| A Brief Look at Plant Development |
Despite evolving multicellularity independently, plants and animals share some common features in their respective development. These shared features include homeotic mutations and the use of transcription factors. Research in plant development also started with model organisms - in this case, the mustard grass Arabidopsis thaliana. In Arabidopsis and other plants, the developing flower is composed of four concentric whorls. The outermost whorl (Whorl 1) is fated to become the sepals, the outer floral leaves. It surrounds Whorl 2, which is fated to become the petals, the white inner floral leaves. Whorl 3 is fated to become stamens, which contains the male organs. The innermost whorl (Whorl 4) is fated to become the carpels, which will form the ovary (Fig. 9).
There are several homeotic mutations in flowers where different parts replace others. For example, in one class of mutations, sepals develop where petals should and carpals develop where stamens should. These mutations have been identified as defects of a family of genes that all encode a particular class of transcription factor, called the MADS box family. MADS box transcription factors occur in both plants and, to a lesser extent, in animals and contain a conserved fifty-eight amino acid sequence.
Plants and animals differ in one important feature: the maintenance of totipotent cells. Cells, like the fertilized egg, which can make all of the cells of the organism, are said to be totipotent. In the process of animal development, the competence of the cells to become different cell types declines. But as cells become more differentiated, they continue to lose competence: In animals, pluripotent cells can produce most, but not all, types of cells, while multipotent cells can produce only a defined set of mature cells. Plants, however, have an apical meristem located at the tip of every root and stem that remains totipotent. They have other meristems that are also totipotent. Moreover, under the right conditions many differentiated plant cells are able to "de-differentiate to the embryonic state and subsequently redifferentiate to new cell types."3