Mendelian Genes Define the Committment to Flowering
Mendelian Genes Define the Committment to FloweringA series of Arabidopsis mutants have been identified in which normal flowers are replaced with structures that resemble infloresce meristems and the shoots that normally develop from them. One such mutant is LEAFY. This mutant does not contain any normal flowers. Instead, the early flower structures that develop appear as infloresence shoots, whereas the later flowers partially resemble normal flowers. These later developing flowers contain sepal and carpel-like structures; that is they lack petals and stamens. This suggests that LEAFY has two functions --- committing the plant to floral meristem development and defining petals and stamens.
How is this conclusion regarding the function of the gene reached? If we believe that a gene defines a phenotype, when that gene is not functioning normally, that is when the gene is mutated, its phenotype is not expressed. When we are considering genes involved in development, then the developmental stage that is altered is controlled at least partially by the gene that is mutated. LEAFY mutants often do not develop floral meristems, and when the committment to a floral meristem is made the flowers lack petals and stamens. From these observations we conclude that the LEAFY gene is necessary for the conversion of the inflorescence meristem to a floral meristem. Likewise, because later flowers of LEAFY mutant lack petals and stamens, this gene must be involved in the development of these flower organs.
Snapdragon mutants have also been quite helpful in defining the steps involved in normal flower development. The analagous gene in snapdragon to LEAFY is floricaula (flo). flo mutants also fail to undergo the transition from inflorescence to floral meristem, and the flowers have the appearance of an inflorescence shoot. Clearly this gene is absolutely necessary for the conversion of the inflorescence meristem into a floral meristem. flo does differ from LEAFY with regards to organ development in that it does not appear to affect petal and stamen development. Therefore, two genes can share one function, in this case the committment to floral mersistem development, but not necessarily all functions. This clearly shows a functional deviation during the evolution of the two species.
Flowers of APETALA1 mutants are not altered as dramatically as LEAFY mutants. These mutants express a partial inflorescence meristem phenotype where secondary floral meristems appear in the axis region of the sepal. But when the APETALA1 and LEAFY mutants are combined, the flowers appears as an inflorescence shoot. The snapdragon analog to the APETALA1 gene, SQUAMOSA is much more severe, and the flowers appear as inflorescence shoots. APETALA1 also affects the normal development of sepals and petals. The image below shows the phenotype of the APETALA1 mutant.
Another gene, CAULIFLOWER, does not express its effects unless coupled with another mutant. CAULIFLOWER and APETALA1 double mutants have inflorescence meristems developing in place of floral meristems. Because the CAULIFLOWER phenotype only appears when it is coupled with APETALA1, it can be concluded that the phenotypic functions maintained by the CAULIFLOWER gene are duplicated by APETALA1. Conversely, the observation that APETALA1 floral meristems are generally normal suggests that CAULIFLOWER contains APETALA1 functions. But not all APETALA1 functions. Remember, APETALA1 mutants also have abnormal sepal and petal development, and these functions are not rescued by the wild type CAULIFLOWER gene. Therefore, APETALA1 has two genetic functions, but only one of these is duplicated by the CAULIFLOWER gene. The image below shows the phenotype of the APETALA1 CAULIFLOWER double mutant.
Copyright © 1998. Phillip McClean