The Molecular Expression of Floral Committment Genes
The Molecular Expression of Floral Committment GenesBefore we discuss the expression of these genes, we need to explain more terminology. The early steps in flower development has been broken into four stages. The floral meristem is identified during the first two stages, stage 1 and 2. During stages 3 and 4, the floral organ identity genes are first expresed. Floral development continues through stage 15.
In accordance with its genetic role, LEAFY is the first gene expressed. Its spatial pattern of expression also fits its proposed role. The LEAFY mRNA first appears in the flanks of the inflorescence meristem, the exact location from which the floral meristem will develop. Its expression rises through stages 1 and 2, and disappears in stages 3 and 4.
Remember that APETALA1, another gene involved in the committment to flowering, also plays a role in sepal and petal development. Its expression is consistent with these genetic conclusions. As LEAFY, APETALA1 is expressed in cells at the margin of the inflorescence meristem during stage 1 and 2. Because whorls 1 and 2 give rise to sepals and petals, respectively, it is not surprising that APETALA1 contains to be expressed in these whorls during stages 3 and 4, while its expression disapppers from whorls 3 and 4 during these stages. Furthermore, APETALA1 continues to be expressed throughout all the floral developmental stages.
As stated above, APETALA1 and CAULIFLOWER share redundant genetic functions. This is supported by sequence data that showed that these genes are about 75% identical. The floral expression pattern of CAULIFLOWER also mimics that of APETALA1. What is unanswered though, is why CAULIFLOWER cannot rescue the the sepal and petal development functions of APETALA1 mutants. Because CAULIFLOWER is expressed in whorls 1 and 2, its inability to complement APETALA1 mutants is not related to a lack of expression in appropriate cells. Possibly, the 25% of the sequence that is not identical between the two genes, is necessary for sepal and petal development. Further research certainly is needed to answer this question.
Copyright © 1998. Phillip McClean