Ectopic Expression Of A Polycomb Group Protein Gene, BML1, Leads To Early Flowering In Arabidopsis

Plant flowering is tightly controlled by both endogenous signals and environmental cues. In Arabidopsis, genetic analysis has revealed dozens of genes which are involved in different flowering pathways. Among them, FLC, encoding a MADS box transcriptional factor, is the central repressor gene of flowering. FLC coordinates the flowering signals from vernalization pathway, autonomous pathway and FRI and PAF1 complexes to quantitatively and precisely mediate flowering time.Polycomb group (PcG) protein was initially identified as the repressor of HOX gene expression in Drosophila, and now is believed to be the one of components in the PRC complexes to repress the expression of master control genes by epigenetically modification of chromatin of these master genes in mammals, fly and worm.The model plant Arabidopsis genome also encodes the homologous genes of animal PRC components including PcG. The previous results have suggested the involvement of the PRC2 complex in the multiple developmental processes from gametegenesis to flowering time control in Arabidopsis. However, the presence and function of PRC 1 complex are still controversial in plant system.We identified three genes encoding the homologues of mammal PRC1 core component BMI1 from Arabidopsis genome by blast using the BMII sequence against Arabidopsis genome. In order to investigate the function of these three genes in Arabidopsis, we characterized all available T-DNA insertion lines from the stock centers. However, none of them is their knock-out line. We then used artificial microRNA approach to knock-down the endogenous expression level of one of the three genes, BML1. Again, although the endogenous BML1 expression level is significantly knock-down, no obvious developmental defect is observed in the transgenic lines. Thus, we used ectopic expression approach instead to reveal the function of BMLl in Arabidopsis developmental processes.For this purpose, both constitutively and tissue specific expressed promoters were chosen to drive BML1 expressed in wild type plants. The BML1 overexpression lines exhibit pleiotropic phenotypes, including early flowering, rounder leaf shape, dwarf, and terminal flower. The early flowering and rounder leaf shape phenotypes are common for the transgenic plants with all promoter types we used to drive BML1 overexpression. However, terminal flower phenotype is unique only for those transgenic plants with constitutively and phloem tissue specific expressed promoters.To understand the molecular mechanism of BML1 in the control of the above mentioned developmental processes, we use flowering as a model system to investigate the relationship between BML1 activity and the expression of master flowering time control gene. We observed that the expression of FLC is significantly repressed, while the expression of FT, the downstream FLC-repressed flowering activator, is significantly increased. Therefore, these results suggested that BML1 is a functional PcG in Arabidopsis.BML1 directly represses the expression of FLC, and indirectly releases the expression of FT, thus leads to early flowering in Arabidopsis.