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 dream, which is influenced by various factors, is inherited from his mother, as researchers from the University of Geneva (UNIGE), Switzerland, had shown. Today, they reveal in the magazineeLife.how this maternal fingerprint is transmitted through small fragments of so-called "interfering" RNAs, which inactivate certain genes.
Biologists also reveal that a similar mechanism allows another impression to be transmitted, that of the temperatures present during the development of the seed. The lower this temperature, the higher the dormancy level of the seed. This mechanism allows the seed to optimize its germination time. The information is then erased in the germinated embryo, so that the next generation can store new data in its environment.
Dormancy is implemented during seed development in the mother plant. This property allows seeds to germinate during the appropriate season, to prevent all a plant's young from developing in the same place and competing for limited resources, and to promote plant dispersal. The seeds also lose their dormancy at variable times. "Subspecies of the same plant can have different levels of latency depending on the latitudes in which they occur, and we wanted to understand why," explains Luis López-Molina, professor at the Department of Botany and Plant Biology at the Faculty of UNIGE . Science.
The paternal gene is silenced
Like all organisms with sexual reproduction, the seed receives two versions of each gene, a maternal allele and a paternal allele, which can have different levels of expression. UNIGE biologists demonstrated in 2016 that the latency levels of Arabidopsis thaliana, a model organism used in laboratories, are inherited from the mother. In fact, in the seed, the level of expression of a latency regulator gene called allantoinase (ALN ) is the same as the maternal allele. This implies that it is the maternal allele of theALN the which is expressed mainly, to the detriment of the paternal allele.
In the current study, the researchers show that this maternal fingerprint is transmitted by an epigenetic mechanism, which influences the expression of certain genes without altering their sequence. The paternal allele ofALN it is "silenced" by biochemical modifications called methylations, which are carried out in the promoter region of the gene to inactivate it.
"These methylations are in themselves the result of a process in which different complexes of factors and enzymes participate, as well as small fragments of the so-called" interfering "RNA. This is a unique example of genomic imprinting, because it is manufactured in the absence of the enzyme that is usually responsible for methylation, ”says Mayumi Iwasaki, a researcher in the Geneva group and first author of the article.
The imprint of the cold past prevents the seed from awakening
The environmental conditions present during seed formation also leave their mark, as their dormancy level increases with decreasing temperatures. "We have discovered that, in this case, both alleles of the geneALN they are strongly repressed in the seed. This is due to a similar epigenetic mechanism, but not all the actors are the same as those used to silence the paternal allele ”, says Luis López-Molina.
This imprint of the cold allows the seed to keep information on past temperatures, to include them in the choice of the optimal germination moment. After germination, the geneALN it is reactivated in the embryo. The cold memory will be erased, 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 rising temperatures could reduce the latency of the seed bank and thus modify the distribution of plant species at a given latitude. This would have multiple consequences, both direct and indirect, for native plant and animal species.
Source: University of Geneva (UNIGE)
Reference: Non-canonical RNA-directed DNA methylation participates in maternal and environmental control of seed dormancy. Mayumi Iwasaki et al. eLife 2019; 8: e37434. DOI: 10.7554 / eLife.37434