Interaction of Polycomb-group proteins controlling flowering in Arabidopsis

In Arabidopsis, the EMBYRONIC FLOWER2 (EMF2), VERNALISATION2 (VRN2) and FERTILISATION INDEPENDENT ENDOSPERM2 (FIS2) genes encode related Polycomb-group (Pc-G) proteins. Their homologues in animals act together with other Pc-G proteins as part of a multimeric complex, Polycomb Repressive Complex 2 (PRC2), which functions as a histone methyltransferase. Despite similarities between the fis2 mutant phenotype and those of some other plant Pc-G members, it has remained unclear how the FIS2/EMF2/VRN2 class Pc-G genes interact with the others. We have identified a weak emf2 allele that reveals a novel phenotype with striking similarity to that of severe mutations in another Pc-G gene, CURLY LEAF (CLF), suggesting that the two genes may act in a common pathway. Consistent with this, we demonstrate that EMF2 and CLF interact genetically and that this reflects interaction of their protein products through two conserved motifs, the VEFS domain and the C5 domain. We show that the full function of CLF is masked by partial redundancy with a closely related gene, SWINGER (SWN), so that null clf mutants have a much less severe phenotype than emf2 mutants. Analysis in yeast further indicates a potential for the CLF and SWN proteins to interact with the other VEFS domain proteins VRN2 and FIS2. The functions of individual Pc-G members may therefore be broader than single mutant phenotypes reveal. We suggest that plants have Pc-G protein complexes similar to the Polycomb Repressive Complex2 (PRC2) of animals, but the duplication and subsequent diversification of components has given rise to different complexes with partially discrete functions.