This is a seed of Arabidopsis thaliana at the beginning of germination.
Credit: © UNIGE / Sylvain Loubéry
Seeds remain in a dormant state -- a
temporary blockage of their germination -- as long as environmental
conditions are not ideal for germination. The depth of this sleep, which
is influenced by various factors, is inherited from their mother, as
researchers from the University of Geneva (UNIGE), Switzerland, had
previously shown. Today, they reveal in the journal eLife how
this maternal imprint is transmitted through small fragments of
so-called 'interfering' RNAs, which inactivate certain genes. The
biologists also reveal that a similar mechanism enables to transmit
another imprint, that of the temperatures present during the development
of the seed. The lower this temperature was, the higher the seed's
dormancy level will be. This mechanism allows the seed to optimize the
timing of its germination. The information is then erased in the
germinated embryo, so that the next generation can store new data on its
environment.
Dormancy is implemented during seed development in the mother plant.
This property allows the seeds to germinate during the appropriate
season, to prevent all the offspring of a plant from developing in the
same place and competing for limited resources, and to promote plant
dispersal. Seeds also lose their dormancy at variable times. "Subspecies
of the same plant can have different levels of dormancy depending on
the latitudes at which they are produced, and we wanted to understand
why," explains Luis Lopez-Molina, Professor at the Department of Botany
and Plant Biology of the UNIGE Faculty of Science.
The paternal gene is silenced
Like all organisms with sexual reproduction, the seed receives two
versions of each gene, a maternal and a paternal allele, which may have
different levels of expression. The UNIGE biologists had shown in 2016
that the dormancy levels of Arabidopsis thaliana, a model organism used
in laboratories, are inherited from the mother. Indeed, in the seed, the
level of expression of a dormancy regulating gene called allantoinase
(ALN) is the same as that of the maternal allele. This implies that it
is the maternal allele of ALN that is mainly expressed, to the detriment
of the paternal allele.
In the current study, the researchers show that this maternal imprint
is transmitted by an epigenetic mechanism, which influences the
expression of certain genes without altering their sequence. The
paternal allele of ALN is 'silenced' by biochemical modifications called
methylations, which are carried out in the promoter region of the gene
in order to inactivate it.
"These methylations are themselves the result of a process in which
different enzymatic and factor complexes are involved, as well as small
fragments of so-called 'interfering' RNA. This is a unique example of
genomic imprinting, because it is made in the absence of the enzyme
usually responsible for methylation," says Mayumi Iwasaki, researcher in
the Geneva group and the first author of the article.
The imprint of past cold prevents the seed from awakening
The environmental conditions present during the seed formation also
leave their mark, as its dormancy level increases with decreasing
temperatures. "We have discovered that, in this case, both alleles of
the ALN gene are strongly repressed in the seed. This is due to a
similar epigenetic mechanism, but not all of the actors are the same as
those used to silence the paternal allele," says Luis Lopez-Molina.
This imprint of the cold enables the seed to keep information on past
temperatures, in order to include them in the choice of the optimal
time of germination. After germination, the ALN gene is reactivated in
the embryo. The memory of the cold will then be cleared, allowing the
counters to be reset for the next generation.
"Studying how maternal and environmental factors cause dormant seeds
to awaken is of crucial importance for agriculture, especially to
prevent early germination in an environment subject to climate change,"
concludes Mayumi Iwasaki. The ecological stakes are also high, because
increasing temperatures could reduce the dormancy of the seed bank and
thus modify the distribution of plant species under a given latitude.
This would have multiple consequences, both direct and indirect, for
native animal and plant species.
Journal Reference:
- Mayumi Iwasaki, Lena Hyvärinen, Urszula Piskurewicz, Luis Lopez-Molina. Non-canonical RNA-directed DNA methylation participates in maternal and environmental control of seed dormancy. eLife, 2019; 8 DOI: 10.7554/eLife.37434
Courtesy: ScienceDaily
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