An ortholog of CURLY LEAF/ENHANCER OF ZESTE like-1 is required for proper flowering in Brachypodium distachyon

Many plants require prolonged (持续很久的) exposure to cold to acquire the competence (能力) to flower. The process by which cold exposure results in competence is known as vernalization (春化). In Arabidopsis thaliana, vernalization leads to the stable repression of the floral repressor FLOWERING LOCUS C via chromatin modification (染色质修饰), including an increase of trimethylation (三甲基化) on lysine 27 of histone H3 (H3K27me3) by Polycomb Repressive Complex 2 (PRC2; 多梳抑制复合物). Vernalization in pooids (禾本科植物) is associated with the stable induction of a floral promoter, VERNALIZATION 1 (VRN1). 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 A. thaliana CURLY LEAF 1, a gene that encodes the catalytic subunit (催化亚基) of PRC2. B. distachyon ezl1 mutants flower rapidly without vernalization in long-day (LD) photoperiods (光周期); thus, EZL1 is required for the proper maintenance of the vegetative state prior to vernalization. Transcriptomic studies in ezl1 revealed mis-regulation of thousands of genes, including ectopic expression (异位表达) of several floral homeotic genes (同源异型基因) 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 knock-down 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.

许多植物需要长时间的低温处理以获得能够开花的能力，这个过程叫做春化。在拟南芥中，春化会导致开花抑制基因FLOWERING LOCUS C的抑制，该过程主要通过染色质修饰完成，包括多梳抑制复合物PRC2增加组蛋白H3第27号亮氨酸的三甲基化（H3K27me3）。禾本科植物中的春化主要与促进开花基因VRN1的稳定诱导相关。通过对二穗短柄草的突变体库表现出春化减弱的特征进行筛选，作者鉴定到了EZL1基因上的两个有害等位基因。EZL1基因与拟南芥中的CURLY LEAF 1同源，该基因编码PRC2的一个催化亚基。在长日照的光照条件下，二穗短柄草的ezl1突变体会表现出快速开花的表型，并且是在没有春化的前提下，这表明EZL1基因对于营养器官的稳定维持发挥作用。对于ezl1突变体的转录组分析显示数千个基因被错误调控，包括一些原先在花中表达的同源异型基因在叶中异位表达。EZL1基因的丢失会导致H3K27me3和H3K27me2整体水平的降低，这与该基因在二穗短柄草中对于PRC2活性至关重要的结论一致。此外，在ezl1突变体中，开花基因VRN1和AG会出现异位表达，并且会降低H3K27me3水平。在ezl1突变体中，通过人工microRNA敲除VRN1或者AG基因均可以部分恢复在非春化植株中开花提前表型，表明在ezl1突变体中基因的异位表达可能是导致其快速开花表型的原因所在。

doi: 10.1111/tpj.13815