SUPPRESSOR OF FRIGIDA (SUF4) supports gamete fusion via regulating Arabidopsis EC1 gene expression

The EC1 gene family of Arabidopsis thaliana comprises five members that are specifically expressed in the egg cell and redundantly control gamete fusion during double fertilization. We investigated the activity of all five EC1 promoters in promoter-deletion studies and identified SUF4 (SUPPRESSOR OF FRIGIDA 4), a C2H2 transcription factor, as a direct regulator of the EC1 gene expression. In particular, we demonstrated that SUF4 binds to all five Arabidopsis EC1 promoters, thus regulating their expression. The down regulation of SUF4 in homozygous suf4-1 ovules results in reduced EC1 expression and delayed sperm fusion, which can be rescued by expressing SUF4-GUS under control of the SUF4 promoter. To identify more gene products able to regulate EC1 expression together with SUF4, we performed co-expression studies that led to the identification of MOM1 (Morpheus' Molecule 1), a component of the silencing mechanism independent of DNA methylation marks. In mom1-3 ovules both SUF4 and EC1 genes are down regulated and EC1 genes show higher levels of Histone 3 lysine 9 acetylation, suggesting that MOM1 contributes to the regulation of SUF4 and EC1 gene expression.