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Thu May 18, 2017, 09:11 PM

Insertion of the wheat gene for oxalic acid oxidase into American Chestnuts induces blight...


For twenty years, I had an American Chestnut (Castanea Dentata) growing in my front yard, and I hoped it was a member of a blight resistant variety.

Unfortunately I was wrong. Blight killed the tree last summer.

The American Chestnut, called the "Sequoia of the East" once dominated Eastern Forests from Georgia to Maine. In 1904, a disease in the tree was noted which rapidly spread - it was imported with Asian Chestnut trees which had evolved blight resistance and thus were relatively immune to the disease - and by the early 1950's the American Chestnut was essentially extinct, except for a few trees grown outside of its native range, and a few trees thought to be resistant.

Efforts have been underway for many decades to try to cross breed the American tree - shoots can survive long enough to yield seeds, as my now dead tree did - with the Asian tree, in order to provide a tree that was better than 90% American but still resistant. This effort was undertaken by the American Chestnut Foundation and has had some success.

However, the tree was still not the pure American; the American tree was noted for its massive size, its extremely valuable lumber, and its flavorful if small nuts.

I've followed this story for many years; and as gene mapping tools were developed, I hoped that the genome of the American Chestnut could be compared with the Asian and the specific genes incurring resistance could be identified.

I knew that the parasite Cryphonectria parasitica was able to gain entry into the bark cells, biochemically via the secretion of oxalic acid, but I had no idea how the Asian tree was able to defeat that capability.

Both trees have now been fully sequenced, but since mapping the genome does not necessarily define the function of the coded proteins - and moreover the coded proteins may require specific post translational modifications to become functional - it wasn't clear to me (I'm not a biologist) whether this approach would take many, many, many years to come to fruition.

The folks at the State University of New York College of Environmental Science & Forestry are smarter than I am, however, and they decided to use a more simple approach. They recognized that many plants utilize a particular enzyme that prevents fungi like Cryophonetria from attacking them while they are alive. This enzyme is oxalic acid oxidase and it is found in wheat, among many other species. In 2007 the gene was successfully transferred to Castanea Dentata embryos. This enzyme oxidizes oxalic acid to carbon dioxide.

Drum roll please: Transgenic American chestnuts show enhanced blight resistance and transmit the trait to T1 progeny (Plant Science Volume 228, November 2014, Pages 8897)

Some excerpts from the paper:

The American chestnut (Castanea dentata [Marsh.] Borkh.) was ecologically and economically significant throughout the eastern United States until it was nearly eliminated by an invasive fungus (Cryphonectria parasitica [Murr.] Barr.) that causes chestnut blight. The blight was first identified in 1904 [1] and had spread through the American chestnut's range by the 1950s [2] ; [3]. Treatment and control efforts attempted while the blight was spreading were generally unsuccessful. However, surviving American chestnut stump sprouts have left interested parties with a partially intact gene pool and therefore some potential for restoration. The backcross breeding program implemented by The American Chestnut Foundation aims to produce a chestnut tree with primarily American chestnut growth and nut characteristics, while retaining blight resistance traits from the Chinese chestnut (Castanea mollissima) [3]. However, the Chinese chestnut's blight resistance is quantitative, involving three primary resistance loci (of up to seven total) [4], which makes breeding a challenge. Another approach to producing an American chestnut tree with enhanced blight resistance is to directly transfer resistance-enhancing genes via Agrobacterium-mediated transformation [5]. In contrast to traditional breeding, genetic transformation offers complete control over which genes are inserted into a host organism. In addition to some transgenic agricultural crops which have become almost ubiquitous in the US, a variety of transgenic trees has been reported for several purposes including timber production and wood properties [6], fruit production [7], freeze tolerance [8], and disease resistance [9]; [10] ; [11]. American chestnut specifically has previously been transformed with marker genes and regenerated into sexually mature trees [12]...

The authors state that the transgenic trees do, in fact, show blight resistance. Some metabolic screening is required before this magnificent tree with this wonderful gene can be released into the environment thus restoring the Eastern forests to a state they once enjoyed.

From the conclusion:

4. Conclusion
Production of a blight-resistant transgenic American chestnut tree would be a noteworthy success for historical, ecological, economic, and technological reasons. While the most resistant of the events described here is not as resistant as the Chinese chestnut, significantly enhanced blight resistance to an intermediate level (as seen in Darling4) is a major step toward that goal. New transgenic events are currently being produced that express the OxO gene at higher levels, and leaf assay results indicate they may be considerably more resistant than Darling4, approaching or even exceeding the blight resistance of Chinese chestnuts [15]. Additionally, genes isolated from Chinese chestnut are currently being evaluated for potential use as stacked transgenes, and insights into the chestnut genome [43] will continue to facilitate research in this direction. The fact that transgenes are inherited and expressed in seedling offspring means that blight resistance can be stable across generations, and it would also facilitate many other aspects of a restoration program. While there are still numerous considerations to be addressed before restoration can begin on a large scale [44], an American chestnut tree with enhanced blight resistance is an absolute necessity before restoration can occur. We believe, based on the data presented here, that transgenic American chestnuts expressing OxO can meet that need.

I'm extremely excited by the potential for this tree to be restored.

My neighbor, by the way, gave me seeds from a cross breed of two naturally resistant seeds, but unfortunately they have not yielded any trees yet in the pots in which I have on my deck.

Have a nice Friday tomorrow.

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Reply Insertion of the wheat gene for oxalic acid oxidase into American Chestnuts induces blight... (Original post)
NNadir May 2017 OP
DK504 May 2017 #1
eppur_se_muova May 2017 #2

Response to NNadir (Original post)

Thu May 18, 2017, 09:48 PM

1. I love seeing stories like this.

Maybe we will be able to get out of our own way and try to keep some forests alive.

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Response to NNadir (Original post)

Fri May 19, 2017, 12:35 AM

2. Very exciting development !

Years ago, my mother told me that her parents had, on their farm, a chestnut which died years ago, but that a new one had grown up since then. I assumed it was an American chestnut, killed by blight. But when I finally got a chance to examine it, it turned out to be a European chestnut. I had really hoped that there might be a surviving American chestnut in that rather isolated valley in WV.

Lots of folks interested in chestnut developments on DU, to judge by a quick DUgle.

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