THE ABC MODEL OF FLOWER DEVELOPMENT

THE ABC MODEL OF FLOWER DEVELOPMENT

18.          THE ABC MODEL OF FLOWER DEVELOPMENT

Arabidopsis thaliana is an angiosperm belongs to Brassicaceae family. Arabidopsis thaliana possess four concentric whorls or verticals (a circular arrangement of the flower which is growing around a central point) which follow an acropetal development and responsible for the formation of sepals, petals, stamen and carpels. As floral meristem determined, its cells will no longer are to divide after getting differentiated. ABC model in Arabidopsis contains three classes of the homeotic gene called ABC genes which express among four whorls. These three classes of genes specify floral organ identity in the developing flower: Cells of two adjacent floral whorls govern by the activity of each class of genes. Gene belongs to each class is expressed in specific parts of the meristem. yet, there are areas of overlap present in gene expression. Gene which causes the transition of flower meristematic cell into different parts of the flower, activated by ABC gene. ABC genes product are MADS-box contain transcription factors, except AP2, MADS-box binds to DNA and a K-box sequence responsible for dimerization. Therefore, these proteins form dimers on DNA. ABC genes activate the expression of other genes that cause the transition of flower meristematic cell into different parts of the flower.

18.1.      Class A gene

Class A genes encoded two genes APETALA 1 (AP1) and APETALA 2 (AP2), whose product act as a transcription factor and responsible for the formation of sepal and petal within whorls 1 and 2 respectively. Both genes also act in the floral meristem and AP2 also involve in ovule as well as leaves formation.

18.2.      Class B gene

Class B genes encoded two genes APETALA 3 (AP3) and PISTILLATA (PI), whose product act as a transcription factor and responsible for the formation of petals and stamens within whorls 2 and 3 respectively. Both genes also act in the floral meristem. Mutation of these genes results in the conversion of petals into sepals and of stamens into carpels.

18.3.      Class C gene

Class C genes encoded one gene AGAMOUS (AG), whose product act as a transcription factor and responsible for the formation of stamens and carpel within whorls 3 and 4 respectively. Genes also act in the floral meristem. the AG mutants show the conversion of androecium or stamen and gynoecium or carpel into petals and sepals, in their places.

The functional proteins class A inhibit C gene expression in the first two whorls and functional protein of C inhibit class A gene expression in the third and fourth whorl. Thus, mutually antagonistic relationship between class A and class C gene, the remaining gene get extend due to the mutation in class A or class C gene.

When chemical signal florigen produces by leaves, it is passed to the apical vegetative meristem and turn on the Flowering Locus T gene, as a result, the expression of LEAFY gene takes place. Leafy act as a transcription factor for ABC gene and binds to the promoter of ABC genes as the dimer, It switches on the ABC gene expression and causes undifferentiated cells in the meristem to change fate and develop as flowers.

A leafy plant result due to the mutation in LEAFY because the developmental ground state of a floral organ is a leaf. If there are no ABC transcription factors to switch on floral genes then vegetative meristem carries on making leaves, as a result, no transition from vegetative growth to flowering.

Short terminal flower formation takes place due to overexpression of LEAFY. Terminal Flower 1 (TFL1) cause repression of LEAFY activity. A single flower development takes place due to mutation of TFL1 and here LEAFY is no longer repressed.   Conversely, flowering is delayed, if TFL1 is constitutively expressed.

Mutational effect

ABC mutant genes plant produce homeotic mutant flowers.

Class A Mutants

Sepals and petals get transformed into stamens and carpels. This type of mutant called APETALA-1 and APETALA-2. Class A gene does not cause inhibition of Class C genes activity, consequently, it gets spread into the other whorls.

Class B Mutants

Petals and stamens get transformed into sepals and carpels respectively. This type of mutant called APETALA 3. ABC genes cannot work together in the absence of class B gene, thus petal and stamen development take place.

Class C Mutants

Stamens and carpels get transformed into petals and sepals. Class C gene does not cause inhibition of Class A gene activity consequently it gets spread into the other whorls. This type of mutant called agamous.

Double mutant

Only sepals formation takes place in all four whorls if the mutation occurs in both class B gene and class C gene.

Only carpels formation takes place in all four whorls if the mutation occurs in both classes A gene and class B gene.

Class A gene and class C gene mutant: leaves formation takes place in whorls first and fourth along with petal and stamen intermediates formation takes place in whorls second and third if the mutation occurs in both classes A gene and class C gene.


Next Previous