Pollen-specific expression of DEFH125, a MADS-box transcription factor in Antirrhinum with unusual features.

MADS-box genes encode transcription factors that regulate different processes of early and late floral development. A novel type of MADS-box gene, DEFH125, was isolated from a stamen specific cDNA library from Antirrhinum majus. The DEFH125 protein shows extensive similarity over the entire length to AGL17, a root-specific MADS-box protein of Arabidopsis. By sharing amino acid deviations from the consensus MADS-box sequence not found in other MADS-box families, these two proteins constitute a novel MADS-box subfamily. However, in contrast to members of other subfamilies the overall structural similarity between the DEFH125 and AGL17 proteins does not coincide with a similarity of expression patterns and functions. The DEFH125 gene is expressed at detectable levels only in the third whorl when the meiotic division of the pollen mother cell is already accomplished. The DEFH125 protein has been located in the cytoplasm of the vegetative cell within the maturing pollen. Surprisingly, after pollination, the DEFH125 protein is also found in nuclei of cells within the transmitting tract of the carpel. The intriguing role of DEFH125, the first MADS-box transcription factor of this type, in aspects of fertilization, such as pollen maturation, pollen tube formation or pollen tube guidance in the carpel, is discussed.