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In flowering plants, the founder cells from which reproductive organs form reside in structures called floral meristems. Recent molecular genetic studies have revealed that the specification of floral meristems is tightly controlled by regulatory networks that underpin several coordinated programmes, from the integration of flowering signals to floral organ formation. A notable feature of certain regulatory genes that have been newly implicated in the acquisition and maintenance of floral meristem identity is their conservation across diverse groups of flowering plants. This review provides an overview of the molecular mechanisms that underlie floral meristem specification in Arabidopsis thaliana and, where appropriate, discusses the conservation and divergence of these mechanisms across plant species.

We describe allelic series for three loci, mutations in which result in homeotic conversions in two adjacent whorls in the Arabidopsis thaliana flower. Both the structure of the mature flower and its development from the initial primordium are described by scanning electron microscopy. New mutations at the APETALA2 locus, ap2-2, ap2-8 and ap2-9, cause homeotic conversions in the outer two whorls: sepals to carpels (or leaves) and petals to stamens. Two new mutations of PISTILLATA, pi-2 and pi-3, cause second and third whorl organs to differentiate incorrectly. Homeotic conversions are petals to sepals and stamens to carpels, a pattern similar to that previously described for the apetala3-l mutation. The AGAMOUS mutations, ag-2 and ag-3, affect the third and fourth whorls and cause petals to develop instead of stamens and another flower to arise in place of the gynoecium. In addition to homeotic changes, mutations at the APETALA2, APETALAS and PISTILLATA loci may lead to reduced numbers of organs, or even their absence, in specific whorls. The bud and flower phenotypes of doubly and triply mutant strains, constructed with these and previously described alleles, are also described. Based on these results, a model is proposed that suggests that the products of these homeotic genes are each active in fields occupying two adjacent whorls, AP2 in the two outer whorls, PI and AP3 in whorls two and three, and AG in the two inner whorls. In combination, therefore, the gene products in these three concentric, overlapping fields specify the four types of organs in the wild-type flower. Further, the phenotypes of multiple mutant lines indicate that the wild-type products of the AGAMOUS and APETALA2 genes interact antagonistically. AP2 seems to keep the AG gene inactive in the two outer whorls while the converse is likely in the two inner whorls. This field model successfully predicts the phenotypes of all the singly, doubly and triply mutant flowers described.