Research
The role of teosinte glume architecture (tga1) in coordinated
regulation and evolution of grass glumes and inflorescence axes
Jill C. Preston1,2, Huai Wang3, Lisa Kursel3, John Doebley3 and Elizabeth A. Kellogg1
1
Department of Biology, University of Missouri – St. Louis, Research 223, One University Boulevard, St. Louis, MO 63121, USA; 2Department of Ecology and Evolutionary Biology,
The University of Kansas, 8009 Haworth Hall, 1200 Sunnyside Avenue, Lawrence, KS 66045, USA; 3Laboratory of Genetics, University of Wisconsin, Madison, WI 53706, USA
Summary
Author for correspondence:
Jill Preston
Tel: +1 785 864 5837
Email: jcpxt8@ku.edu
Received: 29 June 2011
Accepted: 10 August 2011
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doi: 10.1111/j.1469-8137.2011.03908.x
Key words: gene expression evolution,
glume architecture, grass, inflorescence
architecture, Zea mays.
• Hardened floral bracts and modifications to the inflorescence axis of grasses have been
hypothesized to protect seeds from predation and ⁄ or aid seed dispersal, and have evolved
multiple times independently within the family. Previous studies have demonstrated that
mutations in the maize (Zea mays ssp. mays) gene teosinte glume architecture (tga1) underlie
a reduction in hardened structures, yielding free fruits that are easy to harvest. It remains
unclear whether the causative mutation(s) occurred in the cis-regulatory or protein-coding
regions of tga1, and whether similar mutations in TGA1-like genes can explain variation in the
dispersal unit in related grasses.
• To address these questions TGA1-like genes were cloned and sequenced from a number
of grasses and analyzed phylogenetically in relation to morphology; protein expression was
investigated by immunolocalization.
• TGA1-like proteins were expressed throughout the spikelet in the early development of all
grasses, and throughout the flower of the grass relative Joinvillea. Later in development,
expression patterns differed between Tripsacum dactyloides, maize and teosinte (Z. mays
ssp. parviglumis).
• These results suggest an ancestral role for TGA1-like genes in early spikelet development,
but do not support the hypothesis that TGA1-like genes have been repeatedly modified to
affect glume and inflorescence axis diversification.
Introduction
Flowering plants have evolved a tremendous number of strategies
to increase fitness through modification of their inflorescence and
floral structures. The grass family has particularly complex inflorescences and structures surrounding the flowers (Cheng et al.,
1983; Clifford, 1987; Ikeda et al., 2004). Each flower (floret) is
made up of a gynoecium, androecium, modified inner tepals
(lodicules), an adaxial bract (palea) and an abaxial bract (lemma).
The flowers are then aggregated into short spikes (spikelets),
which are subtended by two more bracts (glumes). Although the
gynoecium and androecium are very similar in most grasses, the
structure of the paleas, lemmas and glumes, and the arrangement
of the spikelets in the inflorescence, all vary extensively among
the 12 000 species of grasses.
A few grass species have independently evolved an inflorescence structure in which the spikelets are embedded in the inflorescence axis (rachis). In these species, the axis develops so that it
surrounds the developing spikelet, forming a cup-like depression
(Clayton & Renvoize, 1986; E. A. Kellogg, pers. obs.). One or
both glumes then cover the outside of the spikelet, similar to a
trap door hinged at the bottom; the glumes spread outwards from
the inflorescence axis at anthesis and then close again after
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pollination. In inflorescences such as this, the glumes are often
leathery or hardened.
Inflorescences with embedded spikelets have originated in
several of the grass subfamilies, but are particularly common in
the tribe Andropogoneae, subfamily Panicoideae (Clayton &
Renvoize, 1986; E. A. Kellogg, pers. obs.). In most members of
Andropogoneae, the rachis of the inflorescence breaks apart
(disarticulates) at the node below each spikelet, such that the
dispersal unit includes the spikelet plus its adjacent internode. In
species in which the spikelet is embedded in the rachis, and in
which the glumes are hardened, the mature fruit is fully enclosed
in a case made up of the rachis internode plus the glume(s). This
structure has been postulated to protect the reproductive structures from damage and ⁄ or to facilitate seed dispersal (Wilkes,
1967), although its selective value has never been tested. Despite
the important ecological and economic implications for such
structures, little is known about how their development is controlled at the genetic level.
The best-studied species with spikelets embedded in the rachis
and covered with hardened glumes is teosinte in the genus Zea.
In Zea (tribe Andropogoneae, subfamily Panicoideae), variation
in the SQUAMOSA PROMOTER BINDING PROTEIN-LIKE
(SPL) gene, teosinte glume architecture (tga1), underlies a major
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quantitative trait locus (QTL) for the domestication of maize
(Zea mays ssp. mays) from its progenitor teosinte (Z. mays
ssp. parviglumis) (Dorweiler et al., 1993; Wang et al., 2005).
SPL genes have a wide variety of developmental roles in angiosperms – including the regulation of vegetative and inflorescence
phase change, vegetative and inflorescence branching and fruit
development – and are known to target the regulation of SQUAMOSA ⁄ FRUITFULL (SQUA ⁄ FUL)-like genes, commonly
involved in inflorescence and flower development (Mandel &
Yanofsky, 1995; Klein et al., 1996; Manning et al., 2006;
Schwarz et al., 2008; Wang et al., 2009; Jiao et al., 2010; Preston
& Hileman, 2010).
Phenotypic differences associated with the maize tga1 QTL
determine the ease with which the fruit can be separated from the
surrounding inflorescence structures. Female inflorescences (ears)
of teosinte have remarkably hard glumes with high levels of silica
deposition and a high ratio of small to large cells in the mesophyll
at maturity, deeply invaginated rachis internodes and little elongation of the floral branch (rachilla). Conversely, maize ears have
glumes with fewer small cells, more lignin and less silica deposition, little invagination of the rachis and greater elongation of the
rachilla (Clayton & Renvoize, 1986; Dorweiler & Doebley,
1997). Introgression of the maize allele into a teosinte background results in less internode invagination, such that the fruit
is no longer enclosed by the rachis. Conversely, introgression of
the teosinte allele into a maize background causes increased internode invagination and thickening of the outer glume (Wang
et al., 2005). These differences suggest that tga1 is important for
controlling both the hardness of the glumes and the growth of
the inflorescence axis to enclose the fruit.
The tga1 alleles of teosinte and maize differ by only seven nucleotides (Wang et al., 2005). One of these differences encodes a
nonconservative amino acid substitution from lysine to asparagine at position six in the protein, and has been hypothesized to
affect protein stability or function. This is supported by the teosinte-like phenotype resulting from ethyl methanesulfonate
mutagenesis of maize, causing a nonconservative mutation at a
neighboring amino acid (position five) (Wang et al., 2005). The
remaining six changes are located in the promoter region, and
have been hypothesized to affect gene regulation. In early- to
mid-stage female spikelets, tga1 mRNA transcript levels are
equivalent between maize and teosinte (Wang et al., 2005). The
gene is expressed in the floret meristem, gynoecium, lodicules,
paleas, lemmas and glumes. By contrast, at the same time points,
levels of protein accumulation, as measured by western blots,
appear to be markedly different between the two subspecies
(Wang et al., 2005). Protein levels are significantly higher in both
early and mid-stages of ear development in teosinte and in maize
lines carrying the teosinte tga1 allele (Wang et al., 2005).
The available data suggest that changes to TGA1 protein
stability, translational efficiency or function are more probable
explanations for the maize phenotype than are changes in the regulation of the tga1 gene (Wang et al., 2005). Thus, the lysine to
asparagine amino acid substitution may be one of the causative
sites underlying the morphological diversification of the maize
inflorescence. This amino acid change hypothesis predicts that
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patterns of mRNA expression will be similar between teosinte
and maize post-pollination (> 22 mm ear or style elongation
(silk) stage), the stage at which hardening of the outer glume and
invagination of the rachis occur in teosinte (Dorweiler & Doebley,
1997). Furthermore, if changes in TGA1 protein stability or
translational efficiency, rather than protein function, underlie
phenotypic differences, distinct patterns of protein accumulation
can be predicted between the two.
The developmental role of tga1 must be more complex than
just described for the female inflorescences of maize and teosinte.
Although the female spikelets of teosinte are embedded in the
rachis and have rock-hard glumes, the male (tassel) spikelets are
not embedded and the glumes are firm, but leaf-like. Furthermore, the TGA1-like sequences of two distantly related grasses,
rice (Oryza sativa) and wheat (Triticum aestivum), share the lysine
residue of teosinte at position six (Wang et al., 2005), suggesting
that this is the ancestral state for most grasses. If this residue were
perfectly correlated with glume and inflorescence phenotype, rice
and wheat should have hard glumes and embedded spikelets, like
teosinte. However, the glumes of wheat are leaf-like (membranous to coriaceous), the glumes of rice are tiny and flap-like and
neither species has spikelets embedded in the inflorescence axis;
in this respect, they are similar to most other grasses. These observations suggest that different developmental pathways are responsible for glume and rachis architecture in male and female
teosinte inflorescences, and that the role of TGA1 in glume and
rachis architecture may have diverged more than once within the
tribe Andropogoneae.
The sister genus of Zea is Tripsacum (Lukens & Doebley,
2001; Mathews et al., 2002; Bomblies & Doebley, 2005), which
is, like Zea, monoecious. Female spikelets of Tripsacum have
thick hardened glumes that are sunken into the equally hard
rachis, although neither the glumes nor the rachis is as solid as
those of teosinte. Male spikelets of Tripsacum have leathery
glumes that are somewhat firmer than those of Zea, but much
more flexible than the glumes of the female spikelets. Other
Andropogoneae species with invaginated internodes and thickened glumes include Rhytachne, Coelorachis and various other
genera in the tribe (sensu Clayton & Renvoize, 1986; Mathews
et al., 2002). Unfortunately, previous phylogenetic analyses have
found little support for relationships within Andropogoneae (e.g.
Lukens & Doebley, 2001; Mathews et al., 2002), making the
evolutionary history of glume hardening and rachis internode
invagination difficult to reconstruct.
In this study, we test the hypothesis that TGA1-like genes have
an ancestral role in spikelet development by assessing the pattern
of TGA1 protein expression in the early stages of inflorescence
development across representative grass species and a closely
related grass outgroup. In addition, to evaluate the role of TGA1like gene evolution in glume thickening and rachis invagination,
we generated a phylogeny of TGA1-like genes, and compared patterns of amino acid sequence variation and protein expression with
inferred shifts in inflorescence morphology. Finally, to distinguish
between alternative hypotheses for tga1 diversification between
teosinte and maize, we compared mRNA and protein expression
patterns in early- and late-stage inflorescences of both subspecies.
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Materials and Methods
Plant material and growth conditions
Seeds of maize W22 and Mo17, Avena strigosa (Preston 18),
sorghum (Sorghum bicolor) (Malcomber 3116), Eleusine indica
(PI217609), green millet (Setaria viridis) (PI204624) and Tripsacum dactyloides (Kellogg 4) were obtained from the United States
Department of Agriculture (USDA), or collected in the field
(T. dactyloides), and grown at the University of Missouri – St.
Louis or the University of Wisconsin – Madison (USA) (maize
Mo17) at 20–22C under constant light. Seeds of Streptochaeta
angustifolia (Malcomber 3123) were collected by Lynn G. Clark
and grown at the University of Missouri – St. Louis on vermiculite in a humid chamber under constant light. Inflorescence material from teosinte (PI384065, PI384071, Ames21814) was
harvested from plants grown at the University of Missouri –
Columbia by Sherry Flint-Garcia. Other species included in the
phylogenetic and expression analyses were grown from seed
stocks at the University of Missouri – St. Louis under standard
glasshouse conditions.
Sequencing and phylogenetic analysis
Partial TGA1-like, ndhF, phyB and waxy sequences were isolated
from genomic DNA of representative Andropogoneae and nonAndropogoneae panicoids, nonpanicoid grasses and the nongrass
outgroup Joinvillea ascendens (Joinvilleaceae) listed in Supporting
Information Tables S1 and S2. Total DNA extractions were performed following Malcomber & Kellogg (2006). ndhF, phyB and
waxy genes were amplified as described previously (Mathews
et al., 2002). TGA1-like genes were amplified using the forward
primers 482F (5¢-AGTGCAGCAGGTTCCATCTACT-3¢),
681F (5¢-GATSAAAACCGAGGAGAGYCC-3¢) or 1000F (5¢GACTCSGAYTGTGCTCTCTCTC-3¢) and the reverse primer
1368R (5¢-TACTGCCAYGAGAASGGC-3¢). Each primer combination amplified exons two and three, part of exon two and
exon three, and part of exon three, respectively. Cycling parameters were 94C for 5 min, followed by 30 cycles of 94C for
30 s, 55C for 30 s and 72C for 1 min, with a final extension of
72C for 10 min. PCR fragments were purified and cloned into
the pGEM-T easy vector (Promega, Madison, WI, USA).
Plasmid DNA was cleaned by alkaline lysis and sequenced using
the plasmid primers T7 and SP6. Sequencing reactions were carried out using the BigDye 3.1 terminator cycle sequencing protocol (Applied Biosystems, Foster City, CA, USA), and analyzed
on an ABI capillary sequencer (Applied Biosystems). Sequence
quality was assessed using phred (Ewing et al., 1998).
Sequences with phred scores above 20 were manually aligned
in MacClade 4 (Maddison & Maddison, 2003). Phylogenetic
analyses were performed on the TGA1 nucleotide alignment and
a combined alignment of ndhF, waxy, phyB, tb1 and TGA1-like
genes using GARLI 0.951 and MrBayes 3.1.2 following model
optimization in MrModelTest (Ronquist & Huelsenbeck, 2003;
Nylander, 2004; Zwickl, 2006). Maximum likelihood (ML)
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analyses were run using 10 random addition sequences with 500
bootstrap replicates, implementing the HKY + I + C (TGA1) or
GTR + I + C (combined) model of evolution based on the
results from MrModelTest 2.2. Bayesian analyses were run twice
for ten million generations, sampling every 1000 generations,
with 25% of trees discarded as burn-in. Gaps were treated as
missing data. Trees were rooted with orthologous sequences from
the nongrass outgroup J. ascendens (TGA1-like) or Pennisetum
alopecuroides (Paniceae) and Axonopus fissifolius (Paniceae) (combined) and supplemented with previously generated sequences
from GenBank (Tables S1, S2).
To determine the identity of residue six (the amino acid that
varies between teosinte and maize tga1), a 460-bp fragment was
amplified from maize, teosinte, Bouteloua gracilis (K-1976-198)
(Chloridoideae) and Rottboellia aurita (Panicoideae) using the
primers Tga1-position1F (5¢-ATGGATTGGGATCTCAA-3¢)
and Tga1-SBP-Rev (5¢-TTGGAGTGCGSCTCGCACACCTTGT-3¢). PCR conditions were as described above. To identify
potential SPL protein consensus binding sites, putative promoter
and coding regions were obtained from rice (Ehrhartoideae),
Brachypodium distachyon (Pooideae), sorghum (Panicoideae),
Setaria italica (Panicoideae) and maize (Panicoideae) genomic
sequences corresponding to the duplicated grass FUL-like genes
(Fig. S1). These sequences were used to search for the core
consensus sequence CCGTAC (Birkenbihl et al., 2005; Liang
et al., 2008).
Character state reconstructions
Glume hardness and the extent of rachis internode invagination
were determined for species available in the herbarium collection
of the Missouri Botanical Gardens. Glumes were scored as
membranous to coriaceous (0) or hard (1), based on the ease of
penetration with a needle, whereas rachis internodes were scored
as flat to terete (0) or concave (1), based on their overall shape
and invagination. In order to reconstruct character state transitions, the likelihood of each glume and internode state was estimated under the Mk1 (single rate of change between states)
model of evolution for each node of the best ML tree by tracing
characters over the 312 most likely tree topologies in Mesquite
version 2.01 (Maddison & Maddison, 2006, 2007).
Expression analysis
Inflorescences at different stages of development were fixed in
FAA (47.5% (v ⁄ v) ethanol, 5% (v ⁄ v) acetic acid, 3.7% (v ⁄ v)
formaldehyde (Sigma, St. Louis, MO, USA)) using vacuum infiltration. To increase the definition of the cell walls, tissue was
stained with 1% eosin Y in 95% ethanol, and dehydrated into
paraffin wax, following Jackson (1991). Ribbons of 8-lm longitudinal sections were cut, mounted on Probe-On-Plus microscope slides (Fisher Scientific, Pittsburg, PA, USA) and left to dry
at 37C overnight.
In situ hybridization was performed as described by Jackson
et al. (1994) using a 500-bp mRNA probe from the 3¢ end of
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maize tga1. As the sequence of this probe was 96% identical to a
closely related maize paralog (see Results), hybridization shows
expression of both paralogous genes. Sense and antisense riboprobes were generated using T7 and SP6 Megascript in vitro
transcription kits (Ambion Inc., Austin, TX, USA) with digoxygenin-labeled UTP (Roche, Indianapolis, IN, USA), according
to the manufacturer’s instructions. Probe hydrolysis followed
Jackson (1991). Probe hybridization, washing, immunolocalization and photography followed Jackson et al. (1994) and
Malcomber & Kellogg (2004). Immunolocalization was performed following Lucas et al. (1995) using the TGA1 antibody
described in Wang et al. (2005). Photographs were imported into
Adobe Photoshop and adjusted for contrast, brightness and color
balance.
Allele-specific expression assay
We employed an allele-specific assay of not1 and tga1 accumulation in the F1 hybrids of inbred maize and teosinte lines. Seven
teosinte inbreds (TIL1, TIL3, TIL5, TIL9, TIL11, TIL14 and
TIL25) were used as pollen parents to two or more maize inbreds
(B73, CML103, Ki3, Mo17, Oh43 and W22), creating a total of
27 F1 hybrids (Tables S3, S4). Total cellular RNA was isolated
from 5–10 immature ears from one or two plants of each cross;
5-lg aliquots of each of the RNA samples were DNase treated
and reverse transcribed (RT) using a polyT primer and Superscript III reverse transcriptase (Invitrogen, Carlsbad, CA, USA),
following the manufacturer’s instructions. The integrity of the
cDNA was checked using 0.5 ll of the RT reactions as the template for PCRs with the actin primers WH182 (5¢-CCAAGGCCAACAGAGAGAAA-3¢) and WH183 (5¢-CCAAACGGAGAATAGCATGAG-3¢). The same actin primers were also used to
check for genomic DNA contamination. In all cases, the actin
primer check was negative for genomic DNA contamination.
The RT reactions were diluted and 1-ll aliquots were employed
as the template for PCRs using two sets of fluorescently labeled
primers: TNW120F (5¢ FAM labeled; 5¢-ATCCTGCCCCGCCGTGCAG-3¢) and TNW121R (5¢-CACGAGAAGGGCATCGACGACGAG-3¢) to amplify not1, and TNW124F (5¢
FAM labeled; 5¢-GCGATTCTCACCATTTGCGCATC-3¢)
and TNW126R (5¢-AGGCGTGGCGGCTCCCAG-3¢) to
amplify tga1 (TAQ Core Kit, Qiagen, Valencia, CA, USA). PCR
products were assayed on an ABI 3700 fragment analyzer and
peak areas were determined using Gene Marker version 1.70.
In order to perform the allele-specific expression assay, we utilized the fact that the maize and teosinte parents have different
allele sizes for both not1 and tga1. Both pairs of TNW primers
flank an indel in the respective gene and amplify different sized
products for the maize and teosinte parents. The relative message
level associated with the two chromosomes in each sample was
calculated as the ratio of the area under teosinte ⁄ maize allele
peaks. The same assay was also performed with the DNA from
each of the plants used for RNA extraction to assess any bias in
allele amplification in PCR. This analysis revealed that the ratio
of the maize ⁄ teosinte allele was on average 1.63 for not1 and 1.10
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represented in both cases. Measurements for the area of the teosinte alleles were therefore multiplied by the factor determined
by DNA from the particular plant, and the final message level
ratios were calculated with the corrected teosinte areas.
Results
Phylogenetic analysis and amino acid diversity of TGA1-like
genes
Homologs of tga1 were amplified and sequenced from 26 taxa,
comprising 25 grasses and one nongrass outgroup (J. ascendens,
Joinvilleaceae) (Table S1). In genes of all species, the binding site
for microRNA156 (Corngrass1 (Cg1)) (Chuck et al., 2007) was
fully conserved. ML and Bayesian phylogenetic analyses were largely concordant. Well-supported relationships of TGA1-like
genes outside the Andropogoneae (ML bootstrap above 70%,
posterior probabilities above 95%) largely followed known generic relationships (GPWG, 2001; Sánchez-Ken et al., 2007)
(Fig. 1), except that Orthoclada laxa (Panicoideae) was sister to
PACMAD grasses, although without support; we interpret this as
an artifact created by sparse sampling outside Panicoideae.
Within Andropogoneae there was little resolution, and internal
branches were relatively short (Fig. 1). However, the predicted
sister relationship between the genes of maize and teosinte was
strongly supported (100% ML bootstrap; 100% posterior probability), and there was moderate to strong support for an Andropogoneae clade excluding Coix lacryma-jobi and S. bicolor (70%
ML bootstrap; 100% posterior probability) (Fig. 1).
In contrast with all other sampled taxa, maize and teosinte tga1
had a duplicate locus that was found to be located on the same
chromosome, c. 270 kbp from tga1. Given the proximity of
this tga1 duplicate, we have named it neighbor of tga1 (not1).
tga1 (designated GRMZM2G10511) and not1 (designated
AC233751.1_FG002) are located at positions 44 508 235–
44 512 898 and 44 779 972–44 784 721, respectively, on
chromosome 4 of the maize B73 Reference Genome version 2.
Sequencing from the maize inbred line W22 revealed 87% amino
acid identity between tga1 and not1, and these fell into two wellsupported sister clades, each containing a sequence from maize
and teosinte (Fig. 1). Thus, the two Zea TGA1-like genes are
inferred to have arisen from a duplication event after the divergence of Zea and Tripsacum, but before the divergence of teosinte
from modern maize.
A section of TGA1-like genes spanning the start codon and
part of the highly conserved SBP domain was amplified from
maize (not1), teosinte (not1), B. gracilis and R. aurita, and
aligned with previously sequenced genes from O. sativa,
S. italica, S. bicolor, teosinte (tga1) and maize (tga1). This region
contains the lysine to asparagine change (position six) between
tga1 orthologs of maize and teosinte, and the amino acid (position five) that was mutated in maize by ethyl methanesulfonate
mutagenesis, resulting in a teosinte-like phenotype (Wang et al.,
2005). In all cases, residue five was a phenylalanine, as in both
maize and teosinte tga1, and residue six was a lysine, as in teosinte
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Oryza sativa OsTGA1
Orthoclada laxa OlTGA1
Danthonia spicata DsTGA1
Eleusine coracana EcTGA1
97/*
*/*
Eleusine indica EiTGA1
79/Eragrostis pilosa EpTGA1
85/97
Panicum miliaceum PmTGA1
Setaria italica SiTGA1
Pennisetum glaucum PgTGA1
89/*
Teosinte TGA1
Maize TGA1
82/94
*/*
Teosinte NOT1
98/* Maize NOT1
Chrysopogon gryllus CgTGA1
Phacelurus digitatus PdTGA1
Rottboellia aurita RaTGA1
89/*
Phacelurus huillensis PhTGA1
Rottboellia selloana RsTGA1
82/95
Hemarthria sp. HsTGA1
70/*
Coelorachis lepidura ClTGA1
-/97
Tripsacum dactyloides TdTGA1
Rhytachne rottboellioides RrTGA1
Elionurus muticus EmTGA1
Heteropholis sulcata HsTGA1
93/*
Oxyrhachis gracillima OgTGA1
Coix lacryma-jobi ClTGA1
Sorghum bicolor SbTGA1
Joinvillea ascendens JaTGA1
Andropogoneae
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0.05
Fig. 1 Maximum likelihood (ML) phylogram showing relationships of grass TGA1-like genes. A gene duplication event is inferred to have occurred in the
common ancestor of Zea, giving rise to the paralogous genes tga1 and not1. The remaining relationships largely track the well-supported species phylogeny
(GPWG, 2001). ML bootstrap values > 70% (left) and Bayesian posterior probabilities > 90% (right) are denoted below the branches; 100% support
values are indicated with an asterisk. Dashes indicate no support where applicable.
tga1, indicating that this region is highly conserved across
Andropogoneae and grasses as a whole.
To determine whether TGA1-like proteins have the potential
to regulate spikelet development through the direct regulation of
genes known to be involved in spikelet development, FUL-like
genes from multiple grasses were examined for the SPL protein
consensus binding sequence CCGTAC (Birkenbihl et al., 2005;
Liang et al., 2008). For every target species, one or more consensus binding sites were found within the putative promoter and ⁄ or
intronic region of each FUL1 and FUL2 gene (Fig. S1). For
example, in the rice FUL1 gene (OsMADS14) sensu Preston &
Kellogg (2006), three binding sites (italic type) were found, one
(TTCCGTACGA) 4410 bp upstream of the protein-coding start
site and two (CCCCGTACGA and CTCCGTACTT) within the
first intron; in the rice FUL2 gene (OsMADS15), three (GACCGTACGA, GCCCGTACCA and GCCCGTACCA) binding sites
were found 394, 3965 and 4028 bp upstream, respectively, of
the protein-coding start site. By contrast, a putative SPL binding
site was only found for one of the FUL3 genes examined.
The rice FUL3 gene (OsMADS18) had one binding site
(CACCGTACCC) 5232 bp upstream of the protein-coding start
site; no consensus binding sites were found within the putative
promoter and intronic regions of S. italica, S. bicolor or Z. mays
FUL3 orthologs.
Andropogoneae phylogeny and inflorescence trait
evolution
In order to estimate the number and direction of shifts in glume
hardening and rachis internode invagination within Andropogoneae, we supplemented previously generated ndhF, waxy, phyB
and tb1 sequences with newly generated TGA1 sequences, and
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inferred species relationships (Spangler et al., 1999; Lukens &
Doebley, 2001; Mathews et al., 2002) (Table S2; Fig. S2). We
found no strongly supported topological differences between
trees on the basis of individual markers, and therefore carried out
phylogenetic analyses based on combined sequences (Spangler
et al., 1999; Lukens & Doebley, 2001; Mathews et al., 2002; this
study). The best ML tree based on combined analyses resolved
some of the internal relationships within Andropogoneae with
high support, including a close relationship between Coelorachis
afraurita, Coelorachis selloana, Rottboellia species and Phacelurus
huillensis, and between Hemarthria species and Heteropholis
sulcata (Fig. S2). Similar to previous analyses based on individual
markers, deep branches of the tree were very short and showed
little support between groups (Spangler et al., 1999; Lukens &
Doebley, 2001; Mathews et al., 2002). For example, the position
of Elionurus muticus was unstable within the larger Coelorachis ⁄ Zea
clade (Fig. S2; data not shown). This supports the hypothesis of a
rapid radiation during the diversification of Andropogoneae
subtribes (Celarier, 1956; Spangler et al., 1999).
We used the best ML tree based on the combined dataset to
reconstruct the evolution of glume and inflorescence axis traits
within Andropogoneae based on ML (Fig. 2). In the case of
glume morphology, membranous to coriaceous glumes are
inferred as the ancestral state of the Andropogoneae. However,
hard glumes are inferred to have evolved between four to seven
times independently, followed by four to five independent secondary reductions in glume hardness (Fig. 2). Similar to maize,
the secondary reduction of glume hardness in P. huillensis, Urelytrum digitatum and the Hemarthria ⁄ Heteropholis clade was
strongly supported by both morphological and phylogenetic data;
it remains unclear whether coriaceous glumes were secondarily
derived in E. muticus and Chionachne koenigii.
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Membranous to coriaceous glumes
Hard glumes
Ambiguous
Andropogon eucomus
Schizachyrium scoparium
Andropogon perligulatus
Andropogon chinensis
Andropogon gayanus
Diheteropogon amplectans
Hyparrhenia hirta
Andropogon ternarius
Cymbopogon flexuosus
Dichanthium aristatum
Botriochloa odorata
Capillipedium parviflorum
Heteropogon contortus
Hackelochloa granularis
Sorghum bicolor
Saccharum officinarum
Miscanthus japonicus
Cleistachne sorghoides
Eulalia villosa
Microstegium nudum
Coix aquatica
Coix lacryma jobi
Sehima nervosum
Apluda mutica
Andropterm stolzii
Dimeria lawsonii
Ischaemum afrum
Eriochrysis pallida
Phacelurus digitatus
Chrysopogon fulvus
Chrysopogon gryllus
Tripsacum dactyloides
Zea mays ssp mays
Rhytachne rottboellioides
Urelytrum digitatum
Oxyrhachis gracillima
Elionurus muticus
Chionachne koenigii
Rottboellia aurita
Rottboellia selloana
Phacelurus huillensis
Coelorachis afraurita
Coelorachis selloana
Coelorachis lepidura
Hemarthria compressa
Hemarthria altissima
Heteropholis sulcata
Arundinella hirta
Homolepis glutinosa
Ichnanthus pallens
Axonopus fissifolius
Leptocoryphium lanatum
Pennisetum alopecuroides
Flat to terete internodes
Invaginated internodes
Ambiguous
Fig. 2 Glume (left) and rachis internode (right) character state reconstructions within a morphologically variable monophyletic Andropogoneae clade.
Character states were reconstructed on the best maximum likelihood (ML) tree from the combined analysis inferred from a variable set of 312 ML trees.
Pie charts indicate the proportional likelihood that an ancestor had a particular character state, and are shown only when the probability of a character state
is inferred to have changed.
Similar analyses for inflorescence axis morphology reconstructed the ancestor of Andropogoneae as having flat to terete internodes, with four to six transitions to invaginated internodes
across the phylogeny (Fig. 2). Morphological data clearly support
the independent loss of rachis internode invagination in maize
and E. muticus. However, topological uncertainty makes it
equally likely that flat internodes are ancestral or, instead, evolved
secondarily in E. muticus.
TGA1-like protein expression
In order to test whether TGA1-like proteins have an ancestral
role in spikelet ⁄ flower development, and whether changes in
their expression correlate with changes in glume hardening and
invagination of the inflorescence axis, we compared expression
patterns across representative grasses and a closely related outgroup (J. ascendens) that vary in inflorescence morphology
(Figs 3, 4). Expression patterns were consistent with an ancestral
role for TGA1-like genes in spikelet development, but did not
support the hypothesis that modification of TGA1-like protein
expression underlies shifts in glume and rachis internode
morphology.
Joinvillea ascendens has conventional monocot flowers comprising outer tepals, inner tepals, stamens and a gynoecium (Preston
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et al., 2009). Expression of JaTGA1 was evident in all organs of
the J. ascendens flower, but was absent from the subtending floral
bract and branch (Fig. 3a). Unlike J. ascendens and other grasses,
the early diverging species S. angustifolia does not have tepals and
lacks true spikelets and glumes. However, previous studies have
inferred bracts 1–5 to be transformationally homologous to
glumes, and bracts 6–8 to be transformationally homologous to
outer tepals and lemmas ⁄ paleas (Preston et al., 2009). In early
S. angustifolia flower development, SaTGA1 was detected in
bracts 1–11, but was absent from the inflorescence branch base,
presumed to be homologous to a bract subtending the floral unit
(Preston et al., 2009) (Fig. 3b). In mid- to late stages of inflorescence development, SaTGA1 was no longer detectable in bracts
1–6, but was strongly expressed in bracts 7–11 and reproductive
structures (Fig. 3c).
In grasses with true spikelets, TGA1-like protein was detected
in both outer and inner glumes, and within all floret organs at
early stages of development, regardless of glume and inflorescence
axis morphology (Fig. 3). In the pedicellate spikelets of A. strigosa,
AsTGA1 was expressed in the membranous glumes, and in all
organs of the bisexual proximal florets and reduced distal florets
(Fig. 3d). Likewise, in mid-stage development of E. indica and
S. viridis spikelets, EiTGA1 (Fig. 3e) and SvTGA1 (Fig. 3f) were
expressed in all floral organs. However, at this stage, very little
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(a)
(f)
(j)
(b)
(c)
(g)
(k)
(d)
(i)
(h)
(l)
(e)
(m)
(n)
Fig. 3 TGA1-like protein immunolocalization in early- to mid-stage spikelets and flowers of grasses and a grass outgroup. (a) Typical monocot flower of
the nongrass outgroup, Joinvillea ascendens, with associated floral bracts and branches. JaTGA1 expression is confined to the floral organs. (b, c) Spikelet
equivalents of Streptochaeta angustifolia; SaTGA1 is expressed in floral bracts 1–9 (numbered) and in the floral meristem during early development (b),
and in floral bracts 7–11, stamens and the emerging gynoecium at the mid-stage of development (c). (d) AsTGA1 is expressed in all floret organs and
glumes of Avena strigosa. (e) EiTGA1 is expressed in all floret organs, but not glumes, during mid-stage Eleusine indica spikelet development. (f) SvTGA1 is
expressed in all floret organs, but not glumes, during mid-stage Setaria viridis spikelet development. (g) Immature spikelet of Coix lacryma-jobi; ClTGA1 is
detectable in all organs of developing spikelets. (h, i) SbTGA1 is expressed in all organs of both pedicellate and sessile spikelets at early to mid-stages of
Sorghum bicolor development. (j) TdTGA1 is expressed in all floral organs and glumes of female Tripsacum dactyloides spikelet pairs. (k, l) Female (k) and
male (l) inflorescences of maize show the same pattern of TGA1 ⁄ NOT1 expression in all spikelet organs and in the rachis wall. (m, n) Male (m) and female
(n) inflorescences of teosinte show the same pattern of TGA1 ⁄ NOT1 expression as maize. fb, floral bract; fm, floral meristem; g1, outer glume; g2, inner
glume; gy, gynoecium; it, inner tepal; le, lemma; ot, outer tepal; pa; palea; ps, pedicellate spikelet; ss, sessile spikelet; st, stamen.
protein was detectable in the membranous subtending glumes of
both species.
Within Andropogoneae, all sampled species, with the exception of S. bicolor, are monoecious. In S. bicolor, SbTGA1 was
expressed in all organs of both the bisexual sessile spikelets that
have very hard glumes, and the male ⁄ sterile pedicellate spikelets
that have coriaceous glumes, at early to mid-stages of spikelet
development (Fig. 3h,i). In C. lacryma-jobi, ClTGA1 was
expressed in the coriaceous glumes and floral organs of male and
female C. lacryma-jobi inflorescences (Fig. 3g). Similarly,
TdTGA1 protein was detectable in hard glumes and all floral
organs of young T. dactyloides female spikelets (Fig. 3j). Finally,
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cumulative TGA1 ⁄ NOT1 expression was evident in young to
mid-stage glumes, inflorescences axes and all floral organs of both
female and male maize (Fig. 3k,l) and teosinte (Fig. 3m,n)
spikelets.
In contrast with early- to mid-stage development, late-stage
patterns of protein expression differed markedly among
T. dactyloides, teosinte and maize (Fig. 4). In late-stage spikelet
development of T. dactyloides, little TdTGA1 protein was
detected in the glumes of male (Fig. 4a) or female (Fig. 4b)
spikelets, or within maturing floral organs of female spikelets
(Fig. 4b); expression was still detectable in late-stage male flowers
in the lodicules and stamens (Fig. 4a). Furthermore, at both early
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(a)
(d)
(f)
(c)
(b)
(e)
(g)
(h)
Fig. 4 TGA1-like protein immunolocalization in late-stage spikelet development. (a) In male spikelets of Tripsacum dactyloides, TdTGA1 is only expressed
in the anthers and lodicules. (b) In silk-stage T. dactyloides female spikelets, TdTGA1 is expressed at very low levels, if at all, in glumes, florets and the
rachis. (c–e) In silk-stage (c, e) and fruit-stage (d) spikelets of teosinte ears, TGA1 ⁄ NOT1 is abundantly expressed in the abaxial cells of glumes, is undetectable in the gynoecia and is strongly expressed in the outer epidermal layers of the rachis. (f, g) Silk-stage spikelets of maize ears; TGA1 ⁄ NOT1 is undetectable in the upper and lower florets, but is moderately to weakly expressed in the abaxial cells of the rachis wall and glumes, respectively. (h) No staining is
detectable in teosinte ears in control experiments lacking the TGA1 ⁄ NOT1 antibody. g1, outer glume; g2, inner glume; gyn, gynoecium.
and late stages of development, TdTGA1 protein expression was
barely detectable in the rachis (Figs 3j, 4a,b). In silk-stage ears of
maize, TGA1 ⁄ NOT1 protein was expressed within the wall of
the invaginated rachis, but was only weakly detectable in glumes,
and was undetectable in gynoecia (Fig. 4f,g). By contrast,
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although only expressed at low levels in gynoecia, lodicules, paleas and lemmas, TGA1 ⁄ NOT1 in silk-stage teosinte ears was
strongly expressed in the small cells of the abaxial mesophyll of
glumes and the rachis (Fig. 4c–e). This pattern of teosinte
TGA1 ⁄ NOT1 expression was maintained following fruit set. No
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staining was detectable in control sections lacking the TGA1 ⁄
NOT1 antibody (Fig. 4h) and the results were consistent
between three independent experiments.
parenchymal cells of the adaxial glume mesophyll and the rachis,
but was undetectable in the small cells of the abaxial glume mesophyll (Fig. 5e,f).
In order to differentiate between elevated tga1 vs not1 expression in teosinte relative to maize ears, an allele-specific expression
assay was carried out (Tables S3, S4). For tga1, the average
corrected ratio of the teosinte ⁄ maize allele expression was 1.06
with a standard deviation of 0.21 (Table S3). Thus, the expression of tga1 was not significantly different between teosinte and
maize (P = 0.122). By contrast, for not1, the average corrected
ratio of the teosinte ⁄ maize allele expression was 2.02 with a standard deviation of 1.23 (Table S4). Although the F1 values varied
substantially in allele-specific expression, on average, teosinte
not1 expression was significantly higher than maize not1 expression (P = 3.51 · 10)5).
tga1 ⁄ not1 mRNA expression in Zea
To determine whether tga1 ⁄ not1 expression is distinct from the
expression of the corresponding proteins, a single antisense
mRNA probe was used to assess the cumulative expression
pattern of tga1 ⁄ not1 mRNA in maize and teosinte. The expression of tga1 ⁄ not1 was similar for maize and teosinte ears at early
stages of development (Fig. 5a,d). Before floral differentiation,
tga1 ⁄ not1 mRNA was detected in the inner and outer glumes,
and within the spikelet and floret meristems. At later stages,
tga1 ⁄ not1 was expressed throughout all floral organ primordia,
but was excluded from the expanded region at the base of the
outer glume (Fig. 5a,d).
At the silk stage, coincident with thickening of the glumes and
rachis invagination in teosinte, tga1 ⁄ not1 expression was markedly different between maize and teosinte (Fig. 5b,e,f). In maize,
tga1 ⁄ not1 was barely detectable in all well-developed spikelet
organs and in the invaginated rachis (Fig. 5b). However, in teosinte, tga1 ⁄ not1 was abundantly expressed in the large
(a)
(d)
Discussion
Several species of Andropogoneae develop hard indigestible structures that surround the growing fruit, potentially allowing them
to escape high levels of seed predation and to optimize seed dispersal (Wilkes, 1967). In some species, protection is afforded by
a hardened (indurate) lower glume that wraps around the
(b)
(e)
(c)
(f)
Fig. 5 mRNA in situ hybridization of tga1 ⁄ not1 in immature and silk-stage ears of maize and teosinte. (a) Immature female spikelets of maize; tga1 ⁄ not1
is expressed throughout all organs of the spikelet. (b) Silk-stage spikelet of maize; tga1 ⁄ not1 expression is low in glumes and inflorescence axes, and is
undetectable in floral organs. (c) Sense control of tga1 ⁄ not1 in silk-stage spikelets of maize showing little to no hybridization. (d) Immature female spikelets
of teosinte; tga1 ⁄ not1 is expressed in glume primordia and in the spikelet meristem. (e) Silk-stage spikelet of teosinte; tga1 ⁄ not1 is expressed in the large
parenchymal cells of the inflorescence axis and the adaxial side of the inner and outer glumes, but not in the gynoecium. (f) Rachis (Rach) and glumes of
teosinte ear; tga1 ⁄ not1 is strongly expressed in large, but not small, cells. g1, outer glume; g2, inner glume; gyn, gynoecium.
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developing fruit (e.g. Sorghum), whereas, in other species, the
fruit is shielded by both an indurate glume and a strongly invaginated (sunken) rachis internode (e.g. Tripsacum, Coelorachis and
teosinte) (Clayton & Renvoize, 1986; E. A. Kellogg, pers. obs.).
Together with previous studies, our morphological and phylogenetic analyses suggest that indurate glumes have evolved one to
three times in the clade containing Tripsacum and Heteropholis,
with at least four secondary transitions back to coriaceous glumes
(Urelytrum, P. huillensis, maize and Hemarthria sp.) (Spangler
et al., 1999; Lukens & Doebley, 2001; Mathews et al., 2002;
Watson & Dallwitz, 1992; Teerawatananon et al., 2011; this
study). Similar character reconstructions for invaginated rachis
internodes are less apparent. However, they clearly support at
least two independent origins of flat to terete internodes, once at
the base of Arundinella plus Andropogoneae and once at the base
of maize, and demonstrate that internode invagination and glume
hardness can be uncoupled (Fig. 2).
Evolution and function of TGA1-like genes across grasses
Morphological and phylogenetic evidence suggests that hardened
glumes and invaginated inflorescence axes have been gained and
lost more than once within the Andropogoneae. However, it is
unknown whether mutations in orthologous genes are responsible for the independent losses of these traits. In the case of maize,
mutations in tga1 are tightly associated with the evolution of
glumes that are membranous, at least apically, and flat inflorescence axes (Dorweiler et al., 1993; Wang et al., 2005). Furthermore, various lines of evidence suggest that the causative
mutation underlying the evolution of these traits is within the
coding region of tga1 (Wang et al., 2005). Analysis of the TGA1like gene from multiple species that vary in glume and rachis
morphology has revealed a conserved lysine at amino acid position six, as previously found in the TGA1-like gene of rice and
representative species of Zea (except maize) (Wang et al., 2005).
Similarly, the microRNA binding site is fully conserved in all
TGA1-like genes sampled, suggesting purifying selection and
conservation of the negative interaction between miR156 (Cg1)
and TGA1 (Chuck et al., 2007). Together, these findings suggest
that variation in TGA1-like amino acid sequences is not correlated with independent losses of hard glumes and invaginated
internodes within Andropogoneae. However, as we obtained only
partial sequences for many grass species, we cannot rule out the
possibility that changes in TGA1-like genes have been important
for morphological evolution in these species.
An alternative mechanism to explain the reduction in glume
hardness and invaginated internodes is evolution at the level of
TGA1-like gene regulation. This hypothesis predicts differences
in protein expression between species that vary in glume and
rachis internode morphology. Analyses of protein expression
across grasses outside of Zea that have membranous to coriaceous
glumes (S. angustifolia, A. strigosa, E. indica, S. viridis, C.
lacryma-jobi, pedicellate S. bicolor and male T. dactyloides), indurate glumes (sessile S. bicolor and female T. dactyloides), flat or
terete rachis internodes (S. angustifolia, A. strigosa, E. indica,
S. viridis and C. lacryma-jobi) and invaginated rachis internodes
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(T. dactyloides) revealed no consistent pattern between morphology and protein expression. Except for maize, this is consistent
with functional conservation for TGA1-like proteins across
grasses.
The fact that a single amino acid change can simultaneously
affect multiple traits suggests that tga1 is an upstream regulator
of the Z. mays inflorescence developmental pathway (Wang et al.,
2005), and that the development of Z. mays glumes and inflorescence axes is tightly coupled. However, the exact function of grass
TGA-like genes is unclear. Our data show that TGA1-like genes
have a conserved expression pattern in grass spikelets, being
expressed early in spikelet and floral meristems, and at mid-stages
of development in glumes and floret organ primordia. Furthermore, in the grass outgroup J. ascendens, which has more typical
monocot flowers, JaTGA1 is present in all developing floral
organs, but is not detectable in the subtending floral bract.
Together, these protein expression patterns are similar to the
combined expression patterns of FUL-like genes, MADS-box
transcription factors related to the floral meristem and floral
organ identity genes APETALA1 (AP1) and FUL of Arabidopsis
thaliana (Mandel et al., 1992; Mandel & Yanofsky, 1995;
Ferrándiz et al., 2000; Preston & Kellogg, 2007; Preston et al.,
2009). Indeed, in A. thaliana and Antirrhinum majus, tga1 (SPL)
homologs directly regulate FUL-like genes in leaves and shoot
apical meristems (Klein et al., 1996; Wang et al., 2009; Yamaguchi et al., 2009; Preston & Hileman, 2010). Because SPL protein
binding site sequences were found in all examined FUL1 and
FUL2 genes from across the grass family, we posit that the regulatory interaction between SPL and FUL-like genes is conserved
between core eudicots and grasses. As rice has 19 SPL proteins,
most or all of which are expressed in inflorescences (Xie et al.,
2006; Yang et al., 2008), future analyses of FUL-like gene expression in tga1-like silenced lines will be required to specifically test
the hypothesis of TGA1–FUL interaction.
TGA1 and domestication of the maize ear
Probably the most striking example of glume and rachis evolution is between the ears of maize and its ancestor teosinte.
Although teosinte has indurate glumes and strongly invaginated
inflorescence axes, reduced hardness and flat axes have been
selected for in maize glumes. Previous studies have revealed an
important role for tga1 in the domestication of maize (Dorweiler
et al., 1993; Wang et al., 2005). These morphological differences
were associated with six fixed differences in the promoter and one
amino acid difference in the protein-coding region of the SPL
gene tga1 (Wang et al., 2005).
Several lines of evidence suggest that the amino acid difference
in tga1 underlies the inflorescence differences between maize and
teosinte. First, the lysine of teosinte in this position has been conserved through grass evolution, including taxa as disparate as rice
(Ehrhartoideae), wheat (Pooideae) and R. aurita (Panicoideae)
(Wang et al., 2005; this study). Indeed, extensive sequencing
from multiple teosinte populations found this to be the only fixed
difference between maize and teosinte; the six promoter polymorphisms are variable within teosinte (Zhao, 2006). Second, a
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single nonconservative mutation in an amino acid adjacent to the
potentially causative amino acid of maize tga1 results in teosintelike glume and rachis internode structures (Wang et al., 2005).
Finally, the identification of a tga1 paralog (not1) explains the
apparent differences between the tga1 gene and TGA1 protein
expression patterns in maize and teosinte reported in Wang et al.
(2005). Specifically, quantitative (q)RT-PCR analyses previously
showed no difference in tga1 RNA levels between early- to silkstage maize and teosinte ears (Wang et al., 2005). By contrast, we
found a quantitative difference in RNA levels between maize and
teosinte lower glumes and rachis internodes by in situ hybridization (Wang et al., 2005). Protein levels were also different
between the two, as measured by western blots and immunolocalization (Wang et al., 2005). The qRT-PCR primers are specific
to tga1 alone, and therefore we conclude that gene expression is
indeed similar between maize and teosinte. However, the in situ
probe and antibody probably capture both tga1 ⁄ TGA1 and
not1 ⁄ NOT1 expression, so that elevated RNA and protein levels
in late-stage teosinte ears reflect higher expression of not1 ⁄ NOT1
rather than tga1 ⁄ TGA1. This interpretation is strongly supported
by our allele-specific expression data. We infer that the expression
of the tga1 gene and its protein product are thus quantitatively
similar between the two species. Therefore, our data do not support the hypothesis that differences in the teosinte and maize
glume and rachis are a result of variation in protein stability or
translational efficiency. Instead, we prefer the hypothesis that the
replacement of either residue five or six of TGA1 changes its biochemical function and causes the glume and rachis differences
between maize and teosinte.
Acknowledgements
We thank Sherry Flint-Garcia for providing flowering material of
teosinte, and Shelby Kleweis, Jimena Nores and Chris Gillespie
for help with sequencing. This research was funded by the
National Science Foundation grants DBI-0820619 (J.D.) and
DBI-0110189 (E.A.K.).
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Supporting Information
Additional supporting information may be found in the online
version of this article.
Fig. S1 Phylogenetic relationships of grass FUL-like genes and
number of associated SPL binding sequences in the putative promoters and introns.
Fig. S2 Andropogoneae species relationships based on combined
analyses of waxy, ndhF, phyB, tb1 and tga1-like genes.
Table S1 Species sampled for phylogenetic analyses of tga1 with
vouchers and GenBank accession numbers.
Table S2 Species sampled for combined phylogenetic analyses
with vouchers and GenBank accession numbers.
Table S3 tga1 allele-specific expression assay.
Table S4 not1 allele-specific expression assay.
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