Genetic Mechanisms and Development

Development in plants refers to the formation of shape and pattern in the multicellular organism. While development can be influenced by environmental factors such as light or temperature, the major factor controlling development of any plant is, of course, its genes. Genes determine the overall shape and size of the mature plant, its branching pattern and leaf type, the extent and arrangement of vascular tissue in root and shoot, and the timing of flowering and the form of flowers produced.

In plants the genetic mechanisms that control developmental events are best understood in the case of flower development. A normal (wild type) flower of most angiosperms (flowering plants) has four distinct types of organs that are arranged in four concentric rings (whorls). The outermost whorl has green, leaflike organs called sepals . The second whorl from outside consists of brightly colored organs called petals. The third whorl consists of stamens, which are male reproductive structures that make pollen. The innermost whorl consists of female reproductive structures called carpels. Although the number and shape of these organs differ from species to species, they are genetically determined and develop sequentially from outside to inside (sepals, petals, stamens, and carpels). All floral organs develop from a small group of undifferentiated cells known as the floral meristem.

The genetic basis for this pattern formation was not known until recently. During the 1990s enormous progress was made in identifying the genes that determine the floral organ identity. Much of this information came from genetic studies with Arabidopsis thaliana (mouse ear cress or thale cress), which belongs to the mustard family, and Antirrhinum majus (snap-dragon). A normal flower of Arabidopsis has four sepals, four petals, six stamens, and two carpels. Genetic studies with Arabidopsis and snapdragon indicate that three classes of genes (called class A, B, and C) work together to determine the organ identity and are responsible for the development of the right floral organs in the right place.

Each class of genes acts in two adjacent whorls in a combinatorial fashion to specify organ identity. Whether the cells in the floral meristem develop into a particular organ will depend on the expression of one or two of these classes of genes. Class A genes are active in whorls 1 and 2, Class B genes function in whorls 2 and 3, and class C genes are active in whorls 3 and 4. The activity of class A genes alone leads to the development of sepals, expression of A and B in cells leads to petal development, B and C class genes are necessary for stamen development, and the activity of C class alone allows carpel development. In addition, A activity inhibits the expression of C class and vice versa such that C activity is found in all four whorls of A mutants whereas A activity is found in all four whorls in C mutants.

This model has been supported by several kinds of genetic tests by creating single, double, or triple mutations in ABC genes, expression analysis of these genes, and also by artificially expressing A, B, and/or C genes in wrong whorls. Because A inhibits C, inactivation of A results in expansion of C activity into the first and second whorls, and to the development of carpel-like structures in place of sepals (C acting alone) and stamens in place of petals (C acting with B). Where B is inactivated, stamens are converted to carpels and petals to sepals. When C is inactivated, the presence of A activity in the third and fourth whorls leads to conversion of stamens to petals and carpels to sepals. In plants lacking both B and C genes, class-A genes are expressed in all four whorls, leading to a flower with sepals in all four whorls. Overexpression of class-B genes in all four whorls results in a flower consisting of petals in the first and second whorls and stamens in third and fourth whorls. Inactivation of all three classes of genes results in a flower that has leaves in all four whorls, indicating that the floral organs are modified leaves consistent with theories of floral evolution.

Although the mechanisms through which the activity of ABC genes specifies the floral organ identity are not clear, it is likely that they regulate the expression of other genes that are involved in the development of a specific floral organ. This speculation is supported by the fact that four of the five genes that belong to ABC classes encode transcription factors . How these organ identity genes are turned on at the right time and in the right cells of the floral meristem is not completely understood.

see also Differentiation and Development; Embryogenesis; Flowers; Germination and Growth; Hormonal Control and Development; Hormones; Molecular Plant Genetics; Senescence.

A. S. N. Reddy

Bibliography

Clark, Steve E., and Elliot M. Meyerowitz. "Arabidopsis Flower Development." InArabidopsis, eds. Elliot Meyerowitz and Chris Somerville. New York: Cold Spring Harbor Press, 1994.

Coen, Enrico S., and Elliot M. Meyerowitz. "The War of the Whorls: Genetic Interactions Controlling Flower Development." Nature 353 (1991): 31-37.

Meyerowitz, Elliot M. "The Genetics of Flower Development." Scientific American 271 (1994): 56-65.

In Arabidopsis, A-class genes include two genes (APETALA1, AP1; APETALA2, AP2 ); B-class has two genes (APETALA3, AP3; PISTILLATA, PI ); and C-class has one gene (AGAMOUS, AG ).