Can Genetically Modified Chestnut Trees Fight the Blight? The fifth in a series

More than a century ago, the Appalachian forests from Georgia to Maine contained billions of giant American chestnuts (Castanea dentate). Their nuts supported forest animals and people harvested them by the wagonload to make into flour, beer or to roast. American chestnut wood was abundant, beautiful, and did not rot. These trees are now almost all gone.

The large American chestnut and its nuts (from Google Images).  

What happened? The Asian Chestnut was imported in the 1880s and it carried a fungus to which it had evolved resistance over millennia of human cultivation in China and Japan. The battle between the Asian trees and the fungus (Cryphonectria parasitica) evolved to a near standstill. The Asian trees, which are smaller and grew in orchards, rarely succumbed to the fungus. Yet the fungus still grew on them and formed tough airborne spores by the billions. The American chestnut had evolved no defense and by 1900 the fate of the immense American chestnut forest was blowing in the wind.

 

Billions of chestnuts in this range were affected. The roots and small shoots remain and form the basis of rescue efforts

Imagine this: A spore lands on the trunk of an American tree, perhaps in a small wound where there is a little sap to nourish it. It germinates and makes a fungus that grows and burrows into the tree, forming a canker, a wound that slowly expands around the tree and kills the conducting tissue, the cambium. The infection girdles the tree as effectively as an ax.  Slowly, blowing north and south from the site of the importation on Long Island, the infection spread, killing 4 billion trees and a rich ecosystem and economy.

The growing fungus secretes oxalic acid and creates an acid environment in which it thrives, but the tree's tissue do not. Eventually the conducting elements of the tree die and when the tree is girdled, it dies.

What was to be done? Plant breeders had a strategy – breed the resistant Asian trees with the American trees and select for offspring that are resistant to Cryphonectria parasitica. This continued for generations, always backcrossing to the American tree so that gradually the resistance genes of the Asian trees moved into the American ones. Since breeding started in the1930s, there have been about eight generations and there are now relatively resistant trees whose DNA is 15/16 American.  Each generation takes about eight years - work for patient and dedicated people and fortunately, there were many. One of these researchers is the redoubtable Sandra Anagnostakis, about whom I have written before. Dr. Anagnostakis is in charge of the American chestnut program at the Connecticut Agricultural Research Station.  

There is another approach and that is to study the details of the infection. It turns out that the fungus softens up the tree by secreting a strong acid. Oxalic acid has a benign function in all cells, but in excess, it can also be a nasty piece of work. It is the irritant in Poinsettia leaves and it is the major component of kidney stones. Cryphonectria parasitica secretes it around the site of infection. This acidification blocks the tree’s defenses but allows the fungus to grow because its enzymes evolved to work in acid environments.

Dr. William Powell and Dr. Charles Maynard and their colleagues asked what would happen if an American Chestnut tree had an enzyme that destroyed oxalic acid. A gene from wheat produces such an enzyme, called oxalate oxidase. Enzymes are protein catalysts that carry out chemical reactions. Powell, Maynard and their colleagues engineered the wheat gene into cells of the American chestnut growing in petri dishes. Sure enough, the cells produced oxalate oxidase.

When there were enough cells, they were induced to make small shoots, essentially small plants. The hope was that destroying the parasite’s oxalic acid would give the shoots a fighting chance. So far, the shoots have resisted the fungus. Dr. Powell explains the work in an interesting TEDx talk, while on the same site, Dr. Maynard shows time-lapse videos of infected plants that either have the oxalic acid oxidase gene or do not. The gene seems to protect the few shoots shown. It is too early to claim success - we have to wait a while for trees to test.

The trees produced by decades of plant breeding would be candidates for an oxalate oxidase gene, to create even more resistant trees, since the two mechanisms of resistance may be different and additive.  I have the impression that the two schools do not want to trample on each other’s territory for the moment. The American Chestnut Foundation (I am a member) supports both approaches.

Many environmentalists hope that the forests can be restored without genetic modification of the trees. Some may feel that this would be hubris, that it is a step too far. I cannot see a reason to stop experimenting with these genetically modified trees. We eat that wheat enzyme oxalic acid oxidase in every sandwich.  Humans caused this blight by importing the fungus. Human made methods may be needed to surmount it.

Thinking about Genetically Modified Crops: The fourth in a series

Crops fall victim to viruses, bacteria, insects, fungi, and exotic organisms like the potato blight that wreaked havoc in 19th century Ireland. Our large chestnut tree are gone from fungal infections and our elms hang on in isolation or heavily sprayed reserves. A virus has wiped out the papaya crop in Hawaii. A fungus threatens the coffee crop of small farmers in Central America. How can society deal with these disasters?

The effects of the Papaya Ringspot Virus. The fruit shown on the upper right is  from a GMO modified plant. It is completely protected.

The effects of the Papaya Ringspot Virus. The fruit shown on the upper right is  from a GMO modified plant. It is completely protected.

We have learned to breed resistant crops, and had great success with an older green revolution. We can take other precautions against plant diseases, whether they involve crop rotation or chemicals.  Against foreign pathogens, to which native plants have little resistance, the classical tools of plant breeding may not be enough.

There have been many advances recently in the study of infectious diseases of plants and animals. They have given scientists a better understanding of the offensive capacity of the invader and the defenses of the host. Every organism that attacks a plant (or an animal) has evolved methods to subvert the resistance of the host. We are learning what those tricks are and how to counter them. By comparing the DNA of resistant potatoes, say, with the varieties killed by blight, we get a good idea of which genes protect the plants.

Genetic techniques can provide resistance to various pests, including the virus that kills papayas, the blight that ruins potatoes or eventually, the fungal blight that kills our chestnut trees. The same molecular techniques can make rice or sugarcane resistant to flooding or drought or create a strain of rice that makes vitamin A to fight endemic blindness in Asia. There are many other examples.  The techniques permanently change the genetic constitution of the plant and therein lies the controversy.

There is visceral and dedicated opposition to GMO crops. Some people feel that the new creations will give rise to unpredictable monsters if released into the environment. That is a worry that biologists don’t usually have.  We know the nature of the genes that are being introduced into plant cells, often to unlock normal plant resistance that has been suppressed by an invader. Usually only a few genes are transferred to a plant and they are not involved with making toxins. Millions of genes have now been studied from many species and those linked to human disease are known.

Some people fear that the producers of these crops are giant corporations, like Monsanto, whose motive is profit and not a concern for farmers or the land and a balanced ecosystem.  This is a real concern; power corrupts, after all. Part of the answer lies in regulation, part in competition.  Some GMO crops are not made by large corporations - the laboratories that made the virus resistant papaya and the blight resistant potato are academic institutions.

A third group feels that God created plants that should not be altered by arrogant scientists. Scientists have little sympathy for the idea that nature is best because nature is seething with parasites of all kinds that consume our food or kill us. People reasonably fear what they do not understand, but with a little study they can understand what is being done and why.

The Economist points out that GMO crops designed to grow in marginal lands could be a great benefit to the poor people who live there, especially in times of climate change. Thirty million children die of starvation or malnutrition each year in areas of the world where people live on less that $2 a day. GMO crops will not reverse this disaster but there is a good argument that they will help reduce that number.

Giving up GMO crops does not return us to The Garden of Eden – it returns us to fungicides, insecticides and plants that die in drought or flood. Papaya farmers have a plant that is immune to the Papaya Ringspot Virus, but an organization to keep Hawai’i GMO free is fighting the new crop. It troubles me when people are casual about other people’s livings, destroying a major cash crop, or increasing the threat to the diets of poor people. Blanket denunciations of all GMO crops reveal passion, but not knowledge.

Like many conflicts in American life, the GMO argument can quickly erupt into anger that sheds no light. Here are questions I ask when thinking about genetically modified crops: What, exactly, is the modification? Does it activate a plant to fight an invader? If successful, will it reduce or increase the use of chemicals? Will it increase the food supply? Who controls the distribution of the plants or seed and what is their attitude toward fair distribution and regulation? Above all, we should ask whether modern agriculture should reject a useful tool in the face of climate change?

The Famine of Men

What would happen if, all of a sudden, a few men stopped making testosterone? Did a virus cause the syndrome? Would it spread? What would men do? Would the young virologist who found the first answers be overwhelmed?  In this realistic portrayal of scientists under pressure, the story follows a young scientist as she attacks the problem. The noted University of Chicago microbiologist, Professor Howard Shuman, wrote, “There is no reason that such a virus could not appear. I loved this story and the people in it, even the nasty ones, and I didn’t predict the end.”  You don’t have to be a scientist to enjoy this story – it has been read by novelists, a former lieutenant in the NYPD, professional women and men and lots of other people.  

This is the my first novel, and while it touches on many areas of science and sexuality, it is also my homage to all of the PhD students, post-doctoral fellows and faculty members with whom I have worked for forty years. Forty years ago, a young woman scientist like the hero of this novel would have been rare. Now things are better. As this story will show, that is a very good thing. 

The Famine of Men is available in text and ebook form through AuthorHouse and the author’s website. It can also be ordered on Amazon or most other online sites.

Genetically Modified Potatoes and The Irish Potato Failure

In 1845 blight attacked the potato crop of Ireland. A million people died of starvation and another million or more with enough money to emigrate left for the United States and other countries. The blight knew no borders and never disappeared. It did not cause the famine – the failure of British authorities to divert other food to feed starving farmers caused it.  As readers of Jonathan Swift’s A Modest Proposal may recall, it was not the first time.  For the effects of biological blight and the bizarre, but enduring belief that feeding the starving ruins their souls, read Colum McCann’s novel TransAtlantic.

It may seem prosaic to turn from the chronicles of great writers to the blight itself but science can prevent such disasters.  The plants are destroyed by Phytophthora infestans, which resembles a fungus in that it grows rapidly and produces spores that blow on the wind and spread the disease, but is slightly different. Certain wild strains of potato that are constantly under attack have evolved resistance, but Phytophthora thrives on commercial potatoes in the wet weather of northern Europe and New England.  Today, potato growers apply chemical inhibitors 15-25 times to their potato crops late in the growing season to save them, depending on how many wet days there have been. This practice is expensive, leaves chemicals on the plants and soil and compacts the ground as the sprayers move though the fields.

Plant geneticists at the John Innes Institute in Norfolk, UK have shown that there is another way to avoid the blight. The non-scientist can understand a lot of their paper. Using their knowledge of the circuitry of plant resistance to pathogens, they have transferred several genes that provide resistance in wild potatoes into potato cells growing in a petri dish – in this case cells of a potato variety called Desiree.  Plants cells are astonishing in that they can be broken loose from their cell walls and then grown in a stew of nutrients where they divide indefinitely. In this state, small numbers of genes can be injected into the cells by bacterium called Agrobacterium that has evolved a syringe that transfers DNA to the plant.  When these clusters of transformed cells are put on a jelly like surface and hormones are added, they convert into small plants that produce leaves, stems, flowers, and become normal plants with a few extra protein molecules added to the many thousands of different proteins in the plant cell. Grow them in the soil and they make potatoes.

What did the geneticists do to these plants? The potato plant has hundreds of genes that provide the information to make chemicals that are noxious to invaders – think of nicotine and tobacco. Plants are hugely resourceful if they can mobilize their defenses. When the wind-borne spores of Phytophthora infestans, land on the leaves, the plant has a way to detect them. Special proteins protrude through the membrane of the living plant cells and are shaped to bind to molecules on the surface of the invader. When that happens, a signal is transferred by a series of steps to the genes of the plant and it goes into a defensive mode.  It may thicken its cell wall, produce anti-invader chemicals or kill the infected cells so that the infection does not spread.

The Phytophthora and the Solanum (potato) have been at this battle for millions of years. It is an endless game of serve and return. Sometimes the plant evolves defenses and sometimes the invader overcomes them.  The blight cells outwit the commercial plant by injecting interfering molecules that short circuit the alarm system of the potato plant. (These defenses may have been bred out in the development of the commercial varieties.) It is the botanical equivalent of cyberwarfare and left unchecked, the blight wins, as it did with such terrible consequences in 1845.

Wild potatoes, of which there are many varieties in Peru alone, resist Phytophthora. Recall that the fungus-like blight transfers proteins into the plant cells that silence their alarm system. The wild potatoes are one up on the blight and produce counter proteins that inactivate the Phytophthora proteins so that the natural alarm system of the potato cell functions again. 

Rpi-vnt1.1-transgenic and non-transgenic Desiree in field trials: The genetically modified plants are on the left, the standard are on the right. Rpi stands for Resistance to Phytophthera infestans. Vnt-1 is the name of the gene that provides the instructions for the protein that soaks up the injected Phyophthera toxin.

Jones J D G et al. Phil. Trans. R. Soc. B 2014;369:20130087. The Philosophical Transactions of the Royal Society is an open access journal and has been since the 17th century. 

The scientists in Norfolk identified the genes that produce the proteins that protect wild potato plants.  They transferred them into the commercial variety Desiree. Perhaps they restored a status quo ante. They planted these genetically modified plants next to unmodified plants and waited though a wet summer.  The modified plants lived and produced potatoes and the unmodified ones died. The GMO potatoes could not be distinguished from disease-free normal potatoes. British and European Community regulations require that the new GMO potatoes be immediately burned, but knowing scientists as I do, I bet they found some and ate them. If I had used wild potato genes to protect the food supply, I surely would have eaten them.

Does Genetically Modified Corn Cause Cancer in Rats?

The last post introduced forms of genetically modified crops – why they were made and some details of just what the genetic modification is. This is a good moment to discuss this subject because part of the conversation has focused on a particular paper entitled: Long Term toxicity of a Roundup herbicide and a Roundup-tolerant genetically modified maize by a laboratory in France. Roundup is an herbicide discussed in a previous post. Opponents of genetically modified crops acclaimed the paper, thinking at last there was definitive proof of the harm caused by modified corn.  Many scientists found the paper inconclusive and were harsh in their criticism of the Journal. In the end, the Editor-in-chief of The Journal of Food and Chemical Toxicology, retracted the paper. He probably never imagined that he would be in the eye of this particular storm. Some people think the retraction reflected skullduggery by agribusiness lobbyists. No matter how it came about, retraction is a big deal in the scientific (or the journalistic) world.

So I downloaded and read the paper. You might say that I am not a plant biologist but the paper had little to do with plants. It had to do with rats that were fed genetically modified maize. You might say that I am a geneticist and prejudiced in favor of constructive use of genetics. Maybe so, but I have no vested interest in this debate. A former researcher in my laboratory is now head of research for a very large GMO seed company, but he is a large-minded guy and would not be angry if  I supported the Caen scientists. I have reviewed many such papers over the years so reading this one was not a problem. The problem is to explain it without putting readers to sleep.

Let’s start with the rats: The group in Caen ordered a large number of Sprague-Dawley rats from a breeder in France. They are an in-bred strain so that all of the rats were almost identical twins, except that half were male and half female. That eliminates variability, which is good. Unfortunately, individuals from this particular inbred line are known to be susceptible to cancer as they age, and the plan was to keep them alive for two years. This is a problem because cancer is a disease of aging in rats as well as humans. If you want to measure the effect of something (GM corn and Roundup in this case) it is better to measure a change from a low background. A small change is more likely to be random.

Let’s look at the data:  A group of 10 rats got non-GMO corn. Three out of ten of eventually died of cancer. Another group got relatively low doses of GMO modified corn diluted in normal corn. Five out of ten of them died. Interesting, you might say. But the next group got twice as much GMO corn and only one of them died. A third group got three times as much GMO corn and only one of them died. This result does not support the thesis that GMO crops cause cancer in rats. Adding Roundup to the drinking water, improved the results slightly, but there was still no convincing dose dependence, which we would expect. From a statistical point of view, ten rats receiving each treatment is too small a number to give definitive results. And the cancers almost all occurred at the end of the rats’ normal lifespan, when cancer usually occurs.

At various points in the experiment, which lasted two years, the researchers took blood samples and put them through a battery of tests much like a person’s blood would receive at a thorough annual physical. Astonishingly, the authors did not measure Roundup concentration. If they are making the case that the GMO crops cause cancer because of the Roundup, would they not want to see if it or the detergent in which it is often dissolved got into their blood?  For that matter, would you not want to know how much is in their feed? If there is none, how could it cause cancer?

The data are presented in a confusing way and then touted as definitive in a video, but they are not definitive. They are not even good experiments, although showing cancer -laden rats in a video scares people to death. Why not show cancer ridden control rats?

The biggest problem is to explain just how a few new proteins that do not look like toxins could cause the proposed effects. The study proved nothing. See their claims in a video.

Did GMO corn cause this rat's tumor? Probably not!

Are there problems with GM crops? Sure there are, but for a dispassionate view, read Nature Magazine’s analysis of a year ago, which assesses the problem of Round-up resistant weeds.

What is Modified in a Genetically Modified Crop?

Genetically modified organisms are coming in for bad press, but some of the criticism is distorted.  The implication is that if we give this stuff up we will be fine; but we won’t necessarily. Let’s hold the polemic and explain what is afoot in the world of genetically modified organisms.  Many Universities with an agricultural faculty maintain websites on GMO crops, including Colorado State University and they are useful to consult.

One of the earliest GMO products was corn that carried a gene for a toxin called Bt, which comes from the spore forming bacterium Bacillus thuringiensis. The toxin is actually a crystalline protein that is activated in the intestine of some insects. The toxin made by the bacteria paralyses the gut of feeding insects and they die of starvation, before they consume the plant. The digestive tracts of insects are at relatively high pH, those of animals are acid, which kills the toxin. Bacillus thuringiensis can be found almost everywhere in nature, including most hardware stores or on the Internet.

Farmers are not gardeners; they work on a huge scale in a very risky venture. Their crops must survive drought, flooding, insects, fungi, and weeds if we want to eat.  To help with insect pests, the piece of DNA that provides the instructions for the Bt toxin was inserted into the DNA of corn, cotton, soybeans and other plants. This works.  The insect pests of corn, (or maize), are killed by the Bt toxin that is produced in the leaves.  Plant molecular techniques are now so developed that the toxin is not expressed in the flowers or pollen of the plants, but only in the leaves. When the Bt toxin gene is expressed in corn or cotton, the plant is not considered organic.

Top: Lesser cornstalk borer larvae extensively damaged the leaves of this unprotected peanut plant. (Wikepedia Commons, Image Number K8664-2)-Photo by Herb Pilcher). Bottom: After a few bites of peanut leaves of this genetically engineered plant the corn borer larvae are dead. 

Worm resistant corn, in this case a strain made by Syngenta. Resistant corn is on the left. 

The Bt toxin was one of many genetic modifications put into plants. Another inserts a gene that makes the plants resistant to glyphosate  (Roundup), also available at Home Depot.  Spraying Roundup on a field reduces weeds, without killing the genetically modified corn, soybeans or cotton. When the leaves absorb glyphosate, it inhibits the production of three amino acids, and the weed slowly dies for lack of component parts for its essential proteins. Roundup does not inhibit the enzyme that makes these amino acids in other organisms, so if that gene is put into crop plants, the weeds die and the crops don’t. In both cases the new protein made by the plant has a known function and the other 20,000 or more proteins that a plant requires for life are not affected.

Sometimes both systems are put into the same plant, such as sugarcane. In 2010, 70% of corn, 78% of cotton and 93% of soybeans grown in the United States were genetically modified to resist insects, weeds or both. Wheat is not modified, largely because it is a major export crop and some importing countries will not accept it.

This is not to say there are no problems, such as insect resistance. Anyone who has studied evolution knows that there will be.  There may be effects on beneficial insects. Or, to my mind the most important problem is that such a technology gives too much power to a few gigantic corporations. Their ability to dictate to farmers is a big worry.

But to return to safety issues. Recently, the controversy has coalesced around a disputed study by a group at the University of Caen in France. The group, led by Dr. Gilles-Eric Séralini, did a massive study, lasting two years, in which groups of rats were fed a diet of unmodified maize or one of three concentrations of genetically modified maize.  Some rats also got Roundup in their drinking water.  Enormous numbers of tests were done on the rats and the conclusion was that the genetically modified corn and Roundup caused cancer.

The paper was published in the journal Food and Chemical Technology, and was immediately attacked by critics. Some protests may have been agribusiness induced, some were not.  Opponents of GMO crops were outraged at the criticism of Dr. Séralini and his colleagues, especially when Food and Chemical Technology’s editor-in-chief retracted the paper.

This blog will try to explain the science behind such controversies, even if the passions around an issue have so polarized people that it is hard to explain where the evidence points. So I read the Séralini paper. My conclusions will appear in the next blog.

Demystifying Science

My blog tries to explain the basic mechanisms of science to non-scientists. Many of the blogs come from columns written for The Lakeville Journal in Lakeville CT. These columns have a number of themes, but among the most important is the value of basic science. It is curiosity driven, may have no immediate use and costs money, but it has given rise to everything that is useful in science and medicine. We also encourage skepticism.

We often write about vaccines to explain how they work and what progress science is making. The current blogs are focused on Genetically Modicifed Organisms and according to our theme, try to explain mechanism. Another focus if the value of vaccines.

Science has evolved methods that are the best way for us to understand the natural world, whether it concerns the death of chestnut trees or the death of neurons. Scientists search for theories to explain natural phenomena and these can be disruptive – as with evolution or cosmology or climate change. 

When writing for non-scientists it helps to have a non-scientist as a critic and for this we thank Mrs. Susan Maclin of Houston, Texas. If she doesn’t understand a column it gets rewritten.