Editing Chestnut blight (section)

==Conservation efforts in North America==
[[File:Tower Hill Botanic Garden - American chestnuts.jpg|thumb|right|250px|Experimental trials by [[The American Chestnut Foundation]] at [[Tower Hill Botanic Garden]] in [[Massachusetts]]]]
[[File:castanea dentata-field trial2009.jpg|thumb|upright|American chestnut field trial sapling from the American Chestnut Cooperators Foundation]]
There are approximately 2,500 chestnut trees growing on {{convert|60|acre|ha}} near [[West Salem, Wisconsin]], which is the world's largest remaining stand of American chestnut. These trees are the descendants of those planted by Martin Hicks, an early settler in the area. In the late 1800s, Hicks planted fewer than a dozen chestnuts. Planted outside the natural range of American chestnut, these trees escaped the initial wave of infection by chestnut blight, but in 1987 scientists found blight also in this stand. There is a program to bring American chestnut back to the Eastern forest funded by the [[American Chestnut Foundation]], [[Wisconsin Department of Natural Resources]], [[USDA Forest Service]], [[West Virginia University]], [[Michigan State University]], and [[Cornell University]].<ref name="Chestnut's-Last-Stand" />

Removing blighted trees to control the disease was first attempted when the blight was discovered, but this proved to be an ineffective solution. Scientists then set out to introduce a [[hyperparasite|hyperparasitic]] [[hypovirus]] into the chestnut blight fungus. The trees infected with virus-treated fungus responded immediately and began to heal over their cankers. However, the virus was so efficient at attenuating fungal growth that it prevented the spreading of the virus from an infected fungus growing on one tree to that growing on another tree. Only the virus-treated trees recovered. Scientific opinion regarding the future of the stand varies.<ref name="Chestnut's-Last-Stand"/>

===Hybrid chestnut trees===
Current efforts are underway by the Forest Health Initiative to use modern breeding techniques and genetic engineering to create resistant tree strains, with contributions from [[SUNY College of Environmental Science and Forestry]], [[Pennsylvania State University]], the [[University of Georgia]], and the [[United States Forest Service]]. One of the most successful methods of breeding is to create a [[Backcrossing|back cross]] of a resistant species (such as one from China or Japan) and American chestnut. Researchers identified two or three genes that allow for blight resistance, and are focusing on giving the American chestnut hybrids only those genes from the Chinese or Japanese chestnut.<ref>{{Cite web|url=http://www.acf.org/pdfs/about/restoration.pdf|archive-url=https://web.archive.org/web/20071108102851/http://www.acf.org/pdfs/about/restoration.pdf|archive-date=2007-11-08|title=Restoring the American Chestnut|website=The American Chestnut Foundation}}</ref>

The two species are first bred to create a 50/50 hybrid. After three backcrosses with American chestnut, the remaining genome is approximately 1/16 that of the resistant tree and 15/16 American. The strategy is to [[Selection (biology)|select]] blight-resistance genes during the backcrossing while preserving the more wild-type traits of American chestnut as the dominant [[phenotype]]. Thus, the newly bred hybrid chestnut trees should reach the same heights as the original American chestnut. Many of these 15/16 American chestnut hybrids have been planted along the East Coast, including in the Jefferson National Forest and on the [[Flight 93 National Memorial]]. Some of these sites have had researchers check on the saplings that have been planted to see their survival rate. For the hybrids to do well, they need areas with decent drainage and abundant sunlight.<ref>{{Cite web |title=The American Chestnut Foundation - Mission & History |url=https://tacf.org/about-us/ |archive-url=https://web.archive.org/web/20080516030808/http://acf.org/history.php |archive-date=2008-05-16 |access-date=2016-11-13 |website=www.tacf.org}}</ref> Meeting these needs can be hard to do, so not all restoration areas have been successful with hybrid survival.

===Transgenic blight-resistant chestnut trees===
A 1983 study on [[hypovirulence]] had shown that chestnut blight infected with hypovirus produced less oxalic acid when attacking the [[cambium]].<ref name="Havir-Anagnostakis-1983" /><ref name="Chen-et-al-2010" /> Meanwhile, a [[plant pathologist]], Dr. William Powell, had been trying to figure out how to transfer all of the Asian chestnut's resistance genetics to its American relatives. In the 1990s, he had the idea to give up on the more complicated and larger job, and instead look around for a single gene elsewhere.<ref name="Sierra-Club-transgenic" /> (In related work, in 2001 Liang, Mayard, Allen, and Powell successfully inserted an [[oxalate oxidase]] (OxO) gene from [[wheat]] into ''[[Populus × canadensis|Populus × euramericana]]'' ("Ogy") for ''[[Septoria musiva]]'' resistance.<ref name="Liang-et-al-2001" /> This enzyme breaks down the [[oxalic acid]] secreted by the fungus into carbon dioxide and hydrogen peroxide.) In 2007, Welch, Stipanovic, Maynard, and Powell showed that transgenic ''C. dentata'' expressing a wheat OxO indeed had lower [[lignin]] degradation by oxalic acid, and suggested this was the path to take.<ref name="Welch-et-al-2007" /><ref name="Chen-et-al-2010" />

A few years later this line of research culminated in the final product: Powell<ref name="Templeton-Powell" /><ref name="Plantopia" /><ref name="Plantopia-Powell" /> and another plant pathologist, Dr. Charles Maynard, working at the [[State University of New York College of Environmental Science and Forestry]] developed [[American chestnut]]s which had heightened [[blight resistance]]. Heightened resistance was attained by introducing a wheat OxO gene into the American chestnut genome. (Because an unrelated gene was transferred, this did not make the chestnut trees produce [[gluten]], and the nuts remain [[gluten-free diet|gluten free]].)<ref name="Cornell-Powell-gluten" /> The [[transgenic]] trees have blight resistance either equal to or surpassing that of ''[[Castanea mollissima]]'', Chinese chestnuts.<ref name="Zhang-et-al-2013" /> 
In 2013, SUNY ESF had over 100 individual events being tested, with more than 400 slated to be in the field or in the lab for various assay tests in the next several years.  By 2014, more than 1,000 trees were growing in several field sites.<ref>{{cite web|url=http://www.geneticliteracyproject.org/2015/06/04/government-approval-next-step-in-approving-gm-revival-of-american-chestnut/|title=Government approval next step in GM revival of American chestnut - Genetic Literacy Project|date=2015-06-04|website=www.geneticliteracyproject.org|access-date=9 November 2018}}</ref> Government approval will be required before returning any of these blight resistant trees to the wild.<ref>{{cite web|url=http://theplate.nationalgeographic.com/2015/06/02/can-we-engineer-an-american-chestnut-revival/|archive-url=https://web.archive.org/web/20160722024438/http://theplate.nationalgeographic.com/2015/06/02/can-we-engineer-an-american-chestnut-revival/|url-status=dead|archive-date=July 22, 2016|title=Can We Engineer an American Chestnut Revival?|date=2 June 2015|website=nationalgeographic.com|access-date=9 November 2018}}</ref> The [[New York Botanical Garden]] has planted several of the transgenic trees for public display.<ref>{{cite web|url=http://phenomena.nationalgeographic.com/2013/03/11/resurrecting-a-forest/|archive-url=https://web.archive.org/web/20130314024548/http://phenomena.nationalgeographic.com/2013/03/11/resurrecting-a-forest/|url-status=dead|archive-date=March 14, 2013|title=Resurrecting A Forest|date=11 March 2013|website=nationalgeographic.com|access-date=9 November 2018}}</ref> 
At the start, there were few such engineered chestnut trees. For seed multiplication, [[grafting]] could work.<ref name="Plantopia" /> Normal tree growth requires 6, 7, or even 8 years before a chestnut will flower.<ref name="Plantopia" /> However [[orchard]] management may accelerate pollen production to 2–3 years (although still without fruiting).<ref name="Plantopia" /> Powell's lab had been able to use [[Environmental chamber|growth chamber]]s with higher light inputs to get duration to pollen production down to less than a year.<ref name="Plantopia" />

[[The American Chestnut Foundation]] (TACF) once worked close with SUNY ESF to utilize the [[Darling 58]] in their mission to restore the American chestnut to its native range in the eastern United States.<ref>{{Cite web |title=Darling 58 |url=https://tacf.org/darling-58/ |access-date=2023-08-25 |website=The American Chestnut Foundation |language=en-US}}</ref> However, in December 2023, TACF withdrew its petition for use as a restorative species due to poor performance and high mortality in Darling 54 saplings. <ref>{{Cite web |title=Press Release: TACF Discontinues Development of Darling 58 |url=https://tacf.org/tacf-discontinues-development-of-darling-58/ |access-date=2023-12-08 |website=The American Chestnut Foundation |date=8 December 2023 |language=en-US}}</ref>