INITIATION PATTERNS OF FLOWER AND FLORAL ORGAN DEVELOPMENT IN ARABIDOPSIS-THALIANA

Sector boundary analysis has been used to deduce the number and orient ation of cells initiating flower and floral organ development in Arabi dopsis thaliana. Sectors were produced in transgenic plants carrying t he Ac transposon from maize inserted between the constitutive 35S prom oter and the GUS reporter gene, Excision of the transposon results in a blue-staining sector, Plants were chosen in which an early arising s ector passed from vegetative regions into the inflorescence and throug h a mature flower. The range of sector boundary positions seen in matu re flowers indicated that flower primordia usually arise from a group of four cells on the inflorescence flank, The radial axes of the matur e flower are apparently set by these cells, supporting the concept tha t they act as a structural template, Floral organs show two patterns o f initiation, a leaf-like pattern with eight cells in a row (sepals an d carpels), or a shout-like pattern with four cells in a block (stamen s), The petal initiation pattern involved too few cells to allow assig nment, The numbers of initiating cells were close to those seen when o rgan growth commenced in each case, indicating that earlier specificat ion of floral organ development does not occur, By examining sector bo undaries in homeotic mutant flowers in which second whorl organs devel op as sepal-like organs rather than petals, we have shown that their p attern of origin is position dependent rather than identity dependent.