Growth, fecundity and competitive ability of transgenic Trifolium subterraneum subsp. subterraneum cv. Leura expressing a sunflower seed albumin gene

摘要

Over the past decade, advances in molecular biology have made possible a wide range of transgenic plants that express agronomically beneficial traits. However, concerns associated with the risk of releasing transgenic plants into the environment have remained at the forefront of the debate surrounding GM technology. Although many risks associated with transgenic plant use in agricultural or pasture situations have been suggested (Hails 2000; Wolfenbarger and Phifer 2000), the probability of transgenic plants becoming “superweeds” (Gressel 1999) has received considerable attention. In part this is due to the considerable economic and environmental costs of invasive plants (Lonsdale 1994; Ramakrishnan and Vitousek 1989; Jones et al. 2000; Mack et al. 2000), but also owing to an increased appreciation of the economic value of intact natural systems (Bockstael et al. 2000; Bullock et al. 2001).Recently, a large number of pasture and turf plants have been genetically modified for agricultural purposes (Khan et al. 1996; Christiansen et al. 2000; Jayasena et al. 2001). Given the propensity for pasture plants to become weeds (Lonsdale 1994), investigation into the invasiveness or competitiveness of new transgenic plants intended for future release is a necessary component of any comprehensive risk assessment. Ideally, risk assessment of transgenic plants should include a combination of field and glasshouse trials performed under a variety of experimental conditions, including, if possible, those favouring the selection of the transgene itself (Kjellsson and Simonsen 1994). The nature of the specific experiments undertaken to quantify these risks will depend on the characteristics of both the transgene itself and the recipient plant (Dear et al. 1997), and in particular the ecological factors, such as competition, that currently limit the growth rate of populations of the target species (Schmitt and Linder 1994). Kjellsson and Simonsen (1994) recommend the use of competition trials for assessing whether transgenic and non-transgenic plants differ in competitive ability across a range of demographic stages of development.In this paper we report the results of a glasshouse trial aimed at determining whether transgenic Trifolium subterraneum L. subsp. subterraneum cv. Leura containing the high-sulphur sunflower seed albumin (ssa) gene (Khan et al. 1996) exhibits a competitive advantage over the non-transgenic commercial line in mixtures of different planting density. We also compared the morphology, fecundity and demographic characteristics of transgenic and non-transgenic subclover grown in mixtures and monocultures, quantifying differences in mortality, growth, mean and maximum above-ground plant weight, seed production and intrinsic rate of population increase across both genotypes. The transgenic subclover line used in the trial was developed to increase the rumen-protected sulphur content of forage consumed by sheep in improved pastures, since increased rumen stable sulphur intake enhances wool production (Khan et al. 1996). The ssa gene has been shown to alter the concentration of sulphur-compounds and associated biochemical pathways in a range of plants (Tabe and Droux 2002; Hagan et al. 2003), and since many important physiological processes are linked to inorganic and organic sulphur-containing compounds (e.g. ethylene production and methyl donation; Bright et al. 1980) it is possible that the ecological fitness of subterranean clover and other plants could be altered by ssa gene expression (Dear et al. 2003). The trial reported here comprises the first step in a phased process of assessing the ecologial risk posed by transgenic subterranean clover containing the ssa gene under controlled and field conditions. Subsequent studies will investigate the growth, fecundity and invasiveness of transgenic subclover containing this construct in a native grassland ecosystem considered prone to invasion by subclover and other exotic weeds.