New research into plant grafting has shown how genetic information may be transferred from a donor plant to the host. There is optimism in relation to how this knowledge may be used to improve crop quality and yields.
Plant grafting is a practice where a cut branch from one plant is joined onto the stalk of another, the technique works by joining vascular tissues of two plants; one for its roots (the stock) and the other for its fruit, flowers, leaves or stems (the scion). The scion contains the desired characteristics to be reproduced in the future reproduction of the new plant. This may result in many specific advantages for the plant including; earlier fruiting of a tree, to induce dwarfing, easier propagation, the creation of hybrids, to provide a pollen source, to increase hardiness, to improve disease resistance and to repair wounds. Today it is widely used by the horticultural and agricultural trades and most food crops are now cultivars, few plants existing in their original natural form are used for food production. Plant grafting has been practiced for more than 3 thousand years with evidence spanning from the times of ancient China and Greece.
With centuries of anecdotal evidence regarding plant grafting, adequate explanations of how this technique is effective were scarce until as recently as 2013 and regarded as a mystery. Earlier, Charles Darwin coined the term “graft hybrid” to describe the benefits attained by a grafted plant, the intimation was; somehow the two plants were sharing inherent characteristics. At this time the idea of changes which were sexually heritable met substantial scepticism, the popular explanation of these phenomena were the transformations represent a type of chimera (a mythological creature). With only observational evidence, a full explanation of the technique had to wait.
In Recent times the discovery of the transmissible nature of genetic material between cells and plants make a fully integrated understanding of plant grafting possible. Previous research demonstrates how RNA moves within the grafted plant changing the activity of the host plant. Additionally, it had been speculated the changes may occur due to transposable elements (aka; jumping genes) where fragments of a cells genetic code become detached, travel and transpose in the genome of the cell or of foreign cells. This may result in an increase in size of the particular cells genome.
Subsequently, new research by Lewsey and colleagues at the university of Cambridge focused on the flow of epigenetic information and in particular small RNAs, which silence certain genes during DNA methylation; regulating which genes are active. The team showed how sRNAs move across grafted plants from the shoots to the roots. Three types of Arabidopsis thaliana (thale cress) were used to confirm this, one bred specifically to have an absence of any type of inherent sRNAs. After plant grafting the team analysed plant tissue to look for changes in methylation in individual genomes, assuredly the plant impaired in production of sRNAs clearly contained these molecules within its cells, giving support to the idea of transmission from one plant to another.
Matthew Lewsey commented, “This set-up allowed [the team] to observe something quite unique, the plants were actually transmitting the epigenetic equivalent of alleles, called epialleles.” Alleles are genes which have two variant forms located in the same position within a chromosome, one is inherited by each parent. The team searched for sites along the epigenome of the plants where alleles had been altered by DNA methylation, identifying thousands of loci along the plant genome which were silenced after grafting. Co-author David Baulcombe commented, “What was unexpected, however, was the scale of the changes due to mobile RNA.”
The mobile sRNAs were also observed to obstruct jumping genes from affecting the expression of genes and genome. Despite this silencing of transposons being evident, the wild plant and mutant plant lacking sRNAs showed very little differences in the level of gene expression after grafting, a remarkable reprogramming and amalgamation took place between the plants. Matthew Lewsey first author commented, “In the future, this research might allow growers to exploit epigenetic information to improve crops and yields.”
How might this research, support the idea of crop growth for emergency stock?