Regulation of Phytohormone Biosynthesis Genes by Polycomb-Mediated Histone H3 Lysine 27 Trimethylation in Arabidopsis

Histone H3 lysine 27 trimethylation (H3K27me3) catalyzed by polycomb proteins plays a negative role in the regulation of gene expression. Several key developmental genes have been reported to be enriched with H3K27me3 and repressed by polycomb proteins. The objective of this study was to analyze the role of polycomb-mediated H3K27me3 in regulating expression of hormone biosynthesis genes in Arabidopsis. First of all, enrichment of H3K27me3 on hormone biosynthesis genes was investigated systematically. The results showed that genes encoding crucial enzyme tended to be enriched with H3K27me3 while the others were not. And then expression of cytokinin biosynthesis genes in curly leaf (clf) and like heterochromatin protein1 (lhp1) was detected by qPCR. Most of cytokinin biosynthesis genes expression with basal promoter activity was increased in clf and lhp1. Chromatin Immunoprecipitation (ChIP) analysis indicates that H3K27me3 levels on these genes were decreased in clf, suggesting repressive role of CLF and LHP1 in the regulation of these genes. To reveal the role of CLF1 and LHP1 in the dynamics of gene expression, we tested expression of various genes at different time point after IAA treatment. We found that expression of CYP735A2 was increased in clf1 and lhp1 but could be repressed to the levels as in wild type plants, which indicates that CLF1 and LHP1 are not required for IAA-stimulated repression of CYP735A2. The induction of gibberellin 20-oxidase1 (AtGA20Ox1) and gibberellin 20-oxidase2 (AtGA20Ox2) by IAA application was also affected in clf and lhp1. Expression of AtGA20Ox1 and AtGA20Ox2 was firstly induced by IAA treatment and decreased later in wild type plants. However, decrease of AtGA20Ox1 and AtGA20Ox2 expression was earlier in clf and lhp1 which might be caused by overproduction of gibberellins in the mutants. Thus, we propose that the role of the polycomb group proteins is to reduce expression levels of crucial enzymes to minimum, which might stop hormone production