Gene imprints and seed developmentSeed Physiology

(PP-608)

Prepared byJagadish Jena

Dept. of Agronomy

I.G.K.V., RAIPUR

CONTENTS

Gene imprinting

Seed development

Parental conflict theory

Types of genetic imprinting

Angiospermic gene imprinting mechanism

Conclusion

Gene imprinting Genomic imprinting is an epigenetic phenomenon leading to a change of gene expression dependent on whether the gene was inherited from the maternal or the paternal parent (Reik and Walter, 2001).

Imprinted genes are expressed predominantly one allele in a parent of origin specific manner.

The parental conflict theory has been proposed to explain imprinting in plants and mammals. The reciprocal seed phenotypes observed in intra-ploidy and inter-ploidy crosses are the result of conflict among maternal, paternal and offspring interests over resources allocation from mother to seed (Haig and Westoby, 1989,1991).

Table shows there are least effects of imprinted genes on embryo, in contrast more effect on endosperm. There are about 100-300 genes regulated by genomic imprinting in Arabidopsis, rice and maize.

MEGs: Maternally expressed genes PEGs: Paternally expressed genes

Jiang and Kohler, 2012

The imprint is placed during male or female gametogenesis and determines the differential expression state of the alleles in post fertilization tissues.

This parent-of-origin specific change in gene expression is not connected with the change in DNA sequence and does, therefore, represent one form of epigenetic inheritance where heritable changes in genome function can occur without a change in DNA sequence (Bird, 2007).

Genomic imprinting in mammals occurs both in placenta and embryo as well as in adult tissues, whereas imprinting in angiosperms occurs predominately in the endosperm and in the early embryo, but not in adult tissues.

Overview of known gene-specific imprinting genes in plants

Garnier et al., 2008

Seed development Seed development begins upon double fertilization, where one sperm fertilizes the egg cell forming embryo (2n) and 2nd one fertilizes the polar nuclei forming endosperm (3n).

The endosperm, a terminally differentiated tissue that nourishes the embryo during seed development is the only known site of imprinting in plants.

Imprinting is important here, because yet the evolutionary origin of endosperm is just a puzzle.

Arabidopsis endosperm development begins with the division of the triploid primary endosperm nucleus, which precedes the division of zygote by several hours followed by continual division of endospermic nucleus without cytokinesis to create cyncytium of nuclei organized into three distinct mitotic domains: micropylar endosperm(MCE), peripheral endosperm(PEN) and chalazal endosperm(CZE) (Boisnard-Lorig et al., 2001).

The three domains also are distinct morphologically and cytologically.

CZE has larger nuclei than the rest of the syncytial endosperm, perhaps as a result of DNA replication without nuclear division (Endo-reduplication).

Imprinting is particularly important in the CZE because of its direct connection with the female parent.

According to Brink and Cooper (1947) endosperm dis-function is the primary reason for hybrid incompatibility, with embryo death being a subsequent event.

According to Lin (1982), the paternal source of endosperm is the determinant of seed size.

a. Normal double fertilization in a sexual angiospermb. Seed from hypothetical engineered apomixis in a sexual species abort because of an

unbalanced endosperm genomic ratioc. Unreduced gametophytes that are 4 nucleate, with one rather than 2 polar nuclei.d. Karyogamy of a single unfused polar nucleus with one reduced sperm nucleuse. Fertilisation of two polar nuclei with both sperm cells delivered by the pollen tube.

Grossniklaus et al., 2001

Scott et al (1998) concluded that seeds with excess maternal genomic (4x X 2x) weigh less than seed produced by diploid (2x X 2x).

Bushel et al (2003) reviewed the failure of inter-specific crosses in Arabidopsis lies in the endosperm.

According to Johnston et al (1980) the endosperm of intra-specific , inter-ploidy crosses also must be in a 2m:1p balance to achieve seed viability in potato.

To explain this apparent violation of the 2m:1p rule, Johnston et al (1980) proposed the endospermic balance number (EBN) hypothesis.

Parental conflict theory The parental conflict theory has been proposed to explain imprinting in plants and mammals. According to this hypothesis the reciprocal seed phenotypes observed in intra-ploidy and inter-ploidy crosses are the result of a conflict among maternal, paternal and offspring interests over resource allocation from mother to seed.

Genes that promote maternal resource allocation should be expressed paternally. As a counter balance genes that restrict resource distribution from mother to seed will be expressed maternally but not paternally.

The parental conflict theory doesn’t explain the ability of functional endosperm to be formed in autonomous apomicts, which don’t require fertilization by a male parent.

Types of genetic imprinting

Two types of gene imprinting can be described as;

Allelic imprinting: only alleles from a certain background are subjected to parent-of-origin specific gene expression and

Locus imprinting: All known alleles from different backgrounds are under parent-of-origin control.

Examples:

R-allele in Maize plant responsible for anthocyanin accumulation in aleurone layer is an example of allelic imprinting.

Imprinting of Arabidopsis MEDEA (MEA) isolated responsible for reproductive development and seed abortion is an example of locus imprinting.

How are two alleles reside in the same nucleus and

share the same DNA sequence distinguishable from one another ?

Angiosperm imprinting mechanisms Imprinting cycle in plants would be different from that in mammals if imprinting occurs only in the endosperm.

The endosperm is a terminally differentiated tissue that doesn’t pass on its genetic and epigenetic information to the next generation rendering imprint erasure unnecessary. If imprinting does occur in other parts of the plant, imprints would have to be erased and reset when the lineages that give rise to the male and female gametes diverge.

There are different mechanisms involved in gene imprinting;

DNA methylation

Differences in chromatin structure

DNA methylation and imprinting

Three classes of methyl transferase genes present in Arabidopsis viz. MET1 (methyl transferase), CMT (Chromomethylase) and DRM ( Domain arranged methyl transferase) families. (Cao et al., 2000)

Plants carrying antisense transgene of MET1 (MET1 a/s) show developmental abnormalities due to hypomethylation as MET1 is necessary for maintenance of methylation during gametogenesis. (Finnegan et al., 1996)

When the hypomethylated parent is male, the endosperm produces less peripheral endosperm nuclei and cellularizes earlier but in-contrast when the parent is female, more peripheral nuclei are produced and the cellularization is delayed. (Adams et al.,2000)

Chromatin structure and imprinting

DNA methylation and chromatin structure are intimately connected, for which chromatin structure imprinting also has a significant influence on endosperm character and seed development.

Histone methylation at histone H3 Lys 9 is associated with silent chromatin. Methylation of histone H3 Lys 4 is associated with transcriptionally competent regions of the genome.

The imprinting mechanism in plant is fundamentally different from that in animals, in which the epigenetic modification of imprinted genes are reset at every generation. (Xiao et al., 2003)

Conclusion Double fertilization is the prime mechanism of seed development in angiosperms, where the endosperm which is the nutrition pool for early embryo development, is the site of gene imprinting.

Gene imprinting is the phenomenon in which a set of gene is expressed according to their parent of origin according to the parental conflict theory.

There are certain mechanisms of gene imprinting, among which DNA methylation is most important one.

There are certain abnormalities found in seeds if there is lack of proper ratio of paternal and maternal endospermic characters which shows the importance for its study.