Probable transcription factor involved in the genetic control of flower development. Is required for normal development of petals and stamens in the wild-type flower. Forms a heterodimer with APETALA3 that is required for autoregulation of both AP3 and PI genes. AP3/PI heterodimer interacts with APETALA1 or SEPALLATA3 to form a ternary complex that could be responsible for the regulation of the genes involved in the flower development. AP3/PI heterodimer activates the expression of NAP. AP3/PI prevents GATA22/GNL and GATA21/GNC expression (PubMed:18417639). Forms a heterodimer with APETALA3, capable of binding to CArG-box sequences. AP3/PI heterodimer binds AP1 or SEP3 to form a ternary complex. Positively regulated by the meristem identity proteins APETALA1 and LEAFY with the cooperation of UFO. Repressed by silencing mediated by polycomb group (PcG) protein complex containing EMF1 and EMF2. Mutations in PI cause transformation of petals into sepals and stamina into carpels.