NUCLEAR-LOCALIZATION OF THE PETUNIA MADS BOX PROTEIN FBP1

The petunia floral binding protein FBP1, is a putative transcription factor, with a MADS box DNA binding domain, controlling the identity of whorl 2 and 3 organ primordia. We describe here the nuclear immunolocalization of the product of the fbp1 floral homeotic gene. The FBP1 protein is present in the nucleus of petal cells and mainly located in the outer and inner petal epidermis. Analysis of the accumulation of FBP1 in different developmental stages revealed that it is present from very early (petal primordial to mature stages of petal development in wild-type flowers. In sepaloid petals of the homeotic mutant green petals, the FBP1 protein was not detected. The fbp1 mRNA accumulates throughout the process of floral organ initiation to anthesis. In situ hybridization experiments demonstrated the accumulation of fbp1 mRNA in whorls 2 and 3 of wild-type flower buds at different stages of development. In contrast to this, the FBP1 protein was immunologically not detectable in stamens of flower buds larger than 0.5 cm, whereas the FBP1 protein was present in nuclei of stamen primordia cells and in very early stages of anther development. This suggests that fbp1 gene expression is post-transcriptionally regulated in this organ at later stages of stamen development. Our results correspond with the function of fbp1 as a floral regulatory gene required for the correct initiation and determination of petals and stamens. The nuclear localization of the FBP1 protein is in agreement with the role of FBP1 as a transcriptional activator.