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Altering the DNA of oranges to save them from psyllid-infested Huanglongbing (HLB) disease

Posted on July 30th, 2013 in ecology,Food & Wine,Technology by Robert Miller

Burning Florida orange trees infested with Asian Citrus Psyllid (From New York Times)

Few of us are aware of the destructive forces that have plagued the Florida citrus industry over the last few years. But a few days ago, Amy Harmon, writing in the New York Times published a major article on the problems facing orange growers in Florida, with similar threats beginning to appear in the citrus trees of California and other southern states. This new threat has been created by  the Asian Citrus Psyllid (Diaphorina citri), a new invasive species which lays its eggs in citrus tree leaves and sometimes carries a tree-fatal bacterium: Candidatus Liberibacter asiaticus, that ultimately causes the citrus disease “Huanglongbing” (HLB), characterized by half green oranges hanging on trees that are obviously diseased and doomed. Trees infected must be destroyed and the accompanying  image illustrates this on-going strategy to combat the infestation. Estimates are that Huanglongbing (HLB) disease has already cost the citrus industry in Florida and is beginning to mount such a threat that many growers can visualize the end of Florida orange juice if something isn’t done quickly to stop this raging contagion.

This new threat began in Asia and India and was first discovered in Florida in 1998. By 2001 the psyllid invasion had spread to 31 counties in Florida, primarily due to the movement of infested nursery plants. In 2001, the psyllid infestation spread to the Rio Grande Valley in Texas on nursery stock (orange jessamine); it also was detected in Louisiana. The insect subsequently spread to other states and now is found in Alabama, Georgia, Mississippi, South Carolina, Arizona, California, and Hawaii as well as Mexico. California has been aggressive in trying to identify and destroy trees infested with psyllids, which by themselves inhibit tree growth when the eggs hatch and nymphs feed from tree sap, damaging the tree without killing it. But if the bacterium Candidatus is also injected when the adult feeds and exchanges the bacterium for sap, then the tree will develop HLB disease and eventually die, though it takes several years for this disease to run its full course. HLB is the most serious threat to the citrus industry in history and projections are that if something isn’t done to halt this disease, the Florida citrus industry could be destroyed within a decade.

Closeup of the small psyllid insects on the leaves of an orange tree

Once the devastating nature of this disease became apparent, the citrus industry, aided by the National Academy of Sciences and the Federal government scoured the earth to find trees that had developed a natural immunity to the disease. If a disease has been around for some time, one can usually find trees that have developed a natural resistance, but in the case of HLB, no naturally resistant strain could be found anywhere in the world. Orange trees are not the only target of psyllid attack; they also infest other Citrus trees, including limes, lemons, grapefruit and mandarins as well as kumquats, cherry orange, orange jasmine, Indian curry leaf, Chinese box orange, limeberry and desert-lime. In recent years Florida orange growers have attempted to apply heavy doses of pesticides, but that strategy has also been unsuccessful and growers are increasingly wary of the public reaction to the heavy use of pesticides. Nurseries that supply citrus trees to growers however are warned to use both a systemic and foliar spray before shipping any trees that have signs of infestation. The insecticide carbaryl (Sevin), applied as a foliar spray, has been effective against adult psyllids. The nymphs however are harder to kill with an insecticide spray, because they are tucked inside the small leaves of new growth, which protects them. That is why nurseries use a systemic insecticide to ward off psyllid nymphs. Carbaryl is very toxic to honey bees and to natural enemies of other citrus pests, so growers are warned to only apply it if they are certain of a psyllid infestation.

Psyllid infestation of orange trees illustrating the small nymphs that feed on orange tree sap and the green oranges that have been ruined by the bacteria that causes HLB disease

Florida oranges provide the major source of the nation’s orange juice supply and globally, Florida is second only to Brazil in orange juice production. It is a $ 9 billion industry which provides 76,000 jobs in the state which hosts the Orange Bowl in NCAA football as a fitting recognition to acknowledge the importance of the industry. Last year alone, orange production fell by nine percent, attributed to HLB disease, but there are still 60 million orange trees planted in Florida and all of them are at risk of HLB.  This threat to the Florida orange growers is so alarming that it forced them into something they never imagined they would agree to: search for a genetically modified organism (G.M.O.) to achieve a possible cure against HLB. Given the controversies surrounding genetically modified organisms, it is not in the grower’s personal DNA to consider this strategy unless it is one of last resort, which apparently is the case. Although orange trees are not native to Florida, it is said that they were brought to the state by Ponce de Leon and have thrived there ever since. Already hundreds of thousands of trees have been destroyed and the failure of traditional methods to modify the advance of this scourge, have stimulated the development of genetic modification of the orange tree DNA to find one or more genes that will combat the disease and remove the threat to an agricultural way of life that has existed in Florida for several hundred years.

Whether Florida orange tree farmers are guilty of over investing in monoculture orange trees remains an issue; perhaps more diversity in the orange tree DNA would have provided a resistant hybrid within the farmer’s orchards. In a way, what the farmers are seeking now is more diversity in the orange tree DNA which they might have provided on their own with more diversity and interbreeding among the trees suitable for producing and harvesting oranges.

Several gene candidates for orange DNA modification have been proposed, but the one furthest along was developed by Erik Mirkov of Texas A&M University. The new orange DNA from Mirkov’s laboratory contains a gene from spinach that generates proteins that help ward off bacterial infections. To speed the evaluation of the transgene, shoots from Dr. Mirkov’s plants were grafted onto normal, uninfected trees and so far the result has been successful in eliminating  psyllid infestation. It will take several more years and testing through the Department of Agriculture before this new orange tree DNA can be evaluated and approved for production, but there is hope that this step in bioengineering might lead to a cure for a disease that has threatened a vibrant form of agrarian economy.

Approval for using trees modified by the spinach gene however is only the beginning of an acceptable solution for Florida orange growers. If the spinach gene insertion passes muster with the Department of Agriculture, will it past muster with the public that has expressed increasing concerns about genetically modified organisms? In the past few months Whole Foods yielded to customer demand and announced that it would avoid stocking most G.M.O. food and require labels by 2018. Europeans are generally opposed to genetically modified foods and the public will certainly want to know whether the spinach gene makes orange juice taste like spinach. One cannot help but feel compassion for the orange growers of Florida. Whether they created their own dilemma by not insisting on more genetic diversity in their orange trees, they have adopted a strategy that will hopefully avoid the heavy use of pesticides even though the addition of the spinach gene to orange tree DNA seems more like satisfaction out of desperation rather than a gift from insightful planning.The National Academy of Sciences and the World Health Organization have both argued that G.M.O. crops are generally safe for consumption, but one worries about secondary actions. For example Monsanto’s genetically modified corn and soy seed produces plants that are immune from their herbicide “Roundup.” In that way farmers can administer the herbicide without fear of damaging their primary crop, but some have argued that this strategy has reduced the milkweed population, the plant source in which the Monarch butterfly lays its eggs and young caterpillars must feed. We are currently experiencing a dramatic reduction in the Monarch butterfly population and while the cause of this decline has yet to be established, many suspect increased use of “Roundup” as the most likely explanation. Are we willing to lose species in order to spread the use of monoculture farming techniques created by a single giant of agribusiness? I have not seen a single Monarch butterfly this summer, though my wife claims to have seen two. We use to see dozens during a summer and our own yard has plenty of milkweed plants, many of which I have searched for evidence of Monarch eggs, but I haven’t seen any such evidence as yet and I have searched beyond my own yard into many regions of the Twin Cities looking for them. No one yet seems to understand the cause of Monarch butterfly depletion.




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Genetically engineered salmon for the new world

Posted on June 26th, 2010 in ecology,Environment,Food & Wine,Nature by Robert Miller

Genetically modified Atlantic salmon are getting closer to our dinner table. The AquaBounty Technologies company, that has bio-engineered the fish, has passed several approval hurdles with the FDA, such that the fish may soon appear in the  marketplace, though a few additional hurdles remain before the green light goes on.  The genetic engineering of the fish is ingenious. These modified Atlantic salmon contain a copy of the growth hormone gene from a Chinook salmon as well as a genetic “on-switch” from another fish that turns the growth hormone gene on. Normally salmon do not make growth hormone in cold weather, but the new genetic makeup produces growth hormone all year, allowing the fish to reach market size in eighteen months rather than the usual period of three years. These genetically-altered fish do not apparently get super-sized, but merely grow faster to reach their normal adult weight. The accompanying figure, taken from the front page of today’s New York Times, shows the size of age-matched genetically modified fish at the top and the normal salmon at the bottom. What a difference a gene or two in the right place can make!  The modified AquaBounty salmon eggs will be sold to salmon farms only for commercial fish development. These animals are female only and they are also sterilized, so that even if they get loose in the environment, they are incapable of species propagation, at least that’s the hope.

Did you pass the calorie-counting quiz?

Posted on August 16th, 2009 in Food & Wine by Robert Miller

Here’s the quiz: two cheeseburgers are fat, juicy, smothered with cheese, accompanied by the same accouterments, deliciously presented and mouthwatering in appearance. They both have the identical number of calories, with one cooked as medium rare and the other is well done. So, do these two cheeseburgers provide us with identical caloric gains? Intuition says yes, right?

But, the answer is NO! The medium rare cheeseburger is actually less caloric to your body. The reason for this is simply that the part of the burger that is not completely cooked, contains proteins that have not been completely denatured and denatured proteins, because they “uncurl” are easier to digest. But, the uncooked proteins that remain in their native state, retain their complex foldings and twists which characterize their natural,  tertiary (3D) structure (provided largely by hydrogen bonding between neighbor regions of amino acids that come near to one another).  Those uncooked proteins require more effort on the part of your digestive system, more secretion of digestive enzymes and more time and activity within the gut in order to digest proteins in their natural state. That is why the development of cooking by our ancestor’s made their food acquisition task more efficient. You simply spend more energy digesting uncooked food because the tertiary structure of the proteins is harder to work on. As a result, the medium rare cheeseburger does not give your body the same number of calories as does the well done burger, because more energy is required to break it down and absorb all the calories–it eventually happens, but not before a greater part of the caloric gain has been spent on the energy of additional digestive effort. For proteins, this is not the only factor that reduces their net caloric value, because it also takes energy to convert ammonia to urea, which is a waste product for proteins that gets generated when we break them down into their amino acid constituents. Thus, the true caloric value of the food is the number of calories we swallow minus the number of calories we spend on getting the food digested and transported to internal sites for nutritional processing. Bijai Trivedi of New Scientist has a nice article on this topic, including a little interesting history of the topic.

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