Terminator Trees

Terminator Trees

By Joe Cummins and Mae-Wan Ho

In view of the serious threats posed by transgenic trees to the world's forest ecosystems, the commercial sale of transgenic trees is widely rejected despite claims that there will be strict containment of transgenes through "terminator trees".

Genetically modified sterile trees cannot contain transgenes, however they have raised concerns about their threat to health and biodiversity

Transgenic trees have been extensively investigated in large plots with little caution about the spread of transgenes. Studies on the dispersal of pollen and seeds from trees have shown that gene flow can be measured in kilometers. It is clear that transgenes from GM trees cannot be stopped once they have been introduced into the environment. For that reason, enormous efforts have been put into developing the necessary genetic modifications - commonly referred to as terminator techniques - to prevent flowering or pollen production.

In view of the serious threats posed by transgenic trees to the world's forest ecosystems, the commercial sale of transgenic trees is widely rejected despite claims that there will be strict containment of transgenes through “terminator trees”.

For the most part, the methods used to control flowering or pollination involve interference in genetic programming that allows flower development or the removal of cells related to flower development.

One group of genes - MADS-box genes - encode transcription factors for proteins that recognize DNA binding sites. The MADS plant genes are related to the extensively studied homeotic animal genes (HOX) that regulate developmental pathways.

The separation of the functions of the MADS genes has allowed flower development to be manipulated. Flowering is prevented by small anti-sense genes or regulatory RNA, which prevent active gene products, such as the MADS box transcription factor, from being formed. Also rearrangement is a type of genetic abortion using a suicide gene.

The preferred suicide gene is the one that codes for ribonuclease barnase from soil bacteria Bacillus amylolquefaciens. The ribonuclease is placed under the control of a specific promoter of flower development or pollen. When activated, the gene product effectively kills the cells in which the gene can be expressed.

Another suicide gene used is the bacterium diphtheria toxin Cornyebacterium diphtheria or the related ADP-Ribosyltransferase toxins from other bacteria; but these toxin genes are used less frequently than the barnase gene.

The preferred barnase gene is part of the genetic construct and was the first to be called a "terminator" by producing artificial sterility. This technology was designed so that seed production is under corporate control.

Professor Steven Strauss of the University of Oregon pioneered the control of flowering and pollen production in the poplar. He and his colleagues led the field of flower control in forest trees. Strauss specified that when complete sterility of the flower is achieved, the plant will require vegetative propagation.

Floral barrenness has started to spread from poplars to shade trees. Strauss has argued that the management of transgenic poplar is comparable to conventional poplar even though he is aware of the dispersal of seeds and pollen from transgenic poplar.

In addition to studies on flower sterility, Strauss has been investigating how to increase the speed of flower development (trees normally take years to develop sexually) to allow for faster reproduction and selection cycles.

But it is obvious that the cycle of rapid reproduction is accompanied by the uncertainty of the subsequent development of the mature tree. Strauss has pioneered the use of the poplar homologue for the flowering genes MADS, the promoter of the poplar gene PTD. The PTD promoter was combined with the diphtheria toxin gene, DTA, to produce a sterile poplar without negative effects in the field as had happened in the past.

The problem of somaclonal variation is hardly mentioned in the discussion of flower control in poplar even though this problem was discussed in a report on a four-year field investigation for a herbicide-tolerant poplar by Strauss's group. . Somaclonal variation results from the cell culture technique used to select and propagate transgenic plants. This produces extremely high levels of mutation and chromosomal instability, which could reverse flower sterility.

Previous studies have shown that poplar cell culture had very high levels of somaclonal variation. In Finland, researchers from Sopanen University studied the control of flowering in silver birch. Those scientists identified that MADS box genes controlled flowering in the birch tree.

When the BpMADS1 promoter from the birch flower specific gene was used to drive the barnase gene, cellular floral removal prevented flowering but there were marked side effects for leaves and branches.

The secondary effects could probably be the product of a pleiotropic effect of the insertion of the gene but also due to somaclonal variation of the cell culture. A recent study altered the name of the MADS box gene from BpMADS to BpFULL1. As in the previous study, flowering was avoided but the genetic modification affected the leaves and branches. The observed pleiotropic effects could be extended to other areas, but they have not yet been detected and need to be studied more extensively.

Ecological and health threats to terminator trees

Trees that neither flower nor bear fruit will not provide food for a multitude of insects, birds and mammals that feed on pollen, nectar, seeds and fruits, which will inevitably have enormous impacts on biodiversity.

The elimination toxins used to create barren trees are themselves an additional hazard. Ribonuclease barnase was found to be toxic to the kidneys of rats. Barnase was cytotoxic in mouse and human cell lines.

Animals are not going to welcome genetically modified plantations.

Diphtheria toxin has been associated with an anaphylactic response. As the song says: "If you go through the (transgenic) forests, you will surely find a great surprise." Even if these trees are sterile, they can spread asexually and genes will certainly be transferred horizontally to bacteria, fungi and other organisms found in the soil and in tree root systems, with unpredictable impacts on biota and soil fertility.

There is a remote possibility that these genes are transferred horizontally to other forest trees making them infertile as well.

Since transgenic traits tend to be unstable, they could disrupt and reverse flower development, causing the spread of transgenes to native trees or the creation of pollen that poisons bees and other pollinators causing potential damage to the plants. human beings.

Finally, the effect of preventing sexual reproduction is the drastic reduction of genetic recombination that generates genetic and evolutionary diversity in nature.

Sterile monocultures have a high probability of succumbing to pests or premature aging, which could potentially eliminate entire plantations.

* Prof. Joe Cummins and Dr. Mae-Wan Ho

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