Organ development
The molecular control of floral
organ identity determination is fairly well understood. In a simple model,
three gene activities interact in a combinatorial manner to determine the
developmental identities of the organ primordia within the floral meristem.
These gene functions are called A, B and C-gene functions. In the first floral
whorl only A-genes are expressed, leading to the formation of sepals. In the
second whorl both A- and B-genes are expressed, leading to the formation of
petals. In the third whorl, B and C genes interact to form stamens and in the
center of the flower C-genes alone give rise to carpels. The model is based
upon studies of homeotic mutants in Arabidopsis thaliana and snapdragon,
Antirrhinum majus. For example, when there is a loss of B-gene function, mutant
flowers are produced with sepals in the first whorl as usual, but also in the
second whorl instead of the normal petal formation. In the third whorl the lack
of B function but presence of C-function mimics the fourth whorl, leading to
the formation of carpels also in the third whorl. See also The ABC Model of
Flower Development.
Most genes central in this model belong to the MADS-box genes and are transcription factors that regulate the expression of the genes specific for each floral organ.
Most genes central in this model belong to the MADS-box genes and are transcription factors that regulate the expression of the genes specific for each floral organ.
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