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Grafts guide gene exchange

When two plants are grafted together, they share much more than water and minerals: They also swap genetic material, according to a linkurl:study;http://www.sciencemag.org/cgi/content/summary/324/5927/649 published in tomorrow's (May 1) issue of __Science__. These findings muddy the distinction between naturally-occurring gene transfer in plants and the human-mediated mechanisms we generally refer to as genetic engineering. Image: Science/AAASEver since Soviet and Western scientists in the 1960

By | April 30, 2009

When two plants are grafted together, they share much more than water and minerals: They also swap genetic material, according to a linkurl:study;http://www.sciencemag.org/cgi/content/summary/324/5927/649 published in tomorrow's (May 1) issue of __Science__. These findings muddy the distinction between naturally-occurring gene transfer in plants and the human-mediated mechanisms we generally refer to as genetic engineering.
Image: Science/AAAS
Ever since Soviet and Western scientists in the 1960s denounced linkurl:Trofim Lysenko;http://en.wikipedia.org/wiki/Trofim_Lysenko -- the infamous Stalinist agronomist who rejected Mendelian genetics and maintained that heredity can be changed by grafting and other non-genetic techniques -- the prevailing dogma has been that genetic materials do not mix at grafting junctions. But linkurl:Ralph Bock;http://www.maxplanck.de/cgi-bin/mpg.de/person.cgi?nav=kontakt&persId=21103251〈=en&inst=molek_pflanzenphysiologie and Sandra Stegemann of the Max Planck Institute of Molecular Plant Physiology in Potsdam, Germany, weren't convinced. In linkurl:2003,;http://www.pnas.org/content/100/15/8828.abstract?ijkey=45a0eebd36a52f3b2a882a8c78deeaf2696d27e7&keytype2=tf_ipsecsha they showed that genes are transferred at a high frequency from chloroplasts to the nucleus within tobacco plants, suggesting that genetic material might be "leaky" and flow between cellular components. So, they wondered, might DNA flow between the cells of grafted plants as well? The researchers reciprocally grafted two transgenic tobacco plant varieties that carried distinct genetic markers in different cellular compartments -- one in the nucleus, the other in the chloroplast. They attempted 74 grafts, and showed that 45 contained cells harboring markers from both original plants -- evidence that intracellular transfer had indeed occurred. Further experiments showed that the hereditary handover always flowed from the chloroplast to the nucleus, which Bock and Stegemann argued is probably mediated by the transfer of large chunks of DNA or even entire chloroplast genomes. The researchers also tested distant leaves but never found any evidence of gene transfer far from the graft sites. "From all we can say, it is a process that is probably restricted to the cells that are in contact with each other," Bock told __The Scientist__. Even so, shoots growing out of the graft site could stably transmit transferred DNA to progeny tobacco plants. Farmers wishing to propagate their crops are not the only purveyors of grafting techniques. Stocks and scions can fuse naturally when the stems or roots of different trees grow into each other, too. And if they then swap genes, "this is a way for nature to produce transgenic plants," Bock said. "It's sort of natural genetic engineering." The findings "further blur the boundary" between natural gene transfer and human-mediated genetic engineering techniques, he added. "It's remarkable that they were able to get as many transfers almost every time that they tried," linkurl:Aaron Richardson,;http://aaron.o.richardson.googlepages.com/ an evolutionary biologist at the University of Georgia who was not involved in the study, told __The Scientist__. But Richardson noted that the study only demonstrated gene transfer between "two things that are very closely related too each other." Thus, it remains to be seen how widespread grafting-mediated genetic exchange is between unrelated species. "In terms of wide transfer, this may not be as important," he said. Bock is currently running experiments to test whether he can use grafting to move genes from tomato into potato and from tobacco into tomato. "In theory it should be possible to transfer genes between different species by grafting," he said. If Bock is correct, then grafting could provide a new tool to manipulate plants that are impervious to traditional genetic engineering techniques. Plus, it could explain evolutionary evidence showing that genes have been shuffled around between unrelated plant species.
**__Related stories:__***linkurl:Plant-to-plant horizontal gene transfer;http://www.the-scientist.com/article/display/21443/
[10th July 2003]*linkurl:No horizontal transfer;http://www.the-scientist.com/article/display/19734/
[21st June 2001]
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Comments

Avatar of: Steven Anderson

Steven Anderson

Posts: 9

May 1, 2009

Has anyone checked possible DNA transfer between parasites,such as dodder and mistletoe, and host? Or, for that matter, gall-forming insects?
Avatar of: Steve Summers

Steve Summers

Posts: 28

May 1, 2009

The great horticulturist Luther Burbank already made the observation that grafting could transfer characteristics, although he regarded it as happening in just one case. This was perhaps a case in which the result was far from the grafting site...passed on to progeny.\n\nHis techniques were something he also characterized thus: "see how this whole plant's lifelong stubbornness is broken simply by blending a new life with it, making, by crossing, a complete and powerful change in its life. Then when the break comes, fix it by these generations of patient supervision and selection, and the new plant sets out upon its new way never again to return to the old, its tenacious will broken and changed at last." \n\nSomething Science has not really taken seriously, quote from Burbank:\n\n"The secret of improved plant breeding, apart from scientific knowledge, is love."\n\nHe was not inclined to discuss this aspect of his work with scientists, for obvious reasons.
Avatar of: anonymous poster

anonymous poster

Posts: 51

May 1, 2009

In many cases mistletoes show very similar morphological traits to those of their hosts such as leaf shape and color to the extent that the parasite cant be distinguished from the host. In this case genes might have been transfered from the host to the parasite through the historium that connect the parasite to the host vascular tissues. Lysenko might be smiling in his grave now!.\n
Avatar of: anonymous poster

anonymous poster

Posts: 7

May 4, 2009

The article states that the genetic exchange was only observed between cells at the graft union and not in cells at a distance from the graft union. How then can this be used to explain genetic changes across generations, or between natural clones? I was taught that seeds and other progeny are generated vial mother cells in shoot apices and not associated with graft unions. I can see it as a potential mechanism for generating clones in vitro but not in vivo.
Avatar of: Elie Dolgin

Elie Dolgin

Posts: 6

May 5, 2009

Good question. I passed along these comments to the lead author Ralph Bock for clarification. Here is what he said in an email:\n\n"It is correct that these genetic changes are not normally transmitted into an apical meristem, from where they could enter the ?germline? and become heritable. Heritability would be confined to those (relatively rare) cases, in which the recombinant cell line becomes part of a meristem, for example, by lateral shoot formation from the graft site. Lateral shoot formation from graft sites can occur spontaneously or can be induced (e.g., by breaking off the scion or cutting it off after graft fusion).\n\n"We just briefly mentioned this in our Science paper, but, due to space constraints, could not explain it in detail."\n\nElie Dolgin -- Associate Editor, The Scientist

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