Epigenetic Regulation in Higher Plants

Xiaofeng CaoInstitute of Genetics and Developmental Biology,

Chinese Academy of Sciences, Beijing, China

2010-12-10

Reference

• Liu, Lu, Cui and Cao (2010) Histonemethylation in higher plant

Annu Review of Plant Biology

Genetics

Genotype Phenotype

Genetic information flow (central dogma):

transcription translationDNA RNA protein

Epigenetics

Current Wu & Morris (2001): “the study of changes in gene function that are heritable and that do not entail a change in DNA sequence”.

Heritable:mitotically heritablemeioticallyheritable

Epigenetics

• When a cell undergoes mitosis or meiosis, the epigenetic information is stably transmitted to the daughter cells or subsequent generation

• Epigenetic controls add an ‘extra layer’of transcriptional control—transcriptional memory

Epiallele—same DNA sequence but different epigenetic state

Epigenome—the epigenetic state of all sequences in a genome

Epigenetic mechanisms involve:

-DNA methylation

-Histone modification/histone variants

-Regulatory non-coding RNAs

•Epigenetic phenomena-Paramutation

-Parent-of-origin effects

•DNA methylation and DNA methyltransferases

•Genetic screening for components in maintenance of RNA-directed transcriptional gene silencing

•Identification of genes involved in RdDM

Outline

Paramutation

Described in maize by A. Brink and E. Coe (1950’s).

An interaction between alleles that leads to a mitotically and meiotically heritable change in the expression of one allele.

Described for a few endogenous genes and transgenes in multiple species including plants, animals and fungi.

Only observed with specific alleles. Most alleles do not participate in paramutation (neutral alleles).

Phenotypes of paramutation in maize

paramutable B-I alleles: encodes an enzyme in the pathway of anthocyanin pigments

null b alleles: lack these pigments, and these balleles are completely recessive to B-I

paramutagenic allele B’:make only a small amount of anthocyaninpigment.

F1

F2

B-I is recessive to B’?

3:1?

Paramutation at B locus

F1

The high expressing B-Istate is unstable, changes to low expressing B’ state at a high frequency (0.1-10%).

When B’ and B-I are crossed paramutationoccurs 100% of the time.

B’ is extremely stable. UNUSUAL!

X

B’ B-I

B’/B’*

X

B-I

B’B’ B’ B’

Characteristic of paramutation

• Paramutation is the directed, heritable alteration of the expression of one allele when heterozygous with another allele.– The newly silenced allele can further silence new targets.– Allelism is not a strict requirement for paramutation.– Paramutation is associated with DNA methylation changes

( both at the trigger and target sequence).– Paramutation can be accounted by RdDM.

mediator of paramutation1 (mop1-1) mutation blocks paramutation at the maize B, R, and Pl1 loci.

•Epigenetic phenomena-Paramutation

-Parent-of-origin effects

•DNA methylation and DNA methyltransferases

•Genetic screening for components in maintenance of RNA-directed transcriptional gene silencing

•Identification of genes involved in RdDM

Outline

Genomic imprinting （基因组印记）

♂ AA x aa ♀

F1 Aa

F1 Phenotype can be A or a

Parent-of-origin effects

(mare) X 驴(jack)♂ ♀驴 (jenny) X 马(stallion)♂

骡(mule) ？( hinny)

An imprinted gene

The copy of gene A inherited from the mother is expressed. The copy inherited from the father is silenced. By convention, gene Ais described as paternally imprinted. For some imprinted genes, the maternal allele is silent and the paternal allele is expressed and the gene is said to be maternally imprinted.

Examples of imprinting genes in mouse

The Igf2/H19 cluster

The Igf2/H19 gene pair is part of a cluster of imprinted genes on mouse chromosome7. ICR: imprinting control region; E: enhancer element

Deletion of the imprinting control region and changing the position of the 3’ enhancer both cause loss of imprinting of the Igf2 and H19 genes

The Igf2 /H19 imprinting control regions (ICR) on the maternal and paternal chromosomes differ in CpG methylation and nuclease sensitivity