Researchers are hoping that biotechnology will accelerate long-stalled efforts to overcome devastating diseases of trees

Melanie D. G. Kaplan

Melanie D. G. Kaplan, a contributing editor for CBS SmartPlanet.com, lives in Washington, D.C.

Summary

● Introducing part of viral pathogen gene into plum trees can protect them against plum pox disease; this advance in technology, like others, awaits regulatory review.
● Efforts to combat several other diseases affecting fruit trees are at various stages of development.
● Chestnut blight, the "poster child" of tree diseases, is being fought on several fronts, including with genomes, transgenes, and a virus that attacks the fungal pathogen.
● In some cases, Dutch elm disease, for example, conventional searches uncover resistant varieties; however, concerns remain that the pathogens will overcome that resistance.
● Molecular diagnostic tests as well as improved communications among plant pathologists are also important for combating the spread of tree diseases.

Tree diseases, caused by microbial pathogens many of which are carried by insect vectors, are spreading faster than ever. "Trees didn't evolve to be able to handle the sort of things we're throwing at them," says Adam Costanza, president of the Institute of Forest Biotechnology (IFB) in Raleigh, N.C. "They developed very slowly, over time, and more or less in isolation."

Although tree diseases circulate along with their hosts, the increased rate of their development is forcing growers and researchers to respond rapidly, ideally before damage occurs. "We need to respond very quickly," says Ralph Scorza of the U.S. Department of Agriculture (USDA), who works at the USDA Appalachian Fruit Research Station in Kernersville, W.V. "Most fruit crop breeding takes a career to develop new varieties. We can't afford that." Plant diseases are facing fruit growers with poorer yields and the prospect of tree loss. Meanwhile, diseases in forest trees are affecting the ecology of wooded areas.

Across the country, microbiologists, plant pathologists, and other experts are working with a variety of tree species, including chestnut, elm, oak, plum, and almonds, evaluating control measures that draw on traditional microbiology as well as bioengineering and biological control strategies. "We really need to use the best technologies that will give us the best solutions in the shortest amount of time, and they have to be safe," Scorza says. "We need to have a full toolbox." Those technologies include new approaches to diagnosing and evaluating tree diseases as well as ways of combating them (box, Applying Molecular Technologies To Diagnose Tree Diseases).

Breeding Fruit Trees To Resist Pathogens

One approach to combating plant pathogens that damage fruit trees depends on conventional breeding, which relies on cross-pollinating and selection over successive generations of plants, according to Scorza. However, because trees take up to a decade to bear fruit, this approach is costly because it can be so time-consuming, he says. However, classical breeding now can be combined with the use of molecular markers, enabling researchers to follow certain traits using DNA. Instead of planting seeds from a cross of two varieties and waiting years to learn whether the fruit is good, it becomes possible to learn a good deal when the trees are only a few centimeters tall.

The urgency to develop tree lines that resist the plum pox virus is a case in point. This virus, which is carried between trees by juice-sucking aphids, affects a range of stone-fruit species, including peaches and apricots as well as plums (Fig. 1). While plum pox virus infections do not kill trees, they lead fruit to drop off trees prematurely, lowering yields. The disease severely reduced fruit production in Eastern Europe before it appeared in Pennsylvania in 1999. While not yet widespread in the United States, the disease has spread into orchards in New York and eastern Canada. Experts fear a disastrous economic impact if it were to reach California.

Before plum pox viruses reached U.S. orchards, Scorza and his collaborators applied genetic engineering technology to develop resistant tree lines. "We took a noninfectious part of the virus and put it into the plum, and it simulated a natural reaction, like a vaccine," he says. "It has completely protected the plants." This transgenic tree line, called HoneySweet, "is the first successful example of this [approach] working," he adds. "Now we have a new genetic resource for our plum growers and breeders. If the virus started breaking out in a big way in the U.S., we could release resistant varieties."

Before HoneySweet or other transgenic trees are grown commercially, however, they likely will face some form of regulatory review. Anticipating such a review, Scorza and his colleagues are testing whether fruits from transgenic trees are changed in composition and are as nutritious as the original fruit or if they produce novel allergens, and also whether such trees affect the local ecology in orchards where they are growing. Noting that genetic engineering provides a fast means to respond to infectious disease threats, Scorza says that, from his experience, it also appears to be a safe approach.

This general approach also should be applicable to other diseases affecting fruit trees, according to Scorza. For instance, fire blight in apples and pears is a bacterial disease, which sometimes is treated with antibiotics. It can wipe out orchard productivity very quickly. Meanwhile, brown rot in peaches is the result of a fungal pathogen that spreads over the surfaces of this fruit. Other fruit tree diseases include laurel wilt in avocadoes, which is also caused by a fungus and is transmitted by invasive ambrosia beetles; citrus canker, a bacterial disease that causes citrus tree leaves and fruit to drop early; and citrus greening, which leads ripe fruits to turn green and shortens the life of citrus trees and is caused by Candidatus Liberibacter asiaticus and is now found throughout Florida.

In Uganda, researchers are testing genetically engineered bananas that resist banana Xanthomonas wilt. The plants were engineered by introducing two genes from a sweet pepper plant to inhibit the spread of this disease.

Reviving Chestnuts, Once Prominent in U.S. Forests

The chestnut blight fungus, Cryphonectria parasitica, nearly wiped out American chestnut trees in the early 1900s (Fig. 2). Before the blight arrived, the chestnut was not only commercially valuable, it was also environmentally important for local wildlife, and its decline led some animal populations to plummet. "There is a big emotional involvement with the chestnut," says Scott Merkle, a professor of forestry at the University of Georgia. Nonetheless, for several decades the U.S. Forest Service gave up on the chestnut after a traditional breeding program proved unsuccessful, he says. "Then around 1980, people started looking into it again, and now what was once a lost cause has hope."

After chestnut blight came to North America from Asia, it damaged as many as 6 billion trees. Despite the blight causing widespread damage, however, chestnuts persist because the fungus cannot out-compete protective microorganisms that live along the tree roots, enabling new shoots to emerge after much of the tree dies back. This Sisyphean process repeats itself-shoots grow for a few years, become diseased, and retreat again. Merkle is trying to adapt somatic embryogenesis, an in vitro propagation method, to produce chestnut trees by the thousands. Although this method works well for yellow poplar and black locust trees, chestnut embryos are larger, making it challenging to apply to this species so far.

Merkle and William Powell and Chuck Maynard of the State University of New York (SUNY) in Syracuse, their collaborators, and other researchers are exploring other approaches to render chestnut trees resistant to blight. One such effort involves determining the genomic sequence of the Chinese chestnut, which is naturally resistant to blight, to figure out which genes are responsible. One goal would be to transfer those genes into American chestnut trees.

Before that genomic sequencing effort took shape, the SUNY researchers began evaluating whether transferring genes from other plants into chestnut trees can enable them to withstand blight. A gene from wheat, for example, encodes an enzyme that breaks down oxalate, a key organic acid that the blight fungus produces as it grows on the trees. "It took about 15 years to develop a reliable technique to put genes into American chestnut and regenerate whole plants," Powell says. "The first transgenic American chestnut tree was planted near Syracuse on 7 June 2006. We have been planting more transgenic trees every year since (Fig. 3)."

Yet another approach entails harnessing a virus that infects the fungus to keep the blight at bay, according to Dennis Fulbright, a plant pathology professor at Michigan State University in East Lansing. "Basically you have the fungus trying to kill the tree, and the virus trying to kill the fungus," he says. The virus being evaluated in Michigan is similar to one that protects European chestnuts. "Our goal is to try to keep chestnut blight at bay, rather than run from it," he says. "We put holes in the sides of the trees around an infection point, called a canker. The virus transfers, and you see the tree producing wound tissue, and then it heals up. I'm optimistic that if we continue to do research, the Midwest will be one of the premiere chestnut-producing locations in the country."

Fighting Other Pathogens Affecting Trees in Forests

In Delaware, Ohio, USDA Forest Service researcher Jim Slavicek is leading a team looking for resistance to protect elms against Dutch elm disease, caused by at least three species of Ophiostoma fungi, which are transmitted by elm bark beetles. The disease, which typically kills trees within two to three years, came to North America on timber in 1928. Slavicek searches for elm trees that evaded the disease, propagates them, and then challenges with the fungal pathogens to see how the trees respond. "We're not manipulating, we're just looking for resistance," he says. Among hundreds of thousands of tested trees, several emerged with high levels of tolerance to Dutch elm. One variety, called Princeton, is being propagated as genetically identical clones in many U.S. nurseries.

"It's great that the American elm is coming back, because we'd like to be able to restore it in forested landscapes, but it's not so great that it's being brought back by one single genotype of tree," says Powell of SUNY Syracuse. "If the fungus were to evolve to a form that could overcome the defense mechanisms of the Princeton, they would all die. It would be better if we had 20 to 25 different genotypes. If you start with too narrow of a genetic base, it would most likely be a genetic dead end."

Another major concern for plant pathologists working with trees in forests is sudden oak death, a disease caused by a plant pathogen that is devastating oak trees throughout California and Oregon. Yet another concern is Elm Yellows, a disease caused by phytoplasmas that infect the inner bark of trees and can kill elms just as quickly as does Dutch elm disease. Elm Yellow is spread by leaf-hoppers or root grafts; resistant varieties of elm are not available.

Determining Whether Tree-Protecting Technologies Have Side Effects

In any forest restoration program, it is critical that newly introduced transgenic trees do not disrupt the environment in which they are planted, according to Powell. "Mycorrhizae are the good fungi that almost all trees need to keep healthy," he says. In the case of chestnuts, he adds, "Since we are putting in genes to fight a fungal pathogen, we don't also want them to harm the good fungi."

In general, finding the funding need to conduct such research is a challenge. However, support for restoring the American chestnut is more substantial because the tree is a "poster child," Merkle says. "It's almost like we're a demonstration. We're funded by the Forest Health Initiative (FHI), part of the IFB in North Carolina. The FHI goal in funding our research is not chestnut restoration; it's to show how biotech can be applied to forest health problems."

The FHI-sponsored transgenic research involving the American chestnut is indeed a test case, according to Costanza. "We're exploring it," he says. "We're not 100% positive that the science is there yet, we don't know if the regulatory framework in the U.S. is there yet, and we don't know if the public wants this sort of technology."

"We like to support projects that we think might not have commercial value but have huge value to society, like rescuing the chestnut," says Maud Hinchee of ArborGen, a biotechnology company based in Summerville, S.C. that helps to support Powell, Maynard, and Merkle's transgenic research on this tree species. "It allows the public to see the use of the technology and understand the benefits and risks in something they care about. Chestnuts are a noble cause."

Although the public might express doubts about applying biotechnology to protect chestnuts or other tree species, a bigger obstacle is the regulatory process, according to Powell. "Biotechnology is highly regulated," he says. "It's just the same as it is for food-you need approval from the USDA, Environmental Protection Agency, and Food and Drug Administration. We could have a resistant tree, and it could be years before we could give it to the public."

A big argument in favor of using biotechnology to combat tree disease is speed, according to Costanza. "This is why biotechnology in forest health has started to gain attention," he says. "It allows you to identify the vulnerability in the tree, look at potential solutions, and engineer the tree quickly so it can fight off the disease. Traditional approaches aren't working and, if they do, they take too long. We'll just lose the battle."

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