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[SANET-MG] ban terminator trees
http://www.i-sis.org.uk/TerminatorTrees.phpISIS Press Release 01/03/05
Sterile GM trees cannot contain transgenes, instead, they raise special
safety concerns for health and biodiversity Prof. Joe Cummins and Dr.
A fully referenced version of this article is posted on ISIS members’
website. Details here
Transgenic or genetically modified (GM) trees have been tested
extensively in large open plots with little concern over the spread of
transgenes. Studies on the dispersal of pollen and seeds from forest
trees have shown that gene-flow can be measured in kilometres. It is
clear that the transgenes from GM trees cannot be contained once
released into the environment. For that reason, a great deal of effort
has been devoted to developing genetic modifications – commonly referred
to as terminator techniques - that prevent flowering or pollen production.
In view of the serious threats posed by GM forest trees to the forest
ecosystems of the world (see "GM forest trees – the ultimate threat",
this series), commercial release of transgenic trees is widely rejected
unless strict containment of transgenes can be assured, it is hoped,
through engineering such ‘terminator trees’.
For the most part, the methods used to control flowering or pollination
involved interfering with the genetic programme for floral development
or for deleting cells involved in floral development. A group of genes -
MADS-box genes - code for the protein transcription factors that
recognize DNA binding domains (See "View from MADS house", this series).
The plant MADS genes are related to the extensively studied animal
homeotic (HOX) genes that regulate developmental pathways. Unraveling
the functions of MADS genes has allowed flower development to be
Flowering is prevented by anti-sense genes, or small regulatory RNA to
prevent active gene products such as the MADS box transcription factor
from being formed. Also deployed is a kind of genetic abortion using a
suicide gene. The preferred suicide gene is the barnase ribonuclease
from the soil bacterium Bacillus amylolquefaciens. The ribonuclease is
placed under the control of a promoter specific to floral or pollen
development. When activated, the gene product effectively kills the
cells in which the gene is expressed. Another suicide gene used is the
diphtheria toxin from the bacterium Cornyebacterium diphtheria or
related ADP-Ribosyltransferase toxins from other bacteria; but these
toxin genes are less commonly used than the barnase gene. The preferred
barnase gene is a part of the genetic construction that first attracted
the label "terminator" for engineered sterility, designed to place seed
production under corporate control.
Professor Steven Strauss of Oregon State University pioneered flower and
pollen control in poplar. He and his colleagues have led in the area of
flowering control in forest trees. Strauss pointed out that when
complete floral sterility is achieved, the plant would require
vegetative propagation. Floral sterility has begun to be extended from
poplar to shade trees. Strauss has argued that management of GM poplar
is comparable to conventional poplar even though he is well aware of the
seed and pollen dispersal with transgenic poplar. Along with the
exploration of floral sterility, Strauss has investigated speeding
flower development (trees normally take years to develop sexually) to
allow rapid breeding and selection cycles. Of course the rapid breeding
cycle is fraught with uncertainty regarding the subsequent development
of the mature tree. Strauss has pioneered the use of the poplar
homologue to the floral MADS box genes, the poplar promoter gene PTD.
The PTD promoter was combined with the diphtheria-toxin gene, DTA, to
produce sterile polar without the detrimental effects on yield
encountered earlier. The problem of somaclonal variation is hardly
mentioned in the discussion of flower control in poplar even though the
problem was discussed in a report on a four-year field trial of
herbicide tolerant poplar carried out by the Strauss group. Somaclonal
variation results from the cell culture technique used to select and
propagate transgenic plants. It results in extremely high levels of
mutation and chromosome instability, which could reverse floral
sterility. Earlier reports showed that poplar cell culture resulted in
extremely high levels of somaclonal variation.
In Finland, investigators from Sopanen University have studied the
control of flowering in silver birch. Those investigators identified the
MADS box genes controlling flowering in the birch tree. When a flower
specific birch promoter gene BpMADS1 was used to drive the barnase gene,
floral cell ablation prevented flowering but there were marked side
effects affecting leaves and branching. The side effects were likely a
pleiotropic effect of the gene insertion but could, as well have been
affected by somaclonal variation from cell culture. A recent report
altered the name of the MADS box gene from BpMADS to BpFULL1. As in the
previous study flowering was prevented but the gene modification
affected leaves and branching. The pleiotropic effects observed may
extend into areas not yet detected and they require more extensive study.
Ecological and health hazards of terminator trees
Trees that do not flower and fruit will provide no food for the
multitude of insects, birds and mammals that feed on pollen, nectar,
seed and fruit, and will inevitably have huge impacts on biodiversity.
The ablation toxins used to create sterile trees are themselves an
additional hazard. Barnase ribonuclease proved toxic to the kidneys of
rats. Barnase was cytotoxic in mice and in human cell lines. Animals may
not find the GM forests welcoming. Diphtheria toxin has been associated
with anaphylactic response. As the song goes: "If you go down in the
(transgenic) woods today, You’re sure of a big surprise."
Even if these trees are sterile, they can still spread by asexual means
and certainly, the genes can spread horizontally to soil bacteria, fungi
and other organisms in the extensive root system of the forest trees,
with unpredictable impacts on the soil biota and fertility. There is a
remote chance that such genes could also spread horizontally to other
forest trees, making those also infertile.
As transgenic traits tend to be unstable, they could break down and
revert to flower-development, thereby spreading transgenes to native
trees, or create pollen that poison bees and other pollinators as well
as causing potential harm to human beings.
Finally, the effect of preventing sexual reproduction is to drastically
reduce genetic recombination that generates genetic diversity and
evolutionary novelty in nature. The sterile monocultures are much more
likely to succumb to disease or senescence, which could potentially wipe
out entire plantations.
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