How does plant grafting work?

Question

Plant grafting is a process whereby a piece of one plant is inserted into another and results in a change of the original plant. For example, grafting a piece of a lemon tree into a bitter orange tree will cause that tree to produce lemons instead of oranges for the rest of its life.

How does this work? I mean, at the genetic level. I guess the grafted plant is now a chimera of some sort but if so

1. How does the grafted DNA affect the host cells?

2. Are all of the host's cells affected?

3. If we were to sequence the grafted tree which genome would we find?

4. Is it perhaps that the host DNA is not affected at all and the effects come via a change in expression levels? If that is the case I would assume grafting will only work between very closely related species. Is that so?

Basically I know very little about plant genetics and physiology. I remember from my undergraduate days that plant genetics are just plain weird. Most, if not all, have various levels of polyploidy. Is that relevant?

Answer 1

This article analyses the exchange of DNA during the process of grafting

Exchange of Genetic Material Between Cells in Plant Tissue Grafts - Stegemann and Bock, Science 2009

Quoting from the article:

Although the grafted tissues fuse and establish vascular connections, the stock (the lower part of the graft) and scion (the upper part, usually supplying solely aerial parts to the graft) are thought not to exchange their genetic materials. But grafting (whether natural or assisted) provides a path for horizontal gene transfer. Gene transfer is confined to the graft site and no long-distance transfer may occur. Analyzes indicating that large DNA pieces or even entire plastid genomes are transferred. Only plastid genes may be transferred, no transfer of nuclear genes occur. Plant cells are connected via plasmatic bridges called plasmodesmata, but the passage of large macromolecules requires the action of specific plasmodesmata-widening proteins. Whether large DNA pieces or even entire organelles can travel through plasmodesmata requires further investigation.

Finally, although our data demonstrate the exchange of genetic material between grafted plants, they do not lend support to the tenet of Lysenkoism that “graft hybridization” would be analogous to sexual hybridization. Instead, our finding that gene transfer is restricted to the contact zone between scion and stock indicates that the changes can become heritable only via lateral shoot formation from the graft site. However, there is some reported evidence for heritable alterations induced by grafting and, in light of our findings, these cases certainly warrant detailed molecular investigation.