Figure 3. Mechanistic model for transcriptional and epigenetic control of the floral meristem stem cell pool.

Serial floral stages with the expression of WUS, AG, and KNU are shown. Underneath, the genomic regions of these genes are shown schematically with likely binding of transcription factors, expression status, and histone modifications. At the initial stage of flower development (stage ≤1), the KNU locus is covered by the repressive mark H3K27me3. WUS is activated at stage 2, but the transcriptional activation of AG by WUS is inhibited by H3K27me3. At stage 3, the AG locus undergoes activation by progressive ULT1/ATX1-dependent H3K4me3 and likely demethylation of H3K27. WUS binds in the large second intron of AG. AG expression is maintained by autoactivation as well as by H3K4me3. WUS expression terminates at stage 6. The activation of KNU by AG is initially inhibited by H3K27me3 at the KNU locus between stages 3 to 5. The level of H3K27me3 at the KNU locus is gradually depleted, perhaps by dilution via successive rounds of cell division, and at stage 6 KNU is expressed, causing WUS repression. The punctual termination of WUS expression needs additional PRC1 activity, which suggests that the WUS locus accumulates H3K27me3 and H2AK120Ub. AG, AGAMOUS; ATX1, ARABIDOPSIS HOMOLOG OF TRITHORAX 1; KNU, KNUCKELS; ULT1, ULTRAPETALA1; WUS, WUSCHEL.