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• W4210496524 abstract "Plants that bear flowers are called Angiosperms. With around 300 000species, they represent the largest and most successful group in the green kingdom. Flowers contain one or more female organs, or carpels, in their center. Carpels enclose the ovules, and later the seeds. Stamens, the male pollen-bearing organs, surround the carpels. Sexual organs are in turn surrounded by vegetative organs (the perianth) usually of two types — petals to attract pollinators, and sepals to protect all other organs in the bud. In flowering plants, each shoot carries a cluster of undifferentiated pluripotent cells at its growing tip called a meristem. As the shoot grows, the meristem generates the stalk that supports it, and a succession of leaves on its flanks. Eventually the shoot produces flowers, at which stage it is called an inflorescence. Flowers develop where bracts (small leaf-like organs) join the stem, or directly on the flanks of the shoot meristem. They arise as small meristems that are similar in many ways to shoot meristems. However, instead of producing leaves on their flanks, they produce floral organs which arise as undifferentiated primordia that grow and differentiate into one of the four types of mature organ. Each organ type arises in defined positions and numbers that are relatively constant within the flowers of related species. As recognised by Linnaeus, this constancy means that floral organ numbers can be used as key characters in plant classification. In the last decade we have begun to understand how gene products control the mechanisms of flower development. Success has depended upon the use of representative model species with convenient molecular genetics. These species span the range of Angiosperm diversity, and include the grasses maize and rice, the weedy mustard species Arabidopsis thaliana, the decorative Petunia hybrida and the edible tomato (both members of the tobacco family), and the snapdragon Antirrhinum majus (Fig. 1). In the development of flowers, the best known genetic functions are those that specify floral organ identity (Table 1). These functions are disrupted in homeotic mutants, where organs develop an identity inappropriate for their position. For example, in several such mutants the organs in petal positions develop instead as sepals. In the same mutants, organs arising in stamen positions are carpel-like. In fact, two adjacent organ types are affected in each mutant category. Based on such patterns, and the outcomes of combining different mutant genes in the one plant, three organ identity functions have been recognised. For convenience, these have been called A, B and C functions.Table 1Genes that control early steps in flower developmentFlower development processArabidopsis (Antirrhinum) genes*Arabidopsis and Antirrhinum genes in the same line are orthologs.Function†Unless otherwise noted, all genes encode transcription factors of the type specified.Vegetative and floralMeristem maintenanceCLAVATA1, 2ReceptorCLAVATA3?LigandWUSCHELHDRepression of primordium initiationSHOOT MERISTEMLESSHDPromotion of primordium growthAINTEGUMENTAAP2Separation of adjacent organsCUP-SHAPED COTYLEDONS1, 2?, NACOrgan polarityAbaxialFILAMENTOUS FLOWER, YABBY2, 3YABBYPHABULOSA?AdaxialPINHEAD/ZWILLE, ARGONAUTETranslation(PHANTASTICA)MYBFloral specificActivation of organ identity genesLEAFY (FLORICAULA) UNUSUAL FLORALLFYORGANS (FIMBRIATA)Protein degradationOrgan identityA functionAPETALA2AP2APETALA1 (SQUAMOSA)MADSB functionAPETALA3 (DEFICIENS)MADSPISTILLATA (GLOBOSA)MADSC functionAGAMOUS (PLENA)MADSFloral identity (for B, C function)SEPALLATA1, 2, 3MADSEstablishment of floral ground planPERIANTHIAbZIPOrgan separation between whorlsSUPERMANZnFFloral asymmetry(CYCLOIDEA, DICHOTOMA)TCP* Arabidopsis and Antirrhinum genes in the same line are orthologs.† Unless otherwise noted, all genes encode transcription factors of the type specified. Open table in a new tab Sepal identity depends upon a proposed A function, petal identity upon A and B functions combined, stamens arise under the joint influence of B and C functions, while carpels are the outcome of C function. Furthermore, A and C functions are mutually antagonistic. A function represses C function within the perianth, whereas C function excludes A function from the reproductive organ zone. The ABC model envisaged three overlapping concentric fields of function within the newly developing flower primordium, acting in combination to set in train the developmental program appropriate to each organ's spatial position. Once representative ABC genes were cloned, their expression patterns were, in most cases, remarkably close to the prediction of concentric overlapping fields. Furthermore, they were found to function as regulators of transcription of other genes. Most ABC genes encode transcription factors that carry a MADS box DNA binding domain. They function as dimers, and higher order interactions have also been observed. Despite a sustained search, very few genes that are direct targets of ABC functions have been identified. On the other hand, in Arabidopsis at least, several ABC genes are now known to be activated directly by the upstream flowering protein LEAFY, or indirectly by the UNUSUAL FLORAL ORGANS protein. The ABC group of functions was first identified in Arabidopsis and Antirrhinum, but their roles (with some variations) seem likely to extend to all Angiosperms. Despite its attractive simplicity, the ABC model requires several qualifications. Firstly, A function has not been well defined except in Arabidopsis. Secondly, all ABC mutants disrupt aspects of organ growth as well as organ identity. In B function mutants, for example, reduced growth means that the converted stamens (now carpels) are usually joined with the normal carpels, or the latter may even be absent. On the other hand, in C function mutants, growth of the flower meristem does not stop, and a new flower meristem arises where carpels would normally appear. Recently, three additional MADS box genes (SEPALLATA1–3) have been shown to act redundantly to set the stage for B and C function in the Arabidopsis flower primordium. In sepallata triple mutants, all floral organs develop as sepals. Orthologous genes have been identified in tomato and Petunia, and in Antirrhinum where they have been called ‘identity mediating’ genes. In an exciting recent development, two research groups (Honma and Goto, 2001, and one reported in this issue, Pelaz et al., 2001) have shown that expression of one of these genes, SEPALLATA3, in developing leaves of Arabidopsis, along with A and B, or B and C genes, is sufficient to turn the leaves into floral organs. For example, the ‘leaves’ of A+B+SEP3 plants look much like petals. We can conclude from these results that the SEPALLATA genes provide the floral context that is required for B and C function genes to work. Furthermore, the ability to switch leaves down the petal or stamen developmental pathway by adding the function of just three or four genes elegantly shows how petals and stamens might have evolved from leaf-like organs. Genes must also be involved in defining the blueprint (or bauplan) of the developing flower — the number and relative positions of floral organ primordia. We know much less about these. We do know that a suite of genes defines properties of the shoot meristem, and these have parallel functions within the flower meristem. In Arabidopsis, the balance between proliferation of the central stem cells and the peripheral cells (where primordia arise) is controlled by three CLAVATA putative signal transduction proteins together with the homeodomain transcription factor WUSCHEL. In clavata mutants, more floral organs arise than normal whereas in wuschel mutants there are fewer. The initiation of primordia in general is apparently repressed by another homeodomain protein SHOOT MERISTEMLESS, and the general growth of all primordia seems to be supported by AINTEGUMENTA. Finally, boundaries between radially adjacent primordia are apparently defined by the action of two CUP-SHAPED COTYLEDON genes. Organs may be fused when these genes are mutated, and their normal role may be to suppress growth, generating gaps between individual primordia. Several genes seem to play a more specific role in establishing the floral ground plan. The PERIANTHIA gene in Arabidopsis may be involved in defining perianth organ number since perianthia mutant flowers frequently have five sepals and five petals (i.e. they are pentamerous) rather than the usual four plus four (tetramerous). The gene encodes a basic zipper transcription factor, but its targets in this process are unknown. The SUPERMAN gene encodes a zinc finger protein which seems to be involved in establishing the boundary between stamens and carpels in Arabidopsis. In superman mutants, stamen primordia proliferate inwards at the expense of carpel primordia. Whereas most plant species have bisexual flowers, 10% have separate male and female flowers, present either on the same plant (known as monoecy, e.g. maize) or on different plants (dioecy, e.g. date palm). Do flowers of different sex within a species have different ground plans? In most cases it seems not, because male and female flowers are indistinguishable early in development. Maleness only becomes apparent as newly arising carpels abort, and femaleness when stamen primordia abort. In maize, a gene controlling carpel loss is involved in the metabolism of a steroid-like molecule, whereas one controlling abortion of stamens is associated with gibberellin biosynthesis. Thus abortion may be induced by specific chemicals, although the sporadic occurrence of unisexual flowers across families suggests that unisexuality has arisen many times, not necessarily involving the same mechanism. Many flowers are radially symmetrical (e.g. Arabidopsis and tomato in Fig. 1). In more advanced taxa, however, flowers are frequently bilaterally symmetrical. In Antirrhinum (see Fig. 1), this asymmetry has been attributed to the action of at least two genes, CYCLOIDEA and DICHOTOMA. These related genes encode transcription factors that are expressed in the region closest to the flowering stem in the young flower primordium. It seems that they inhibit growth in this region, so that petals and stamens that arise here are morphologically different from those that arise in uninhibited regions. Another form of asymmetry seen in some flowers is chirality. Organs, especially petals, may be folded within the bud in either a right-handed or a left-handed spiral. When this is present in rosids (e.g. Hibiscus), the direction is usually not fixed, even within an individual plant, and seems to arise largely due to chance. However, when present in the more advanced asterids (e.g. Convulvulus), all flowers on all plants within a species spiral the same way. The direction of the spiral is genetically controlled and will be transmitted from generation to generation. Different types of asymmetry, or polarity, occur within floral organs. For example, genes have recently been uncovered that seem to regulate which side of an organ is which. In mutants of genes that control the identity of the inner side (i.e. the adaxial side, that closest to the center of the flower), cells with this identity fail to arise. Genes controlling the outer (abaxial) side show the reciprocal loss. An associated change is that the affected organs are usually rod-shaped rather than flattened, indicating that organ polarity is intimately linked with lateral outgrowth. Leaves as well as floral organs are affected, again highlighting their likely common origin. Many flower development genes, mostly encoding transcription factors, have been discovered in the last decade. But we still do not know much about the genes they regulate, especially ‘realisator’ genes that control organ differentiation. There are also gaps in our understanding about how genes control the sites of organ initiation, and how different regions within developing flowers communicate. Now that we have available a dictionary of all plant genes, at least for Arabidopsis, we can dissect the roles of all members of gene families. In many cases these functions have not been uncovered using mutant screens because of redundancy within the family. Comparative studies across the phylogenetic spectrum are an exciting prospect and will soon allow us to deduce when individual gene functions arose, and how they have changed during the 130million years of flowering plant evolution. I thank Stan Alvarez and Marcus Heisler for discussions and comments on the manuscript." @default.
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• W4210496524 date "2001-02-01" @default.
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• W4210496524 title "Flower development" @default.
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