An international research team has uncovered the answer to a decades-old question - how do plants forget? Like humans, plants have memories too, although they do it differently. For example, many plants sense and remember prolonged cold during winter to ensure that they flower in spring. This "epigenetic memory" occurs by modifying specialized proteins called histones, which are important for packaging and indexing DNA in the cell.

One such histone modification called H3K27me3 marks genes that are turned off. In the case of flowering, cold conditions cause H3K27me3 to accumulate at genes that control flowering. Previous work has shown how H3K27me3 is faithfully transmitted from cell to cell so that in spring, plants remember that winter is over, allowing them to flower at the right time. Once they've flowered and made seeds, the seeds need to forget this ‘memory' of the cold so that they do not flower too soon when winter comes around again. Since H3K27me3 is faithfully copied from cell to cell, how do plants go about forgetting this memory in seeds?

The research team led by Dr. Michael Borg in the lab of Dr. Frédéric Berger at the Gregor Mendel Institute of Molecular Plant Biology (GMI) of the Austrian Academy of Sciences, analyzed histones in pollen, hypothesizing that the process of forgetting would most likely occur in the embedded sperm. The researchers were surprised to find that H3K27me3 completely disappeared in sperm. They also found that sperm accumulate a special histone unable to carry H3K27me3. This ensures that the modification is erased from hundreds of genes, not only those that prevent flowering but also ones which control a large array of important functions in seeds, which are produced once the sperm is carried by the pollen to fuse with the plant egg cell. This phenomenon is called "epigenetic resetting" and is more like erasing and reformatting data on a hard drive.

For more details, read the article on the GMI website.

Figure: Source from Borg M, Jacob Y, Susaki D, et al. (2020) Targeted reprogramming of H3K27me3 resets epigenetic memory in plant paternal chromatin. Nature Cell Biology.