Fig. 6. Models for regulation of AP3 and PI expression by LFY, ASK1 and UFO. (A) Model I. ASK1 and UFO facilitate the degradation of a negative modulator (X) of LFY protein activity. In the wild type, the level of X is low due to the function of ASK1, UFO and other ASK and F-box proteins, together with cullin, the third subunit of SCF. In the ask1-1 mutant, the level of X may increase slightly but other ASK genes can still provide some needed function. Wild-type LFY protein is not obviously affected by the slight increase of X. In the lfy-5 mutant, the mutant protein has reduced activity, leading to decreased AP3 and PI transcription. In the lfy-5 ask1-1 double mutant, the combination of a weak LFY protein and an increased level of X causes a dramatic reduction of AP3 and PI expression. If LFY activity is completely eliminated, as in lfy-6 (not shown here), then there is little AP3 and PI expression regardless of the level of X. (B) Model II, ASK1 and UFO facilitate the degradation of a transcriptional repressor (Y) of AP3 and PI genes. Again, the combination of ask1-1 and lfy-5 mutations would result in both an increase in the Y repressor and a decrease in the LFY activator, and a severe reduction in AP3 and PI expression, whereas either single mutation would have less pronounced effects. In the absence of LFY activator (lfy-6), the Y repressor would have no effect.