ThePolycomb-group protein MEDEA regulates seed development by controlling expression of the MADS-box genePHERES1

Abstract

ThePolycomb-group (PcG) proteins MEDEA, FERTILIZATION INDEPENDENT ENDOSPERM, and FERTILIZATION INDEPENDENT SEED2 regulate seed development inArabidopsisby controlling embryo and endosperm proliferation. All three of these FIS-class proteins are likely subunits of a multiprotein PcG complex, which epigenetically regulates downstream target genes that were previously unknown. Here we show that the MADS-box genePHERES1(PHE1) is commonly deregulated in thefis-class mutants. PHE1 belongs to the evolutionarily ancient type I class of MADS-box proteins that have not yet been assigned any function in plants. Both MEDEA and FIE directly associate with the promoter region ofPHE1, suggesting thatPHE1expression is epigenetically regulated by PcG proteins.PHE1is expressed transiently after fertilization in both the embryo and the endosperm; however, it remains up-regulated in thefismutants, consistent with the proposed function of theFISgenes as transcriptional repressors. Reduced expression levels ofPHE1inmedeamutant seeds can suppressmedeaseed abortion, indicating a key role ofPHE1repression in seed development.PHE1expression in a hypomethylatedmedeamutant background resembles the wild-type expression pattern and is associated with rescue of themedeaseed-abortion phenotype. In summary, our results demonstrate that seed abortion in themedeamutant is largely mediated by deregulated expression of the type I MADS-box genePHE1.