Plants have memories they learn to forget

An international research team is describing the ability of plants to store important environmental cues in the form of an epigenetic memory. 

The plants use this information to “remember” prolonged cold weather. This way, the plants realize when the conditions subside so they can produce flowers at the right time in the spring.

After they flower and seed, however, the memory is erased to reset the timing of blooming for the next season.

Humans have epigenetic memories as well. Liftstyle and environmental factors, including diet, pollution, and stress, modify a person’s DNA through specialized proteins called histones. These epigenetic changes are passed along to children for generations.

In plants, one specific histone modification called H3K27me3 tends to mark genes that are switched off. Cold weather conditions cause H3K27me3 to accumulate at the genes that regulate flowering. 

Previously, in the lab of Dr. Frédéric Berger of the Gregor Mendel Institute of the Austrian Academy of Sciences, experts demonstrated that H3K27me3 is faithfully transmitted from cell to cell. As a result, in the springtime, the plants remember that it was cold and that winter is now over. 

But once the plants have flowered, they need to erase the cold memory from their seeds. 

In the current study, a research team led by Jörg Becker of the Instituto Gulbenkian de Ciência set out to investigate how plants manage to “forget” the epigenetic memory in the seeds.

The experts analyzed histones in pollen to examine whether the process of forgetting may take place in the embedded sperm. 

“The study led us to identify a phenomenon, the so-called ‘epigenetic resetting,’ akin to erasing and reformatting data on a hard drive,” said Becker.

The researchers discovered that H3K27me3 completely disappeared in sperm. This is because the sperm had accumulated a special histone that is unable to carry H3K27me3. 

The epigenetic memory of cold is subsequently erased from hundreds of genes – not only those that prevent flowering, but also genes which control a variety of important functions in seeds.

“This actually makes a lot of sense from an ecological perspective” said study first author Dr. Michael Borg. “Since pollen can spread over long distances, by wind or bees for example, and much of the ‘memory’ carried by H3K27me3 is related to environmental adaptation, it makes sense that seeds should ‘forget’ their dad’s environment and instead remember their mother’s, since they are most likely to spread and grow next to mom.”

Dr. Berger noted that animals also erase this epigenetic memory in sperm, but they do it by replacing histones with a completely different protein. 

“This is one of the first examples of how a specialized histone variant can help reprogram and reset a single epigenetic mark while leaving others untouched,” said Dr. Berger.

“There are many more unstudied histone variants in both plants and animals, and we expect that aspects of this resetting mechanism we have discovered will be found in other organisms and developmental contexts.”

The study is published in the journal Nature Cell Biology.

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By Chrissy Sexton, Earth.com Staff Writer