ABC model of flower development
The ABC model of flower development is a
An external
The identity of the organs present in the four floral verticils is a consequence of the interaction of at least three types of
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 erklären").[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]
Floral transition
The
There are many signals that regulate the molecular biology of the process. The following three genes in Arabidopsis thaliana possess both common and independent functions in floral transition: FLOWERING LOCUS T (FT), LEAFY (LFY), SUPPRESSOR OF OVEREXPRESSION OF CONSTANS1 (SOC1, also called AGAMOUS-LIKE20).[5] 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
Floral architecture
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 sexual reproduction in flowering plants. The flower arises from the activity of three classes of genes, which regulate floral development:[8]
- Meristem identity genes, which 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, which 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, which 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.
The ABC model
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. Type A and C genes are reciprocally antagonistic.[10]
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.
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 ovule, as a separate reproductive function from the development of the carpels, which occurs after their determination.[12] Function E relates to a physiological requirement that is a characteristic of all floral verticils, although, it was initially described as necessary for the development of the three innermost verticils (Function E sensu stricto).[13] However, its broader definition (sensu lato) suggests that it is required in the four verticils.[14] Therefore, when Function D is lost the structure of the ovules becomes similar to that of leaves and when Function E is lost sensu stricto, the floral organs of the three outer most verticils are transformed into sepals,[13] while on losing Function E sensu lato, all the verticils are similar to leaves.[14] The gene products of genes with D and E functions are also MADS-box genes.[15]
Genetic analysis
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
Analysis of mutants
There are a great many
- 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.
-
AGAMOUS mutation resulting in a double-flowered Arabidopsis
Techniques for detecting differential expression
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.
The nature of these genes corresponds to that of
Genes exhibiting type-A function
In A. thaliana, function A is mainly represented by two genes APETALA1 (AP1) and APETALA2 (AP2)
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.[19]
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
Genes exhibiting type-B function
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 either of these genes causes the homeotic conversion of petals into sepals and of stamens into carpels.
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.
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 family Liliaceae have two nearly identical external petaloid verticils (the tepals). In order to explain the floral morphology of the Liliaceae, van Tunen et al. proposed a modified ABC model in 1993. This model suggests that class B genes are not only expressed in verticils 2 and 3, but also in 1. It therefore follows that the organs of verticils 1 and 2 express class A and B genes and this is how they have a petaloid structure. This theoretical model has been experimentally proven through the cloning and characterization of homologs of the Antirrhinum genes GLOBOSA and DEFICIENS in a Liliaceae, the tulip Tulipa gesneriana. These genes are expressed in verticils 1,2 and 3.[26] The homologs GLOBOSA and DEFICIENS have also been isolated and characterized in Agapanthus praecox ssp. orientalis (
Genes exhibiting type-C function
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.
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
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).[28]
Genes exhibiting type-D and E functions
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,[29] where they are also involved in controlling the development of carpels and the ovule and even with structures related to seed dispersal.
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]
See also
References
- ISBN 978-84-486-0258-1.[page needed]
- .
- ^ Goethe J.W. von (1790) Versuch die Metamorphose der Pflanzen zu erklaren. Gotha, Ettlinger; paragraph 120."
- ^ PMID 9596637.
- S2CID 4373900.
- PMID 10915624.
- PMID 12070094.
- ^ ISBN 978-0-87893-823-0.[page needed]
- .
- ^ PMID 1726485.
- PMID 10411495.
- ^ PMID 8535139.
- ^ S2CID 4391070.
- ^ S2CID 8122306.
- PMID 15862102.
- ^ PMID 2535466.
- ^ PMID 0007919989.
- ^ PMID 23034631.
- PMID 12606571.
- PMID 11226182.
- PMID 2535466.
- PMID 1968830.
- PMID 8643482.
- PMID 14973163.
- PMID 9611190.
- S2CID 24366077.
- S2CID 28657835.
- ^ PMID 10406807.
- PMID 14555696.
Sources
General texts
- Soltis, DE; ISBN 978-0-12-005944-7.
- Wolpert, Lewis; Beddington, R.; Jessell, T.; Lawrence, P.; Meyerowitz, E.; Smith, W. (2002). Principles of Development (Second ed.). Oxford: Oxford University Press. ISBN 978-0-19-879291-8.
External links
- Murai, K. (2013). "Homeotic Genes and the ABCDE Model for Floral Organ Formation in Wheat". Plants. 2 (3): 379–395. PMID 27137382.
| Subdisciplines | |||||||||||
|---|---|---|---|---|---|---|---|---|---|---|---|
| Plant groups | |||||||||||
| Plant anatomy |
| ||||||||||
| Plant physiology Materials | |||||||||||
| Plant growth and habit | |||||||||||
| Reproduction | |||||||||||
| Plant taxonomy | |||||||||||
| Practice | |||||||||||
| |||||||||||
