Abstract
Winter annual habit of Arabidopsis thaliana requires active alleles of FLOWERING LOCUS C (FLC), which encodes a potent flowering repressor, and FRIGIDA (FRI), an activator of FLC. FLC activation by FRI is accompanied by histone methylations, such as trimethylation of histone H3 at Lys 4 (H3K4me3). Such H3K4 methylation at the FLC locus requires two known classes of H3K4 methyltransferases, Drosophila Trithorax-class ARABIDOPSIS TRITHORAX1 (ATX1) and yeast Set1-class ATX-RELATED7 (ATXR7). Here, we revealed that ATXR3, which encodes a member of a novel class of histone methyltransferase, also contributes to H3K4 methylation and the FLC activation. An ATXR3 lesion suppressed the delayed flowering caused by FRI and FLC. The suppression was accompanied by decreased FLC expression and H3K4me3 levels at FLC chromatin. Rapid flowering of atxr3 was epistatic to that of atxr7, suggesting that ATXR3 is indispensable for the ATXR7 function in the FLC activation and flowering repression. These data indicate that a novel-class SET domain protein, ATXR3, is a transcriptional activator of FLC that plays a key role in establishing the winter-annual habit.
