Box 12.1The genetic basis of flower development

Developmental processes in plants are, in most respects, very different from those of fruit flies and other animals, but at the genetic level there are certain similarities, sufficient for the knowledge gained about Drosophila development to be of value in interpreting similar research carried out with plants. In particular, the recognition that a limited number of homeotic selector genes control the Drosophila body plan has led to a model for plant development which postulates that the structure of the flower is determined by a small number of homeotic genes.

All flowers are constructed along similar lines, made up of four concentric whorls, each comprising a different floral organ. The outer whorl, number 1, contains sepals, which are modified leaves that envelop and protect the bud during its early development. The next whorl, number 2, contains the distinctive petals, and within these are whorls 3 (stamens, the male reproductive organs) and 4 (carpels, the female reproductive organs).

Most of the research on plant development has been carried out with Antirrhinum (the snapdragon) and Arabidopsis thaliana, a small vetch that has been adopted as a model species, partly because it has a genome of only 125 Mb (see Table 2.1), one of the smallest known among flowering plants. Although these plants do not appear to contain homeodomain proteins, they do have genes which, when mutated, lead to homeotic changes in the floral architecture, such as replacement of sepals by carpels. Analysis of these mutants has led to the ‘ABC model’, which states that there are three types of homeotic genes - A, B and C - controlling flower development:

  • Whorl 1 is specified by A-type genes: examples in Arabidopsis are apetala1 and apetala2.
  • Whorl 2 is specified by A genes acting in concert with B genes, examples of the latter including apetala3 and pistillata.
  • Whorl 3 is specified by the B genes plus the C gene, agamous.
  • Whorl 4 is specified by the C gene acting on its own.Image ch12fb2.jpg

As anticipated from the work with Drosophila, the A, B and C homeotic gene products are transcription activators. All except the APETALA2 protein contain the same DNA-binding domain, the MADS box, which is also found in other proteins involved in plant development, including SEPALLATA1, 2 and 3, which work with the A, B and C proteins in defining the detailed structure of the flower (Theissen and Saedler, 2001). Other components of the flower development system include at least one master gene, called floricaula in Antirrhinum and leafy in Arabidopsis, which controls the switch from vegetative to reproductive growth, initiating flower development, and also has a role in establishing the pattern of homeotic gene expression (Ma, 1998; Parcy et al., 1998). In Arabidopsis there is also a gene, called curly leaf, whose product acts like Polycomb of Drosophila (Section 12.2.2), maintaining the differentiated state of each cell by repressing those homeotic genes that are inactive in a particular whorl (Goodrich et al., 1997).

From: Chapter 12, Regulation of Genome Activity

Cover of Genomes
Genomes. 2nd edition.
Brown TA.
Oxford: Wiley-Liss; 2002.
Copyright © 2002, Garland Science.

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