PETAL AND STAMEN FORMATION IN PETUNIA IS REGULATED BY THE HOMEOTIC GENE-FBP1

For Arabidopsis and Antirrhinum, the so-called ABC model has been deve loped, which postulates that the determination of floral organ primord ia is controlled by the action of three classes of homeotic genes. A n umber of these ABC genes encode putative transcription factors with th e MADS box DNA binding motif. This paper reports on the functional ana lysis of the petunia MADS box gene fbp1. The temporal and spatial expr ession of fbp1 has been investigated in detail in transgenic plants co ntaining the beta-glucuronidase (GUS) reporter gene fused to an fbp1 p romoter fragment. fbp1-driven GUS activity was specifically detected i n emerging petal and stamen primordia, suggesting a function of fbp1 i n the control of second and third floral whorl identity. To test this hypothesis, transgenic petunia plants were generated in which fbp1 exp ression was inhibited by a co-suppression approach. The flowers of suc h plants exhibited homeotic conversions of petals towards sepals and s tamens towards carpels. Occasionally, the third whorl carpels are fuse d forming a pentalocular gynoecium. This dominant fbp1 mutation acted as a single Mendelian trait in genetic crosses. These results strongly indicate that fbp1 is a petunia class B homeotic gene which is requir ed for the correct initiation and determination of petals and stamens.