The ULT1 and ULT2 trxG Genes Play Overlapping Roles in Arabidopsis Development and Gene Regulation

The epigenetic regulation of gene expression is critical for ensuring the proper deployment and stability of defined genome transcription programs at specific developmental stages. The cellular memory of stable gene expression states during animal and plant development is mediated by the opposing activities of Polycomb group （PcG） factors and trithorax group （trxG） factors. Yet, despite their importance, only a few trxG factors have been characterized in plants and their rotes in regulating plant development are poorly defined. In this work, we report that the closely related Arabidopsis trxG genes ULTRAPETALA1 （ULT1） and ULT2 have overlapping functions in regulating shoot and floral stem cell accumula- tion, with ULT1 playing a major role but ULT2 also making a minor contribution. The two genes also have a novel, redun- dant activity in establishing the apical-basal polarity axis of the gynoecium, indicating that they function in differentiating tissues. Like ULT1 proteins, ULT2 proteins have a dual nuclear and cytoplasmic localization, and the two proteins physically associate in planta. Finally, we demonstrate that ULT1 and ULT2 have very similar overexpression phenotypes and regulate a common set of key development target genes, including floral MADS-box genes and class I KNOX genes. Our results reveal that chromatin remodeling mediated by the ULT1 and ULT2 proteins is necessary to control the development of mer- istems and reproductive organs. They also suggest that, like their animal counterparts, plant trxG proteins may function in multi-protein complexes to up-regulate the expression of key stage- and tissue-specific developmental regulatory genes.

The LEAFY floral regulator displays pioneer transcription factor properties

Pioneer transcription factors(TFs)are a special category of TFs with the capacity to bind to closed chromatin regions in which DNA is wrapped around histones and may be highly methylated.Subsequently,pioneer TFs are able to modify the chromatin state to initiate gene expression.In plants,LEAFY(LFY)is a master floral regulator and has been suggested to act as a pioneer TF in Arabidopsis.Here,we demonstrate that LFY is able to bind both methylated and non-methylated DNA using a combination of in vitro genomewide binding experiments and structural modeling.Comparisons between regions bound by LFY in vivo and chromatin accessibility data suggest that a subset of LFY bound regions is occupied by nucleosomes.We confirm that LFY is able to bind nucleosomal DNA in vitro using reconstituted nucleosomes.Finally,we show that constitutive LFY expression in seedling tissues is sufficient to induce chromatin accessibility in the LFY direct target genes APETALA1 and AGAMOUS.Taken together,our study suggests that LFY possesses key pioneer TF features that contribute to launching the floral gene expression program.