Abiotic stress and transgenics: Implications for reproductive success and crop-to-wild gene flow in Brassicas
Introduction
Diurnal background concentration of tropospheric ozone (O3) already exceeds 40 ppb in many regions and trends towards increasing levels are predicted to continue for the coming decades (Sitch, Cox, Collins, & Huntingford 2007). This can make O3 an important contributor on plant competitive dynamics. Oxidative stress by elevated O3 is a severe environmental challenge for plants: phytotoxic symptoms arise and this affects photosynthetic processes, phenological development and yield (Booker et al. 2009). Most wild plant species are classified as susceptible to O3 compared with crop species, which have been selected through breeding to be more robust under variable environments (Pleijel and Danielson, 1997, Davison and Barnes, 1998, Biswas et al., 2008). Genetically regulated O3 tolerance (Biswas et al. 2008), but also differences in phenology (e.g. maturation age), life-histories (Pleijel & Danielson 1997) and phenotypes (Overmyer et al. 2008) affect O3 responses of plants and could alter the competitive dynamics of co-species under elevated O3.
The introduction of transgenic crops has initiated wide ecological research of potential gene escape into native species (Stewart, Halfhill, & Warwick 2003). The probabilities for transgene flow have been extensively studied on Brassicas (e.g. Warwick et al., 2003, Halfhill et al., 2004, Kelly et al., 2005, Warwick et al., 2008), since wild Brassica species occur worldwide and exist commonly in agroecosystems. They are capable of hybridizing with cultivated Brassicas, which enables segregation of common genomic material among these plants (Wilkinson et al., 2003a, Halfhill et al., 2004). Unwanted introgression of transgenes into wild species could have severe ecological consequences, although the pathway required for a transgene to be fully introgressed into a wild-plant genotype is a complicated one in which the success of the hybrids are crucial (Wilkinson, Sweet, & Poppy 2003). Environmental stresses, including elevated O3, are candidates for affecting the performance and competitiveness of introgressed plant individuals and thus are critical in assessing the environmental risk of a transgene introgressing into the genome of a crop's wild relatives. Our study is the first to compare the performance of nontransgenic and transgenic crops, wild relatives and introgressed transgene-carrying back-cross hybrid plants under elevated O3.
Bacillus thuringiensis (Bt) Cry1Ac-transgenic oilseed rape (Brassica napus ssp. oleifera), insecticidal against numerous key Lepidopteran Brassica pests, is a model plant used widely in ecological risk assessment studies for crop-to-wild gene flow (e.g. Halfhill et al., 2001, Warwick et al., 2003). Previously, we have shown that Bt toxin concentration in Bt-producing B. napus is not compromised by, but increased under high atmospheric O3 (Himanen et al. 2009), and the plants exhibit similar responses to chronic and acute O3 elevation during vegetative growth as their nontransgenic parent plants (Himanen et al. 2008). Here, our first aim was to test for a trade-off, as a result of intrinsic costs of constitutive Bt toxin production, among reproduction and O3 tolerance in Bt B. napus. Secondly, we assessed whether elevated O3 affects certain physiological characteristics (growth, allocation to reproduction, seed size) and therefore the performance of sexually compatible wild B. rapa and crop B. napus plants, representing different ecological life-history strategies (Moles and Westoby 2006), in a different way. Finally, we evaluated the O3 responses and within-plant allocation patterns of introgressed Bt-transgene-carrying B. rapa × B. napus BC2F2 back-crossed hybrid plants to reveal whether elevated O3 could affect the probability of transgene escape through altered performance or reproduction of hybrids. Our results could reveal important aspects for assessing competitive advantage of introgressed transgene-carrying hybrids, wild relatives and crop plants in future O3-enriched atmospheres.
Section snippets
Materials and methods
Plants used in the experiments were: 1) non-transgenic Brassica napus ssp. oleifera (oilseed rape) cv. Westar (parent line), 2) its Bt-transgenic line GT1 F4 (containing a synthetic Bt Cry1Ac gene and a green fluorescent protein (gfp) marker gene under CaMV 35S promoters, as described by Halfhill et al. 2001), 3) Brassica rapa wild accession 2974 (Milby, Québec, Canada; Halfhill et al. 2005) and 4) wild B. rapa (described above) × crop (Bt-transgenic GT1 B. napus) BC2F2 hybrid carrying the
Vegetative growth stage responses
At 22 days after sowing, transgene-harbouring BC2F2 hybrid and wild B. rapa plants had higher leaf DW than the B. napus genotypes (Table 1, Table 2). By day 33, O3 reduced leaf DW in B. rapa , whereas there was no difference in leaf DW between genotypes at this time-point. Photosynthesis rates were similar in all plant genotypes and at both O3 levels at day 22 (Table 1, Table 2). By days 28 and 35, the photosynthesis rate was lower in hybrid and B. rapa plants than in B. napus
Ozone tolerance of wild and cultivated Brassicas: implications for crop–weed population dynamics
The high percentage of O3 lesions and reduced photosynthesis rates in wild B. rapa and transgene-carrying back-crossed hybrid plants suggested high O3 sensitivity, which is typical for native species (Davison & Barnes 1998). B. rapa plants had earlier senescence under elevated O3 than crop B. napus plants, and O3-induced differences in phenology might reduce overlap of their flowering periods and, hence, cross-fertilization. Important from the aspect of reproduction was also the increased
Acknowledgments
We thank Timo Oksanen for technical help and Virpi Tiihonen, Jaana Rissanen, Juuso Heinonen and Maria Saastamoinen for assistance in experiments. This work was supported by the Academy of Finland (grant no. 105209) (S.J.H., A-M.N. and J.K.H.), the Finnish Cultural Foundation (S.J.H.), ISONET (MRTN-CT-2003-504720) (J.K.H.), USDA Biotechnology Risk Assessment Program (C.N.S.), BBSRC and NERC (G.M.P.).
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