from developing bde zag1
double mutant ears. SMs are normal in bde zag1 double mutants (H). bde
zag1 initiates a meristem in the correct position for the LFM (I), but no
floral organs are initiated. Instead, meristems continue to initiate ectopic
Florets in the grasses are organized in spikelets, and the number
of florets in a spikelet depends on the species. Maize and other
members of the Andropogoneae contain two florets, which
although seemingly identical, develop at different rates (Clifford,
1987). Interestingly, bde mutants affect the UFM and LFM
differently. In the upper floret, bde is required to specify floral
organ number and fate, and in the lower floret, bde is required to
specify FM identity. One explanation for the difference between
UFM and LFM could be differential gene expression. For example, a gene that functions redundantly with bde to promote FM
fate might be expressed in the UFM, but not the LFM. Indeed,
previous work suggests that many genes are differentially expressed in the upper and lower floret. Two MADS box genes,
ZMM8 and ZMM14, are expressed in the UFM and their floral
organs but not in the lower floret (Cacharrón et al., 1999).
Microarray analysis indicates that ;9% of anther-expressed
genes are differentially expressed in anthers from the upper and
lower floret (Skibbe et al., 2008). If MADS box genes are differentially expressed in the UFM and LFM, BDE might participate in
distinct complexes in the two florets. A second possibility is that
the upper and lower florets have different developmental potentials based on the different history of these two meristems.
Although the UFM and LFM are identical morphologically, their
origins differ. For example, one model suggests the SM first
initiates the LFM and then converts to the UFM (Irish, 1998). It is
possible that the chromatin state may be different in the UFM and
LFM and that some genes are unavailable for transcriptional
meristems (black arrowheads) and form branch-like structures ([J] to
[M]). In some spikelets, a carpel ridge is formed ([L]; white arrowhead).
Bars = 2 mm in (B) to (G) and 100 mM in (H) to (M).
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Figure 8. BDE and ZAG1 Protein Interactions
(A) BDE and ZAG1 interact in yeast two-hybrid assays. Left, BD-BDE autoactivates in a B-gal assay but not in a growth assay; BD-BDEDC and BDZAG1 do not autoactivate in either assay. Right, BD-BDE+AD-ZAG1, BD-ZAG-1+AD-BDE, and BD-BDEDC+AD-ZAG1 are able to grow on selective
media, but BD-BDE+AD-BDE and BD-BDEDC+AD-BDE are not. The ability to grow in the absence of histidine and adenine indicate activation of
reporter genes and a positive protein–protein interaction. Media lacking Trp and Leu select for bait and prey plasmids, respectively.
(B) BiFC indicates that ZAG1 and BDE interact in planta. Top, yellow fluorescence indicates that ZAG1-nYFP and BDE-cYFP interact in the nucleus of
epidermal onion cells. Middle, ZAG1-nYFP does not interact with cYFP. Bottom, nYFP does not interact with BDE-cYFP.
(C) Model for ZAG1 and ZAG3 function. BDE and ZAG1 interact in Complex A to control floral organ number. BDE is present in a complex that does not
contain ZAG1 (Complex B) to control floral organ development. BDE and ZAG1 redundantly promote FM identity. BDE and ZAG1 might be able to
compensate for each other in the same complex (Complex C) or might be present in two different complexes that both promote FM identity.
activation by BDE. Differentiating these two possibilities requires
determining which mRNAs are expressed in the upper and lower
floret and determining BDE target genes.
Links between Sex Determination and
Meristem Determinacy
In addition to FM determinacy, bde is required for sex determination. Interestingly, this aspect of the phenotype is also expressed
differently in the upper and lower floret, since almost all ectopic
silks are produced from the lower floret in bde tassels. Characterization of bde and other mutants suggest that meristem determinacy and sex determination are linked. The recessive mutant
tasselseed4 (ts4) and the dominant mutant Ts6 fail to abort carpels
in the tassel and have indeterminate SPM and SM (Irish et al.,
1994; Chuck et al., 2007). ts4 encodes a miRNA that targets the
SM determinacy gene indeterminate spikelet1 (ids1). Ts6 harbors a
mutation in the miRNA binding site of ids1 mRNA, rendering it
immune to miRNA regulation (Chuck et al., 2007). One possible link
between sex determination and meristem determinacy is the plant
hormone, gibberellin (GA). GA has a well-established role in maize
sex determination, as application of exogenous GA results in
failure of carpel abortion in the tassel (Nickerson, 1959), and
mutants in the GA biosynthetic pathway do not abort stamens in
the ear (Bensen et al., 1995). GA is also required to promote FM
identity in Arabidopsis (Okamuro et al., 1996, 1997). Furthermore,
the MADS box genes AP3, PI, and AG are targets of GA signaling
(Yu et al., 2004), providing another possible link between MADS
box proteins, sex determination, and meristem identity.
bde Provides Insight into the Function of the AGL6 Clade
MADS box genes have well-studied roles in plant development,
particularly in floral development. In this work, we show that bde
encodes the MADS box gene previously described as zag3. bde
is a member of the AGL6 family of MIKC-type transcription
factors, which is sister to the SEP clade (Becker and Theissen,
2003; Parenicova et al., 2003). Although AGL6-like genes are
conserved among diverse angiosperms, their role in development has remained elusive. Multiple lines of evidence, however,
suggest that AGL6-like genes function specifically in floral development. In petunia, AGL6 functions redundantly with the sep
genes FBP2 and FBP5, although single AGL6 mutants have no
phenotype (Rijpkema et al., 2009). Overexpression of orchid
bde Controls Maize Floral Development
(Oncidium Gower Ramsey) and hyacinth (Hyacinthus orientalis)
AGL6 in Arabidopsis yields an early flowering phenotype and
defects in floral organ identity (Hsu et al., 2003; Fan et al., 2007).
In Arabidopsis, expression of AGL6 mRNA is enriched in flowers
(Ma et al., 1991), and the related gene AGL13 is expressed in a
subset of cells in the developing ovule (Rounsley et al., 1995).
Finally, AGL6-like genes are conserved in the grasses and are
expressed in the FM and integument, similar to bde (Reinheimer
and Kellogg, 2009).
MADS Box Regulation of Maize Floral Development
Mutants in zag1, the maize homolog of the C-class gene AG,
make extra carpels in ear florets, similar to the upper floret of bde
mutants (Mena et al., 1996), suggesting they may function in a
common genetic pathway. In addition, bde and zag1 are expressed in overlapping domains (Figure 6; Schmidt et al., 1993),
and double mutants exhibit a novel ear phenotype, in which FM
are converted to branch-like or inflorescence-like meristems. In
contrast with the ear, bde zag1 double mutant tassels resemble
bde tassels. Although zag1 is expressed in the tassel, zag1
mutant tassel florets are normal, suggesting other genes function
redundantly with zag1 in the tassel (Mena et al., 1996).
MADS box proteins physically interact and form dimers and
higher-order complexes to regulate the expression of downstream target genes (Fan et al., 1997; Honma and Goto, 2001; de
Folter et al., 2005). In Arabidopsis and petunia, the BDE and
AGL6 homologs physically interact, although the functional
significance of this interaction is unknown (Fan et al., 1997;
Rijpkema et al., 2009). We also found that BDE and ZAG1 interact
both in yeast and in planta.
The protein interaction data and double mutant analysis suggest
that BDE functions in at least three distinct complexes in the ear
(Figure 8C). BDE is likely to function in many more complexes,
based on the numerous protein interactions of AGL6 in Arabidopsis
(de Folter et al., 2005) and the pleiotropy of the bde mutant
phenotype. The observations that bde and zag1 mutants both
produce extra carpels in female florets and that BDE and ZAG1
physically interact suggest that BDE and ZAG1 interact in a
complex that regulates floral organ number (Complex A). In addition, bde mutants have phenotypes that are not shared with zag1,
including floral organ development and phyllotaxy defects. Thus,
BDE is also likely to function in a second complex that does not
contain ZAG1 but that is required to promote floral organ fate
(Complex B). Finally, the bde zag1 double mutant phenotype
suggests that BDE and ZAG1 redundantly promote FM identity
and function in a complex required to promote FM fate (Complex C).
BDE and ZAG1 may be able to compensate for the lack of the other
in a single complex that promotes FM identity, or alternatively, BDE
and ZAG1 may function in two separate complexes that promote
FM fate. The different phenotypes of bde and zag1 in the ear and
tassel also indicate that different MADS box complexes are present
in the two inflorescences.
The Branch Meristem Is a Default Meristem Fate
The bde zag1 double mutants are reminiscent of ifa zag1 double
mutants, which also produce branch-like meristems from ear
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florets (Laudencia-Chingcuanco and Hake, 2002). In ifa zag1
double mutants, however, branches are initiated from the carpel
whorl of the floret, and other floral organs are specified normally.
Analysis of both double mutants suggests that a branch-like
meristem might be a default state in the ear if the floral development program cannot be executed. In bde zag1 double
mutants, the FM cannot adopt FM fate and reverts to a default
branch-like meristem. A similar situation occurs in ifa zag1
double mutants, although in this case, FM identity is maintained
until the carpel whorl develops, at which point, the FM reverts
to a BM.
Analysis of other mutants also supports the idea that a branch
meristem is a default meristem in the ear. ids1 is required for SM
meristem determinacy, and in the absence of ids1, spikelets
produce extra florets (Chuck et al., 1998). In ifa ids1 double mutant ears, SM are converted to BM, indicating that ifa and ids1 are
redundantly required for SM identity (Laudencia-Chingcuanco
and Hake, 2002). Similarly, bd1 is also required for SM identity,
and in the absence of bd1, SMs are converted to BMs (Chuck
et al., 2002).
In contrast with the ear, a branch meristem is not likely to be a
default meristem in the tassel. In the tassel of bde zag1, ifa zag1,
and ifa ids1 double mutants, FMs and SMs do not convert to BM.
In ifa zag1 double mutants, FMs are converted to SMs, while in ifa
ids1 double mutants, SMs are converted to SPMs (LaudenciaChingcuanco and Hake, 2002). In bd1 mutant tassels, SMs are
converted to meristems with some branch-like characteristics,
although the conversion is not as complete as in the ear (Chuck
et al., 2002). Thus, a common theme between the tassel and ear
is that determinate meristems are converted to less determinate
meristems, but a complete conversion to a BM does not occur in
the tassel.
ABCDE Model in Grass Floral Development
The simple yet elegant ABCDE model provides a mechanism to
help understand how floral organ identity is determined in
Arabidopsis and other dicot species. B and C class function is
conserved in the grasses, although it is unclear if A and E class
functions are also conserved or if other genes function in floral
development specifically in the grasses. For example, in rice, the
YABBY gene DROOPING-LEAF1 is a carpel identity gene, a
function not conserved in Arabidopsis (Yamaguchi et al., 2004).
In this work, we described bde functions in maize floral
development. bde is most closely related to AGL6 in Arabidopsis, which has no known biological function. In several respects,
bde resembles Arabidopsis Class E or SEP genes. First, like the
SEP genes, bde affects organ development in all four whorls.
Second, BDE likely functions in multiple complexes (Figure 8C).
BDE interacts with the C class protein ZAG1 to promote FM
determinacy and likely interacts with B and C class proteins to
regulate floral organ development. We also propose that BDE
functions in a third complex with unknown members to promote
FM identity. Similarly, SEP proteins act in multiple complexes
with class A, B, and C proteins (Honma and Goto, 2001) as well
as complexes important for ovule development (Favaro et al.,
2003). Third, Arabidopsis SEP genes, particularly SEP4, have a
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The Plant Cell
role in promoting FM identity, similar to bde (Pelaz et al., 2000;
Ditta et al., 2004). The conclusion that bde resembles a Class E
gene is consistent with the finding that agl6 functions redundantly with sep genes in petunia (Rijpkema et al., 2009).
Although aspects of the ABCDE model clearly apply to the
grasses, there are multiple differences in the regulatory networks
governing floral development in Arabidopsis and maize. A more
complete understanding of maize floral development will require
further understanding of MADS box protein interactions and if
and how these interactions map onto the ABCDE model, as well
as identifying new genes that function specifically in maize.
METHODS
Genetics
To map the bde mutation, we followed the basic strategy for position
based cloning in maize (Zea mays) outlined by Bortiri et al. (2006).
Mapping populations for bde-McC were developed by crossing bde to
B73 and Mo17 inbred lines and self-pollinating. Using bulked segregate
analysis (Michelmore et al., 1991), we localized bde on chromosome 5,
bin 7. We defined flanking markers and further refined the interval using
markers from the IBM neighbors map. We designed additional single
nucleotide polymorphism markers using maize ESTs in the region (based
on synteny in rice [Oryza sativa]). We sequenced the coding region of the
candidate gene, zag3, in three alleles, bde-McC, bde-N868, and bde03IL-A619TR-864 (bde-TR864); we also sequenced ;1 kb of 59 flanking
sequence in bde-N868 to define the insertion/deletion first observed by
PCR analysis.
For phenotypic characterization of single bde mutants, we used bdeMcC introgressed three times in the A619 inbred background. To construct the bde zag1 double mutant, we used stocks of bde and zag1
introgressed in the B73 inbred background four or more times. bde-McC
zag1-mum1 double mutants were genotyped using primers BT235
(59-CAGCAGCATGGATCACAGAT-39) and BT237 (59-TGCCATGGAGAGAGAACCTC-39) for bde-McC and the simple sequence repeat marker
umc 1187 for zag1-mum1.
In situ hybridization was done as described previously (Bortiri et al.,
2006), except tissue was fixed overnight in FAA. The template for the
bde probe was generated by cloning a fragment of bde cDNA lacking
the MADS box into the pGEM-T Easy vector (Promega) (primers used
zag3-5F, 59-CAGGCAACTCAAACACAAG-39, and zag3-1R, 59-ATTCTATGCACCGGGCTATG-39). This fragment was amplified using M13 forward and reverse primers and used as a template for in vitro transcription
(T7 RNA polymerase; Promega) using DIG RNA labeling mix (Roche
Diagnostics).
Protein Interaction Analysis
BDE and ZAG1 interactions were tested by cloning bde and zag1 cDNAs
into pBDGAL4 and pADGAL4 vectors (Stratagene) and transforming bait
and prey plasmids into Y187 and AH109 yeast strains, respectively. To
create double transformants, Y187 and AH109 yeast were mated overnight and plated on selective media. To avoid BDE autoactivation in
pBDGAL4, we created a truncated version of BDE that lacked the
C-terminal domain by digesting pBDGAL4-ZAG3 with PstI (removes
amino acids 184 to 257) and religating the vector.
For BiFC assays, full-length zag1 and bde were cloned into pSAT4nEYFP-N1 and pSAT4-cEYFP-N1 vectors (Citovsky et al., 2006), respectively, in frame with YFP. For microbombardment, 2 mg of each test
plasmid and 250 ng pRecA-RFP (a kind gift from Ian Small), as an internal
control, was absorbed onto S1000d gold beads according to the manufacturer’s instructions (Seashell Technology) and bombarded onto
onion epidermal peels using a Helios gene gun system (model PDS1000/He; Bio-Rad) at a pressure of 1100 p.s.i. After 26 to 30 h, samples
were mounted and examined for RFP and YFP expression. A total of four
replicates were performed for each plasmid combination. For ZAG1- and
BDE-expressing cells, 60 to 100 RFP-expressing cells were examined for
each replicate. For negative controls, 30 to 70 RFP-expressing cells were
examined for each replicate. Images were processed using Adobe
Photoshop CS.
Accession Number
Sequence data from this article can be found in GenBank/EMBL data
libraries under accession number NM_001111862 (bde [zag3]).
Scanning Electron Microscopy
Supplemental Data
Inflorescence primordia for scanning electron microscopy were fixed in
FAA (50% ethanol, 5% glacial acetic acid, and 3.7% formaldehyde)
overnight at 48C and dehydrated in an ethanol series to 100% ethanol.
Samples were critical point dried, and glumes were manually dissected to
reveal developing florets. Samples were sputter coated with palladium for
60 s and viewed on a Hitachi S-4700 at an accelerating voltage of 2 kV.
Images were processed using Adobe Photoshop CS.
The following materials are available in the online version of this article.
Supplemental Figure 1. Ectopic Meristems in bde Mutants.
Supplemental Figure 2. Additional bde Mapping Data.
Supplemental Figure 3. The LFM Retains FM Identity in bde
Mutants.
Expression Analysis
ACKNOWLEDGMENTS
RNA for quantitative PCR was isolated from approximately five ear
primordia of bde-McC, bde-TR864, bde-N868, and A619, using Trizol
(Invitrogen) according to the manufacturer’s instructions. Approximately
1 mg of total RNA was used as a template for cDNA synthesis, using an
oligo(dT) primer according to the manufacturer’s recommendations (Superscript III first-strand synthesis system for RT-PCR; Invitrogen). Quantitative PCR was performed on a MyIQ single-color real-time detection
system (Bio-Rad), and data were processed in MyIQ and Excel. Data are
from at least three RT-PCR and quantitative PCR replicates. Data were
normalized against gapdh levels (primers used are ZmGAPDH-5F,
59-CCTGCTTCTCATGGATGGTT-39, and ZmGAPDH-6R, 59-TGGTAGCAGGAAGGGAAACA-39).
We thank members of the Hake lab for helpful discussions and technical
advice. Nathalie Bolduc, China Lunde, and Renata Reinheimer gave
helpful comments on the manuscript. Nathalie Bolduc provided gapdh
primers and technical assistance for quantitative PCR, Mily Ron and
Helena Pires provided technical assistance with bombarding, and Bryan
Thines provided technical assistance with yeast two-hybrid experiments. We thank David Hantz and Julie Calfas for plant care in the
greenhouse, Stan Gelvin for the BiFC vectors, and Ian Small for the
pRecA-RFP plasmid. We thank De Wood for use of the scanning
electron microscope at Western Region Research Center. This work
was supported by National Science Foundation Grant 0604923 to S.H.
bde Controls Maize Floral Development
and R.S. and National Institutes of Health–National Research Service
Award postdoctoral fellowship F32GM082002 to B.E.T.
Received April 8, 2009; revised August 4, 2009; accepted August 25,
2009; published September 11, 2009.
REFERENCES
Ambrose, B.A., Lerner, D.R., Ciceri, P., Padilla, C.M., Yanofsky, M.F.,
and Schmidt, R.J. (2000). Molecular and genetic analyses of the
silky1 gene reveal conservation in floral organ specification between
eudicots and monocots. Mol. Cell 5: 569–579.
Becker, A., and Theissen, G. (2003). The major clades of MADS-box
genes and their role in the development and evolution of flowering
plants. Mol. Phylogenet. Evol. 29: 464–489.
Bensen, R.J., Johal, G.S., Crane, V.C., Tossberg, J.T., Schnable,
P.S., Meeley, R.B., and Briggs, S.P. (1995). Cloning and characterization of the maize An1 gene. Plant Cell 7: 75–84.
Bortiri, E., Chuck, G., Vollbrecht, E., Rocheford, T., Martienssen, R.,
and Hake, S. (2006). ramosa2 encodes a LATERAL ORGAN BOUNDARY domain protein that determines the fate of stem cells in branch
meristems of maize. Plant Cell 18: 574–585.
Cacharrón, J., Saedler, H., and Theissen, G. (1999). Expression of
MADS box genes ZMM8 and ZMM14 during inflorescence development of Zea mays discriminates between the upper and the lower
floret of each spikelet. Dev. Genes Evol. 209: 411–420.
Cheng, P.C., Greyson, R.I., and Walden, D.B. (1983). Organ initiation
and the development of unisexual flowers in the tassel and ear of Zea
mays. Am. J. Bot. 70: 450–462.
Chuck, G., Meeley, R., Irish, E., Sakai, H., and Hake, S. (2007). The
maize tasselseed4 microRNA controls sex determination and meristem cell fate by targeting Tasselseed6/indeterminate spikelet1. Nat.
Genet. 39: 1517–1521.
Chuck, G., Meeley, R.B., and Hake, S. (1998). The control of maize
spikelet meristem fate by the APETALA2-like gene indeterminate
spikelet1. Genes Dev. 12: 1145–1154.
Chuck, G., Muszynski, M., Kellogg, E., Hake, S., and Schmidt, R.J.
(2002). The control of spikelet meristem identity by the branched
silkless1 gene in maize. Science 298: 1238–1241.
Citovsky, V., Lee, L.Y., Vyas, S., Glick, E., Chen, M.H., Vainstein, A.,
Gafni, Y., Gelvin, S.B., and Tzfira, T. (2006). Subcellular localization
of interacting proteins by bimolecular fluorescence complementation
in planta. J. Mol. Biol. 362: 1120–1131.
Clifford, H.T. (1987). Spikelet and floral morphology. In Grass Systematics and Evolution, T.R. Soderstrom, K.W. Hilu, C.S. Campbell, and
M.E. Barkworth, eds (Washington, DC: Smithsonian Institution Press),
pp. 21–30.
Coen, E.S., and Meyerowitz, E.M. (1991). The war of the whorls:
Genetic interactions controlling flower development. Nature 353:
31–37.
Colombo, L., Franken, J., Koetje, E., van Went, J., Dons, H.J.,
Angenent, G.C., and van Tunen, A.J. (1995). The petunia MADS box
gene FBP11 determines ovule identity. Plant Cell 7: 1859–1868.
de Folter, S., Immink, R.G., Kieffer, M., Parenicova, L., Henz, S.R.,
Weigel, D., Busscher, M., Kooiker, M., Colombo, L., Kater, M.M.,
Davies, B., and Angenent, G.C. (2005). Comprehensive interaction
map of the Arabidopsis MADS box transcription factors. Plant Cell 17:
1424–1433.
Ditta, G., Pinyopich, A., Robles, P., Pelaz, S., and Yanofsky, M.F.
(2004). The SEP4 gene of Arabidopsis thaliana functions in floral organ
and meristem identity. Curr. Biol. 14: 1935–1940.
2589
Dreni, L., Jacchia, S., Fornara, F., Fornari, M., Ouwerkerk, P.B., An,
G., Colombo, L., and Kater, M.M. (2007). The D-lineage MADS-box
gene OsMADS13 controls ovule identity in rice. Plant J. 52: 690–699.
Egea-Cortines, M., Saedler, H., and Sommer, H. (1999). Ternary
complex formation between the MADS-box proteins SQUAMOSA,
DEFICIENS and GLOBOSA is involved in the control of floral architecture in Antirrhinum majus. EMBO J. 18: 5370–5379.
Fan, H.Y., Hu, Y., Tudor, M., and Ma, H. (1997). Specific interactions
between the K domains of AG and AGLs, members of the MADS
domain family of DNA binding proteins. Plant J. 12: 999–1010.
Fan, J., Li, W., Dong, X., Guo, W., and Shu, H. (2007). Ectopic expression
of a hyacinth AGL6 homolog caused earlier flowering and homeotic
conversion in Arabidopsis. Sci. China C Life Sci. 50: 676–689.
Favaro, R., Pinyopich, A., Battaglia, R., Kooiker, M., Borghi, L., Ditta,
G., Yanofsky, M.F., Kater, M.M., and Colombo, L. (2003). MADSbox protein complexes control carpel and ovule development in
Arabidopsis. Plant Cell 15: 2603–2611.
Goto, K., and Meyerowitz, E.M. (1994). Function and regulation of the
Arabidopsis floral homeotic gene PISTILLATA. Genes Dev. 8: 1548–
1560.
Honma, T., and Goto, K. (2001). Complexes of MADS-box proteins are
sufficient to convert leaves into floral organs. Nature 409: 525–529.
Hsu, H.F., Huang, C.H., Chou, L.T., and Yang, C.H. (2003). Ectopic
expression of an orchid (Oncidium Gower Ramsey) AGL6-like gene
promotes flowering by activating flowering time genes in Arabidopsis
thaliana. Plant Cell Physiol. 44: 783–794.
Irish, E.E. (1998). Grass spikelets: A thorny problem. Bioessays 20:
789–793.
Irish, E.E., Langdale, J.A., and Nelson, T.M. (1994). Interactions
between tassel seed genes and other sex-determining genes in
maize. Dev. Genet. 15: 155–171.
Jack, T., Brockman, L.L., and Meyerowitz, E.M. (1992). The homeotic
gene APETALA3 of Arabidopsis thaliana encodes a MADS box and is
expressed in petals and stamens. Cell 68: 683–697.
Jackson, D., Veit, B., and Hake, S. (1994). Expression of maize
KNOTTED1 related homeobox genes in the shoot apical meristem
predicts patterns of morphogenesis in the vegetative shoot. Development 120: 405–413.
Jeon, J.S., Jang, S., Lee, S., Nam, J., Kim, C., Lee, S.H., Chung, Y.Y.,
Kim, S.R., Lee, Y.H., Cho, Y.G., and An, G. (2000). leafy hull sterile1
is a homeotic mutation in a rice MADS box gene affecting rice flower
development. Plant Cell 12: 871–884.
Jofuku, K.D., den Boer, B.G., Van Montagu, M., and Okamuro, J.K.
(1994). Control of Arabidopsis flower and seed development by the
homeotic gene APETALA2. Plant Cell 6: 1211–1225.
Kaufmann, K., Melzer, R., and Theissen, G. (2005). MIKC-type MADSdomain proteins: Structural modularity, protein interactions and network evolution in land plants. Gene 347: 183–198.
Laudencia-Chingcuanco, D., and Hake, S. (2002). The indeterminate
floral apex1 gene regulates meristem determinacy and identity in the
maize inflorescence. Development 129: 2629–2638.
Ma, H., Yanofsky, M.F., and Meyerowitz, E.M. (1991). AGL1-AGL6, an
Arabidopsis gene family with similarity to floral homeotic and transcription factors. Genes Dev. 5: 484–495.
Mandel, M.A., Gustafson-Brown, C., Savidge, B., and Yanofsky,
M.F. (1992). Molecular characterization of the Arabidopsis floral
homeotic gene APETALA1. Nature 360: 273–277.
Mena, M., Mandel, M.A., Lerner, D.R., Yanofsky, M.F., and Schmidt,
R.J. (1995). A characterization of the MADS-box gene family in maize.
Plant J. 8: 845–854.
Mena, M., Ambrose, B.A., Meeley, R.B., Briggs, S.P., Yanofsky, M.F.,
and Schmidt, R.J. (1996). Diversification of C-function activity in
maize flower development. Science 274: 1537–1540.
2590
The Plant Cell
Michelmore, R.W., Paran, I., and Kesseli, R.V. (1991). Identification of
markers linked to disease-resistance genes by bulked segregant
analysis: a rapid method to detect markers in specific genomic
regions by using segregating populations. Proc. Natl. Acad. Sci.
USA 88: 9828–9832.
Nagasawa, N., Miyoshi, M., Sano, Y., Satoh, H., Hirano, H., Sakai, H.,
and Nagato, Y. (2003). SUPERWOMAN1 and DROOPING LEAF
genes control floral organ identity in rice. Development 130: 705–718.
Nickerson, N.H. (1959). Sustained treatment with gibberellin acid of five
different kinds of maize. Ann. Mo. Bot. Gard. 47: 19–37.
Okamuro, J.K., den Boer, B.G., Lotys-Prass, C., Szeto, W., and
Jofuku, K.D. (1996). Flowers into shoots: Photo and hormonal control
of a meristem identity switch in Arabidopsis. Proc. Natl. Acad. Sci.
USA 93: 13831–13836.
Okamuro, J.K., Szeto, W., Lotys-Prass, C., and Jofuku, K.D. (1997).
Photo and hormonal control of meristem identity in the Arabidopsis
flower mutants apetala2 and apetala1. Plant Cell 9: 37–47.
Parenicova, L., de Folter, S., Kieffer, M., Horner, D.S., Favalli, C.,
Busscher, J., Cook, H.E., Ingram, R.M., Kater, M.M., Davies, B.,
Angenent, G.C., and Colombo, L. (2003). Molecular and phylogenetic analyses of the complete MADS-box transcription factor family
in Arabidopsis: New openings to the MADS world. Plant Cell 15:
1538–1551.
Pelaz, S., Ditta, G.S., Baumann, E., Wisman, E., and Yanofsky, M.F.
(2000). B and C floral organ identity functions require SEPALLATA
MADS-box genes. Nature 405: 200–203.
Prasad, K., Parameswaran, S., and Vijayraghavan, U. (2005).
OsMADS1, a rice MADS-box factor, controls differentiation of specific
cell types in the lemma and palea and is an early-acting regulator of
inner floral organs. Plant J. 43: 915–928.
Reinheimer, R., and Kellogg, E.A. (2009). Evolution of AGL6 genes in
grasses (Poaceae): Ovule expression is ancient, palea expression is
new. Plant Cell, in press.
Rijpkema, A.S., Zethof, J., Gerats, T., and Vandenbussche, M.
(2009). The petunia AGL6 gene has a SEPALLATA-like function in
floral patterning. Plant J., in press.
Rounsley, S.D., Ditta, G.S., and Yanofsky, M.F. (1995). Diverse roles
for MADS box genes in Arabidopsis development. Plant Cell 7: 1259–
1269.
Schmidt, R.J., Veit, B., Mandel, M.A., Mena, M., Hake, S., and
Yanofsky, M.F. (1993). Identification and molecular characterization
of ZAG1, the maize homolog of the Arabidopsis floral homeotic gene
AGAMOUS. Plant Cell 5: 729–737.
Skibbe, D.S., Wang, X., Borsuk, L.A., Ashlock, D.A., Nettleton, D.,
and Schnable, P.S. (2008). Floret-specific differences in gene expression and support for the hypothesis that tapetal degeneration of
Zea mays L. occurs via programmed cell death. J. Genet. Genomics
35: 603–616.
Theissen, G. (2001). Development of floral organ identity: Stories from
the MADS house. Curr. Opin. Plant Biol. 4: 75–85.
Thompson, B.E., and Hake, S. (2009). Translational biology: From
Arabidopsis flowers to grass inflorescence architecture. Plant Physiol.
149: 38–45.
Whipple, C.J., Ciceri, P., Padilla, C.M., Ambrose, B.A., Bandong,
S.L., and Schmidt, R.J. (2004). Conservation of B-class floral homeotic gene function between maize and Arabidopsis. Development 131:
6083–6091.
Yamaguchi, T., Lee, D.Y., Miyao, A., Hirochika, H., An, G., and
Hirano, H.Y. (2006). Functional diversification of the two C-class
MADS box genes OSMADS3 and OSMADS58 in Oryza sativa. Plant
Cell 18: 15–28.
Yamaguchi, T., Nagasawa, N., Kawasaki, S., Matsuoka, M., Nagato,
Y., and Hirano, H.Y. (2004). The YABBY gene DROOPING LEAF
regulates carpel specification and midrib development in Oryza sativa.
Plant Cell 16: 500–509.
Yanofsky, M.F., Ma, H., Bowman, J.L., Drews, G.N., Feldmann, K.A.,
and Meyerowitz, E.M. (1990). The protein encoded by the Arabidopsis homeotic gene agamous resembles transcription factors. Nature
346: 35–39.
Yu, H., Ito, T., Zhao, Y., Peng, J., Kumar, P., and Meyerowitz, E.M.
(2004). Floral homeotic genes are targets of gibberellin signaling in
flower development. Proc. Natl. Acad. Sci. USA 101: 7827–7832.
bearded-ear Encodes a MADS Box Transcription Factor Critical for Maize Floral Development
Beth E. Thompson, Linnea Bartling, Clint Whipple, Darren H. Hall, Hajime Sakai, Robert Schmidt and
Sarah Hake
Plant Cell 2009;21;2578-2590; originally published online September 11, 2009;
DOI 10.1105/tpc.109.067751
This information is current as of June 15, 2017
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References
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