![ABC Model](https://upload.wikimedia.org/wikipedia/commons/6/6e/ABC_Model.svg)
The ABC model of flower development is a
scientific model
Scientific modelling is a scientific activity, the aim of which is to make a particular part or feature of the world easier to understand, define, quantify, visualize, or simulate by referencing it to existing and usually commonly accepted ...
of the process by which
flowering plant
Flowering plants are plants that bear flowers and fruits, and form the clade Angiospermae (), commonly called angiosperms. The term "angiosperm" is derived from the Greek words ('container, vessel') and ('seed'), and refers to those plants th ...
s produce a pattern of
gene expression in
meristem
The meristem is a type of tissue found in plants. It consists of undifferentiated cells (meristematic cells) capable of cell division. Cells in the meristem can develop into all the other tissues and organs that occur in plants. These cells conti ...
s that leads to the appearance of an organ oriented towards
sexual reproduction
Sexual reproduction is a type of reproduction that involves a complex life cycle in which a gamete ( haploid reproductive cells, such as a sperm or egg cell) with a single set of chromosomes combines with another gamete to produce a zygote th ...
, 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
An inflorescence is a group or cluster of flowers arranged on a Plant stem, stem that is composed of a main branch or a complicated arrangement of branches. Morphology (biology), Morphologically, it is the modified part of the shoot of sperma ...
; 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
Development of the human body is the process of growth to maturity. The process begins with fertilization, where an egg released from the ovary of a female is penetrated by a sperm cell from a male. The resulting zygote develops through mitosi ...
genetics.
An external
stimulus
A stimulus is something that causes a physiological response. It may refer to:
*Stimulation
**Stimulus (physiology), something external that influences an activity
**Stimulus (psychology), a concept in behaviorism and perception
*Stimulus (economi ...
is required in order to trigger the
differentiation of the meristem into a flower meristem. This stimulus will activate
mitotic
In cell biology, mitosis () is a part of the cell cycle in which replicated chromosomes are separated into two new nuclei. Cell division by mitosis gives rise to genetically identical cells in which the total number of chromosomes is maintai ...
cell division in the apical meristem, particularly on its sides where new
primordia
A primordium (; plural: primordia; synonym: anlage) in embryology, is an organ or tissue in its earliest recognizable stage of development. Cells of the primordium are called primordial cells. A primordium is the simplest set of cells capable o ...
are formed. This same stimulus will also cause the meristem to follow a
developmental
Development of the human body is the process of growth to maturity. The process begins with fertilization, where an egg released from the ovary of a female is penetrated by a sperm cell from a male. The resulting zygote develops through mitosi ...
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
In botany, phyllotaxis () or phyllotaxy is the arrangement of leaves on a plant stem. Phyllotactic spirals form a distinctive class of patterns in nature.
Leaf arrangement
The basic arrangements of leaves on a stem are opposite and alterna ...
, 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
sepal
A sepal () is a part of the flower of angiosperms (flowering plants). Usually green, sepals typically function as protection for the flower in bud, and often as support for the petals when in bloom., p. 106 The term ''sepalum'' was coine ...
s,
petals,
stamens and
carpel
Gynoecium (; ) is most commonly used as a collective term for the parts of a flower that produce ovules and ultimately develop into the fruit and seeds. The gynoecium is the innermost whorl of a flower; it consists of (one or more) '' pistils' ...
s. 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
A gene product is the biochemical material, either RNA or protein, resulting from expression of a gene. A measurement of the amount of gene product is sometimes used to infer how active a gene is. Abnormal amounts of gene product can be correlated ...
, 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
The perianth (perigonium, perigon or perigone in monocots) is the non-reproductive part of the flower, and structure that forms an envelope surrounding the sexual organs, consisting of the calyx (sepals) and the corolla ( petals) or tepals when ...
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
Johann Wolfgang von Goethe (28 August 1749 – 22 March 1832) was a German poet, playwright, novelist, scientist, statesman, theatre director, and critic. His works include plays, poetry, literature, and aesthetic criticism, as well as tr ...
'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:
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
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. 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
gibberellin
Gibberellins (GAs) are plant hormones that regulate various developmental processes, including stem elongation, germination, dormancy, flowering, flower development, and leaf and fruit senescence. GAs are one of the longest-known classes of plan ...
s having a particularly important role.
There are many
signals
In signal processing, a signal is a function that conveys information about a phenomenon. Any quantity that can vary over space or time can be used as a signal to share messages between observers. The ''IEEE Transactions on Signal Processing'' ...
that regulate the
molecular biology
Molecular biology is the branch of biology that seeks to understand the molecular basis of biological activity in and between cells, including biomolecular synthesis, modification, mechanisms, and interactions. The study of chemical and physi ...
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
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 /Tsub>6G ...
-type gene, which integrates responses to photoperiod,
vernalization
Vernalization (from Latin ''vernus'', "of the spring") is the induction of a plant's flowering process by exposure to the prolonged cold of winter, or by an artificial equivalent. After vernalization, plants have acquired the ability to flower, ...
and gibberellins.
Formation of the floral meristem or the inflorescence
The
meristem
The meristem is a type of tissue found in plants. It consists of undifferentiated cells (meristematic cells) capable of cell division. Cells in the meristem can develop into all the other tissues and organs that occur in plants. These cells conti ...
can be defined as the tissue or group of plant tissues that contain undifferentiated
stem cells
In multicellular organisms, stem cells are undifferentiated or partially differentiated cells that can differentiate into various types of cells and proliferate indefinitely to produce more of the same stem cell. They are the earliest type of ...
, 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 Within the field of developmental biology, one goal is to understand how a particular cell develops into a final cell type, known as fate determination. Within an embryo, several processes play out at the cellular and tissue level to create an organ ...
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
Feedback occurs when outputs of a system are routed back as inputs as part of a chain of cause-and-effect that forms a circuit or loop. The system can then be said to ''feed back'' into itself. The notion of cause-and-effect has to be handled c ...
, 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
Cell division is the process by which a parent cell divides into two daughter cells. Cell division usually occurs as part of a larger cell cycle in which the cell grows and replicates its chromosome(s) before dividing. In eukaryotes, there ar ...
in the daughter cells, which, had they been allowed to differentiate, would have given rise to distinct organs.
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
Sexual reproduction is a type of reproduction that involves a complex life cycle in which a gamete ( haploid reproductive cells, such as a sperm or egg cell) with a single set of chromosomes combines with another gamete to produce a zygote th ...
in
flowering plant
Flowering plants are plants that bear flowers and fruits, and form the clade Angiospermae (), commonly called angiosperms. The term "angiosperm" is derived from the Greek words ('container, vessel') and ('seed'), and refers to those plants th ...
s. The flower arises from the activity of three classes of genes, which regulate floral development:
* Meristem identity genes, which code for the
transcription factor
In molecular biology, a transcription factor (TF) (or sequence-specific DNA-binding factor) is a protein that controls the rate of transcription of genetic information from DNA to messenger RNA, by binding to a specific DNA sequence. The f ...
s 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. It was first used as a model to describe the collection of genetic mechanisms that establish floral organ identity in the
Rosids
The rosids are members of a large clade (monophyletic group) of flowering plants, containing about 70,000 species, more than a quarter of all angiosperms.
The clade is divided into 16 to 20 orders, depending upon circumscription and classificati ...
, as exemplified by ''Arabidopsis thaliana'', and the
Asterids
In the APG IV system (2016) for the classification of flowering plants, the name asterids denotes a clade (a monophyletic group). Asterids is the largest group of flowering plants, with more than 80,000 species, about a third of the total flowe ...
, 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 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. Homeotic genes often encode transcription factor proteins ...
s 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
Reverse genetics is a method in molecular genetics that is used to help understand the function(s) of a gene by analysing the phenotypic effects caused by genetically engineering specific nucleic acid sequences within the gene. The process pr ...
techniques to produce transgenic plants that contain a mechanism for
gene silencing
Gene silencing is the regulation of gene expression in a cell to prevent the expression of a certain gene. Gene silencing can occur during either transcription or translation and is often used in research. In particular, methods used to silence ge ...
through
RNA interference
RNA interference (RNAi) is a biological process in which RNA molecules are involved in sequence-specific suppression of gene expression by double-stranded RNA, through translational or transcriptional repression. Historically, RNAi was known by ...
. In other studies, using
forward genetics
Forward genetics is a molecular genetics approach of determining the genetic basis responsible for a phenotype. Forward genetics provides an unbiased approach because it relies heavily on identifying the genes or genetic factors that cause a partic ...
techniques such as
genetic mapping
Genetic linkage is the tendency of DNA sequences that are close together on a chromosome to be inherited together during the meiosis phase of sexual reproduction. Two genetic markers that are physically near to each other are unlikely to be separ ...
, it is the analysis of the
phenotype
In genetics, the phenotype () is the set of observable characteristics or traits of an organism. The term covers the organism's morphology or physical form and structure, its developmental processes, its biochemical and physiological pr ...
s of flowers with structural anomalies that leads to the
cloning
Cloning is the process of producing individual organisms with identical or virtually identical DNA, either by natural or artificial means. In nature, some organisms produce clones through asexual reproduction. In the field of biotechnology, c ...
of the gene of interest. The flowers may possess a non-functional or
over expressed allele
An allele (, ; ; modern formation from Greek ἄλλος ''állos'', "other") is a variation of the same sequence of nucleotides at the same place on a long DNA molecule, as described in leading textbooks on genetics and evolution.
::"The chro ...
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
In seed plants, the ovule is the structure that gives rise to and contains the female reproductive cells. It consists of three parts: the '' integument'', forming its outer layer, the ''nucellus'' (or remnant of the megasporangium), and the ...
, 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 product
A gene product is the biochemical material, either RNA or protein, resulting from expression of a gene. A measurement of the amount of gene product is sometimes used to infer how active a gene is. Abnormal amounts of gene product can be correlate ...
s of genes with D and E functions are also MADS-box genes.
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
phenotype
In genetics, the phenotype () is the set of observable characteristics or traits of an organism. The term covers the organism's morphology or physical form and structure, its developmental processes, its biochemical and physiological pr ...
s for the relative characteristics of the flowers, which allows the characterization of the
homeotic genes implicated in the process.
Analysis of mutants
There are a great many
mutation
In biology, a mutation is an alteration in the nucleic acid sequence of the genome of an organism, virus, or extrachromosomal DNA. Viral genomes contain either DNA or RNA. Mutations result from errors during DNA replication, DNA or viral repl ...
s that affect floral
morphology
Morphology, from the Greek and meaning "study of shape", may refer to:
Disciplines
* Morphology (archaeology), study of the shapes or forms of artifacts
* Morphology (astronomy), study of the shape of astronomical objects such as nebulae, galaxies ...
, 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
''Drosophila'' () is a genus of flies, belonging to the family Drosophilidae, whose members are often called "small fruit flies" or (less frequently) pomace flies, vinegar flies, or wine flies, a reference to the characteristic of many speci ...
''. In ''Arabidopsis'' and ''
Antirrhinum
''Antirrhinum'' is a genus of plants commonly known as dragon flowers, snapdragons and dog flower because of the flowers' fancied resemblance to the face of a dragon that opens and closes its mouth when laterally squeezed. They are native t ...
'', 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:
* 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
The perianth (perigonium, perigon or perigone in monocots) is the non-reproductive part of the flower, and structure that forms an envelope surrounding the sexual organs, consisting of the calyx (sepals) and the corolla ( petals) or tepals when ...
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.
Image:Arabidopsis mutants.jpg , AGAMOUS mutation resulting in a 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 ablat ...
Arabidopsis
Techniques for detecting differential expression
Cloning
Cloning is the process of producing individual organisms with identical or virtually identical DNA, either by natural or artificial means. In nature, some organisms produce clones through asexual reproduction. In the field of biotechnology, c ...
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
Serial Analysis of Gene Expression (SAGE) is a transcriptomic technique used by molecular biologists to produce a snapshot of the messenger RNA population in a sample of interest in the form of small tags that correspond to fragments of those tra ...
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
In molecular biology, a transcription factor (TF) (or sequence-specific DNA-binding factor) is a protein that controls the rate of transcription of genetic information from DNA to messenger RNA, by binding to a specific DNA sequence. The fun ...
, which, as expected, have analogous structures to a group of factors contained in
yeast
Yeasts are eukaryotic, single-celled microorganisms classified as members of the fungus kingdom. The first yeast originated hundreds of millions of years ago, and at least 1,500 species are currently recognized. They are estimated to constit ...
s and
animal cells
The cell is the basic structural and functional unit of life forms. Every cell consists of a cytoplasm enclosed within a membrane, and contains many biomolecules such as proteins, DNA and RNA, as well as many small molecules of nutrients an ...
. 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.
Genes exhibiting type-A function
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
In molecular biology, a transcription factor (TF) (or sequence-specific DNA-binding factor) is a protein that controls the rate of transcription of genetic information from DNA to messenger RNA, by binding to a specific DNA sequence. The fun ...
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
Post-transcriptional regulation is the control of gene expression at the RNA level. It occurs once the RNA polymerase has been attached to the gene's promoter and is synthesizing the nucleotide sequence. Therefore, as the name indicates, it occ ...
exists, which controls its 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
Sequence homology is the biological homology between DNA, RNA, or protein sequences, defined in terms of shared ancestry in the evolutionary history of life. Two segments of DNA can have shared ancestry because of three phenomena: either a sp ...
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.
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.
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
A dimer () ('' di-'', "two" + ''-mer'', "parts") is an oligomer consisting of two monomers joined by bonds that can be either strong or weak, covalent or intermolecular. Dimers also have significant implications in polymer chemistry, inorganic ch ...
. 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
In biology, phylogenetics (; from Greek φυλή/ φῦλον [] "tribe, clan, race", and wikt:γενετικός, γενετικός [] "origin, source, birth") is the study of the evolutionary history and relationships among or within groups o ...
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
The lily family, Liliaceae, consists of about 15 genera and 610 species of flowering plants within the order Liliales. They are monocotyledonous, perennial, herbaceous, often bulbous geophytes. Plants in this family have evolved with a fair a ...
have two nearly identical external petaloid verticils (the
tepal
A tepal is one of the outer parts of a flower (collectively the perianth). The term is used when these parts cannot easily be classified as either sepals or petals. This may be because the parts of the perianth are undifferentiated (i.e. of very ...
s). 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
Tulips (''Tulipa'') are a genus of spring-blooming perennial herbaceous bulbiferous geophytes (having bulbs as storage organs). The flowers are usually large, showy and brightly coloured, generally red, pink, yellow, or white (usually in warm ...
''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 frame
In molecular biology, open reading frames (ORFs) are defined as spans of DNA sequence between the start and stop codons. Usually, this is considered within a studied region of a prokaryotic DNA sequence, where only one of the six possible readin ...
s that code for proteins with 210 to 214
amino acid
Amino acids are organic compounds that contain both amino and carboxylic acid functional groups. Although hundreds of amino acids exist in nature, by far the most important are the alpha-amino acids, which comprise proteins. Only 22 alpha a ...
s. Phylogenetic analysis of these sequences indicated that they belong to B gene family of the
monocotyledon
Monocotyledons (), commonly referred to as monocots, (Lilianae ''sensu'' Chase & Reveal) are grass and grass-like flowering plants (angiosperms), the seeds of which typically contain only one embryonic leaf, or cotyledon. They constitute one of ...
s.
In situ hybridization
''In situ'' hybridization (ISH) is a type of hybridization that uses a labeled complementary DNA, RNA or modified nucleic acids strand (i.e., probe) to localize a specific DNA or RNA sequence in a portion or section of tissue (''in situ'') o ...
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
''Agapanthus'' is a genus of plants, the only one in the subfamily Agapanthoideae of the family Amaryllidaceae. The family is in the monocot order Asparagales. The name is derived from Greek: ἀγάπη (''agapē'' – "love"), ἄνθος ('' ...
'' also follows the modified ABC model.
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.
Therefore, the ''AG'' mutants are devoid of
androecium
The stamen (plural ''stamina'' or ''stamens'') is the pollen-producing reproductive organ of a flower. Collectively the stamens form the androecium., p. 10
Morphology and terminology
A stamen typically consists of a stalk called the fila ...
and
gynoecium
Gynoecium (; ) is most commonly used as a collective term for the parts of a flower that produce ovules and ultimately develop into the fruit and seeds. The gynoecium is the innermost whorl of a flower; it consists of (one or more) ''pistils' ...
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
Maize ( ; ''Zea mays'' subsp. ''mays'', from es, maíz after tnq, mahiz), also known as corn (North American and Australian English), is a cereal grain first domesticated by indigenous peoples in southern Mexico about 10,000 years ago. The ...
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'').
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'').
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
A seed is an embryonic plant enclosed in a protective outer covering, along with a food reserve. The formation of the seed is a part of the process of reproduction in seed plants, the spermatophytes, including the gymnosperm and angiospe ...
dispersal.
The appearance of interesting phenotypes in
RNA interference
RNA interference (RNAi) is a biological process in which RNA molecules are involved in sequence-specific suppression of gene expression by double-stranded RNA, through translational or transcriptional repression. Historically, RNAi was known by ...
studies in ''Petunia'' and
tomato
The tomato is the edible berry of the plant ''Solanum lycopersicum'', commonly known as the tomato plant. The species originated in western South America, Mexico, and Central America. The Mexican Nahuatl word gave rise to the Spanish word ...
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.
See also
*
Flower
A flower, sometimes known as a bloom or blossom, is the reproductive structure found in flowering plants (plants of the division Angiospermae). The biological function of a flower is to facilitate reproduction, usually by providing a mechani ...
*
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 /Tsub>6G ...
*
Mutation
In biology, a mutation is an alteration in the nucleic acid sequence of the genome of an organism, virus, or extrachromosomal DNA. Viral genomes contain either DNA or RNA. Mutations result from errors during DNA replication, DNA or viral repl ...
*
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 ...
*
Superman (gene)
References
Sources
General texts
*
*
External links
*
{{botany
Developmental genetics
Plant development