The ABC model of flower development is a scientific model of the process by which flowering plants produce a pattern of gene expression in meristems that leads to the appearance of an organ oriented towards sexual reproduction, a flower. There are three physiological developments that must occur in order for this to take place: firstly, the plant must pass from sexual immaturity into a sexually mature state (i.e. a transition towards flowering); secondly, the transformation of the apical meristem’s function from a vegetative meristem into a floral meristem or inflorescence; and finally the growth of the flower’s individual organs. The latter phase has been modelled using the ABC model, which aims to describe the biological basis of the process from the perspective of molecular and developmental genetics.
An external stimulus is required in order to trigger the differentiation of the meristem into a flower meristem. This stimulus will activate mitotic cell division in the meristem, particularly on its sides where new primordia are formed. This same stimulus will also cause the meristem to follow a developmental pattern that will lead to the growth of floral meristems as opposed to vegetative meristems. The main difference between these two types of meristem, apart from the obvious disparity between the objective organ, is the verticillate (or whorled) 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.
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 erklären"). 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".
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 descendants. 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.
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). SOC1 is a MADS-box-type gene, which integrates responses to photoperiod, vernalization and gibberellins.
The meristem can be defined as the tissue or group of plant tissues that contain undifferentiated stem cells, which are capable of producing any type of cell tissue. Their maintenance and development, both in the vegetative meristem or the meristem of the inflorescence is controlled by genetic cell fate determination mechanisms. This means that a number of genes will directly regulate, for example, the maintenance of the stem cell’s characteristics (gene WUSCHEL or WUS), and others will act via negative feedback mechanisms in order to inhibit a characteristic (gene CLAVATA or CLV). In this way both mechanisms give rise to a feedback loop, which along with other elements lend a great deal of robustness to the system. Along with the WUS gene the SHOOTMERISTEMLESS (STM) gene also represses the differentiation of the meristematic dome. This gene acts by inhibiting the possible differentiation of the stem cells but still allows cell division in the daughter cells, which, had they been allowed to differentiate, would have given rise to distinct organs.
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: genes which regulate the identity of the meristem, the identity of the flower organ and finally cadastral genes.
The ABC model of flower development was first formulated by George Haughn and Chris Somerville in 1988. 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.
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. 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.
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. 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). However, its broader definition (sensu lato) suggests that it is required in the four verticils. 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, while on losing Function E sensu lato, all the verticils are similar to leaves. The gene products of genes with D and E functions are also MADS-box genes.
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.
There are a great many mutations that affect floral morphology, although the analysis of these mutants is a recent development. Supporting evidence for the existence of these mutations comes from the fact that a large number affect the identity of floral organs. For example, some organs develop in a location where others should develop. This is called 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:
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 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.
In A. thaliana, function A is mainly represented by two genes APETALA1 (AP1) and APETALA2 (AP2) AP1 is a MADS-box type gene, while AP2 belongs to the family of genes that contains AP2, which it gives its name to and which consists of transcription factors that are only found in plants. AP2 has also been shown to complex with the co-repressor TOPLESS (TPL) in developing floral buds to repress the C-class gene AGAMOUS (AG). However, AP2 is not expressed in the shoot apical meristem (SAM), which contains the latent stem cell population throughout the adult life of Arabidopsis, and so it is speculated that TPL works with some other A-class gene in the SAM to repress AG.AP1 functions as a type A gene, both in controlling the identity of sepals and petals, and it also acts in the floral meristem. AP2 not only functions in the first two verticils, but also in the remaining two, in developing ovules and even in leaves. It is also likely that post-transcriptional regulation exists, which controls it’s A function, or even that it has other purposes in the determination of organ identity independent of that mentioned here.
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.
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. 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 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. This also occurs in its orthologs in A. majus, which are DEFICIENS (DEF) and GLOBOSA (GLO) respectively. 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.
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». 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.
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. The homologs GLOBOSA and DEFICIENS have also been isolated and characterized in Agapanthus praecox ssp. orientalis (Agapanthaceae), which is phylogenetically distant from the model organisms. In this study the genes were called ApGLO and ApDEF, respectively. Both contain 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.
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. 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.
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.
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).
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). It was found that, in Petunia, they are involved in the development of the ovule. Equivalent genes were later found in Arabidopsis, 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.
AP3 may refer to one of the following:
The ABC Model of Flower Development
Andrew Pendelton III, an American professional wrestlerAP-3 may refer to:
USS Hancock (AP-3), a transport ship acquired by the United States Navy in 1902 which saw service during World War I
AP-3C Orion, a variant of the P-3C Orion maritime patrol aircraft used by the Royal Australian Air ForceApetala 2
Apetala 2 (AP2) is a gene and a member of a large family of transcription factors, the AP2/EREBP family. In Arabidopsis thaliana AP2 plays a role in the ABC model of flower development. It was originally thought that this family of proteins was plant-specific; however, recent studies have shown that apicomplexans, including the causative agent of malaria, Plasmodium falciparum encode a related set of transcription factors, called the ApiAP2 family.In the A. thaliana transcription factor RAV1 the N-terminal AP2 domain binds 5'-CAACA-3' sequence, while the C-terminal highly conserved B3 domain binds 5'-CACCTG-3' sequence.Arabidopsis thaliana
Arabidopsis thaliana, the thale cress, mouse-ear cress or arabidopsis, is a small flowering plant native to Eurasia and Africa. A. thaliana is considered a weed; it is found by roadsides and in disturbed land.
A winter annual with a relatively short life cycle, A. thaliana is a popular model organism in plant biology and genetics. For a complex multicellular eukaryote, A. thaliana has a relatively small genome of approximately 135 megabase pairs (Mbp). It was the first plant to have its genome sequenced, and is a popular tool for understanding the molecular biology of many plant traits, including flower development and light sensing.Double-flowered
"Double-flowered" describes varieties of flowers with extra petals, often containing flowers within flowers. The double-flowered trait is often noted alongside the scientific name with the abbreviation fl. pl. (flore pleno, a Latin ablative form meaning "with full flower"). The first abnormality to be documented in flowers, double flowers are popular varieties of many commercial flower types, including roses, camellias and carnations. In some double-flowered varieties all of the reproductive organs are converted to petals — as a result, they are sexually sterile and must be propagated through cuttings. Many double-flowered plants have little wildlife value as access to the nectaries is typically blocked by the mutation.Evolutionary history of plants
The evolution of plants has resulted in a wide range of complexity, from the earliest algal mats, through multicellular marine and freshwater green algae, terrestrial bryophytes, lycopods and ferns, to the complex gymnosperms and angiosperms of today. While many of the earliest groups continue to thrive, as exemplified by red and green algae in marine environments, more recently derived groups have displaced previously ecologically dominant ones, e.g. the ascendance of flowering plants over gymnosperms in terrestrial environments.There is evidence that cyanobacteria and multicellular photosynthetic eukaryotes lived in freshwater communities on land as early as 1 billion years ago, and that communities of complex, multicellular photosynthesizing organisms existed on land in the late Precambrian, around 850 million years ago.Evidence of the emergence of embryophyte land plants first occurs in the mid-Ordovician (~470 million years ago), and by the middle of the Devonian (~390 million years ago), many of the features recognised in land plants today were present, including roots and leaves. By Late Devonian (~370 million years ago) some free-sporing plants such as Archaeopteris had secondary vascular tissue that produced wood and had formed forests of tall trees. Also by late Devonian, Elkinsia, an early seed fern, had evolved seeds.
Evolutionary innovation continued throughout the rest of the Phanerozoic eon and still continues today. Most plant groups were relatively unscathed by the Permo-Triassic extinction event, although the structures of communities changed. This may have set the scene for the appearance of the flowering plants in the Triassic (~200 million years ago), and their later diversification in the Cretaceous and Paleogene. The latest major group of plants to evolve were the grasses, which became important in the mid-Paleogene, from around 40 million years ago. The grasses, as well as many other groups, evolved new mechanisms of metabolism to survive the low CO2 and warm, dry conditions of the tropics over the last 10 million years.Flower
A flower, sometimes known as a bloom or blossom, is the reproductive structure found in flowering plants (plants of the division Magnoliophyta, also called angiosperms). The biological function of a flower is to effect reproduction, usually by providing a mechanism for the union of sperm with eggs. Flowers may facilitate outcrossing (fusion of sperm and eggs from different individuals in a population) or allow selfing (fusion of sperm and egg from the same flower). Some flowers produce diaspores without fertilization (parthenocarpy). Flowers contain sporangia and are the site where gametophytes develop. Many flowers have evolved to be attractive to animals, so as to cause them to be vectors for the transfer of pollen. After fertilization, the ovary of the flower develops into fruit containing seeds.
In addition to facilitating the reproduction of flowering plants, flowers have long been admired and used by humans to bring beauty to their environment, and also as objects of romance, ritual, religion, medicine and as a source of food.History of research on Arabidopsis thaliana
Arabidopsis thaliana is a first class model organism and the single most important species
for fundamental research in plant molecular genetics.
A. thaliana was the first plant for which a high-quality reference genome sequence was determined (see below), and a worldwide research community has developed many other genetic resources and tools.
The experimental advantages of A. thaliana have enabled many important discoveries.
These advantages have been extensively reviewed,
as has its role in fundamental discoveries
about the plant immune system,
and other areas.Homeotic gene
In evolutionary developmental biology, homeotic genes are genes which regulate the development of anatomical structures in various organisms such as echinoderms, insects, mammals, and plants. This regulation is done via the programming of various transcription factors by the homeotic genes, and these factors affect genes through regulatory genetic pathways.Mutations in homeotic genes cause displaced body parts (homeosis), such as antennae growing at the posterior of the fly instead of at the head. Mutations that lead to such ectopic placements are usually lethal.Homology (biology)
In biology, homology is the existence of shared ancestry between a pair of structures, or genes, in different taxa. A common example of homologous structures is the forelimbs of vertebrates, where the wings of bats, the arms of primates, the front flippers of whales and the forelegs of dogs and horses are all derived from the same ancestral tetrapod structure. Evolutionary biology explains homologous structures adapted to different purposes as the result of descent with modification from a common ancestor. The term was first applied to biology in a non-evolutionary context by the anatomist Richard Owen in 1843. Homology was later explained by Charles Darwin's theory of evolution in 1859, but had been observed before this, from Aristotle onwards, and it was explicitly analysed by Pierre Belon in 1555.
In developmental biology, organs that developed in the embryo in the same manner and from similar origins, such as from matching primordia in successive segments of the same animal, are serially homologous. Examples include the legs of a centipede, the maxillary palp and labial palp of an insect, and the spinous processes of successive vertebrae in a vertebral column. Male and female reproductive organs are homologous if they develop from the same embryonic tissue, as do the ovaries and testicles of mammals including humans.
Sequence homology between protein or DNA sequences is similarly defined in terms of shared ancestry. Two segments of DNA can have shared ancestry because of either a speciation event (orthologs) or a duplication event (paralogs). Homology among proteins or DNA is inferred from their sequence similarity. Significant similarity is strong evidence that two sequences are related by divergent evolution from a common ancestor. Alignments of multiple sequences are used to discover the homologous regions.
Homology remains controversial in animal behaviour, but there is suggestive evidence that, for example, dominance hierarchies are homologous across the primates.Inflorescence
An inflorescence is a group or cluster of flowers arranged on a stem that is composed of a main branch or a complicated arrangement of branches. Morphologically, it is the modified part of the shoot of seed plants where flowers are formed. The modifications can involve the length and the nature of the internodes and the phyllotaxis, as well as variations in the proportions, compressions, swellings, adnations, connations and reduction of main and secondary axes.
Inflorescence can also be defined as the reproductive portion of a plant that bears a cluster of flowers in a specific pattern.
The stem holding the whole inflorescence is called a peduncle and the major axis (incorrectly referred to as the main stem) holding the flowers or more branches within the inflorescence is called the rachis. The stalk of each single flower is called a pedicel. A flower that is not part of an inflorescence is called a solitary flower and its stalk is also referred to as a peduncle. Any flower in an inflorescence may be referred to as a floret, especially when the individual flowers are particularly small and borne in a tight cluster, such as in a pseudanthium.
The fruiting stage of an inflorescence is known as an infructescence.
Inflorescences may be simple (single) or complex (panicle). The rachis may be one of several types, including single, composite, umbel, spike or raceme.MADS-box
The MADS box is a conserved sequence motif. The genes which contain this motif are called the MADS-box gene family. The MADS box encodes the DNA-binding MADS domain. The MADS domain binds to DNA sequences of high similarity to the motif CC[A/T]6GG termed the CArG-box. MADS-domain proteins are generally transcription factors. The length of the MADS-box reported by various researchers varies somewhat, but typical lengths are in the range of 168 to 180 base pairs, i.e. the encoded MADS domain has a length of 56 to 60 amino acids. There is evidence that the MADS domain evolved from a sequence stretch of a type II topoisomerase in a common ancestor of all extant eukaryotes.Petal
Petals are modified leaves that surround the reproductive parts of flowers. They are often brightly colored or unusually shaped to attract pollinators. Together, all of the petals of a flower are called a corolla. Petals are usually accompanied by another set of special leaves called sepals, that collectively form the calyx and lie just beneath the corolla. The calyx and the corolla together make up the perianth. When the petals and sepals of a flower are difficult to distinguish, they are collectively called tepals. Examples of plants in which the term tepal is appropriate include genera such as Aloe and Tulipa. Conversely, genera such as Rosa and Phaseolus have well-distinguished sepals and petals. When the undifferentiated tepals resemble petals, they are referred to as "petaloid", as in petaloid monocots, orders of monocots with brightly coloured tepals. Since they include Liliales, an alternative name is lilioid monocots.
Although petals are usually the most conspicuous parts of animal-pollinated flowers, wind-pollinated species, such as the grasses, either have very small petals or lack them entirely.Plant evolutionary developmental biology
Evolutionary developmental biology (evo-devo) is the study of developmental programs and patterns from an evolutionary perspective. It seeks to understand the various influences shaping the form and nature of life on the planet. Evo-devo arose as a separate branch of science rather recently. An early sign of this occurred in 1999.Most of the synthesis in evo-devo has been in the field of animal evolution, one reason being the presence of elegant model systems like Drosophila melanogaster, C. elegans, zebrafish and Xenopus laevis. However, since 1980, a wealth of information on plant morphology, coupled with modern molecular techniques has helped shed light on the conserved and unique developmental patterns in the plant kingdom also.Superman (gene)
Superman is a plant gene in Arabidopsis thaliana, that plays a role in controlling the boundary between stamen and carpel development in a flower. It is named for the comic book character Superman, and the related genes kryptonite (gene) and clark kent were named accordingly (although the latter turns out to just be another form of superman). It encodes a transcription factor (specifically a C2H2 type zinc finger protein). Homologous genes are known in the petunia and snapdragon, which are also involved in flower development, although in both cases there are important differences from the functioning in Arabidopsis. Superman is expressed early on in flower development, in the stamen whorl adjacent to the carpel whorl. It interacts with the other genes of the ABC model of flower development in a variety of ways.
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