Science Society Sustainability
http://www.i-sis.org.uk
ISIS Press Release 02/03/05
Terminator Trees
Sterile GM trees cannot contain transgenes, instead, they raise
special safety concerns for health and biodiversity
<mailto:[EMAIL PROTECTED]>Prof. Joe Cummins and
<mailto:[EMAIL PROTECTED]>Dr. Mae-Wan Ho
A <http://www.i-
sis.org.uk/full/TerminatorTreesFull.php>fully referenced version of
this article is posted on ISIS membersâ website. <http://www.i-
sis.org.uk/membership.php>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 "<http://www.i-sis.org.uk/GMFTTUT.php>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
"<http://www.i- sis.org.uk/VFMH.php>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 manipulated.
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, Youâre 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.
This article can be found on the I-SIS website at
http://www.i-sis.org.uk/TerminatorTrees.php
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