Ethiopia goes organic to feed herself - The Institute of Science In ...

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.
Low Lignin GM Trees
and Forage Crops
The plant cell is protected by a cell
wall that has a structure analogous
to reinforced concrete represented
by lignin. Lignin determines the
rigidity, strength and resistance of a
plant structure.
When wood fibre 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 fibre
and forage leads to greatly reduced
costs of preparing fibre 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 defence 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 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
SCIENCE IN SOCIETY 23, AUTUMN 2004