Characterization of competitive interactions in the coexistence of Bt-transgenic and conventional rice

Investigating the presence of compensatory evolution in dicamba resistant IAA16 mutated kochia (Bassia scoparia)†

BACKGROUND

Lack of fitness costs has been reported for multiple herbicide resistance traits, but the underlying evolutionary mechanisms are not well understood. Compensatory evolution that ameliorates resistance costs, has been documented in bacteria and insects but rarely studied in weeds. Dicamba resistant IAA16 (G73N) mutated kochia was previously found to have high fecundity in the absence of competition, regardless of significant vegetative growth defects. To understand if costs of dicamba resistance can be compensated through traits promoting reproductive success in kochia, we thoroughly characterized the reproductive growth and development of different G73N kochia biotypes. Flowering phenology, seed production and reproductive allocation were quantified through greenhouse studies, floral (stigma-anthers distance) and seed morphology, as well as resulting mating and seed dispersal systems were studied through time-course microcopy images.

RESULTS

G73N covaried with multiple phenological, morphological and ecological traits that improve reproductive fitness: (i) 16-60% higher reproductive allocation; (ii) longer reproduction phase through early flowering (2-7 days); (iii) smaller stigma-anthers separation (up to 60% reduction of herkogamy and dichogamy) that can potentially promote selfing and reproductive assurance; (iv) 'winged' seeds with 30-70% longer sepals that facilitate long-distance seed dispersal.

CONCLUSION

The current study demonstrates that costs of herbicide resistance can be ameliorated through coevolution of other fitness penalty alleviating traits. As illustrated in a hypothetical model, the evolution of herbicide resistance is an ongoing fitness maximization process, which poses challenges to contain the spread of resistance. © 2020 The Authors. Pest Management Science published by John Wiley & Sons Ltd on behalf of Society of Chemical Industry.

Transcriptomic and proteomic responses to brown plant hopper (Nilaparvata lugens) in cultivated and Bt-transgenic rice (Oryza sativa) and wild rice (O. rufipogon)

Strategies are still employed to reduce insect damage in crop production, including conventional breeding with wild germplasm resources and transgenic technology with foreign genes' insertion. Cultivated and Bt-transgenic rice (Oryza sativa) and two ecotypes of wild rice (O. rufipogon) were treated by a 72 h feeding of brown plant hopper (Nilaparvata lugens). Under the feeding of N. lugens, compared with the cultivated rice (568 and 4), more differentially expressed genes (DEGs) and differentially accumulated proteins (DAPs) were identified in transgenic rice (2098 and 11) and two wild ecotypes (1990, 39 and 1932, 25, respectively). The iTRAQ analysis showed 79 DAPs and confirmed the results of RNA-seq, which showed the least GO terms and KEGG pathways responding to herbivory in the cultivated rice. DAPs significantly enriched two GO terms that are related with Bph14 and Bph33 genes in rice. Most of DEGs and DAPs were related to plant biological processes of plant-pathogen interaction and plant hormone signal transduction, and hormone signaling and transcription factors regulate the immune response of rice to BPH. Our results demonstrated the similarity in the wild rice and Bt-transgenic rice for their transcriptomic and proteomic response to herbivory, while cultivated rice lacked enough pathways in response to herbivory. STATEMENT OF SIGNIFICANCE OF THE STUDY: The iTRAQ analysis and RNA-seq were employed 39 to identify differentially expressed genes (DEGs) and differentially accumulated proteins (DAPs) in seedlings of cultivated, Bt-transgenic and two wild rice ecotypes under feeding of brown plant hopper. Wild rice showed DEGs and DAPs related to biochemical pathways of plant pathogen interactions and plant hormone signal transductions, while cultivated rice lacked enough pathways in response to herbivory. Crop domestication weakened the response of plants to herbivory, while the insertion of Bt gene might promote the response of plants to herbivory. Growing environment plays an important role in regulating gene networks of plant response to herbivory. Our results highlighted the importance of conservation of crop wild species. SIGNIFICANCE: Insect damage is one of main factors in reducing agricultural production, and technologies and methods were employed to control insect pests in agricultural systems. Transgenic technology is developed to produce insect-resistant crops, but receive concerns on biosafety risks. Alternatively, crop wild species are important genetic resource in crop breeding to produce trait-specific varieties. Here, we investigated the molecular mechanisms of plant response to herbivory in wild, Bt-transgenic and cultivated rice, and found crop domestication weakened the response of plants to herbivory. The insertion of foreign Bt gene may promote the expression of other genes. In addition, our results showed growing environment plays an important role in regulating gene networks of plant response to herbivory. These results highlight the importance of wild species conservation, with the strategy of in situ conservation.

Expression of Bt Protein in Transgenic Bt Cotton Plants and Ecological Fitness of These Plants in Different Habitats

Fitness is one of the key parameters to evaluate the effects of transgenic plants on the ecological environment. To evaluate the ecological risk of transgenic Bt cotton plants growing in different habitats, we determined the expression of the exogenous Bt gene and the fitness of transgenic and non-transgenic cotton plants in three habitats (farmland, grassland, and shrub). We observed that the expression of Bt protein in the farmland was significantly higher than that in the natural habitat, and when the growth environment was suitable, the Bt protein expression level showed a downward trend with the advancement of the growth. There were no significant differences in plant height, aboveground biomass, and seed yield between the Bt transgenic and non-transgenic cotton plants at the same growth stage under the same habitat. Nevertheless, in different habitats, the fitness of the same cotton line showed significant differences. In the farmland habitat, the plant height, aboveground biomass, and seed yield of both transgenic cotton and its non-transgenic isoline were significantly higher than that in the other two natural habitats. The results indicate that the expression of Bt protein does not increase the fitness of the parent plants and would not cause the weeding of the recipient cotton plants.

Enhanced yield performance of transgenic cry1C* rice in saline-alkaline soil

China has a large area of saline-alkaline land that can be utilized for the cultivation of transgenic rice. Therefore, the growth and reproductive behavior of transgenic rice are not only a problem for production that needs to be resolved, but also an important aspect of environmental risk assessment for saline alkali soil. In the present study, an insect-resistant transgenic cry1C* rice, T1C-19, was grown in farmland and saline-alkaline soils. The transcription and translation of the exogenous cry1C*, and vegetative and reproductive fitness, such as plant height, tiller number, biomass, filled grain number and weight per plant, were assessed. Our findings indicated that the transcription and translation of exogenous cry1C* gene in T1C-19 rice grown in saline-alkaline soil were lower than that grown in farmland; however, the correlation was not significant. The vegetative and reproductive growth abilities of T1C-19 were lower than that of the parental rice, Minghui63 (MH63), in farmland. In alkaline-saline soil, except for tiller number and biomass, there were no significant differences between T1C-19 and MH63 in other vegetative indices. In contrast, the reproductive indices of T1C-19 were significantly higher than those of MH63. The results suggested that T1C-19 had a strong reproductive capacity, and significantly reduced the loss of yield caused by insects, thereby leading to a higher yield than that of MH63 grown in saline-alkaline soils. This may promote the cultivation of saline-alkaline soil to permit farming of T1C-19 in China in the future, despite the possible increased ecological risks.

The application of gene splitting technique for controlling transgene flow in rice

Controlling transgene flow in China is important, as this country is part of the center of origin of rice. A gene-splitting technique based on intein-mediated trans-splicing represents a new strategy for controlling transgene flow via biological measures. In this study, the G2-aroA gene which provides glyphosate tolerance was split into an N-terminal and a C-terminal region, which were then fused to intein N and intein C of the Ssp DnaE intein, ultimately forming EPSPSn:In and Ic:EPSPSc fusion genes, respectively. These fusion genes were subsequently transformed into the rice cultivar Zhonghua 11 via the Agrobacterium-mediated method. The two split gene fragments were then introduced into the same rice genome by genetic crossings. Glyphosate tolerance analysis revealed that the functional target protein was reconstituted by Ssp DnaE intein-mediated trans-splicing and that the resultant hybrid rice was glyphosate tolerant. The reassembly efficiency of the split gene fragments ranged from 67 to 91% at the molecular level, and 100% of the hybrid F1 progeny were glyphosate tolerant. Transgene flow experiments showed that when the split gene fragments are inserted into homologous chromosomes, the gene-splitting technique can completely avoid the escape of the target trait to the environment. This report is the first on the reassembly efficiency and effectiveness of transgene flow containment via gene splitting in rice. This study provides not only a new biological strategy for controlling rice transgene flow but also a new method for cultivating hybrid transgenic rice.

Fitness Cost of Transgenic cry1Ab/c Rice Under Saline-Alkaline Soil Condition

The environmental release and biosafety of transgenic Bt crops have attracted global attention. China has a large area of saline-alkali land, which is ideal for large-scale production of Bt transgenic rice. Therefore an understanding of the fitness of Bt transgenic rice in saline-alkaline soils and the ability to predict its long-term environmental effects are important for the future sustainable use of these crops. In the present study, we aimed to evaluate the fitness of cry1Ab/c transgenic rice in both farmland and natural ecosystems. Transgenic cry1Ab/c rice Huahui1, for which a national biosafety certificate was obtained, was grown on normal farmland and saline-alkaline soils in a glass greenhouse. The expression pattern of exogenous Cry1Ab/c protein, and vegetative and reproductive fitness of rice were assessed. The expression of the exogenous Cry1Ab/c protein in the transgenic rice grown on saline-alkaline soil was lower than that in the strain grown on farmland soil. Under both the soil conditions, vegetative growth abilities, as evaluated by tiller number and biomass, and reproductive growth abilities, as measured by filled grain number and filled grain weight per plant, showed a significantly higher fitness cost for Huahui1 than that for the parental rice Minghui63 grown under the same soil conditions. In saline-alkaline soil, the fitness cost of Huahui1was moderately higher than that of Minghui63. Therefore, the ecological risk of cry1Ab/c transgenic rice is not expected to be higher than that of parental rice Minghui63 if the former escapes into natural saline-alkaline soil. The results of the present study provide a scientific basis to improve environmental safety assessment of the insect-resistant transgenic rice strain Huahui1 before commercialization.

Coexistence of Bacillus thuringiensis (Bt)-transgenic and conventional rice affects insect abundance and plant fitness in fields

BACKGROUND

As genetically modified (GM) crops are cultivated worldwide, concerns are emerging about the ecological consequences of the coexistence of transgenic and non-transgenic crops in fields. We first conducted field experiments using insect-resistant transgenic rice expressing Bacillus thuringiensis (Bt-transgenic rice) and its counterpart conventional rice (Oryza sativa L.) with or without insecticide spraying in 2013 and 2014. In 2015 and 2016, Bt-transgenic and conventional rice plants were employed in pure and mixed cages, with an infestation of the target insect (Chilo suppressalis) and with insecticide spraying as the control treatment to prevent target insect infestation.

RESULTS

The presence of Bt-transgenic rice decreased the abundance of target insects but did not affect non-target insects and predators in fields. Compared with conventional rice, Bt-transgenic rice showed more empty seeds but comparable seed production in cages. The infestation of target insects significantly decreased the plant fitness of conventional rice in pure cages, but did not affect its fitness when conventional rice coexisted with Bt-transgenic rice. In mixed cages, the presence of Bt-transgenic rice decreased the abundance of target insects and the percentage of dead sheaths in conventional rice.

CONCLUSION

The presence of Bt-transgenic rice benefits the growth and reproduction of non-transgenic rice in fields because of a decreased abundance of target insects. © 2018 Society of Chemical Industry.

Influence of intergenotypic competition on multigenerational persistence of abiotic stress resistance transgenes in populations of Arabidopsis thaliana

Reducing crop losses due to abiotic stresses is a major target of agricultural biotechnology that will increase with climate change and global population growth. Concerns, however, have been raised about potential ecological impacts if transgenes become established in wild populations and cause increased competitiveness of weedy or invasive species. Potential risks will be a function of transgene movement, population sizes, and fitness effects on the recipient population. While key components influencing gene flow have been extensively investigated, there have been few studies on factors subsequent to transgene movement that can influence persistence and competitiveness. Here, we performed multiyear, multigenerational, assessment to examine fitness effects and persistence of three mechanistically different abiotic stress tolerance genes: C-repeat binding factor 3/drought responsive element binding factor 1a (CBF3/DREB1a); Salt overly sensitive 1 (SOS1); and Mannose-6-phosphate reductase (M6PR). Transgenic Arabidopsis thaliana overexpressing these genes were grown in pure populations and in competition with wild-type (WT) parents for six generations spanning a range of field environment conditions. Growth, development, biomass, seed production, and transgene frequency were measured at each generation. Seed planted for each generation was obtained from the previous generation as would occur during establishment of a new genotype in the environment. The three transgenes exhibited different fitness effects and followed different establishment trajectories. In comparison with pure populations, CBF3 lines exhibited reduced dry weight, seed yield, and viable seed yield, relative to WT background. In contrast, overexpression of SOS1 and M6PR did not significantly impact productivity measures in pure populations. In competition with WT, negative fitness effects were magnified. Transgene frequencies were significantly reduced for CBF3 and SOS1 while frequencies of M6PR appeared to be subject to genetic drift. These studies demonstrate the importance of fitness effects and intergenotype competition in influencing persistence of transgenes conferring complex traits.

Paternal inheritance of plastid-encoded transgenes in Petunia hybrida in the greenhouse and under field conditions

As already demonstrated in greenhouse trials, outcrossing of transgenic plants can be drastically reduced via transgene integration into the plastid. We verified this result in the field with Petunia, for which the highest paternal leakage has been observed. The variety white 115 (W115) served as recipient and Pink Wave (PW) and the transplastomic variant PW T16, encoding the uidA reporter gene, as pollen donor. While manual pollination in the greenhouse led to over 90% hybrids for both crossings, the transgenic donor resulted only in 2% hybrids in the field. Nevertheless paternal leakage was detected in one case which proves that paternal inheritance of plastid-located transgenes is possible under artificial conditions. In the greenhouse, paternal leakage occurred in a frequency comparable to published results. As expected natural pollination reduced the hybrid formation in the field from 90 to 7.6% and the transgenic donor did not result in any hybrid.

Effects of Bt-transgenic rice cultivation on planktonic communities in paddy fields and adjacent ditches

The non-target effects of transgenic plants are issues of concern; however, their impacts in cultivated agricultural fields and adjacent natural aquatic ecosystems are poorly understood. We conducted field experiments during two growing seasons to determine the effects of cultivating Bacillus thuringiensis (Bt)-transgenic rice on the phytoplankton and zooplankton communities in a paddy field and an adjacent ditch. Bt toxin was detected in soil but not in water. Water quality was not significantly different between non-Bt and Bt rice fields, but varied among up-, mid- and downstream locations in the ditch. Cultivation of Bt-transgenic rice had no effects on zooplankton communities. Phytoplankton abundance and biodiversity were not significantly different between transgenic and non-transgenic rice fields in 2013; however, phytoplankton were more abundant in the transgenic rice field than in the non-transgenic rice field in 2014. Water quality and rice type explained 65.9% and 12.8% of this difference in 2014, respectively. Phytoplankton and zooplankton were more abundant in mid- and downstream, than upstream, locations in the ditch, an effect that we attribute to water quality differences. Thus, the release of Bt toxins into field water during the cultivation of transgenic crops had no direct negative effects on plankton community composition, but indirect effects that alter environmental conditions should be taken into account during the processes of management planning and policymaking.