**8. Prospects**

278 Soybean – Genetics and Novel Techniques for Yield Enhancement

The newly-grown lateral roots of soybean scion may absorb access water and nutrient from the soil, which will weaken the stress, caused by distant grafting and be not conducive to the induction of mutation. As we noticed, the presence or absence of the lateral roots of scions after grafting is related to whether the mutants occur or not and which generation the mutants occur. When removing the lateral roots of scion in time, the mutated traits can be observed on the grafting generation (G0). When retaining a lateral root of scion, the emergence of mutated traits may be postponed to the G1 to G3 generation. When retaining

The retained leaves numbers of scion when grafting may also have an important impact on the mutation rate. In general, the retained leaves numbers of scion negatively correlate with the mutation rate of grafted progeny. According to our experience, retaining 1-2 leaves of

In our research on distant–graft in soybean, we have observed the same morphological mutation as those in mungbean reported by Zhang *et al*. (2002). These mutation consist of the variation of growth habit, growth period, seed size, seed coat color, leaf shape, seed germination capacity and so on, which offer the basic materials for soybean breeding and gene mining. Through the directional selection on the progeny of distant-grafting, we have gained the excellent soybean lines characteristic of good quality, early mature, tolerance to

The results of karyotype analysis revealed that there was ploidy variation of chromosomes in the offspring of distant grafting in soybean. About 20% of the root tip cell showed chromosomal abnormalities (2n=36 or 38) for some individuals in G1 generation (unpublished). Certainly, there will be other variation such as photosynthetic capacity,

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

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

two or more lateral roots of scion, the grafted progeny may not have any mutants.

**5.2.2 The lateral roots of soybean scion** 

**5.2.3 The number of retained leaves of soybean scion** 

soybean scion is the best option for the early 30 days after grafting.

**6. Mutation types induced by distant grafting in soybean** 

drought and chilling, resistance to stress, lodging and diseases.

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

silence the special genes or affect the special genes expression of the scion.

isozymes and genes waiting for further research.

It is of practically important significance for putting distant-graft technology into innovating excellent soybean materials, broadening genetic base of soybean and promoting breeding of new soybean with good quality, high yield and resistance to stress. However, there is yet no comprehensive system of distant grafting mutagenesis up to now. In particular, the affinity mechanism and the mutation mechanism of distant grafting are not clear, which leads many scholars to disbelief in the distant-graft mutagenesis technology and soybean mutants induced by distant-grafting. Thus, the most important task on distant graft now is to check on genotypes of the known existing soybean lines derived from distant-graft through molecular approach, validating the reliability of applying asexual hybridization (grafting) to soybean breeding. On the basis of it, the affinity mechanism and the mutation mechanism of distant grafting will be exploring, highlighting as soon as possible the true nature of distantgrafting mutagenesis. At the same time, the grafting technology is integrated with the conventional identification and screening approaches of the mutated traits to create soybean germplasm resources with good nitrogen fixation ability, resistance to drought, salt, diseases and aging, and to identify the genetic controlling loci of the related traits. Besides, Grafting technique can be applied in revealing the physiological process mechanisms involved by the signal transduction materials and becomes irreplaceable means to deep understand the interaction of different organs of plant. We believe in the coming future, the distant-grafting mutagenesis technology will be paid more attention by scholars, and play an increasing role on the soybean resources innovation, soybean breeding and gene mining and so forth.
