Expression of transcription factor genes determines organ differentiation in plants

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Like animals, plants have organs—for example, leaves and roots. Many plants form flowers, and many flowers are composed of four types of organs: sepals, petals, stamens (male reproductive organs), and carpels (female reproductive organs). These floral organs occur in concentric whorls, with groups of each organ type encircling a central axis. The sepals are on the outside and the carpels are on the inside (Figure 19.11A).

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Figure 19.11 Organ Identity Genes in Arabidopsis Flowers (A) The four organs of a flower—carpels (yellow), stamens (green), petals (purple), and sepals (pink)—grow in whorls that develop from the floral meristem. (B) Floral organs are determined by three classes of organ identity genes whose polypeptide products combine in pairs to form transcription factors. (C) Combinations of polypeptide subunits in transcription factors activate gene expression for specific organs.

Activity 19.3 Genes and Development Simulation

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In Arabidopsis thaliana (thale cress), flowers develop in a radial pattern around the shoot (stem and leaf) apex as it develops and elongates. At the shoot apex and in other parts of the plant where growth and differentiation occur (such as the root tip), there are groups of undifferentiated, rapidly dividing cells called meristems. Each flower begins as a floral meristem of about 700 undifferentiated cells arranged in a dome, and the four whorls develop from this meristem. How is the identity of a particular whorl determined? Three classes of genes called organ identity genes encode proteins that act in combination to produce specific whorl features (Figure 19.11B and C):

  1. Genes in class A are expressed in whorls 1 and 2 (which form sepals and petals, respectively).

  2. Genes in class B are expressed in whorls 2 and 3 (which form petals and stamens).

  3. Genes in class C are expressed in whorls 3 and 4 (which form stamens and carpels).

These genes encode transcription factors that are active as dimers, that is, proteins with two polypeptide subunits. The composition of the dimer determines which genes the transcription factor activates. For example, a dimer made up of two class A monomers activates transcription of the genes that make sepals; a dimer made up of A and B monomers results 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. The name “MADS” comes from the initials of four genes encoding proteins with this domain.

Two lines of experimental evidence support this model for floral organ determination:

  1. Loss-of-function mutations: for example, a mutation in a class A gene results in no sepals or petals.

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  2. Gain-of-function mutations: for example, a promoter for a class C gene can be artificially coupled to a class A gene. In this case, the class A gene is expressed in all four whorls, resulting in only sepals and petals. In any organism, the replacement of one organ for another is called homeosis, and this type of mutation is a homeotic mutation.

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Transcription of the floral organ identity genes is controlled by other gene products, including the LEAFY protein. The wild-type LEAFY protein is a transcription factor that stimulates expression of the class A, B, and C genes so that they produce flowers. Plants with loss-of-function mutations in the LEAFY gene make stems instead of flowers, with increased numbers of modified leaves called bracts. This finding has practical applications. It usually takes 6–20 years for a citrus tree to produce flowers and fruits. Scientists have made transgenic orange trees expressing the LEAFY gene coupled to a strongly expressed promoter. These trees flower and fruit years earlier than normal trees.