The seed stage is an integral part of a plant’s life cycle and necessary for species’ survival. As the dispersal unit of the organism, the seed must be able to sustain extended periods of unfavorable conditions if necessary. It does this by remaining dormant, thereby delaying germination until the environment is ideal.
Biologists at the University of Geneva (UNIGE), Switzerland previously revealed that seed dormancy is inherited from the mother plant. Now, this same team has uncovered how this maternal imprint is passed on. In their new study, they have identified the epigenetic mechanism that inactivates a specific gene linked to plant dormancy. Their findings were recently published in eLife online.
Epigenetics refers to heritable changes in gene expression that do not alter the genetic sequence but determines how the DNA code is read in a cell. Plants rely on epigenetic processes to function correctly. Their epigenetic mechanisms are similar to those found in mammal genomes such as histone marks and DNA methylation at cytosine bases. Although compared to animals, the epigenome of a plant is more responsive to environmental influences. In other articles, we’ve reported on how epigenetics plays a role in protecting plants from drought as well as other extreme weather conditions. A plant’s phenotypic plasticity during development gives them the ability to survive and reproduce in unpredictable situations.
Various environmental and genetic factors contribute to seed dormancy which is initiated by the mother plant during seed development. This property allows for germination to occur in the appropriate season and prevents plant offspring in the same location from competing with each other. Seeds can also abandon their dormancy at different times. According to Luis Lopez-Molina, UNIGE Faculty of Science, “Subspecies of the same plant can have different levels of dormancy depending on the latitudes at which they are produced.”
Seeds acquire two versions of each gene from their parents – one paternal allele and one maternal allele – and both have distinct levels of gene expression. In 2016, the UNIGE researchers demonstrated that dormancy levels of the model plant, Arabidopsis thaliana, are derived from the mother. The gene that regulates this process is called allantoinase (ALN), and it is preferentially expressed on the maternal allele as well as in the seed. Cold temperatures, which enhance dormancy during seed development, were also found to suppress ALN.
In the new study, the team discovered that the paternal allele of ALN is silenced at the promoter region of the gene by biochemical alterations called methylations, more specifically non-canonical RNA-directed DNA methylation (RdDM).
“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,” says first author, Mayumi Iwasaki. “This is a unique example of genomic imprinting because it is made in the absence of the enzyme usually responsible for methylation.”
The environment also affects seed formation, as colder weather can prolong dormancy. According to the researchers, lower temperatures can significantly repress both ALN alleles in the seed. Lopez-Molina says. “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.”
Fortunately, the cold effect permits the seed to retain information longer so that germination occurs at an optimal time. Reactivation of the ALN gene occurs after germination in the seed embryo. All epigenetic modifications that were induced by the temperature are cleared, and the mechanisms are reset. This process ensures that the next generation can adapt to their appropriate flowering time in a new environment.
With global temperatures on the rise, scientists are concerned that abnormal seed dormancy could disrupt the plant distribution of numerous species. The effects from this would no doubt directly or indirectly impact the ecosystem and potentially harm native animal and plant species.
Iwasaki concluded, “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.”
Source: Iwasaki, M. et al. (March 2019). Non-canonical RNA-directed DNA methylation participates in maternal and environmental control of seed dormancy. eLife.
Reference: Université de Genève. “Seeds inherit memories from their mother.” March 2019.