The Physiology of Flowering Plants - KHAM PHA MOI

288 REPRODUCTIVE DEVELOPMENTBox 11.1One of the very first descriptions ofa plant with a mutation in a genecontrolling zygomorphy, in 1744,was by the famous Swedishnaturalist Carl Linnaeus,responsible for the introduction ofthe binomial system for namingspecies. Common toadflax (Linariavulgaris) normally produceszygomorphic flowers but one of hisstudents found an example whichwas normal in every way exceptthat it produced actinomorphicflowers. As the classification systemof Linnaeus relied heavily on floralmorphology, this should haveplaced the plant in an entirelydifferent taxonomic group.Linnaeus recognized that this plantmay have arisen from the commontoadflax but it disturbed himsufficiently to name it Peloria(Greek – monster) and to write‘This is certainly no less remarkablethan if a cow were to give birth to acalf with a wolf’s head’. Fortunatelyfor our peace of mind, we nowknow that this form has a mutationin a gene homologous to CYC(Cubas et al. 1999).homeotic mutants (homeo = like) as one part of the flower has beentransformed into another part. Homeotic mutants have been foundin many organisms including insects, fish and mammals.Fig. 11.11 shows some of the homeotic mutations which havebeen described in Arabidopsis, together with a diagram to show howthe identity of the organs within individual whorls has been modified.It can be seen that these mutations affect organ development inadjacent whorls of organs. Analysis of these mutants led to the developmentof the ‘ABC’ model of flowering (Coen & Meyerowitz 1991).There are a number of features of this model which, together,account for many aspects of floral patterning.11.7.1 The ABC model of floral patterningThe model proposes that three distinct activities are expressed in theflower but that these activities are restricted to adjacent pairs ofwhorls. Activity A is expressed in whorls 1+2, activity B in whorls2+3 and activity C in whorls 3+4. Several different genes may benecessary for each ‘activity’ to be expressed; hence mutations inmore than one gene may give rise to the same phenotype. Thecombination of activities present in any particular whorl determineswhich organ will develop there (Table 11.1, Fig. 11.11). The modelalso proposes that activity A and activity C are mutually antagonistic,i.e. activity A represses activity C and vice-versa. The apetala2 mutantlacks activity A so activity C is now expressed in all four whorls.Likewise, the agamous mutant lacks activity C, so activity A is nowexpressed in all four whorls. The model also proposes that if a carpelis present in whorl 4, further whorls of organs do not develop.This model can account for the homeotic mutants which areobserved in many plant species, and it is supported by a number oflines of evidence. If mutants are crossed together so that two, or eventhree, of the activities are absent, the phenotypes of the double andtriple mutants can largely be predicted by the model. It is interestingthat in the absence of activities A and C an intermediate sepal/petalstructure is produced in whorls 2 and 3. In the absence of all threeTable 11.1 The ABC model of flower development predicts the activities of homeotic genes to be restricted tospecific whorls of organs. In most cases the expression of these genes is also restricted to these same whorls. APETALA2 is an exception – expression occurs in all whorls, but activity is restricted, in the wild-type plant, to whorls 1 and 2.BPISTILLATA, APETALA3AAPETALA1, APETALA2CAGAMOUSA A + B B + C CWhorl 1 2 3 4Organ Sepal Petal Stamen Carpel