The Steps of Flower Development - Genes Are Implicated
Mendelian Genes Define the Committment to Flowering
Mendelian Genes Define Floral Organ Identity
Cloning Committment to Flowering and Flower Organ Genes
Analyzing Gene Expression with In Situ Hybridization
The Molecular Expression of Floral Committment Genes
The Molecular Expression of Floral Organ Genes
A model was developed that attempts to explain the interactions of the different genes that
control floral organ identity. All the genes are classified as expressing
one of three activities, A, B, or C. The A gene activities control the development of
the sepal and petal, B gene activities control petal and stamen development, and C gene activities
control stamen and carpel development. The following diagram depicts these interactions.
The two A function genes are APETALA2 and APETALA1. Alleles of these two genes
have been isolated that show varying degrees of effect, but in general if an A function gene is
mutated, the first whorl develops as a carpelc and the second whorl develops as a stamen.
ovulata is an A function gene of snapdragon similar to APETALA2. The following
image shows the APETALA2 mutant.
The B gene functions are defined by the genes APETALA3 and PISTILLATA. The net effect
of B gene mutations is that whorl 2 develops as a sepal rather than a petal, and whorl 3 develops
as a carpel not a stamen. deficiens and globosa are snapdragon genes that have
homologous functions to the Arabidopsis B function genes. The following image shows the
Finally, C gene functions are defined by the gene AGAMOUS. Mutants of this gene have
the third whorl stamen replaced by a petal, and the fourth whorl develops into a new flower
with the sepal-petal-petal pattern. Furthermore, flower development in AGAMOUS mutants
is indeterminate, not determinate. A snapdragon gene similar to AGAMOUS is pleniflora.
The following image shows the AGAMOUS mutant.
The following table summaries the phenotypic effects of mutations in each of the three
Phenotypic Effects of Mutations in A, B or C Function Floral Identity Genes
Several conclusions can be drawn regarding the functions of these genes by studying single and
double mutants. Because a mutation of an A function gene results in the expressiion of organ
phenotypes controlled by C function genes, it appears that A gene functions repress the expression
of the C gene functions in the whorls giving rise to sepals and petals. Likewise, the appearance
of the petal in the third whorl of C gene mutants, suggest that C genes repress the activities of
A genes in the organs that they control.
These conclusions are based on single mutants. For example, APETALA2 A function mutants
develop C function organs, carpels and stamens, in the first two whorls, respectively. What would
be expected from an A and C double mutant? This mutant would not be expected to have any functions
exclusively controlled by the the A and C function genes. And indeed this is what was seen when the
APETALA2/AGAMOUS double mutant was developed. The first whorl develops as a leaf
and the second whorl has stamen-like petals. This second whorl phenotype of this mutant is the
result of the activities of the B gene functions.
Finally what would be expected with an A, B and C function triple mutant? This mutant would
have no genes functioning that determine normal floral organ development. As expected, the
triple mutants lack any floral organs, and the flower essentially consists of leaves developing
form each of the whorls.
Copyright © 1998. Phillip McClean