petals, and it also acts in the floral meristem. ap2 not only functions in the first two verticils, but also in the remaining two, in developing ovules and even in leaves. it is also likely that post-transcriptional regulation exists, which controls its a function, or even that it has other purposes in the determination of organ identity independent of that mentioned here. in antirrhinum, the orthologous gene to ap1 is squamosa (squa), which also has a particular impact on the floral meristem. the homologs for ap2 are lipless1 (lip1) and lipless2 (lip2), which have a redundant function and are of special interest in the development of sepals, petals and ovules. a total of three genes have been isolated from petunia hybrida that are similar to ap2: p. hybrida apetala2a (phap2a), phap2b and phap2c. phap2a is, to a large degree, homologous with the ap2 gene of arabidopsis, both in its sequence and in its expression pattern, which suggests that the two genes are orthologs. the proteins phap2b and phap2c, on the other hand, are slightly different, even though they belong to the family of transcription factors that are similar to ap2. in addition they are expressed in different ways, although they are very similar in comparison with phap2a. in fact, the mutants for these genes do not show the usual phenotype, that of the null alleles of a genes. a true a-function gene has not been found in petunia; though a part of the a-function (the inhibition of the c in the outer two whorls) has been largely attributed to mirna169 (colloquially called blind)ref. genes exhibiting type-b function in a. thaliana the type-b function mainly arises from two genes, apetala3 (ap3) and pistillata (pi), both of which are mads-box genes. a mutation of either of these genes causes the homeotic conversion of petals into sepals and of stamens into carpels. this also occurs in its orthologs in a. majus, which are deficiens (def) and globosa (glo) respectively. for both species the active form of binding with dna is that derived from the heterodimer: ap3 and pi, or def and glo, dimerize. this is the form in which they are able to function. the glo/pi lines that have been duplicated in petunia contain p. hybrida globosa1 (phglo1, also called fbp1) and also phglo2 (also called pmads2 or fbp3). for the functional elements equivalent to ap3/def in petunia there is both a gene that possesses a relatively similar sequence, called phdef and there is also an atypical b function gene called phtm6. phylogenetic studies have placed the first three within the «euap3» lineage, while phtm6 belongs to that of «paleoap3». it is worth pointing out that, in terms of evolutionary history, the appearance of the euap3 line seems to be related with the emergence of dicotyledons, as representatives of euap3-type b function genes are present in dicotyledons while paleoap3 genes are present in monocotyledons and basal angiosperms, among others. as discussed above, the floral organs of eudicotyledonous angiosperms are arranged in 4 different verticils, containing the sepals, petals, stamen and carpels. the abc model states that the identity of these organs is determined by the homeotic genes a, a+b, b+c and c, respectively. in contrast with the sepal and petal verticils of the eudicots, the perigone of many plants of the family liliaceae have two nearly identical external petaloid verticils (the tepals). in order to explain the floral morphology of the liliaceae, van tunen et al. proposed a modified abc model in 1993. this model suggests that class b genes are not only expressed in verticils 2 and 3, but also in 1. it therefore follows that the organs of verticils 1 and 2 express class a and b genes and this is how they have a petaloid structure. this theoretical model has been experimentally proven through the cloning and characterization of homologs of the antirrhinum genes globosa and deficiens in a liliaceae, the tulip tulipa gesneriana. these genes are expressed in verticils 1,2 and 3. the homologs globosa and deficiens have also been isolated and characterized in agapanthus praecox ssp. orientalis (agapanthaceae), which is phylogenetically distant from the model organisms. in this study the genes were called apglo and apdef, respectively. both contain open reading frames that code for proteins with 210 to 214 amino acids. phylogenetic analysis of these sequences indicated that they belong to b gene family of the monocotyledons. in situ hybridization studies revealed that both sequences are expressed in verticil 1 as well as in 2 and 3. when taken together, these observations show that the floral development mechanism of agapanthus also follows the modified abc model. genes exhibiting type-c function in a. thaliana, the c function is derived from one mads-box type gene called agamous (ag), which intervenes both in the establishment of stamen and carpel identity as well as in the determination of the floral meristem. therefore, the ag mutants are devoid of androecium and gynoecium and they have petals and sepals in their place. in addition, the growth in the centre of the flower is undifferentiated, therefore the petals and sepals grow in repetitive verticils. the plena (ple) gene is present in a. majus, in place of the ag gene, although it is not an ortholog. however, the farinelli (far) gene is an ortholog, which is specific to the development of the anthers and the maturation of pollen. in petunia, antirrhinum and in maize the c function is controlled by a number of genes that act in the same manner. the genes that are closer homologs of ag in petunia are pmads3 and floral-binding protein 6 (fbp6). genes exhibiting type-d and e functions the d function genes were discovered in 1995. these genes are mads-box proteins and they have a function that is distinct from those previously described, although they have a certain homology with c function genes. these genes are called floral binding protein7 (fbp7) and floral binding protein1l (fbp1l). it was found that, in petunia, they are involved in the development of the ovule. equivalent genes were later found in arabidopsis, where they are also involved in controlling the development of carpels and the ovule and even with structures related to seed dispersal. the appearance of interesting phenotypes in rna interference studies in petunia and tomato led, in 1994, to the definition of a new type of function in the floral development model. the e function was initially thought to be only involved in the development of the three innermost verticils, however, subsequent work found that its expression was required in all the floral verticils.