ISIS Report 01/03/05
Terminator Trees
Sterile GM trees cannot contain transgenes, instead, they raise
special safety concerns for health and biodiversity
Prof. Joe Cummins and
Dr. Mae-Wan Ho
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 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, 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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