C

Wild-type

Arabidopsis flower

Whorl 4: carpel

Early flower differentiation (meristems)

Early flower differentiation (meristems)

Whorl 4: carpel

► Loss of function mutations: For example, a mutation in gene A results in no sepals or petals.

► Gain of function mutations: For example, the promoter for gene C can be coupled to gene A. In this case, A is expressed in all four whorls, resulting in only sepals and petals.

Genes A, B, and C code for subunits of transcription factors, which are active as dimers. Gene regulation in these cases is combinatorial—that is, the composition of the dimer determines which other genes will be activated by the transcription factor. For example, a dimer made up only of transcription factor A would activate transcription of the genes that make sepals; a dimer made up of A and B would result in petals, and so forth.

A common feature of the A, B, and C proteins, as well as many other plant transcription factors, is a DNA-binding domain called the MADS box (named for homologous regions found in four genes in yeast and in two in plants and humans, that all encode a similar amino acid sequence). These 200-amino acid proteins also have domains for interaction with other proteins.

In addition to being fascinating to biologists, plant organ identity genes have caught the eye of horticultural and agricultural scientists. Flowers filled with petals instead of stamens and carpels often have mutations of the C genes. Many of the foods that make up the human diet, such as the grains of wheat, rice, and corn, come from fruits and seeds. These fruits and seeds form from the carpels (the female reproductive organs) of the flower. Genetically modifying the number of carpels on a particular plant could increase the amount of grain a crop could produce.

flower

Mutation of class A: No petals or sepals; stamens and carpels instead

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