How different organs are formed from small sets of undifferentiated
precursor cells is a key question in developmental biology. To
understand the molecular mechanisms underlying organ specification
in plants, we studied the function of the homeotic selector
genes APETALA3 (AP3) and PISTILLATA (PI), which control the
formation of petals and stamens during Arabidopsis flower development.
To this end, we characterized the activities of the transcription
factors that AP3 and PI encode throughout flower
development by using perturbation assays as well as transcript profiling
and genomewide localization studies, in combination with
a floral induction system that allows a stage-specific analysis of
flower development by genomic technologies. We discovered considerable
spatial and temporal differences in the requirement for
AP3/PI activity during flower formation and show that they control
different sets of genes at distinct phases of flower development.
The genomewide identification of target genes revealed that
AP3/PI act as bifunctional transcription factors: they activate genes
involved in the control of numerous developmental processes required
for organogenesis and repress key regulators of carpel formation.
Our results imply considerable changes in the composition
and topology of the gene network controlled by AP3/PI during the
course of flower development. We discuss our results in light of
a model for the mechanism underlying sex-determination in seed
plants, in which AP3/PI orthologues might act as a switch between
the activation of male and the repression of female development
