The histone methylation readers MRG1/MRG2 and the histone chaperones NRP1/NRP2 associate in fine‐tuning Arabidopsis flowering time

Abstract

Histones are highly basic proteins involved in packaging DNA into chromatin, and histone modifications are fundamental in epigenetic regulation in eukaryotes. Among the numerous chromatin modifiers identified in Arabidopsis (Arabidopsis thaliana), MORF RELATED GENE 1 (MRG1) and MRG2 have redundant functions in reading histone H3 lysine 36 trimethylation (H3K36me3). Here, we show that MRG2 binds histone chaperones belonging to the NUCLEOSOME ASSEMBLY PROTEIN 1 (NAP1) family, including NAP1‐RELATED PROTEIN 1 (NRP1) and NRP2. Characterization of the loss‐of‐function mutants mrg1 mrg2, nrp1 nrp2 and mrg1 mrg2 nrp1 nrp2 revealed that MRG1/MRG2 and NRP1/NRP2 regulate flowering time through fine‐tuning transcription of floral genes by distinct molecular mechanisms. In particular, the physical interaction between NRP1/NRP2 and MRG1/MRG2 inhibited the binding of MRG1/MRG2 to the transcription factor CONSTANS (CO), leading to a transcriptional repression of FLOWERING LOCUS T (FT) through impeded H4K5 acetylation (H4K5ac) within the FT chromatin. In contrast, NRP1/2 and MRG1/MRG2 act together, likely in a multiprotein complex manner, in promoting transcription of FLOWERING LOCUS C (FLC) via increase of both H4K5ac and H3K9ac in the FLC chromatin. Because the expression pattern of FLC represents the major category of differentially expressed genes identified by genome‐wide RNA‐seq analysis in the mrg1 mrg2, nrp1 nrp2 and mrg1 mrg2 nrp1 nrp2 mutants, it is reasonable to speculate that the NRP1/2‐MRG1/MRG2 complex may be involved in transcriptional activation of genes beyond FLC and flowering time control.