The genes that determine floral organ identity were discovered as floral homeotic mutants (see Chapter 14 on the
1 Also known as SUPPRESSOR OF OVEREXPRESSION OF CONSTANS 1 (SOC1).
web site). As discussed in Chapter 14, mutations in the fruit fly, Drosophila, led to the identification of a set of homeotic genes encoding transcription factors that determine the locations at which specific structures develop. Such genes act as major developmental switches that activate the entire genetic program for a particular structure. The expression of homeotic genes thus gives organs their identity.
As we have seen already in this chapter, dicot flowers consist of successive whorls of organs that form as a result of the activity of floral meristems: sepals, petals, stamens, and carpels. These organs are produced when and where they are because of the orderly, patterned expression and interactions of a small group of homeotic genes that specify floral organ identity.
The floral organ identity genes were identified through homeotic mutations that altered floral organ identity so that some of the floral organs appeared in the wrong place. For example, Arabidopsis plants with mutations in the APETALA2 (AP2) gene produce flowers with carpels where sepals should be, and stamens where petals normally appear.
The homeotic genes that have been cloned so far encode transcription factors—proteins that control the expression of other genes. Most plant homeotic genes belong to a class of related sequences known as MADS box genes, whereas animal homeotic genes contain sequences called home-oboxes (see Chapter 14 on the web site).
Many of the genes that determine floral organ identity are MADS box genes, including the DEFICIENS gene of snapdragon and the AGAMOUS, PISTILLATA1, and APETALA3 genes of Arabidopsis. The MADS box genes share a characteristic, conserved nucleotide sequence known as a MADS box, which encodes a protein structure known as the MADS domain. The MADS domain enables these transcription factors to bind to DNA that has a specific nucleotide sequence.
Not all genes containing the MADS box domain are homeotic genes. For example, AGL20 is a MADS box gene, but it functions as a meristem identity gene.
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