Homeotic Genes and the ABCDE Model for Floral Organ Formation in Wheat - PubMed (original) (raw)

Review

Homeotic Genes and the ABCDE Model for Floral Organ Formation in Wheat

Koji Murai. Plants (Basel). 2013.

Abstract

Floral organ formation has been the subject of intensive study for over 20 years, particularly in the model dicot species Arabidopsis thaliana. These studies have led to the establishment of a general model for the development of floral organs in higher plants, the so-called ABCDE model, in which floral whorl-specific combinations of class A, B, C, D, or E genes specify floral organ identity. In Arabidopsis, class A, B, C, D, E genes encode MADS-box transcription factors except for the class A gene APETALA2. Mutation of these genes induces floral organ homeosis. In this review, I focus on the roles of these homeotic genes in bread wheat (Triticum aestivum), particularly with respect to the ABCDE model. Pistillody, the homeotic transformation of stamens into pistil-like structures, occurs in cytoplasmic substitution (alloplasmic) wheat lines that have the cytoplasm of the related wild species Aegilops crassa. This phenomenon is a valuable tool for analysis of the wheat ABCDE model. Using an alloplasmic line, the wheat ortholog of DROOPING LEAF (TaDL), a member of the YABBY gene family, has been shown to regulate pistil specification. Here, I describe the current understanding of the ABCDE model for floral organ formation in wheat.

Keywords: ABCDE model; MADS-box gene; floral organ; homeotic gene; pistillody; wheat.

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Figures

Figure 1

Figure 1

Wheat inflorescences and floral organs. (a) Developing young spike at the floret differentiation stage. The spikelet primordium (Spp) is indicated. Scale bar = 1 mm; (b) The wheat inflorescence (spike, ear, or head) is composed of spikelets (Sp) attached at the nodes of a zigzag rachis (Rs). Scale bar = 2 cm; (c) A spikelet that has been removed from the rachis. The spikelet consists of multiple (usually six to eight) florets attached at the rachilla (Ra). Two small bract leaves called glumes (Gl) enclose the spikelet. Scale bar = 1 cm; (d) A magnified image of an opened floret. In the floret, the reproductive organs, pistil (Pi) and stamens (St) are enveloped by two leaf-like structures, the lemma (Le) and the palea (Pa). The lemma and palea have been separated to make the reproductive organs visible in the figure. Scale bar = 2 mm; (e) An individual flower containing one pistil (Pi), three stamens (St) and two lodicules (Lo). The palea (Pa) is also indicated. In this figure, the pistil, stamens, lodicules and palea have been removed from the rachilla. Scale bar = 2 mm; (f) A flower from a plant of the pistillody line. The stamens are transformed into pistil-like structure (Pst) with stigmas. Scale bar = 2 mm.

Figure 2

Figure 2

Schematic illustrations of the phytomeric structures of the wheat inflorescence. The spikelets are arranged as two opposite rows of lateral branches from the main axis (rachis). Each spikelet is composed of florets joined at the axis (rachilla) alternately on opposite sides, and enclosed by two glumes. Each floret is composed of a lemma, a palea, two lodicules, three stamens and a pistil. gl, glume; le, lemma; pa, palea; lo, lodicule; st, stamen; pi, pistil.

Figure 3

Figure 3

The ABCDE model of floral organ formation in wheat. In contrast to the Arabidopsis ABCDE model, the wheat ABCDE model involves duplicated genes for class B (_PI_-like) and class C (_AG_-like) functions. Furthermore, class E genes are divided into two groups, WSEP and WLHS1, with sub-functionalization. A YABBY gene TaDL specifies the pistil (carpel) identity. The pistillody line has been valuable for constructing the ABCDE model in wheat. The wheat ABCDE model is similar to that of rice except for the class A genes. The current wheat ABCDE model indicates that class B and TaDL proteins show mutual suppression, which was suggested from analysis of a pistillody line. The mutual suppression between class A and C genes is also postulated here. The wheat ABCDE model probably functions through complex homoeologous gene interactions.

References

    1. Rijpkema A.S., Vandenbussche M., Koes R., Heijmans K., Gerats T. Variations on a theme: Changes in the floral ABCs in angiosperms. Semin. Cell Dev. Biol. 2010;21:100–107. doi: 10.1016/j.semcdb.2009.11.002. -DOI -PubMed
    1. Smaczniak C., Immink R.G.H., Angenent G.C., Kaufmann K. Developmental and evolutionary diversity of plant MADS-domain factors: Insights from recent studies. Development. 2012;139:3081–3098. doi: 10.1242/dev.074674. -DOI -PubMed
    1. Jofuku K.D., den Boer B.G., Montagu M.V., Okamuro J.K. Control of Arabidopsis flower and seed development by the homeotic gene APETALA2. Plant Cell. 1994;6:1211–1225. -PMC -PubMed
    1. Mandel M.A., Gustafson-Brown C., Savidge B., Yanofsky M.F. Molecular characterization of the Arabidopsis floral homeotic gene APETALA1. Nature. 1992;360:273–277. doi: 10.1038/360273a0. -DOI -PubMed
    1. Jack T., Brockman L.L., Meyerowitz E.M. The homeotic gene APETALA3 of Arabidopsis thaliana encodes a MADS box and is expressed in petals and stamens. Cell. 1992;68:683–697. doi: 10.1016/0092-8674(92)90144-2. -DOI -PubMed

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