ISIS Press Release 05/06/04
Low Lignin GM Trees and Forage Crops
Prof. Joe Cummins explains why
genetically modifying trees and forage crops to reduce their lignin content
could make them more susceptible to pests. Other issues related to the GM
construct, such as genetic instability, the persistence of antibiotic
resistance marker genes in the ecosystem and biosafety in general, have also
not been sufficiently considered.
A fully referenced version of this article is posted
on ISIS members’ website. Details here.
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 defence 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 is 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 that 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.
The monomeric structure of lignin influences the properties of the
plant material. There are two main types of lignin, quaiacyl lignin and
guaiacyl-syringyl. Guaiacyl lignin is characteristic of softwoods, which are
resistant to chemical and biological degradation. Guaiacyl-syingyl lignin is
typical of hardwoods such as poplar, which are more readily degraded.
Modifying plants with a gene enhancing the proportion of
guaiacyl-syringyl lignin therefore provides a lignin more readily degraded by
chemicals or enzymes. Reducing lignin content also leads to plants more readily
digested with enzymes or chemicals.
Lignin reduction has been achieved using anti-sense genes to limit
production of key enzymes on the lignin biosynthesis pathway. Multiple genetic
transformations of forest trees have been used to enhance production of
guaiacyl-syringyl lignin and to limit total lignin production. Four
Agrobacterium T-DNA vectors, each with a cauliflower mosaic virus
promoter, two of which included anti-sense to limit undesirable enzymes and two
with sense constructions to enhance desirable enzymes, were used to
simultaneously alter the genome of aspen (Populus tremuloides). This
resulted in reduced lignin content of guaiacyl lignin and increased
guiaicyl-syringyl proportion in the remaining lignin.
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.
Lignin genetic engineering is promoted as a promising strategy to
improve fiber production but the drawbacks of anti-sense manipulation and
transgene stability are not seriously dealt with. Trees genetically modified to
produce low lignin 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.
Lignin content increases as crops age or are stressed. Animal feed rich
in lignin is poorly digestible and considered to be of low quality. Grass,
alfalfa or maize with reduced lignin or lignin with increased guaiacyl-syringyl
proportion (readily digested) may provide a large economic benefit in animal
production, provided that the genetic modifications do not result in
susceptibility to predatory insects, fungi and bacteria and do not compromise
food or feed safety (for example, fungus food contamination may lead to
pollution of food with toxins, causing liver damage and cancer).
The main technique used to produce lignin modifications is anti-sense
genes designed to reduce one or another enzyme level on the pathway to lignin
production. Maize with improved forage quality was produced by down-regulating
the enzyme O-methyl transferase to limit lignin production. Tall fescue pasture
grass with improved forage digestibility was produced using an anti-sense gene
for the lignin precursor enzyme cinnamyl alcohol dehydrogenase. Alfalfa
down-regulated for lignin enzyme caffeoyl coenzyme A 3-O-methyl transferase
produced plants with increased guaiacyl-syringyl lignin proportions leading to
improved rumen digestibility.
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.
Furthermore, smooth brome grass clones selected using conventional
breeding showed that reduced lignin was associated with severe rust fungus
disease. Alfalfa selected for forage quality (including reduced lignin) had
reduced vigour but was not expected to affect levels of disease resistance.
Sudan grass selected for brown- midrib trait (an indicator of reduced lignin)
experienced severe yield reductions and environmental sensitivity, particularly
during cooler growing seasons.
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 great disease
resistance makes forage less digestible and tree pulp more expensive to
process.
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 comparable 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.
Note added by editor: Another consideration is ecological. Wood,
with its naturally high lignin content, generally takes a long time to decay
and recycle in the ecosystem, probably for good reasons. It is a long-term
energy store complementing the shorter-term energy storage depots, which
enables the ecosystem to function most efficiently and effectively (see "Why
are organisms so complex? A lesson in sustainability",
SiS 21).
Slow-decaying wood is also a major carbon sink. Reducing its lignin content to
enhance degradation will end up returning carbon dioxide too rapidly to the
atmosphere, thereby exacerbating climate change (see "Why Gaia needs
rainforests" SiS
20).
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