The Molecular Expression of Floral Organ Genes
The Molecular Expression of Floral Organ GenesThe genetic model predicts that the A organ identity genes will be expressed in the tissues from which sepals and petals are derived. Although APETALA2 is classified as an A function gene, mutants of this genes also affect stamen and carpel development. This gene is shown to be expressed in all four whorls. The expression of the other A gene, APETALA1 appears to be restricted to whorls 1 and 2, which is consistent with mutant patterns.
The genetic model has also suggested that C organ identity genes are negatively regulated by the expression of A genes. This would lead to a hypothesis stating that the expression of C genes such as AGAMOUS would not appear in cells giving rise A function organs. In situ hyridizations with the AGAMOUS genes demonstrated that early expression of this gene is restricted to whorls 3 and 4. Later in development, the expression of AGAMOUS is restricted to specific cell types. In stamens, the gene is not found in any cells that give rise to the pollen, nor is it expressed in the pollen grain itself. And in the carpel cells, AGAMOUS is only epxpressed in the outer cells of the ovule.
B gene function genes have been suggested to control petal and stamen function (whorl 2 and 3, respectively). Both APETALA3 and PISTILLATA are found to be expressed in the appropriate whorls. PISTILLATA though is also found to be expressed in whorl 4 that gives rise to carpels.
The study of the genes which control floral development provides a valuable insight to power of Mendelian or classic genetics. Through the careful study of mutants at the phenotypic level, predictions could be made regarding the molecular expression of the genes. In a large part, these predictions have come true. The geneticist who studies phenotypes brings the art of observation to the science of genetics. Remember that Mendel discovered all of his law by studying the phenotype of the plant. Likewise, Barbara McClinktock predicited the existence of transposable elements strictly from phenotypic studies. What these scientist had is what McClintock called "a feel for the organism". It is this feel for phenotypic studies that has served the development of plant molecular genetics very well.
Copyright © 1998. Phillip McClean