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Title: Trithorax group proteins act together with a polycomb group protein to maintain chromatin integrity for epigenetic silencing during seed germination in Arabidopsis

Author
item XU, FAN - Chinese Academy Of Agricultural Sciences
item KUO, TONY - Academia Sinica
item ROSLI, YENNY - University Of California
item LIU, MAO-SEN - Academia Sinica
item WU, LIMIN - Hangzhou Normal University
item CHEN, LONG-FANG - Academia Sinica
item Fletcher, Jennifer
item SUNG, ZINMAY - University Of California
item PU, LI - University Of California

Submitted to: Molecular Plant
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 1/29/2018
Publication Date: 5/1/2018
Citation: Xu, F., Kuo, T., Rosli, Y., Liu, M., Wu, L., Chen, L., Fletcher, J.C., Sung, Z., Pu, L. 2018. Trithorax group proteins act together with a polycomb group protein to maintain chromatin integrity for epigenetic silencing during seed germination in Arabidopsis. Molecular Plant. 11(5):659-677.

Interpretive Summary: As seeds germinate into seedlings the genes specifying embryo formation and seed maturation processes must be repressed to allow the growth and development of the young plant tissues. Although failure of this process can reduce overall plant yield, how germination is controlled at the molecular level is not well understood. This study investigated the genetic interaction between two genes that induce gene expression, ATX1 and ULT1, and the EMF1 gene that represses gene expression. Seedlings lacking the activity of all three genes displayed abnormal swollen roots, which we showed was because seed gene expression was not repressed after germination. We determined that this lack of repression was caused by improper arrangement of protein-DNA complexes in the nucleus. Our results demonstrate that the ATX1, ULT1 and EMF1 factors function together to turn off seed gene expression during germination, information that may prove useful to improve the germination efficiency and total yield of agronomic crop species.

Technical Abstract: 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 emf1 mutants. Instead, emf1 atx1 ult1 seedlings showed a novel swollen root phenotype and massive deregulation of gene expression. Greater ectopic expression of seed master regulatory genes in emf1 atx1 ult1 triple than in emf1 single mutants indicates that PcG and trxG factors together repress seed gene expression after germination. Furthermore, we found that the widespread gene derepression is associated with reduced levels of H3K27me3, an epigenetic repressive mark of gene expression, and with glob- ally altered chromatin organization. EMF1, ATX1, 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 EMF1-mediated gene silencing. Thus, while ATX1, ULT1, and EMF1 interact antagonistically to regulate flowering, they work together to maintain chromatin integrity and prevent precocious seed gene expression after germination.