EMBRYONIC FLOWER1 and ULTRAPETALA1 Act Antagonistically on Arabidopsis Development and Stress Response

Epigenetic regulation of gene expression is of fundamental importance for eukaryotic development. EMBRYONIC FLOWER1 (EMF1) is a plant-specific gene that participates in PcG-mediated transcriptional repression of target genes such as the flower MADS Box genes AGAMOUS, APETALA3 and PISTILLATA. Here, we investigated the molecular mechanism underlying the curly leaf and early flowering phenotypes caused by reducing EMF1 activity in the leaf primordia of LFYasEMF1 transgenic plants, and propose a combined effect of multiple flower MADS Box gene activities on these phenotypes. ULTRAPETALA1 (ULT1) functions as a trxG factor that counteracts PcG action in Arabidopsis. Removing ULT1 activity rescues both the abnormal developmental phenotypes and most of the mis-regulated gene expression of LFYasEMF1 plants. Reducing EMF1 activity increases salt tolerance, an effect that is diminished by introducing the ult1-3 mutation into the LFYasEMF1 background. EMF1 is required for trimethylating lysine 27 on histone 3 (H3K27me3) and ULT1 associates with ARABIDOPSIS TRITHORAX 1 (ATX1) for trimethylating H3K4 at flower MADS Box gene loci. Reducing EMF1 activity decreases H3K27me3 marks and increases H3K4me3 marks on target gene loci. Removing ULT1 activity has the opposite effect on the two histone marks. Removing both gene activities restores the active and repressive marks to near wild-type levels. Thus, ULT1 acts as an anti-repressor that counteracts EMF1 action through modulation of histone marks on target genes. Our analysis indicates that, instead of acting as OFF and ON switches, EMF1 and ULT1 mediate histone mark deposition and modulate transcriptional activities of the target genes.