Maize Polycomb Protein MEZ1 Controls Histone Modification And DNA Methylation

PcG(Polycomb group)proteins,first identified in Drosophila,are large members of chromatin regulatory factors.PRC2(polycomb repressive complex 2)is a representative example among them and is highly conserved between animals and plants.PRC2 is well-studied in animals and extensive studies of PRC2 in Arabidopsis have been done as well since 2000.However,we knew very little about the role it played in maize development for a long time.This research aims to explore the role of MEZ1,a core and enzymatic component of PRC2,in maize seed development by reverse genetics using Crispr-Cas9.We found that MEZ1 was a maternally expressed imprinting gene(MEG)and loss of MEZ1 exhibited imprinted phenotype.mezl/MEZ1 seeds carrying a maternal-derived mezl allele showed delayed embryo and endosperm development.But it could be restored to the wild type level finally,suggesting that an autoregulation mechanism existed as Arabidopsis did.Meanwhile,homozygous mez1lmezl seeds were smaller and embryo lethal.For the first time,we demonstrated that MEZ1 was a component of PRC2,which also contains polycomb protein FIE1(fertilization-independent endosperm 1)with a mass of about 700 kDa.MEZ1 was a bona fide H3K27me3 methyltransferase,as H3K127me3 was almost undetectable in mez1/mezl endosperm.Although two paralogs of MEZ1,MEZ2 and MEZ3,were expressed in the endosperm,they could not complement the loss of MEZ1,indicating the specific role of MEZ1 in the endosperm.We identified that genes involved in endosperm cellularization,stress response,hormone response and patterning were regulated by MEZ1 using RNA-seq.PEGs(paternally expressed imprinted genes)were preferentially controlled by MEZ1.BETL specific genes were controlled by MEZ1 indirectly.MEZ1 might be the general upstream factor that controlled the expression of BETL specific genes.H3K27me3 ChIP-seq data revealed that MEZ1 target genes were low expressed and showed tissue-specific expression pattern.The target genes involved in transcription regulation,cell wall,stress response,fatty acid metabolism and nitrogen metabolism,as well as transcription factors.MADS box genes that played vital roles in endosperm development were preferentially targeted by MEZ1.Whole genome BS-seq showed that although the methylation level of whole genic region exhibited limited increase in mezl,transcription factors and PEG genes regulated by MEZ1 showed obviously increased methylation level.MEZ1 might be the upstream regulator of CHH island.In the whole genome scale,MEZ1 mainly regulated TE methylation,with the level of CG methylation decreased while that of CHG and CHH methylation increased in mezl.CHH methylation in TE body might be originally inhibited by MEZ1.In general,our study uncovered an indispensable role of MEZ1 in seed development.