Regulation of cell proliferation patterns by homeotic genes during Arabidopsis floral development

The shoot apical meristem of Arabidopsis thaliana consists of three cell la yers that proliferate to give rise to the aerial organs of the plant. By la beling cells in each layer using an Ac-based transposable element system, w e mapped their contributions to the floral organs, as well as determined th e degree of plasticity in this developmental process, We found that each ce ll layer proliferates to give rise to predictable derivatives: the L1 contr ibutes to the epidermis, the stigma, part of the transmitting tract and the integument of the ovules, while the L2 and L3 contribute, to different deg rees, to the mesophyll and other internal tissues. In order to test the rol es of the floral homeotic genes in regulating these patterns of cell prolif eration, we carried out similar clonal analyses in apetala3-3 and agamous-1 mutant plants. Our results suggest that cell division patterns are regulat ed differently at different stages of floral development. In early floral s tages, the pattern of cell divisions is dependent on position in the floral meristem, and not on future organ identity. Later, during organogenesis, t he layer contributions to the organs are controlled by the homeotic genes. We also show that AGAMOUS is required to maintain the layered structure of the meristem prior to organ initiation, as well as having a non-autonomous role in the regulation of the layer contributions to the petals.