ABA合成关键基因NCED随种子萌发进程沉默的分子机制

Seed germination plays a vital role in higher plant development as well as in agricultural production. Abscisic acid (ABA) is an important phytohormone that inhibits seed germination. For after-ripen non-dormant seeds, a rapid reduction of ABA during imbibition is necessary for germination. Seed ABA content results from both synthesis and degradation pathways, and ABA catabolism gene CYP707A2, quickly induced upon imbibition, has been long regarded as the key to ABA reduction in seeds. Nevertheless, overexpression of NCED6 leads to inhibited germination in Arabidopsis, providing evidence for the importance of ABA synthesis repression during early germination as well. Our data show a rapid silencing of ABA synthesis genes (NCED2,5,6,9, etc.) upon seed imbibition. Preliminary results suggest the involvement of PRC2-dependent H3K27me3 histone modification together with activity of one RNA binding protein, indicating a possible interaction between RNA metabolism and H3K27me3 in epigenetic silencing. Here we would like to further explore the molecular silencing mechanism of NCED genes during seed germination and early seedling stage. This study could contribute to our understanding of seed germination, ABA synthesis and gene silencing.

种子萌发不仅是高等植物重要的发育事件，同时也与农业生产存在紧密的实际联系。ABA作为种子萌发过程中主效的植物激素，其在种子吸涨过程中的快速及持续降解是萌发的先决条件。ABA的水平由其合成和降解共同决定，萌发过程中ABA降解基因CYP707A2大量表达，这被认为是ABA含量减少的主因。然而过量表达ABA合成基因NCED6能够引起萌发的延迟，暗示ABA合成基因的抑制对ABA含量下降同样关键。申请人发现ABA合成基因NCED2,5,6,9在种子萌发过程中逐渐被沉默。初步结果显示这种沉默依赖PRC2复合体及其介导的H3K27me3，同时也依赖一个RNA结合蛋白的活性，暗示RNA代谢与H3K27me3可能在基因的表观沉默过程中共同作用。据此，申请人希望深入研究NCED基因在种子萌发及幼苗期被选择性表观沉默的分子机制。通过本研究，希望增进人们对种子萌发、ABA合成以及基因沉默机制的理解。

该课题主要探究了拟南芥的脱落酸合成基因NCED6等在种子萌发过程中逐渐被沉默的分子机制。研究首先发现该过程需要RNA结合蛋白RZ1与H3K27me3修饰复合物PRC2的协同调控：同时失去RZ1与PRC2的核心蛋白可以引起种子吸涨过程中显著的基因表达变化并且抑制萌发的发生。其次，在种子萌发过程早期，一些关键基因（如NCED6）上存在H3组蛋白乙酰化修饰和H3K4me3修饰被去除的过程，而该过程在rz-1b rz-1c双突变体中被明显推迟，且RZ1与去乙酰化酶等蛋白存在相互作用，暗示着RZ1参与了ABA合成基因早期表达关闭的调控；这一过程与相应基因位点上H3K27me3修饰的建立相辅相成，最终导致了基因的协同沉默。相关研究成果于2022年顺利发表在国际重要学术杂志Plant Cell上（见成果附件），并获得了国内外同行的关注和报道。