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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 Understanding is a psychological Psychology is the science of mind and behavior. Psychology inc ...
of the process by which
flowering plant Flowering plants include multiple members of the clade Angiospermae (), commonly called angiosperms. The term "angiosperm" is derived from the Greek#REDIRECT Greek Greek may refer to: Greece Anything of, from, or related to Greece Greec ...

flowering plant
s produce a pattern of
gene expression Gene expression is the process by which information from a gene is used in the synthesis of a functional gene product that enables it to produce end products, protein or non-coding RNA, and ultimately affect a phenotype, as the final effect. The ...

gene expression
in
meristem The meristem is a type of tissue Tissue may refer to: Biology * Tissue (biology), an ensemble of similar cells that together carry out a specific function * ''Triphosa haesitata'', a species of geometer moth found in North America * ''Triphosa du ...
s that leads to the appearance of an organ oriented towards
sexual reproduction Sexual reproduction is a type of reproduction Reproduction (or procreation or breeding) is the biological process Biological processes are those processes that are vital for an organism In biology, an organism (from Ancient Greek, ...
, a flower. There are three
physiological Physiology (; ) is the scientific Science () is a systematic enterprise that builds and organizes knowledge Knowledge is a familiarity or awareness, of someone or something, such as facts A fact is an occurrence in the real world. ...
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 flower A flower, sometimes known as a bloom or blossom Cherry blossoms in Paris in full bloom. In botany, blossoms are the flowers of stone fruit fruit tree, trees (genus ''Prunus'') and of some ...
; 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 A molecule is an electrically Electricity is the set of physical phenomena associated with the presence and motion Image:Leaving Yongsan Station.jpg, 300px, Motion involves a change in position In physics, motion is the phenomenon ...
and
developmental Development of the human body is the process of growth to maturity. The process begins with fertilization Fertilisation or fertilization (see spelling differences), also known as generative fertilisation, syngamy and impregnation, is the ...
genetics. An external
stimulus A stimulus is something that causes a physiological response. It may refer to: *Stimulation Stimulation is the encouragement of development or the cause of activity generally. For example, "The press provides stimulation of political discourse." ...
is required in order to trigger the
differentiation Differentiation may refer to: Business * Differentiation (economics), the process of making a product different from other similar products * Product differentiation, in marketing * Differentiated service, a service that varies with the identity o ...
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 gives rise to genetically identical cells in which the total number of chromosomes is maintained. In gene ...

mitotic
cell division in the apical meristem, particularly on its sides where new
primordia A primordium (; plural: primordia; synonym: anlage) in embryology Embryology (from Ancient Greek, Greek ἔμβρυον, ''embryon'', "the unborn, embryo"; and -λογία, ''-logy, -logia'') is the branch of biology that studies the Prenat ...
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 Fertilisation or fertilization (see spelling differences), also known as generative fertilisation, syngamy and impregnation, is the ...
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 Botany, also called , plant biology or phytology, is the science of plant life and a branch of biology. A botanist, plant scientist or phytologist is a scientist who specialises in this field. The term "botany" comes from the Anci ...
, that is, the absence of
stem Stem or STEM may refer to: Biology * Plant stem, the aboveground structures that have vascular tissue and that support leaves and flowers ** Stipe (botany), a stalk that supports some other structure ** Stipe (mycology), the stem supporting the c ...

stem
elongation among the successive whorls or verticils of the primordium. These verticils follow an acropetal development, giving rise to
sepal A sepal ( or ) is a part of the flower A flower, sometimes known as a bloom or blossom Cherry blossoms in Paris in full bloom. In botany, blossoms are the flowers of stone fruit fruit tree, trees (genus ''Prunus'') and of some other p ...
s,
petal Petals are modified leaves A leaf (plural leaves) is the principal lateral appendage of the vascular plant plant stem, stem, usually borne above ground and specialized for photosynthesis. The leaves, stem, flower and fruit together fo ...

petal
s,
stamen The stamen (plural The plural (sometimes abbreviated An abbreviation (from Latin ''brevis'', meaning ''short'') is a shortened form of a word or phrase, by any method. It may consist of a group of letters, or words taken from the full ve ...
s and
carpel Gynoecium (; ) is most commonly used as a collective term for the parts of a flower A flower, sometimes known as a bloom or blossom Image:Cerisier du Japon Prunus serrulata.jpg, Cherry blossoms in Paris in full bloom. In botany, blossoms ...
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
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 Gene expression, 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 c ...
, 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 (botany), calyx (sepals) and the corolla (flower), ...
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 German(s) may refer to: Common uses * of or related to Germany * Germans, Germanic ethnic group, citizens of Germany or people of German ancestry * For citizens of G ...

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:


Floral transition

The transition from the
vegetative phase Vegetative phase change is the juvenile (organism), juvenile-to-adult transition in plants. This transition is distinct from the Flower#Flowering transition, reproductive transition and is most prolonged and pronounced in Woody plant, woody species ...
to a
reproductive phase
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 Descendant(s) or descendent(s) may refer to: * Lineal descendant A lineal descendant, in legal usage, is a blood relative in the direct line of descent – the children Biologically, a child (plural children) is a human being between the st ...

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 Morphogenesis (from the Greek#REDIRECT Greek Greek may refer to: Greece Anything of, from, or related to Greece Greece ( el, Ελλάδα, , ), officially the Hellenic Republic, is a country located in Southeast Europe. Its population is a ...
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 A leaf (plural leaves) is the principal lateral appendage of the vascular plant plant stem, stem, usually borne above ground and specialized for photosynthesis. The leaves, stem, flower and fruit together form the shoot system. Leaves are ...

leaves
and contain a certain level of total
biomass Biomass is plant or animal material used as fuel to produce electricity Electricity is the set of physical phenomena associated with the presence and motion Image:Leaving Yongsan Station.jpg, 300px, Motion involves a change in position ...

biomass
. Certain environmental conditions are also required such as a characteristic
photoperiod Photoperiodism is the physiological reaction of organisms to the length of night or a dark period. It occurs in plant Plants are predominantly photosynthetic Photosynthesis is a process used by plants and other organisms to Energy trans ...
.
Plant hormone Plant hormones (or phytohormones) are signal molecule In biology, cell signaling (cell signalling in British English), or cell-cell communication, governs the basic activities of cell (biology), cells and coordinates multiple-cell actions. A si ...
s play an important part in the process, with the
gibberellin Gibberellins (GAs) are plant hormone Plant hormones (or phytohormones) are signal molecule In biology, cell signaling (cell signalling in British English), or cell-cell communication, governs the basic activities of cell (biology), cells and ...
s having a particularly important role. There are many signals that regulate the
molecular biology Molecular biology is the branch of biology Biology is the natural science that studies life and living organisms, including their anatomy, physical structure, Biochemistry, chemical processes, Molecular biology, molecular interactions, P ...
of the process. The following three genes in ''
Arabidopsis thaliana ''Arabidopsis thaliana'', the thale cress, mouse-ear cress or arabidopsis, is a small flowering plant Flowering plants include multiple members of the clade Angiospermae (), commonly called angiosperms. The term "angiosperm" is derived from ...

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 In biology, a sequence motif is a nucleotide or amino acid, amino-acid Sequence (biology), sequence pattern that is widespread and usually assumed to be related to biological function of the macromolec ...
-type gene, which integrates responses to photoperiod,
vernalization Vernalization (from Latin Latin (, or , ) is a classical language belonging to the Italic languages, Italic branch of the Indo-European languages. Latin was originally spoken in the area around Rome, known as Latium. Through the power of ...
and gibberellins.


Formation of the floral meristem or the inflorescence

The
meristem The meristem is a type of tissue Tissue may refer to: Biology * Tissue (biology), an ensemble of similar cells that together carry out a specific function * ''Triphosa haesitata'', a species of geometer moth found in North America * ''Triphosa du ...
can be defined as the tissue or group of plant tissues that contain undifferentiated
stem cells In multicellular organisms Multicellular organisms are organism In biology, an organism (from Ancient Greek, Greek: ὀργανισμός, ''organismos'') is any individual contiguous system that embodies the Life#Biology, properties ...
, 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 Developmental biology is the study of the process by which animal Animals (also called Metazoa) are multicellular A multicellular organism is an organism In biology, an organism () i ...
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 Negative feedback (or balancing feedback) occurs when some function Function or functionality may refer to: Computing * Function key A function key is a key on a computer A computer is a machine that can be programmed to carry out se ...
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 A chain is a wikt:series#Noun, serial assembly of connected pieces, called links, typically made of metal, with an overall character similar to that of ...

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 Differentiation may refer to: Business * Differentiation (economics), the process of making a product different from other similar products * Product differentiation, in marketing * Differentiated service, a service that varies with the identity o ...
of the stem cells but still allows
cell division Cell division is the process by which a parent cell (biology), cell divides into two or more daughter cells. Cell division usually occurs as part of a larger cell cycle. In eukaryotes, there are two distinct types of cell division; a vegetative ...

cell division
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 Reproduction (or procreation or breeding) is the biological process Biological processes are those processes that are vital for an organism In biology, an organism (from Ancient Greek, ...
in
flowering plant Flowering plants include multiple members of the clade Angiospermae (), commonly called angiosperms. The term "angiosperm" is derived from the Greek#REDIRECT Greek Greek may refer to: Greece Anything of, from, or related to Greece Greec ...

flowering plant
s. 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. * Meristem identity genes. Code for the
transcription factor In molecular biology Molecular biology is the branch of biology that seeks to understand the molecule, molecular basis of biological activity in and between Cell (biology), cells, including biomolecule, molecular synthesis, modification, m ...
s 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.


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 A clade (), also known as a monophyletic group or natural group, is a group of organisms that are monophyly, monophyletic – that is, composed of a common ancestor and all its lineage (evolution), line ...
, as exemplified by ''Arabidopsis thaliana'', and the
Asterids In the APG IV systemAPG is an abbreviation with several different meanings: * Aberdeen Proving Ground, a United States Army installation in Aberdeen, Maryland, also ** Phillips Army Airfield, the airfield of the above, from its IATA airport co ...

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 geneIn 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 factors, transcr ...
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 Molecular genetics is a sub-field of biology that addresses how differences in the structures or expression of DNA molecules manifests as variation among organisms. Molecular genetics often ...
techniques to produce transgenic plants that contain a mechanism for
gene silencing Gene silencing is the regulation of gene expression Regulation of gene expression, or gene regulation, includes a wide range of mechanisms that are used by cells to increase or decrease the production of specific gene products (protein Pro ...
through
RNA interference RNA interference (RNAi) is a biological process in which RNA Ribonucleic acid (RNA) is a polymer A polymer (; Greek ''wikt:poly-, poly-'', "many" + ''wikt:-mer, -mer'', "part") is a Chemical substance, substance or material consisting ...
. In other studies, using
forward genetics Forward genetics is a molecular genetics Molecular genetics is a sub-field of biology that addresses how differences in the structures or expression of DNA Deoxyribonucleic acid (; DNA) is a molecule File:Pentacene on Ni(111) STM.j ...
techniques such as
genetic mapping Genetic linkage is the tendency of DNA sequences A nucleic acid sequence is a succession of bases signified by a series of a set of five different letters that indicate the order of nucleotides Nucleotides are organic molecules , CH4; is ...
, it is the analysis of the
phenotype In genetics Genetics is a branch of biology Biology is the natural science that studies life and living organisms, including their anatomy, physical structure, Biochemistry, chemical processes, Molecular biology, molecular inter ...

phenotype
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 Asexual reproduction is a type ...

cloning
of the gene of interest. The flowers may possess a non-functional or
allele An allele (, ; ; modern formation from Greek ἄλλος ''állos'', "other") is one of two, or more, forms of a given gene In biology, a gene (from ''genos'' "...Wilhelm Johannsen coined the word gene to describe the Mendelian_inheritance ...
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 plant A seed is an embryonic ''Embryonic'' is the twelfth studio album by experimental rock band the Flaming Lips released on October 13, 2009, on Warner Bros. Records, Warner Bros. The band's first double album, it was released ...

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 product A gene product is the biochemical material, either RNA Ribonucleic acid (RNA) is a polymer A polymer (; Greek ''wikt:poly-, poly-'', "many" + ''wikt:-mer, -mer'', "part") is a Chemical substance, substance or material consisting of very ...
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 Genetics is a branch of biology Biology is the natural science that studies life and living organisms, including their anatomy, physical structure, Biochemistry, chemical processes, Molecular biology, molecular inter ...

phenotype
s for the relative characteristics of the flowers, which allows the characterization of the
homeotic genes 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 factors, transc ...
implicated in the process.


Analysis of mutants

There are a great many
mutation In biology Biology is the natural science that studies life and living organisms, including their anatomy, physical structure, Biochemistry, chemical processes, Molecular biology, molecular interactions, Physiology, physiological mechan ...
s that affect floral
morphology Morphology, from the Greek and meaning "study of shape", may refer to: Disciplines *Morphology (archaeology) In archaeology, morphology is the study of the shape of Artifact (archaeology), artefacts and ecofacts. Morphology is a major consid ...
, 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 fly, flies, belonging to the family (biology), family Drosophilidae, whose members are often called "small fruit flies" or (less frequently) pomace flies, vinegar flies, or wine flies, a reference to the character ...

Drosophila
''. In ''Arabidopsis'' and ''
Antirrhinum ''Antirrhinum'' is a genus of plants commonly known as dragon flowers or snapdragons because of the flowers' fancied resemblance to the face of a European dragon, dragon that opens and closes its mouth when laterally squeezed. They are native ...

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: * 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 (botany), calyx (sepals) and the corolla (flower), ...
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.


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 Asexual reproduction is a type ...

Cloning
studies have been carried out on
DNA Deoxyribonucleic acid (; DNA) is a molecule A scanning tunneling microscopy image of pentacene molecules, which consist of linear chains of five carbon rings. A molecule is an electrically Electricity is the set of physical ...

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 Transcriptomics technologies, 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 corresp ...
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 Molecular biology is the branch of biology that seeks to understand the molecule, molecular basis of biological activity in and between Cell (biology), cells, including biomolecule, molecular synthesis, modification, m ...

transcription factors
, which, as expected, have analogous structures to a group of factors contained in
yeast Yeasts are eukaryotic Eukaryotes () are organism In biology Biology is the natural science that studies life and living organisms, including their anatomy, physical structure, Biochemistry, chemical processes, Molecular ...

yeast
s and
animal cells The cell (from Latin Latin (, or , ) is a classical language A classical language is a language A language is a structured system of communication Communication (from Latin ''communicare'', meaning "to share" or "to be in ...
. 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 Regulation of gene expression, or gene regulation, includes a wide range of mechanisms that are used by cells to increase or decrease the production of specific gene product A gene product is the biochemical material, either RNA Ribonucle ...

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 Molecular biology is the branch of biology that seeks to understand the molecule, molecular basis of biological activity in and between Cell (biology), cells, including biomolecule, molecular synthesis, modification, m ...

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 Post-transcriptional regulation is the control of gene expression Gene expression is the process by which information from a gene is used in the synthesis of a functional gene product that enables it to produce end products, protein or non-co ...
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 homology (biology), biological homology between DNA sequence, DNA, RNA sequence, RNA, or Protein primary structure, protein sequences, defined in terms of shared ancestry in the evolutionary history of life. Two segments ...
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 () (''wikt:di-, di-'', "two" + ''-mer'', "parts") is an oligomer consisting of two monomers joined by bonds that can be either strong or weak, Covalent bond, covalent or Intermolecular force, intermolecular. The term ''homodimer'' is used ...
. 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 Biology is the natural science that studies life and living organisms, including their anatomy, physical structure, Biochemistry, chemical processes, Molecular biology, molecular interactions, Physiology, physiological mechanism ...

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
dicotyledon The dicotyledons, also known as dicots (or more rarely dicotyls), are one of the two groups into which all the flowering plant Flowering plants include multiple members of the clade Angiospermae (), commonly called angiosperms. The term "a ...
s, 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 In human society A society is a Social group, group of individuals involved in persistent Social relation, social interaction, or a large social group sharing the same spatial or social territory, typically subj ...

Liliaceae
have two nearly identical external petaloid verticils (the
tepal A tepal is one of the outer parts of a flower A flower, sometimes known as a bloom or blossom Image:Cerisier du Japon Prunus serrulata.jpg, Cherry blossoms in Paris in full bloom. In botany, blossoms are the flowers of stone fruit fruit tree, ...
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 Genus /ˈdʒiː.nəs/ (plural genera /ˈdʒen.ər.ə/) is a taxonomic rank In biological classification In biology Biology is the natural science that studies life and living organisms, inclu ...

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 frame In molecular biology Molecular biology is the branch of biology that seeks to understand the molecule, molecular basis of biological activity in and between Cell (biology), cells, including biomolecule, molecular synthesis, modification, mech ...

open reading frame
s that code for proteins with 210 to 214
amino acid Amino acids are organic compound In , organic compounds are generally any s that contain - . Due to carbon's ability to (form chains with other carbon s), millions of organic compounds are known. The study of the properties, reactions, a ...

amino acid
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 Embryo#Plant embryos, embryonic leaf, or cotyledon. Th ...
s.
In situ hybridization ''In situ'' hybridization (ISH) is a type of hybridization that uses a labeled complementary DNA In genetics Genetics is a branch of biology Biology is the natural science that studies life and living organisms, including the ...

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 ''Agapanthus'' is the only genus in the subfamily Agapanthoideae of the flowering plant Family (biology), family Amaryllidaceae. The family is in the monocot Order (biology), order Asparagales. The name is derived from Greek: ἀγάπη (''agap ...

Agapanthus
'' 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 The plural (sometimes abbreviated An abbreviation (from Latin ''brevis'', meaning ''short'') is a shortened form of a word or phrase, by any method. It may consist of a group of letters, or words taken from the full ve ...

androecium
and
gynoecium Gynoecium (; ) is most commonly used as a collective term for the parts of a flower A flower, sometimes known as a bloom or blossom Image:Cerisier du Japon Prunus serrulata.jpg, Cherry blossoms in Paris in full bloom. In botany, blossoms ...

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
anther The stamen (plural The plural (sometimes abbreviated An abbreviation (from Latin ''brevis'', meaning ''short'') is a shortened form of a word or phrase, by any method. It may consist of a group of letters, or words taken from the full ve ...
s and the maturation of
pollen Pollen is a powdery substance consisting of pollen grains which are microsporophytes of seed plants The spermatophytes (; ), also known as phanerogams (taxon Phanerogamae) or phaenogams (taxon Phaenogamae), comprise those plant Plant ...

pollen
. In ''Petunia'', ''Antirrhinum'' and in
maize Maize ( ; ''Zea mays'' subsp. ''mays'', from es, maíz after tnq, mahiz), also known as corn (North American North America is a continent in the Northern Hemisphere and almost entirely within the Western Hemisphere. It can also be ...

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'').


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 ''Embryonic'' is the twelfth studio album by experimental rock band the Flaming Lips released on October 13, 2009, on Warner Bros. Records, Warner Bros. The band's first double album, it was released to generally positi ...

seed
dispersal. The appearance of interesting phenotypes in
RNA interference RNA interference (RNAi) is a biological process in which RNA Ribonucleic acid (RNA) is a polymer A polymer (; Greek ''wikt:poly-, poly-'', "many" + ''wikt:-mer, -mer'', "part") is a Chemical substance, substance or material consisting ...
studies in ''Petunia'' and
tomato The tomato is the edible berry A berry is a small, pulpy, and often edible fruit In botany, a fruit is the seed-bearing structure in flowering plants (also known as angiosperms) formed from the ovary after flowering. Fruits are ...

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.


See also

*
Flower A flower, sometimes known as a bloom or blossom Image:Cerisier du Japon Prunus serrulata.jpg, Cherry blossoms in Paris in full bloom. In botany, blossoms are the flowers of stone fruit fruit tree, trees (genus ''Prunus'') and of some other plan ...

Flower
*
MADS-box The MADS box is a conserved sequence motif In biology, a sequence motif is a nucleotide or amino acid, amino-acid Sequence (biology), sequence pattern that is widespread and usually assumed to be related to biological function of the macromolec ...
*
Mutation In biology Biology is the natural science that studies life and living organisms, including their anatomy, physical structure, Biochemistry, chemical processes, Molecular biology, molecular interactions, Physiology, physiological mechan ...
*
Plant evolutionary developmental biology Evolutionary developmental biology Evolutionary developmental biology (informally, evo-devo) is a field of biological research Biology is the natural science that studies life and living organisms, including their anatomy, physical struc ...
* Superman (gene)


References


Sources


General texts

* *


External links

* {{botany Plant development