by The Institute of Science in Society
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.
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
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 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
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
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 [11,12]. 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
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 [21,22].
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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