[en] Many plants require prolonged cold exposure to acquire the competence to flower. The process by which cold exposure results in competence is known as vernalization. In Arabidopsis thaliana, vernalization leads to the stable repression of the floral repressor FLOWERING LOCUS C via chromatin modification including an increase of trimethylation on lysine 27 of histone H3 (H3K27me3) by Polycomb Repressive Complex 2 (PRC2). Vernalization in pooids is associated with the stable induction of a floral promoter, VERNALIZATION 1. From a screen for mutants with a reduced vernalization requirement in the model grass Brachypodium distachyon, we identified two recessive alleles of ENHANCER OF ZESTE-LIKE 1 (EZL1). EZL1 is orthologous to Arabidopsis CURLY LEAF 1, a gene that encodes the catalytic subunit of PRC2. B. distachyon ezl1 mutants flower rapidly without vernalization in long-day (LD) photoperiods; thus, EZL1 is required for proper maintenance of the vegetative state prior to vernalization. Transcriptomic studies in ezl1 revealed mis-regulation of thousands of genes including ectopic expression of several floral homeotic genes in leaves. Loss of EZL1 results in the global reduction of H3K27me3 and H3K27me2, consistent with this gene making a major contribution to PRC2 activity in B. distachyon. Furthermore, in ezl1 mutants, the flowering genes VRN1 and AGAMOUS (AG) are ectopically expressed and have reduced H3K27me3. Artificial microRNA knockdown of either VRN1 or AG in ezl1-1 mutants partially restores wild-type flowering behavior in non-vernalized plants suggesting that ectopic expression in ezl1 mutants may contribute to the rapid-flowering phenotype.
This work was funded in part by the National Science Foundation (grant no. IOS-1258126) and by the DOE Great Lakes Bioenergy Research Center (DOE Office of Science BER DE-FC02-07ER64494). Work in Xuehua Zhong’s laboratory was supported by Alexander von Humboldt Foundation (Alfred Toepfer Faculty Fellow), NSF CAREER award (MCB-1552455), and USDA & National Institute of Food and Agriculture grant (Hatch 1002874). Aaron Lomax, Daniel Woods, and Kevin Mayer were supported in part by a National Institutes of Health sponsored pre-doctoral training fellowship to the University of Wisconsin Genetics Training Program. Frédéric Bouché was supported by Wallonie-Bruxelles international fellowship.
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