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[SANET-MG] low ignin trees and forage
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ISIS Press Release 05/06/04
Low Lignin GM Trees and Forage Crops
Prof. Joe Cummins <mailto:firstname.lastname@example.org> explains why genetically
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
instability, the persistence of antibiotic resistance marker genes in the
ecosystem and biosafety in general, have also not been sufficiently
A fully referenced <http://www.i-sis.org.uk/full/LLGMTFull.php> version
article is posted on ISIS members' website. Details here <http://www.i-
The plant cell is protected by a cell wall that has a structure
reinforced concrete. The cellulose fibrils play the role of steel
rods, while concrete is represented by lignin. Lignin determines the
strength and resistance of a plant structure.
When wood fiber is processed to make paper or composite products, lignin
removed using polluting chemicals and a great deal of energy. Also, the
digestibility of animal feed is influenced by lignin content - the
lignin content, the poorer the food source. Genetic engineering is now
used to fundamentally modify the lignin of forest trees and animal feed.
Reducing lignin content of fiber and forage leads to greatly reduced
preparing fiber and improved digestibility of fodder and forage.
advantages of reduced lignin are offset by the disadvantage of plants with
reduced lignin, which are more readily attacked by predators such as
fungi and bacteria. Indeed, increasing lignin content has been promoted
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
spruce bark beetles, and lignin in the roots of the date palm played a
in defence against the fungus Fusarium. It has been suggested that a
type of lignin subunit) rich lignin was produced as "defence" lignin when
Eucalyptus is wounded by a predator. Lignin content of larch species
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
formation produced abnormal lignin that was associated with severe fungal
attacks. Tobacco plants modified to limit production of lignin subunits
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
fitness including increased winter mortality and decreased biomass.
It seems clear that plant genetic modification leading to reduced
proposed for use in pulp and paper or in livestock production, must be
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
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
enzymes or chemicals.
Lignin reduction has been achieved using anti-sense genes to limit
key enzymes on the lignin biosynthesis pathway. Multiple genetic
of forest trees have been used to enhance production of
and to limit total lignin production. Four Agrobacterium T-DNA vectors,
with a cauliflower mosaic virus promoter, two of which included
limit undesirable enzymes and two with sense constructions to enhance
enzymes, were used to simultaneously alter the genome of aspen (Populus
tremuloides). This resulted in reduced lignin content of guaiacyl lignin
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
in aspen showed extensive DNA sequence scrambling at the insertion
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
production but the drawbacks of anti-sense manipulation and transgene
are not seriously dealt with. Trees genetically modified to produce low
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
abnormal pest damage was not found. However, the pest damage studies were
cursory and not compared with experimental controls, but with norms
The antibiotic resistance markers from the leaves of transgenic aspen
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 persistence of antibiotic resistance genes in the forest ecosystem
to impact not only soil microbes, but human and animal inhabitants of
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,
or maize with reduced lignin or lignin with increased guaiacyl-syringyl
proportion (readily digested) may provide a large economic benefit in
production, provided that the genetic modifications do not result in
susceptibility to predatory insects, fungi and bacteria and do not
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
the enzyme O-methyl transferase to limit lignin production. Tall fescue
grass with improved forage digestibility was produced using an
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
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
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
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.
selected for brown- midrib trait (an indicator of reduced lignin)
severe yield reductions and environmental sensitivity, particularly during
cooler growing seasons.
Lignin modification of trees and forage crops has been a focus of
genetic engineering. But lignin provides both fundamental structural
and resistance to animal and microbial pests. Lignin enhancement that
greater forage or tree pulp quality also leads to susceptibility to
while lignin enhancement that leads to great disease resistance makes
less digestible and tree pulp more expensive to process.
The economic consequences of effective lignin modification could be
but producing forests and rangelands highly susceptible to insects,
bacteria would lead to economic and environmental disaster. The low
is comparable to a loss in immune functions comparable to AIDS in
chemical corporations might well welcome a huge increase in pesticides
disease in forests and pastures. Nevertheless, the best strategy is to
prudently and honestly evaluate the consequences of far reaching genetic
Note added by editor: Another consideration is ecological. Wood, with its
naturally high lignin content, generally takes a long time to decay and
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
so complex? A lesson in sustainability", SiS 21 <http://www.i-
sis.org.uk/isisnews/sis21.php>). Slow- decaying wood is also a major carbon
sink. Reducing its lignin content to enhance degradation will end up
carbon dioxide too rapidly to the atmosphere, thereby exacerbating climate
change (see "Why Gaia needs rainforests" SiS 20 <http://www.i-
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