The ABC model and the diversification of floral organ identity

Litt, Amy; Kramer, Elena M.

doi:10.1016/j.semcdb.2009.11.019

Cited by 169 publications

(147 citation statements)

References 97 publications

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“…2). We therefore focused our analysis on these loci because the Band C-class genes are the most conservative elements of the floral organ identity module in core eudicots (17). Thus, they have greater predictive power for interpreting floral organ homology.…”

Section: Resultsmentioning

confidence: 99%

“…Under these circumstances, only multiple independent lines of inquiry can be used to understand the floral structure and evolution of these charismatic plants, including comparative structural, developmental, and gene-expression data. In terms of this last source of data, the ABC model of floral development provides a predictive framework for understanding the identities of the floral organs of Rafflesiaceae (17,18). The model involves three gene activities: A activity alone specifies sepal identity, A+ B define petals, B+C confer stamen identity, and C determines carpel identity.…”

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confidence: 99%

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Developmental origins of the world’s largest flowers, Rafflesiaceae

Nikolov

Endress

Sugumaran

et al. 2013

Proc. Natl. Acad. Sci. U.S.A.

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Rafflesiaceae, which produce the world's largest flowers, have captivated the attention of biologists for nearly two centuries. Despite their fame, however, the developmental nature of the floral organs in these giants has remained a mystery. Most members of the family have a large floral chamber defined by a diaphragm. The diaphragm encloses the reproductive organs where pollination by carrion flies occurs. In lieu of a functional genetic system to investigate floral development in these highly specialized holoparasites, we used comparative studies of structure, development, and gene-expression patterns to investigate the homology of their floral organs. Our results surprisingly demonstrate that the otherwise similar floral chambers in two Rafflesiaceae subclades, Rafflesia and Sapria, are constructed very differently. In Rafflesia, the diaphragm is derived from the petal whorl. In contrast, in Sapria it is derived from elaboration of a unique ring structure located between the perianth and the stamen whorl, which, although developed to varying degrees among the genera, appears to be a synapomorphy of the Rafflesiaceae. Thus, the characteristic features that define the floral chamber in these closely related genera are not homologous. These differences refute the prevailing hypothesis that similarities between Sapria and Rafflesia are ancestral in the family. Instead, our data indicate that Rafflesia-like and Sapria-like floral chambers represent two distinct derivations of this morphology. The developmental repatterning we identified in Rafflesia, in particular, may have provided architectural reinforcement, which permitted the explosive growth in floral diameter that has arisen secondarily within this subclade.ABC model | comparative gene expression | evo-devo | gigantism | parasitic plants

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Section: Resultsmentioning

confidence: 99%

mentioning

confidence: 99%

Developmental origins of the world’s largest flowers, Rafflesiaceae

Nikolov

Endress

Sugumaran

et al. 2013

Proc. Natl. Acad. Sci. U.S.A.

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“…While the model is not strictly conserved across all angiosperms (Litt, 2007;Litt and Kramer, 2010), it provides a useful baseline against which to gauge morphological divergence, as is present in maize. According to the model, A function alone specifies sepal identity in the first (outermost) floral whorl.…”

Section: Introductionmentioning

confidence: 99%

The Maize PI/GLO Ortholog Zmm16/sterile tassel silky ear1 Interacts with the Zygomorphy and Sex Determination Pathways in Flower Development

et al. 2015

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In monocots and eudicots, B class function specifies second and third whorl floral organ identity as described in the classic ABCE model. Grass B class APETALA3/DEFICIENS orthologs have been functionally characterized; here, we describe the positional cloning and characterization of a maize (Zea mays) PISTILLATA/GLOBOSA ortholog Zea mays mads16 (Zmm16)/ sterile tassel silky ear1 (sts1). We show that, similar to many eudicots, all the maize B class proteins bind DNA as obligate heterodimers and positively regulate their own expression. However, sts1 mutants have novel phenotypes that provide insight into two derived aspects of maize flower development: carpel abortion and floral asymmetry. Specifically, we show that carpel abortion acts downstream of organ identity and requires the growth-promoting factor grassy tillers1 and that the maize B class genes are expressed asymmetrically, likely in response to zygomorphy of grass floral primordia. Further investigation reveals that floral phyllotactic patterning is also zygomorphic, suggesting significant mechanistic differences with the wellcharacterized models of floral polarity. These unexpected results show that despite extensive study of B class gene functions in diverse flowering plants, novel insights can be gained from careful investigation of homeotic mutants outside the core eudicot model species.

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“…Each of these gene classes is involved in the combinatorial determination of the identity of different floral organs: A and E genes specify sepals (Ditta et al, 2004); genes A, B and E determine petals; the combination of genes B, C and E specify stamens (male reproductive organs); genes C and E determine carpels (female reproductive organs); and genes D and E determine ovules (reviewed in : Theißen, 2001; Krizek and Fletcher, 2005;Theißen and Melzer, 2007). The conservation of this system across major plant groups indicates that floral homeotic genes constitute an ancient regulatory network for most angiosperms (reviewed in: Rijpkema et al, 2010;Litt and Kramer, 2010). As the different functional classes of MADS-box genes are members of distinct clades (reviewed in: Becker and Theißen, 2003), studying the expression and function of the genes in the comparative context of their phylogeny has been important for testing hypotheses on the evolution of flower morphology.…”

Section: Introductionmentioning

confidence: 99%

Conserved differential expression of paralogous DEFICIENS‐ and GLOBOSA‐like MADS‐box genes in the flowers of Orchidaceae: refining the ‘orchid code’

2011

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SUMMARYIn flowering plants, class-B floral homeotic genes encode MADS-domain transcription factors, which are key in the specification of petal and stamen identity, and have two ancient clades: DEF-like and GLO-like genes. Many species have one gene of each clade, but orchids have typically four DEF-like genes, representing ancient gene clades 1, 2, 3 and 4. We tested the 'orchid code', a combinatorial genetic model suggesting that differences between the organs of the orchid perianth (outer tepals, inner lateral tepals and labellum) are generated by the combinatorial differential expression of four DEF-like genes. Our experimental test involves highly sensitive and specific measurements, with qRT-PCR of the expression of DEF-and GLO-like genes from the distantly related Vanilla planifolia and Phragmipedium longifolium, as well as from wild-type and peloric Phalaenopsis hybrid flowers. Our findings support the first 'orchid code' hypothesis, in that absence of clade-3 and -4 gene expression distinguishes the outer tepals from the inner tepals. In contrast to the original hypothesis, however, mRNA of both clade-3 and -4 genes accumulates in wild-type inner lateral tepals and the labellum, and in labellum-like inner lateral tepals of peloric flowers, albeit in different quantities. Our data suggest a revised hypothesis where high levels of clade-1 and -2, and low levels of clade-3 and -4, gene expression specify inner lateral tepals, whereas labellum development requires low levels of clade-1 and -2 expression and high levels of clade-3 and -4 expression.

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The ABC model and the diversification of floral organ identity

Cited by 169 publications

References 97 publications

Developmental origins of the world’s largest flowers, Rafflesiaceae

Developmental origins of the world’s largest flowers, Rafflesiaceae

The Maize PI/GLO Ortholog Zmm16/sterile tassel silky ear1 Interacts with the Zygomorphy and Sex Determination Pathways in Flower Development

Conserved differential expression of paralogous DEFICIENS‐ and GLOBOSA‐like MADS‐box genes in the flowers of Orchidaceae: refining the ‘orchid code’

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