The Arabidopsis histone demethylase JMJ28 regulates CONSTANS by interacting with FBH transcription factors

Arabidopsis thaliana CONSTANS (CO) is an essential transcription factor that promotes flowering by activating the expression of the floral integrator FLOWERING LOCUS T (FT). A number of histone modification enzymes involved in the regulation of flowering have been identified, but the involvement of epigenetic mechanisms in the regulation of the core flowering regulator CO remains unclear. Previous studies have indicated that the transcription factors, FLOWERING BHLH1 (FBH1), FBH2, FBH3, and FBH4, function redundantly to activate the expression of CO. In this study, we found that the KDM3 group H3K9 demethylase JMJ28 interacts with the FBH transcription factors to activate CO by removing the repressive mark H3K9me2. The occupancy of JMJ28 on the CO locus is decreased in the fbh quadruple mutant, suggesting that the binding of JMJ28 is dependent on FBHs. Furthermore, genome-wide occupancy profile analyses indicate that the binding of JMJ28 to the genome overlaps with that of FBH3, indicating a functional association of JMJ28 and FBH3. Together, these results indicate that Arabidopsis JMJ28 functions as a CO activator by interacting with the FBH transcription factors to remove H3K9me2 from the CO locus.