The Steps of Flower Development - Genes Are Implicated
Mendelian Genes Define the Committment to Flowering
Mendelian Genes Define Floral Organ Identity
Cloning Committment to Flowering and Flower Organ Genes
Analyzing Gene Expression with In Situ Hybridization
The Molecular Expression of Floral Committment Genes
The Steps of Flower Development - Genes Are Implicated
Molecular geneticists owe a great deal to geneticists who studied genes at the phenotypic or Mendelian level. Careful observations of traits at the phenotypic level often gives clues about how a gene might function in a genetic cascade involving other genes. Molecular geneticist rely upon these observations to form hypotheses regarding how a gene interacts with other genes to control a specific phenotype. The example that we will discuss regards the genes which control the development of flowers. Because the Mendelian and molecular genetics of Arabidopsis flower development has been extensively analyzed, we will use that system as our basis of discussion. For comparative purposes will be mention the genes from Antirrhinum (snapdragon).Plants have two basic growth modes during their life cycles --- vegetative growth and flower and seed growth. The above ground vegetative growth of the plant develops from the apical meristem. This vegetative meristem gives rise to all of the leaves that are found on the plant. The plant will maintain its vegetative growth pattern until the apical meristem undergoes a change. This change actually alters the identity of the meristem from a vegetative to an inflorescence meristem. The inflorescence meristem produce small leaves before it next produces floral meristems. It is the floral meristem from which the flower develops.
The floral meristem under goes a series of developmental changes that eventually give rise to the four basic structures of the flower --- sepals, petals, stamens and carpels. Each of these structures is derived sequentially from a whorl that develops from the floral meristem. Whorl 1 is the first to appear, and it develops into the sepals of the plant. The second whorl develops into petals. The third and fourth whorls define the stamen (male reproductive organs) and carpel (female reproductive organs), respectively. If you move from the base of the flower upwards and inwards you will encounter the four organs in the same order in which they are developed.
From a genetic perspecitive, two phenotypic changes that control vegetative and floral growth are programmed in the plant. The first genetic change involves the switch from the vegetative to the floral state. If this genetic change is not functioning properly, then flowering will not occur. The second genetic event follows the commitment of the plant to form flowers. The observation that the organs of the plant develop in a sequential manner suggests that a genetic mechanism exists in which a series of genes are sequentially turned on and off. This switching is necessary for each whorl to obtain its final unique identity.
Copyright © 1998. Phillip McClean