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Low Lignin GM Trees and Forage Crops
by Joe Cummins
The plant cell is protected by a cell wall that has a structure analogous to reinforced concrete. The cellulose fibrils play the role of steel reinforcing rods, while concrete is represented by lignin. Lignin determines the rigidity, strength and resistance of a plant structure.
When wood fiber is processed to make paper or composite products, lignin must be removed using polluting chemicals and a great deal of energy. Also, the digestibility of animal feed is influenced by lignin content—the greater the lignin content, the poorer the food source. Genetic engineering is now being used to fundamentally modify the lignin of forest trees and animal feed.
Reducing lignin content of fiber and forage leads to greatly reduced costs of preparing fiber and improved digestibility of fodder and forage. However, the advantages of reduced lignin are offset by the disadvantage of plants with reduced lignin, which are more readily attacked by predators such as insects, fungi and bacteria. Indeed, increasing lignin content has been promoted as a defense against pests.
The importance of lignin in disease resistance has been known for well over twenty years. For example, lignification was crucial in reducing predation by spruce bark beetles, and lignin in the roots of the date palm played a key role in defense against the fungus Fusarium. It has been suggested that a guaiacyl (a type of lignin subunit) rich lignin was produced as “defence” lignin when Eucalyptus was wounded by a predator. Lignin content of larch species determined the level of heartwood brown-rot decay. Genetic modification of plants to enhance lignin production is covered in United States Patent 5,728,570.
However, Arabidopsis plants modified in the metabolic pathway leading to lignin formation produced abnormal lignin that was associated with severe fungal attacks. Tobacco plants modified to limit production of lignin subunits were susceptible to virulent fungal pathogens, but it was suggested that the precursors of lignin and not lignin itself protected plants from pathogens. Genetic modifications for reduced lignin level nevertheless resulted in reduced fitness including increased winter mortality and decreased biomass.
It seems clear that plant genetic modification leading to reduced lignin, as proposed for use in pulp and paper or in livestock production, must be fully evaluated for fitness in the environment.
Multiple genetic transformations of forest trees have been used to limit total lignin production. Even though a potentially desirable end product is created, the multiple transformations (gene stacking) are liable to create chromosome instability leading to translocations, duplications and deletions through homologous recombination during germ cell formation and in somatic tissues (mitotic recombination). Independent studies of transgene integration using T-DNA vectors in aspen showed extensive DNA sequence scrambling at the insertion points. DNA sequence scrambling occurring in the cells during growth is a significant complication in long-lived trees.
A burn sterilized the forest in Coconino Plateau near Flagstaff, Arizona. The forest is stunted because fire suppression allowed too many trees to grow. Forest “experts” aim for massive thinning and perhaps planting of Aspen. Photo by Uncle Don Fanning, 2001.
Genetically modified low lignin trees are called “super” trees with little consideration of pest resistance and genetic stability. Field and pulping performance of transgenic poplars with altered lignin was evaluated to be superior by the developers of the poplar, and abnormal pest damage was not found. However, the pest damage studies were cursory and not compared with experimental controls, but with norms reported by government agencies.
The antibiotic resistance markers from the leaves of transgenic aspen have been studied for their persistence in the soil. The field study showed that the marker DNA of the aspen leaves persisted for as much as four months in the soil. The persistence of antibiotic resistance genes in the forest ecosystem is likely to impact not only soil microbes but human and animal inhabitants of the forest as well.
There is little question that the forage and fodder with reduced lignin and lignin with improved composition are more desirable food sources for grazing animals. However, the downside of lignin manipulation—greater disease susceptibility—was not thoroughly considered by developers of crops with modified lignin. The developers seem to ignore safety issues while they promote the modified crops.
Lignin modification of trees and forage crops has been a focus of research in genetic engineering. But lignin provides both fundamental structural features and resistance to animal and microbial pests. Lignin enhancement that leads to greater forage or tree pulp quality also leads to susceptibility to disease, while lignin enhancement that leads to greater disease resistance makes forage less digestible and tree pulp more expensive to process.
. . . low lignin trees are called “super” trees with little consideration of pest resistance and genetic stability.
The economic consequences of effective lignin modification could be tremendous, but producing forests and rangelands highly susceptible to insects, fungi and bacteria would lead to economic and environmental disaster. The low lignin trait is comparable to a loss in immune functions similar to AIDS in mammals. The chemical corporations might well welcome a huge increase in pesticides to fight disease in forests and pastures. Nevertheless, the best strategy is to proceed prudently and honestly evaluate the consequences of far reaching genetic engineering experiments.
Joe Cummins is Professor Emeritus at the University of Western Ontario.
This article can be found on the I-SIS website at http://www.i-sis.org.uk/LLGMT.php
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[2 feb 05]