Direct regulation of the floral homeotic APETALA1 gene by APETALA3 and PISTILLATA in Arabidopsis

The floral homeotic gene APETALA1 (AP1) specifies floral meristem identity and sepal and petal identity in Arabidopsis. Consistent with its multiple roles during floral development, AP1 is initially expressed throughout the floral meristem, and later its expression becomes restricted to sepal and petal primordia. Using chromatin immunoprecipitation, we show that the floral homeotic PISTILLATA (PI) protein, required for petal and stamen development, has the ability to bind directly to the promoter region of AP1. In support of the hypothesis that PI, and its interacting partner APETALA3 (AP3), regulates the transcription of AP1, we show that AP1 transcript levels are elevated in strong ap3-3 mutant plants. Kinetic studies, using transgenic Arabidopsis plants in which both AP3 and PI are under post-translational control, show that AP1 transcript levels are downregulated within 2 h of AP3/PI activation. This implies that the reduction in AP1 transcripts is an early event in the cascade following AP3/PI induction and provides independent support for the hypothesis that AP1 is a direct target of the AP3/PI heterodimer. Together these results suggest a model whereby AP3/PI directly acts, in combination with other factors, to restrict the expression of AP1 during early stages of floral development.