DYNAMIC REGULATION OF HISTONE H3 LYSINE 4 METHYLATION AND ITS REGULATORY ROLE IN PLANT DEVELOPMENT

Abstract

Plant development results from specific patterning of gene expression that is tightly regulated in a spatio-temporal manner. Chromatin modifications play important roles in establishing and maintaining gene expression patterns. One of the important chromatin modifications that regulate transcription is histone H3 Lysine 4 (H3K4) methylation which is tightly coupled to active gene transcription. In Arabidopsis thaliana, how H3K4 methylation is executed and what are the roles of H3K4 methylation in plant development are largely unclear.

To understand the control of H3K4 methylation and its regulatory role in plant development, I identified and characterized components of the H3K4 methyltransferase and demethylase complexes in Arabiodpsis, and investigated their functions in control of the H3K4 methylation state and plant development. The findings presented in this thesis show that the H3K4 methyltransferase complexes (i.e COMPASS-like complexes) are evolutionarily conserved in Arabidopsis. Knocking down the expression of core components in COMPASS-like complexes resulted in reduced H3K4 trimethylation (H3K4me3) levels on FLOWERING LOCUS C (FLC) chromatin, leading to downregulation of FLC expression and subsequently early flowering phenotype. In addition, severe functional disruption of ARABIDOPSIS ASH2 RELATIVE (ASH2R), one of the components of Arabidopsis COMPASS-like complexes, resulted in global reduction of H3K4me3 and pleiotropic developmental defects, indicating the important roles of COMPASS-like complexes in regulating H3K4 methylation and plant development. On the other hand, our results show that H3K4 methylation at FLC locus is ereased by H3K4 demethylases. Loss of H3K4 demethylases such as FLOWERING LOCUS D (FLD), LSD1 LIKE 1 (LDL1) and LSD1 LIKE 2 (LDL2) led to increased H3K4 dimethylation (H3K4me2) and H3K4me3 on FLC chromatin. Moreover, it was found that FLD interacts with HISTONE DEACETYLASE 6 (HDA6), FVE and MULTICOPY SUPPRESSOR OF IRA 5 (MSI5) to form co-repressor like complexes which directly bind on FLC chromatin. The results described in this thesis demonstrate that H3K4 methylation is dynamically controlled by H3K4 methyltransferase and demethylase complexes, and this well balanced regulation is critical for proper plant development such as the floral transition.