Trithorax Group Proteins Act Together with a Polycomb Group Protein to Maintain Chromatin Integrity for Epigenetic Silencing during Seed Germination in Arabidopsis

Polycomb group （PcG） and trithorax group （trxG） proteins have been shown to act antagonistically to epigenetically regulate gene expression in eukaryotes. The trxG proteins counteract PcG-mediated floral repression in Arabidopsis, but their roles in other developmental processes are poorly understood. We investigated the interactions between the trxG genes, ARABIDOPSIS HOMOLOG OF TRITHORAX1 （ATX1） and ULTRAPETALA1 （ULT1）, and the PcG gene EMBRYONIC FLOWER 1 （EMF1） during early development. Unexpectedly, we found that mutations in the trxG genes failed to rescue the early-flowering phenotype of emfl mutants. Instead, emfl atxl ultl seedlings showed a novel swollen root phenotype and massive deregulation of gene expression. Greater ectopic expression of seed master regulatory genes in emfl atxl ultl triple than in emfl single mutants indicates that PcG and trxG factors together repress seed gene expression after germination. Furthermore, we found that the widespread gene derepression is asso- ciated with reduced levels of H3K27me3, an epigenetic repressive mark of gene expression, and with globally altered chromatin organization. EMF1, ATXl, and ULT1 are able to bind the chromatin of seed genes and ULT1 can physically interact with ATX1 and EMF1, suggesting that the trxG and EMF1 proteins directly associate at target gene loci for EMFl-mediated gene silencing. Thus, while ATXl, ULT1, and EMF1 interact antagonistically to regulate flowering, they work together to maintain chromatin integrity and prevent precocious seed gene expression after germination.

The Roles of Different CLE Domains in Arabidopsis CLE Polypeptide Activity and Functional Specificity

The CLE （CLVATA3/ESR-related） family of plant polypeptide signaling molecules shares a conserved 14-aminoacid （aa） motif, designated the CLE motif, which recent studies suggest is sufficient for CLE function in vitro. In this study, we report that Arabidopsis CLE proteins can function in a tissue-specific manner and confirm some CLE factors can act through different receptors. Using domain swapping, we show for the first time that the CLE motif likely determines much of the functional tissue-specificity of the proteins in planta. However, we also provide evidence in support of the new view that sequences outside the CLE motif （14 aa） contribute to CLE function and functional specificity in vivo. Additionally, we report that deletion of the putative signal peptide from different CLE proteins completely inactivates CLE function in vivo, whereas exchanging the CLE signal peptides with a conventional signal peptide from a rice glycine-rich cell wall protein also influences CLE function. We thus propose that the CLE motif itself determines its functional tissue-specificity by dic- tating the direct recognition and interaction of each CLE peptide with its optimal receptor（s）, whereas the receptor（s） may be available in a tissue-specific manner. On the other hand, the sequences outside the CLE motif may influence CLE function by affecting the processing of CLE peptides, resulting in a change in the availability and/or abundance of CLE peptides in specific tissues and/or cells.

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.