The ASK1 gene regulates B function gene expression in cooperation with UFOand LEAFY in Arabidopsis

The Arabidopsis floral regulatory genes APETALA3 (AP3) and PISTILLATA (Pf) are required for the B function according to the ABC model for floral organ identity. AP3 and PI expression are positively regulated by the LEAFY (LFY ) and UNUSUAL FLORAL ORGANS (UFO) genes. UFO encodes an F-box protein, and we have shown previously that UFO genetically interacts with the ASK1 gene encoding a SKP1 homologue; both the F-box containing protein and SKP1 are s ubunits of ubiquitin ligases. We show here that the ask1-1 mutation can enh ance the floral phenotypes of weak lfy and ap3 mutants; therefore, like UFO , ASK1 also interacts with LFY and AP3 genetically. Furthermore, our result s from RNA in situ hybridizations indicate that ASK1 regulates early AP3 an d PI expression. These results support the idea that UFO and ASK1 together positively regulate AP3 and PI expression. We propose that the UFO and ASK1 proteins are components of a ubiquitin ligase that mediates the proteolysi s of a repressor of AP3 and PI expression. Our genetic studies also indicat e that ASK1 and UFO play a role in regulating the number of floral organ pr imordia, and we discuss possible mechanisms for such a regulation.