3 Discussion

As described above, we succeeded in the inter-family grafting between mungbean and sweet potato with careful maintenance. When the seeds harvested from the scion were continuously sowed, we obtained several genetic varieties with improved quality and higher yield from the progenies. Besides the combination between mungbean and sweet potato, we also succeeded in those between mungbean and pumpkin, and wheat and sweet potato with the same method (see Fig. 11 and J for the settled scions). Genetic varieties with distinctly improved traits were obtained as well from these later combinations (data not shown).

However, the molecular mechanism for the emergence of the genetic varieties is unknown. In the present study, we analysed the alternations of cytoplasmic and nuclear DNA upon the mungbean variety. The results showed no RFLP in the cytoplasmic DNA but rich RAPD dimorphism in the nuclear DNA between a variety (SL6) and the original scion (XHJ). Traditional studies have presumed strongly that grafting-induced genetic variation may be caused by direct gene transmission from stock to scion[4, 10]. However, in our examinations, though a few PCR fragments amplified from the variety (SL6) corresponded to those amplified from the stock (sweet potato Fig. 4), there was no hybridization between the fragments when examined with Southern analyses. This result reduces the possibility of the transgenic mechanism at least in nonspecific graft system. Furthermore, although large molecules with the size up to mRNA are found to be moveable from stock to scion[20-22], there is so far no cellular or molecular evidence revealing the possibility or exact mechanism of long distance movement of DNA fragments in the graft system.

Recently, it has been indicated that environmental stress, such as tissue culture conditions activate plant transposons[23, 24]. It has been also suggested that transposition in response to environmental stress is a common phenomenon by which plants acquire genetic diversity[25, 26]. In our system of non-specific grafting, the mungbean scion is exposed to nutrient and hormone stresses throughout the growth. A mature plant of the scion species (XHJ) normally reaches 40 cm in height and produces 540 seeds on average. However, it stops growing at 20 cm in height and fruits less than 80 seeds when grafted. This large inhibition of growth and fruitage may reflect the great stress that the scion receives during non-specific grafting. If large amounts of transposition occurred during grafting, genetic variation and RAPD dimorphism as described in the present study may be explained since plant genomes may contain large amounts of transposons. In a bean species Vicia faba, for example, Ty1-copia elements may build up 1/4 of the genome.[26] Therefore, non-specific graft induced genetic variation is more likely caused by transposition rather than stock-toscion transformation. Our investigations of the transposons in the scion and the variant mungbean are in progress.

Although the exact mechanism remains unknown at the present stage, the fact that graft induces genetic variation should not be ignored in botanical research. In our experiments, genetic traits that have never been seen in the natural population of scion species emerged after nonspecific grafting (such as mungbean with black seed coat or rambling plant and some traits in a wheat variation with data not shown). Such traits can be hardly acquired from traditional cross breeding. Therefore, further studies on the application of the technique may contribute to a new way for plant breeding, while the mechanism investigation may lead to a better understanding of plant genome.

Graft can induce inheritable variations in the progenies of the scion plants. Seedling of mungbean Vigna radiata (L.) Wilczek) was grafted onto the stem of sweet potato (Ipomoea batatas (L.) Lam.). The growth of the scion was maintained until the scion produced selfed seeds. We sowed the seeds for several generations under normal conditions. Distinct genetic variations appeared in the progenies. Similar variations did not appear in the generations of the scion sowed normally without graft.

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Graft can induce inheritable variations in the progenies of the scion plants. Seedling of mungbean Vigna radiata (L.) Wilczek) was grafted onto the stem of sweet potato (Ipomoea batatas (L.) Lam.). The growth of the scion was maintained until the scion produced selfed seeds. We sowed the seeds for several generations under normal conditions. Distinct genetic variations appeared in the progenies. Similar variations did not appear in the generations of the scion sowed normally without graft.