Microscopy image of Arabidopsis thaliana pollen, which contains sperm and companion cells. Image credit: Jingyi Zhang (CC BY 4.0)
In most organisms, the genetic information contains DNA segments called transposable elements which are able to move around in the genome. When transposable elements insert themselves in a new location, this can lead to negative outcomes such as cell death or cancer.
Animals, plants and other organisms have evolved sophisticated mechanisms to ‘lock in’ transposable elements and prevent them from jumping from place to place in the genome. For instance, adding small molecules called methyl groups onto these sequences tightly packages the DNA, which wraps itself around proteins known as histones. The resulting structure is known as heterochromatin, and it limits the movement of the transposable elements.
In certain situations, cells may ‘reactivate’ some of their transposable elements: this is for example the case in plant sperm companion cells, which protect the sperm and deliver them to the egg cell. However, it was not clear how many transposable elements are reactivated in these cells, or how this process works.
Here, He et al. investigate this process in the sperm companion cells of a small weed known as Arabidopsis thaliana. The experiments showed that around 100 transposable elements were reactivated, most of them when an enzyme called DEMETER removed the methyl groups found in heterochromatin. However, this enzyme alone was not enough. It could only access the methyl groups if the tightly packed structure of the heterochromatin had relaxed following the removal of a histone protein called H1.
Taken together, these results indicate that histone H1 and DEMETER cooperate to regulate the activity of transposable elements in the genome. In addition, H1 is known to prevent the addition of methyl groups onto DNA; that it also impedes their removal suggests that this protein plays a complex role in controlling the way genetic information is interpreted. The next step would now be to investigate the impact of the reactivation of transposable elements on the next generation of plants and during evolution.
