Molecular function of the MADS floral organ identity proteins APETALA3 and PISTILLATA in Arabidopsis

APETALA3 (AP3) and PISTILLATA (PI) are floral organ identity genes that are necessary and sufficient for petal and stamen development. Surprisingly little is known about molecular mechanisms by which AP3 and PI proteins direct floral organ identity. The aims of this thesis, therefore, are to analyze the functions of AP3 and PI proteins in planta.;AP3 and PI encode MIKC MADS transcription factors. MADS proteins are highly conserved; representatives can be found in yeast, animal and plant genomes and they bind DNA sequences as dimers. Functional analysis of MIKC MADS proteins indicates that DNA binding and dimerization occur via the M and K domains, while the less conserved C domain is of variable function. The C domain of AP3 and PI proteins contains small motifs at the C-terminal end of the C domain that are conserved among orthologs of AP3 and PI. Whether the C-terminal motifs are necessary for petal and stamen identity was previously unresolved. I demonstrate that truncations that lack the C-terminal domains of AP3 or PI can rescue the floral organ identity defects in ap3 and pi mutants. I also demonstrate that the truncations direct organ identity transformations when ectopically expressed. This data indicates that the C-terminal motifs of AP3 and PI are not required for floral organ identity. Alternative functional roles for these motifs are discussed.;Evidence indicates that AP3 and PI are both required for either protein to function. I have analyzed AP3 and PI protein in various tissues and demonstrate that in certain tissues, AP3 or PI proteins are stably expressed in the absence of its putative partner. If functionally relevant, this might suggests that AP3 or PI can act as a monomer, homodimer, or a heterodimer with another protein in planta.;To further elucidate the function of AP3 and PI, I have analyzed an enhancer of pi-5 named B-class modifer3. In pi-5 flowers, the second whorl organs develop as sepals instead of petals but the third whorl stamens are largely unaffected. In bcm3 pi-5 flowers, stamens are partially converted to carpels. BCM3 maps to a 54 gene interval on chromosome one. The gene EMBRYO DEFECTIVE 2421 lies in this interval and contains a single base pair change in bcm3 pi-5 plants that is absent in the pi-5 parental. This mutation results in a proline to serine amino acid transition in the encoded protein. Therefore, the mutation in BCM3 that enhances the pi-5 phenotype may be a mutation in EMB2421. Further experiments will be required to substantiate that BCM3 is EMB2421.