Abstract
In the winter-annual accessions of Arabidopsis, an active allele of FRIGIDA (FRI) elevates expression of FLOWERING LOCUS C (FLC), a repressor of flowering, and thus confers a vernalization requirement. FLC activation by FRI involves methylation of Lys 4 of histone H3 (H3K4) at FLC chromatin. Many multicellular organisms contain two classes of H3K4 methylases, a yeast Set1 class and a class related to Drosophila Trithorax. Here, we demonstrate that ARABIDOPSIS TRITHORAX-RELATED7 (ATXR7), a putative Set1 class H3K4 methylase, is required for proper FLC expression. The atxr7 mutation partially suppresses the delayed flowering of a FRI-containing line. The rapid flowering of atxr7 is associated with reduced FLC expression and is accompanied by decreased H3K4 methylation at FLC. Previously, it has been reported that lesions in ATX1, which encodes a Trithorax class H3K4 methylase, partially suppress the delayed flowering of winter-annual Arabidopsis. We show that the flowering phenotype of atx1 atxr7 double mutants is additive relative to those of single mutants. Therefore, both classes of H3K4 methylases appear to be required for proper regulation of FLC expression.
