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

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Woody trees decompose slowly and are an important carbon sink
main types of lignin, guaiacyl lignin
and guaiacyl-syringyl. Guaiacyl
lignin is characteristic of softwoods,
which are resistant to
chemical and biological degradation.
Guaiacyl-syringyl 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 guaiacylsyringyl
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 guiaicylsyringyl
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 fibre 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 antisense
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 brownmidrib
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 greater 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). SiS
www.i-sis.org.uk