**7. Possible mechanism of distant-graft mutagenesis in soybean**

It is initially thought that the genetic variation induced by grafting is due to integration of genetic material of rootstock into the genome of scion (Taller *et al*., 1998). However, recent research in molecular biology have identified that though there are indeed great changes for the DNA sequence of scion, the genetic material of rootstock can be detected in the genome of scion. Meanwhile, it is found that the stress-related retrotransposons in the progeny of distant graft have undergone transposition (Xiao, 2005), which as we know is one of the important mechanisms to induce genome rearrangement and gene mutation. In addition, there are also other evidences that the nucleic acid materials of rootstock can be transmitted to the scion through grafting (Stegemann and Bock, 2009), in which the small RNA might silence the special genes or affect the special genes expression of the scion.

Taken our preliminary study and the results of recent studies together, we believe that stock's genetic material is not integrated into the soybean genome, and speculate that distant graft-induced heritable variation in soybean may be the result that the nucleic acid substances of rootstock as a signal molecule is transmitted into the plant of soybean during the distant grafting process, which initiate the special small RNA interfere system and lead to the genetic mutation, or the stress conditions caused by distant grafting stimulate the stress-related transposable element transposition, leading to the genome rearrangement or gene mutation.
