Fred Berger & Pauline
Jullien
From the Special Topic of
Epigenetics
One of the core papers in theArabidopsis
thalliana DNA MethylationResearch Front
Map, which is a part of our Special Topic on Epigenetics,
is "Maintenance of DNA methylation during the
Arabidopsis life cycle is essential for parental
imprinting," (Jullien PE, et al., Plant Cell
18[6]: 1360-72, June 2006). In
Essential Science IndicatorsSMfrom
Thomson
Reuters, this paper has 44 citations to its credit up
to December 31, 2008.
In this interview, ScienceWatch.com talks with two of the
paper's authors, Dr. Frederic Berger, who is the Senior Principal
Investigator, and Dr. Pauline Jullien, who was the major contributor to
this study. Both authors work at Temasek Lifesciences Laboratory, which is
an affiliate of the National University of Singapore and Nanyang
Technological University.
Below, they talk about the paper
and its significance for the research
community.
Would you please describe the significance of your
paper and why it is highly cited?
At the time the paper was published only two genes had been reported for
their monoparental expression (imprinting) in plants. In this paper, we
identified FERTILIZATION INDEPENDENT SEED 2(FIS2) as a third
imprinted gene in the model plant
Arabidopsis. More importantly, our study
clearly showed that the maintenance DNA methyltransferase MET1 is
responsible for silencing of the paternal allele of FIS2 during
the vegetative life cycle, male gametogenesis, and development of the
endosperm, the extra-embryonic tissues where imprinting takes place in
plants. Thus, in this study we described the whole cycle of imprinting
in plants for the first time.
How did you become involved in this research, and were
there any particular successes or obstacles that stand out?
"We want to understand how two
alleles with identical DNA sequences can be
differentially marked by epigenetic
modification, which, in turn, results in
imprinted gene
expression."
Our original main research interest was to study endosperm development.
Amongst mutants affecting endosperm development, fis2 mutations
(originally identified by A. Chaudhury) affected endosperm development when
inherited from the mother. This suggested that FIS2 expression was
restricted to the maternal allele. The lack of typical signature for
targets of DNA methylation on FIS2 was problematic for
identification of the cis-element responsible for imprinting, and our
collaborator Tetsu Kinoshita helped to solve this problem. This study
caused a major shift in the research orientation of our team, which became
focused on investigations of imprinted gene regulation in the endosperm.
Where do you see your research and the broader
field leading in the future?
One of the current aims of our research is to dissect how imprinted gene
expression is established. We want to understand how two alleles with
identical DNA sequences can be differentially marked by epigenetic
modification, which, in turn, results in imprinted gene expression. This
takes place during male and female gametogenesis, and we have recently
shown that the Retinoblastoma pathway plays an essential role in maternal
activation of DNA methylation dependent imprinting genes (Jullien PE,
et al., Retinoblastoma and its binding partner MSl1 control
imprinting in Arabidopsis," PLoS Biology 6[8]: 1693-1705,
August 2008). One of the broader directions of the field will be to
identify new imprinted genes in Arabidopsis in order to draw
general conclusions.
What are the implications of your work for this field?
Two years ago, in parallel with the identification of imprinting of
FIS2, we showed that maternally expressed imprinted genes could be
also controlled by a different silencing mechanism dependent on histone
methylation by Polycomb group activity (Jullien PE, et al.,
"Polycomb group complexes self-regulate imprinting of the polycomb group
gene MEDEA in Arabidopsis," Current Biology
16[5]: 486-92, 7 March 2006). This identification also led to the
understanding of a negative feed-loop involving the Polycomb Group gene
FIS2. Both DNA and histone methylation are also essential for
imprinting of genes in mammals and our results pointed to a convergent
evolution of imprinting mechanisms.
Work in our team and in the teams of T. Kinoshita and R. Fischer has shown
that epigenetic marks establishment during gametogenesis is critical for
imprint establishment for genes regulated by DNA methylation. The same
principle probably holds true for imprinting genes dependent on histone
methylation, and the mechanisms involved in the dynamics of these
epigenetic marks now needs to be addressed. We hope that the concerted
efforts of researchers will lead in the near future to a general view of
imprinting mechanisms in plants. This will provide the basis for
longer-term major questions pertaining to the biological significance of
imprinting and how this remarkable epigenetic mechanism
evolved.
Frederic Berger and Pauline Jullien
Developmental Biology Program
Temasek Lifesciences Laboratory
Republic of Singapore
Jullien PE et al., "Maintenance of DNA methylation
during the Arabidopsis life cycle is essential
for parental imprinting," Plant Cell 18(6):
1360-72, June 2006. Source:
Essential Science Indicators from
Thomson
Reuters.