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ABC model of flower development

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ABC model of flower development

A diagram illustrating the ABC model in Arabidopsis. Class A genes (blue) affect sepals and petals, class B genes (yellow) affect petals and stamens, class C genes (red) affect stamens and carpels. In two specific whorls of the floral meristem, each class of organ identity genes is switched on.
A diagram illustrating the ABC model. Class A genes affect sepals and petals, class B genes affect petals and stamens, class C genes affect stamens and carpels. In two specific whorls of the floral meristem, each class of organ identity genes is switched on.

Flower development is the process by which modelled using the ABC model, which endeavours to describe the biological basis of the process from the perspective of molecular and developmental genetics.

An external phyllotaxis, that is, the absence of stem elongation among the successive whorls or verticils of the primordium. These verticils follow an acropetal development, giving rise to sepals, petals, stamens and carpels. Another difference from vegetative axillary meristems is that the floral meristem is «determined», which means that, once differentiated, its cells will no longer divide.[1]

The identity of the organs present in the four floral verticils is a consequence of the interaction of at least three types of gene products, each with distinct functions. According to the ABC model, functions A and C are required in order to determine the identity of the verticils of the perianth and the reproductive verticils, respectively. These functions are exclusive and the absence of one of them means that the other will determine the identity of all the floral verticils. The B function allows the differentiation of petals from sepals in the secondary verticil, as well as the differentiation of the stamen from the carpel on the tertiary verticil.

Goethe’s «foliar theory» was formulated in the 18th century and it suggests that the constituent parts of a flower are structurally modified leaves, which are functionally specialized for reproduction or protection. The theory was first published in 1790 in the essay "Metamorphosis of Plants" ("Versuch die Metamorphose der Pflanzen zu erklaren").[2] where Goethe wrote:

"...we may equally well say that a stamen is a contracted petal, as that a petal is a stamen in a state of expansion; or that a sepal is a contracted stem leaf approaching a certain stage of refinement, as that a stem leaf is a sepal expanded by the influx of cruder saps".[3]

Contents

  • Floral transition 1
  • Formation of the floral meristem or the inflorescence 2
  • Floral architecture 3
    • The ABC model 3.1
    • Genetic analysis 3.2
      • Analysis of mutants 3.2.1
      • Techniques for detecting differential expression 3.2.2
    • Genes exhibiting type-A function 3.3
    • Genes exhibiting type-B function 3.4
    • Genes exhibiting type-C function 3.5
    • Genes exhibiting type-D and E functions 3.6
  • See also 4
  • References 5
  • Sources 6
    • General texts 6.1
  • External links 7

Floral transition

The transition from the vegetative phase to a reproductive phase involves a dramatic change in the plant’s vital cycle, perhaps the most important one, as the process must be carried out correctly in order to guarantee that the plant produces descendents. This transition is characterised by the induction and development of the meristem of the inflorescence, which will produce a collection of flowers or one flower, where only one is produced. This morphogenetic change contains both endogenous and exogenous elements: For example, in order for the change to be initiated the plant must have a certain number of leaves and contain a certain level of total biomass. Certain environmental conditions are also required such as a characteristic photoperiod. Plant hormones play an important part in the process, with the gibberellins having a particularly important role.[4]

There are many signals that regulate the molecular biology of the process. However, it is worth noting the role of the following three genes in Arabidopsis thaliana: FLOWERING LOCUS T (FT), LEAFY (LFY), SUPPRESOR OF OVEREXPRESSION OF CONSTANS1 (SOC1, also called AGAMOUS-LIKE20).[5] These genes possess both common and independent functions in floral transition. SOC1 is a MADS-box-type gene, which integrates responses to photoperiod, vernalization and gibberellins.[4]

Formation of the floral meristem or the inflorescence

The

  • Genes controlling flower development in plants
  • Flower Development

External links

  • Soltis, DE; Soltis, PS; Leebens-Mack, J, eds. (2006). Advances in botanical research: Developmental genetics of the flower. New York, NY: Academic Press.  
  • Wolpert, Lewis; Beddington, R.; Jessell, T.; Lawrence, P.; Meyerowitz, E.; Smith, W. (2002). Principles of Development (Second ed.). Oxford: Oxford University Press.  

General texts

Sources

  1. ^ Azcón-Bieto; et al. (2000). Fundamentos de fisiología vegetal. McGraw-Hill/Interamericana de España, SAU.  
  2. ^ Dornelas, Marcelo Carnier; Dornelas, Odair (2005). "From leaf to flower: Revisiting Goethe's concepts on the ¨metamorphosis¨ of plants". Brazilian Journal of Plant Physiology 17 (4).  
  3. ^ Goethe J.W. von (1790) Versuch die Metamorphose der Pflanzen zu erklaren. Gotha, Ettlinger; paragraph 120."
  4. ^ a b Blazquez, MA; Green, R; Nilsson, O; Sussman, MR; Weigel, D (1998). "Gibberellins promote flowering of arabidopsis by activating the LEAFY promoter". The Plant cell 10 (5): 791–800.  
  5. ^ Blázquez, Miguel A.; Weigel, Detlef (2000). "Integration of floral inductive signals in Arabidopsis". Nature 404 (6780): 889–92.  
  6. ^ Brand, U.; Fletcher, JC; Hobe, M; Meyerowitz, EM; Simon, R (2000). "Dependence of Stem Cell Fate in Arabidopsis on a Feedback Loop Regulated by CLV3 Activity". Science 289 (5479): 617–9.  
  7. ^ Lenhard, Michael; Jürgens, Gerd; Laux, Thomas (2002). "The WUSCHEL and SHOOTMERISTEMLESS genes fulfil complementary roles in Arabidopsis shoot meristem regulation". Development (Cambridge, England) 129 (13): 3195–206.  
  8. ^ a b Taiz and Zeiger (2002). Plant physiology. Sinauer associates.  
  9. ^ Haughn, George W.; Somerville, Chris R. (1988). "Genetic control of morphogenesis in Arabidopsis". Developmental Genetics 9 (2): 73.  
  10. ^ a b "Expression of the Arabidopsis floral homeotic gene AGAMOUS is restricted to specific cell types late in flower development" 3 (8). August 1991. pp. 749–58.  
  11. ^ Somerville, C.; Somerville, S (1999). "Plant Functional Genomics". Science 285 (5426): 380–3.  
  12. ^ a b Colombo, L; Franken, J; Koetje, E; Van Went, J; Dons, HJ; Angenent, GC; Van Tunen, AJ (1995). "The petunia MADS box gene FBP11 determines ovule identity". The Plant cell 7 (11): 1859–68.  
  13. ^ a b c Pelaz, Soraya; Ditta, Gary S.; Baumann, Elvira; Wisman, Ellen; Yanofsky, Martin F. (2000). "B and C floral organ identity functions require SEPALLATA MADS-box genes". Nature 405 (6783): 200–3.  
  14. ^ a b Ditta, Gary; Pinyopich, Anusak; Robles, Pedro; Pelaz, Soraya; Yanofsky, Martin F. (2004). "The SEP4 Gene of Arabidopsis thaliana Functions in Floral Organ and Meristem Identity". Current Biology 14 (21): 1935–40.  
  15. ^ Ma, Hong (2005). "Molecular Genetic Analyses of Microsporogenesis and Microgametogenesis in Flowering Plants". Annual Review of Plant Biology 56: 393–434.  
  16. ^ a b Bowman, J. L. (1989). "Genes Directing Flower Development in Arabidopsis". The Plant Cell Online 1 (1): 37–52.  
  17. ^ a b Jofuku, KD; Den Boer, BG; Van Montagu, M; Okamuro, JK (1994). "Control of Arabidopsis flower and seed development by the homeotic gene APETALA2". The Plant cell 6 (9): 1211–25.  
  18. ^ Keck, Emma; McSteen, Paula; Carpenter, Rosemary; Coen, Enrico (2003). "Separation of genetic functions controlling organ identity in flowers". The EMBO Journal 22 (5): 1058–66.  
  19. ^ Maes, T; Van De Steene, N; Zethof, J; Karimi, M; d'Hauw, M; Mares, G; Van Montagu, M; Gerats, T (2001). "Petunia Ap2-like genes and their role in flower and seed development". The Plant cell 13 (2): 229–44.  
  20. ^ Bowman, JL; Smyth, DR; Meyerowitz, EM (1989). "Genes directing flower development in Arabidopsis". The Plant cell 1 (1): 37–52.  
  21. ^ Sommer, H; Beltrán, JP; Huijser, P; Pape, H; Lönnig, WE; Saedler, H; Schwarz-Sommer, Z (1990). "Deficiens, a homeotic gene involved in the control of flower morphogenesis in Antirrhinum majus: The protein shows homology to transcription factors". The EMBO Journal 9 (3): 605–13.  
  22. ^ Riechmann, Jose Luis; Allyn Krizek, Beth; Meyerowitz, Elliot M. (1996). "Dimerization Specificity of Arabidopsis MADS Domain Homeotic Proteins APETALA1, APETALA3, PISTILLATA, and AGAMOUS". Proceedings of the National Academy of Sciences of the United States of America 93 (10): 4793–8.  
  23. ^ Vandenbussche, M; Zethof, J; Royaert, S; Weterings, K; Gerats, T (2004). "The duplicated B-class heterodimer model: Whorl-specific effects and complex genetic interactions in Petunia hybrida flower development". The Plant cell 16 (3): 741–54.  
  24. ^ Kramer, EM; Dorit, RL; Irish, VF (1998). "Molecular evolution of genes controlling petal and stamen development: Duplication and divergence within the APETALA3 and PISTILLATA MADS-box gene lineages". Genetics 149 (2): 765–83.  
  25. ^ Kanno, Akira; Saeki, Hiroshi; Kameya, Toshiaki; Saedler, Heinz; Theissen, Günter (2003). "Heterotopic expression of class B floral homeotic genes supports a modified ABC model for tulip (Tulipa gesneriana)". Plant Molecular Biology 52 (4): 831–41.  
  26. ^ Nakamura, Toru; Fukuda, Tatsuya; Nakano, Masaru; Hasebe, Mitsuyasu; Kameya, Toshiaki; Kanno, Akira (2005). "The modified ABC model explains the development of the petaloid perianth of Agapanthus praecox ssp. Orientalis (Agapanthaceae) flowers". Plant Molecular Biology 58 (3): 435–45.  
  27. ^ a b Davies, Brendan; Motte, Patrick; Keck, Emma; Saedler, Heinz; Sommer, Hans; Schwarz-Sommer, Zsuzsanna (1999). "PLENA and FARINELLI: Redundancy and regulatory interactions between two Antirrhinum MADS-box factors controlling flower development". The EMBO Journal 18 (14): 4023–34.  
  28. ^ Favaro, R; Pinyopich, A; Battaglia, R; Kooiker, M; Borghi, L; Ditta, G; Yanofsky, MF; Kater, MM; Colombo, L (2003). "MADS-box protein complexes control carpel and ovule development in Arabidopsis". The Plant cell 15 (11): 2603–11.  

References

See also

The appearance of interesting phenotypes in RNA interference studies in Petunia and tomato led, in 1994, to the definition of a new type of function in the floral development model. The E function was initially thought to be only involved in the development of the three innermost verticils, however, subsequent work found that its expression was required in all the floral verticils.[13]

The D function genes were discovered in 1995. These genes are MADS-box proteins and they have a function that is distinct from those previously described, although they have a certain homology with C function genes. These genes are called FLORAL BINDING PROTEIN7 (FBP7) and FLORAL BINDING PROTEIN1L (FBP1l).[12] It was found that, in Petunia, they are involved in the development of the ovule. Equivalent genes were later found in Arabidopsis,[28] where they are also involved in controlling the development of carpels and the ovule and even with structures related to seed dispersal.

Genes exhibiting type-D and E functions

In Petunia, Antirrhinum and in maize the C function is controlled by a number of genes that act in the same manner. The genes that are closer homologs of AG in Petunia are pMADS3 and floral-binding protein 6 (FBP6).[27]

The PLENA (PLE) gene is present in A. majus, in place of the AG gene, although it is not an ortholog. However, the FARINELLI (FAR) gene is an ortholog, which is specific to the development of the anthers and the maturation of pollen.[27]

In A. thaliana, the C function is derived from one MADS-box type gene called AGAMOUS (AG), which intervenes both in the establishment of stamen and carpel identity as well as in the determination of the floral meristem.[16] Therefore the AG mutants are devoid of androecium and gynoecium and they have petals and sepals in their place. In addition, the growth in the centre of the flower is undifferentiated, therefore the petals and sepals grow in repetitive verticils.

Genes exhibiting type-C function

As discussed above, the floral organs of eudicotyledonous angiosperms are arranged in 4 different verticils, containing the sepals, petals, stamen and carpels. The ABC model states that the identity of these organs is determined by the homeotic genes A, A+B, B+C and C, respectively. In contrast with the sepal and petal verticils of the eudicots, the perigone of many plants of the open reading frames that code for proteins with 210 to 214 amino acids. Phylogenetic analysis of these sequences indicated that they belong to B gene family of the monocotyledons. In situ hybridization studies revealed that both sequences are expressed in verticil 1 as well as in 2 and 3. When taken together, these observations show that the floral development mechanism of Agapanthus also follows the modified ABC model.[26]

The GLO/PI lines that have been duplicated in Petunia contain P. hybrida GLOBOSA1 (PhGLO1, also called FBP1) and also PhGLO2 (also called PMADS2 or FBP3). For the functional elements equivalent to AP3/DEF in Petunia there is both a gene that possesses a relatively similar sequence, called PhDEF and there is also an atypical B function gene called PhTM6. Phylogenetic studies have placed the first three within the «euAP3» lineage, while PhTM6 belongs to that of «paleoAP3».[23] It is worth pointing out that, in terms of evolutionary history, the appearance of the euAP3 line seems to be related with the emergence of dicotyledons, as representatives of euAP3-type B function genes are present in dicotyledons while paleoAP3 genes are present in monocotyledons and basal angiosperms, among others.[24]

In A. thaliana the type-B function mainly arises from two genes, APETALA3 (AP3) and PISTILLATA (PI), both of which are MADS-box genes. A mutation of one of these genes causes the homeotic conversion of petals into sepals and of stamens into carpeloid structures.[20] This also occurs in its orthologs in A. majus, which are DEFICIENS (DEF) and GLOBOSA (GLO).[21] For both species the active form of binding with DNA is that derived from the heterodimer: AP3 and PI, or DEF and GLO, dimerize. This is the form in which they are able to function.[22]

Genes exhibiting type-B function

A total of three genes have been isolated from Petunia hybrida that are similar to AP2: P. hybrida APETALA2A (PhAP2A), PhAP2B and PhAP2C. PhAP2A is, to a large degree, homologous with the AP2 gene of Arabidopsis, both in its sequence and in its expression pattern, which suggests that the two genes are orthologs. The proteins PhAP2B and PhAP2C, on the other hand, are slightly different, even though they belong to the family of transcription factors that are similar to AP2. In addition they are expressed in different ways, although they are very similar in comparison with PhAP2A. In fact, the mutants for these genes do not show the usual phenotype, that of the null alleles of A genes.[19] A true A-function gene has not been found in Petunia; though a part of the A-function (the inhibition of the C in the outer two whorls) has been largely attributed to miRNA169 (colloquially called BLIND)ref.

In Antirrhinum, the orthologous gene to AP1 is SQUAMOSA (SQUA), which also has a particular impact on the floral meristem. The homologs for AP2 are LIPLESS1 (LIP1) and LIPLESS2 (LIP2), which have a redundant function and are of special interest in the development of sepals, petals and ovules.[18]

[17] belongs to the family of genes that contains AP2, which it gives its name to and which consists of AP2 is a MADS-box type gene, while AP1 [16])AP2 (APETALA2 and AP1) (APETALA1, function A is mainly represented by two genes A. thaliana In

Genes exhibiting type-A function

The nature of these genes corresponds to that of transcription factors, which, as expected, have analogous structures to a group of factors contained in yeasts and animal cells. This group is called MADS, which is an acronym for the different factors contained in the group. These MADS factors have been detected in all the vegetable species studied, although the involvement of other elements involved in the regulation of gene expression cannot be discounted.[8]

Cloning studies have been carried out on DNA in the genes associated with the affected homeotic functions in the mutants discussed above. These studies used serial analysis of gene expression throughout floral development to show patterns of tissue expression, which, in general, correspond with the predictions of the ABC model.

Techniques for detecting differential expression

  • Mutations in type A genes, these mutations affect the calyx and corolla, which are the outermost verticils. In these mutants, such as APETALA2 in A. thaliana, carpels develop instead of sepals and stamen in place of petals. This means that, the verticils of the perianth are transformed into reproductive verticils.
  • Mutations in type B genes, these mutations affect the corolla and the stamen, which are the intermediate verticils. Two mutations have been found in A. thaliana, APETALA3 and PISTILLATA, which cause development of sepals instead of petals and carpels in the place of stamen.
  • Mutations in type C genes, these mutations affect the reproductive verticils, namely the stamen and the carpels. The A. thaliana mutant of this type is called AGAMOUS, it possesses a phenotype containing petals instead of stamen and sepals instead of carpels.

There are a great many homeotic mutation, which is analogous to HOX gene mutations found in Drosophila. In Arabidopsis and Antirrhinum, the two taxa on which models are based, these mutations always affect adjacent verticils. This allows the characterization of three classes of mutation, according to which verticils are affected:

Analysis of mutants

The methodology for studying flower development involves two steps. Firstly, the identification of the exact genes required for determining the identity of the floral meristem. In A. thaliana these include APETALA1 (AP1) and LEAFY (LFY). Secondly, genetic analysis is carried out on the aberrant phenotypes for the relative characteristics of the flowers, which allows the characterization of the homeotic genes implicated in the process.

Flowers of Petunia hybrid.
Flowers of A. majus.
Flower of A. thaliana.

Genetic analysis

[15] of genes with D and E functions are also MADS-box genes.gene products It is interesting to note that the [14], all the verticils are similar to leaves.sensu lato while on losing Function E [13] The existence of two supplementary functions, D and E, have also been proposed in addition to the A, B and C functions already discussed. Function D specifies the identity of the

The fact that these homeotic genes determine an organ’s identity becomes evident when a gene that represents a particular function, for example the A gene, is not expressed. In "Arabidopsis" this loss results in a flower which is composed of one verticil of carpels, another containing stamens and another of carpels.[10] This method for studying gene function uses reverse genetics techniques to produce transgenic plants that contain a mechanism for gene silencing through RNA interference. In other studies, using forward genetics techniques such as genetic mapping, it is the analysis of the phenotypes of flowers with structural anomalies that leads to the cloning of the gene of interest. The flowers may possess a non-functional or over expressed allele for the gene being studied.[11]

The ABC model of flower development was first formulated by George Haughn and Chris Somerville in 1988.[9] It was first used as a model to describe the collection of genetic mechanisms that establish floral organ identity in the Rosids, as exemplified by Arabidopsis thaliana, and the Asterids, as demonstrated by Antirrhinum majus. Both species have four verticils (sepals, petals, stamens and carpels), which are defined by the differential expression of a number of homeotic genes present in each verticil. This means that the sepals are solely characterized by the expression of A genes, while the petals are characterized by the co-expression of A and B genes. The B and C genes establish the identity of the stamens and the carpels only require C genes to be active. It should be noted that type A and C genes are reciprocally antagonistic.[10]

Graphic representation of the ABC model. The single or additive expression of the homeotic genes in the left hand column have repercussions for the development of the organs in the central column and determine the nature of the whorl, on the right.

The ABC model

  • Meristem identity genes. Code for the transcription factors required to initiate the induction of the identity genes. They are positive regulators of organ identity during floral development.
  • Organ identity genes. Directly control organ identity and also code for transcription factors that control the expression of other genes, whose products are implicated in the formation or function of the distinct organs of the flower.
  • Cadastral genes. Act as spatial regulators for the organ identity genes by defining boundaries for their expression. In this way they control the extent to which genes interact thereby regulating whether they act in the same place at the same time.

A flower’s anatomy, as defined by the presence of a series of organs (sepals, petals, stamens and carpels) positioned according to a given pattern, facilitate cadastral genes.[8]

Anatomy of a flower.

Floral architecture

[7]

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