Science in Society Archive

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

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 [1,2]. 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 [3]. 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 [5,6].

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 [7]. Floral sterility has begun to be extended from poplar to shade trees [8]. 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 [9]. 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 [10]. 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 [11]. The PTD promoter was combined with the diphtheria-toxin gene, DTA, to produce sterile polar without the detrimental effects on yield encountered earlier [12]. 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 [13]. 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 [14,15].

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 [16,17]. 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 [18]. 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 [19]. 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 [20] Barnase was cytotoxic in mice and in human cell lines [21]. Animals may not find the GM forests welcoming. Diphtheria toxin has been associated with anaphylactic response [22]. 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.

Article first published 01/03/05


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