How plant cells remember what they are
Some plants require a prolonged cold-period before they can flower, a phenomenon termed vernalization. But how do plants remember in the Spring, that it was cold in the Winter? Cold treatment leads to the deposition of a modification H3K27me3 on the histone H3, which is a basic unit of the nucleosomes that package DNA. H3K27me3 contributes to silencing of a gene called FLC, whose expression normally inhibits the transition to flowering. This phenomenon takes place in dividing cells and the replication of DNA dilutes the levels of this important modification. Scientists have thus expected there is a mechanism in place to maintain this modifiction.
Part of this mechanism is reported today in the journal Science from the lab of Frederic Berger at the Gregor Mendel Institute of Molecular Plant Biology, an Institute of the Austrian Academy of Sciences. In an article titled “DNA replication-coupled histone modification maintains Polycomb gene silencing in plants” Dr. Danhua Jiang from Dr. Berger’s lab observed that in plants where the H3.1 variant had been removed, the H3K27me3 mark was reduced, FLC expression remained high, and plants flowered later. This result was expended into a set of experiments that explained how H3K27me3 is maintained following DNA replication.
According to Dr. Jiang “Our work contributes to the understanding of how epigenetic memory is transmitted through cell cycles. Cell fate is specified and maintained by epigenetic information. Although the inheritance of genetic information is well known (by DNA replication), how epigenetic information is propagated from mother cell to daughter cells is not well understood.”
By measuring levels of H3K27me3, Dr. Jiang found that, while the level of this modification drops during replication it is rapidly restored before the cell divides. Using a combination of genetic and biochemical approaches, he went on to show that the mechanism involved depends on two key factors, the association of the protein complex PRC2 that deposits the mark at the DNA replication fork and the histone 3 variant, H3.1 that is specifically deposited at the fork. An enzyme found only in plants specifically recognizes H3.1 and modifies it in such a way that it can be recognized by PRC2, which then turns it into H3K27me3. Through this mechanism, the memory carried by H3K27me3 is faithfully transmitted through after DNA replication.
According to Dr. Berger, “This work clarifies a controversy regarding H3K27me3, and demonstrates that it is an epigenetic mark in the strict sense that it is re-established following cell division. This tight regulation of H3K27me3 in plants may be critical, as plant cell can turn into other cell types much more easily than animal cells – this mechanism is likely important to help plant cells remember what they are.”
The dynamics of H3K27me3 in plants contrasts with dynamics reported in mammals, where H3K27me3 levels are only restored after the next cell division, demonstrating that plants have a specific mechanism to rapidly restore this modification. Unlike many animals, plant cells can readily lose their identity to differentiate into a distinct cell type or even to form different organs which is the reason plants can be propagated from cuttings. The findings reported might impact our ability to promote plant regeneration and also to help crops “remember” and better cope with environmental stress.
Jiang D and Berger F (2017) DNA replication-coupled histone modification maintains Polycomb gene silencing in plants. Science [epub].
Press release (german).