Michurinsky epigenetics
Graft and rootstock were able to regulate each other's genomes
The grafted stem does not just take root on the "host" plant, but also actively exchanges epigenetic information with it. The scale of this process was previously greatly underestimated. Biologists from the UK and the USA talk about this in an article published by the journal PNAS (Lewsey et al., Mobile small RNAs regulate genome-wide DNA methylation). Also about this work is a press release from the Salk University of Biological Research: Grafted plants’ genomes can communicate with each other.
People have learned to improve the beneficial properties of plants with the help of vaccination thousands of years ago. For example, by attaching the shoot of a cultivated plant (graft) to the stem and root system of its wild form (rootstock), you can collect valuable fruits of the first and enjoy high resistance to diseases and other environmental adversities of the second plant. However, it is only in our time that it has become obvious that the interaction of the graft and the seed goes extremely far – up to the level of epigenetics.
A few years ago, a team of Cambridge biologists led by David Baulcombe managed to show that a graft of a mutant line of the rhesus Thaliana (Arabidopsis thaliana) placed on a wild form rootstock exchanges small RNA (sRNA) with it. These small – just over 20 nucleotide bases – molecules are involved in the regulation of genome activity.
They can suppress the transcription of the product with matrix RNA, and also play an important role in determining the positions of DNA methylation, a chemical modification that blocks the possibility of binding translation enzymes. All these mechanisms are especially important for maintaining the "genetic immunity" of a plant: with their help, it can stop the activity of mobile genetic elements – transposons that clog its genome. However, in the same epigenetic way, that is, by changing the activity of genes, and not their nucleotide sequence, many other aspects of an organism's life can be controlled.
In 2010 Boulcomb and co-authors have demonstrated that sRNAs are indeed capable of being transmitted between graft and rootstock by participating in RNA-mediated DNA methylation. However, the scale of this phenomenon remained unknown, so scientists continued their work, and in their new article they report that such an epigenetic exchange occurs extremely widely and can change the activity of thousands of plant genes.
On the rootstock of one of the wild forms of the rhesus, the authors grafted two grafts – another wild form and a mutant one, unable to produce sRNA. As a result, it was found that the sRNAs were safely moving between them all. "Such a statement allowed us to observe something really unique: they really exchanged epigenetic equivalents of alleles, epiallels," says one of the authors, an employee of the Salk Institute for Biological Research Matthew Lucy (Mathew Lewsey).
A colored preparation of the junction of the scion and the rootstock in the rhesus of the Talus.
Photograph: Charles Melnyk at The Sainsbury Laboratory, Cambridge University
However, this could be expected based on the previous results. A real surprise can be called the scale of such an exchange: scientists found that thousands of sections of rhesus DNA were subjected to methylation under the action of sRNA. In the vast majority of cases, these were all the same transposons. However, for other plants, the role of this mechanism in the epigenetic regulation of traits may be much more significant.
The Tal rhesus is distinguished by a relatively small and uncomplicated genome, which made it a popular model object in plant genetics and physiology. The genome of real agricultural species may be thousands of times larger than it, and the extent of the exchange of epigenetic information during vaccination is anyone's guess. At least until similar experiments are put on them. The authors plan such work in the near future, and believe that this will allow more accurate and accurate manipulation of the genome of useful plants, creating more stable and productive varieties.
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25.01.2015