Environmental risk assessment of releases of transgenic plants containing virus-derived inserts

Impact of transgenic wheat with wheat yellow mosaic virus resistance on microbial community diversity and enzyme activity in rhizosphere soil

The transgenic wheat line N12-1 containing the WYMV-Nib8 gene was obtained previously through particle bombardment, and it can effectively control the wheat yellow mosaic virus (WYMV) disease transmitted by Polymyxa graminis at turngreen stage. Due to insertion of an exogenous gene, the transcriptome of wheat may be altered and affect root exudates. Thus, it is important to investigate the potential environmental risk of transgenic wheat before commercial release because of potential undesirable ecological side effects. Our 2-year study at two different experimental locations was performed to analyze the impact of transgenic wheat N12-1 on bacterial and fungal community diversity in rhizosphere soil using polymerase chain reaction-denaturing gel gradient electrophoresis (PCR-DGGE) at four growth stages (seeding stage, turngreen stage, grain-filling stage, and maturing stage). We also explored the activities of urease, sucrase and dehydrogenase in rhizosphere soil. The results showed that there was little difference in bacterial and fungal community diversity in rhizosphere soil between N12-1 and its recipient Y158 by comparing Shannon's, Simpson's diversity index and evenness (except at one or two growth stages). Regarding enzyme activity, only one significant difference was found during the maturing stage at Xinxiang in 2011 for dehydrogenase. Significant growth stage variation was observed during 2 years at two experimental locations for both soil microbial community diversity and enzyme activity. Analysis of bands from the gel for fungal community diversity showed that the majority of fungi were uncultured. The results of this study suggested that virus-resistant transgenic wheat had no adverse impact on microbial community diversity and enzyme activity in rhizosphere soil during 2 continuous years at two different experimental locations. This study provides a theoretical basis for environmental impact monitoring of transgenic wheat when the introduced gene is derived from a virus.

A Novel Moderate Constitutive Promoter Derived from Poplar (Populus tomentosa Carrière)

A novel sequence that functions as a promoter element for moderate constitutive expression of transgenes, designated as the PtMCP promoter, was isolated from the woody perennial Populus tomentosa. The PtMCP promoter was fused to the GUS reporter gene to characterize its expression pattern in different species. In stable Arabidopsis transformants, transcripts of the GUS reporter gene could be detected by RT-PCR in the root, stem, leaf, flower and silique. Further histochemical and fluorometric GUS activity assays demonstrated that the promoter could direct transgene expression in all tissues and organs, including roots, stems, rosette leaves, cauline leaves and flowers of seedlings and maturing plants. Its constitutive expression pattern was similar to that of the CaMV35S promoter, but the level of GUS activity was significantly lower than in CaMV35S promoter::GUS plants. We also characterized the promoter through transient expression in transgenic tobacco and observed similar expression patterns. Histochemical GUS staining and quantitative analysis detected GUS activity in all tissues and organs of tobacco, including roots, stems, leaves, flower buds and flowers, but GUS activity in PtMCP promoter::GUS plants was significantly lower than in CaMV35S promoter::GUS plants. Our results suggested that the PtMCP promoter from poplar is a constitutive promoter with moderate activity and that its function is presumably conserved in different species. Therefore, the PtMCP promoter may provide a practical choice to direct moderate level constitutive expression of transgenes and could be a valuable new tool in plant genetic engineering.

Recombination in the TYLCV Complex: a Mechanism to Increase Genetic Diversity. Implications for Plant Resistance Development

Mutation, reassortment, and recombination are the major sources of genetic variation of plant viruses (García-Arenal et al., 2001; Worobey & Holmes, 1999). During mixed infections, viruses can exchange genetic material through recombination or reassortment of segments (when the parental genomes are fragmented) if present in the same cell context of the host plant. Hybrid progeny viruses might then arise, some of them with novel pathogenic characteristics and well adapted in the population that can cause new emerging diseases. Genetic exchange provides organisms with a tool to combine sequences from different origins which might help them to quickly evolve (Crameri et al., 1998). In many DNA and RNA viruses, genetic exchange is achieved through recombination (Froissart et al., 2005; Martin et al., 2005). As increasing numbers of viral sequences become available, recombinant viruses are recognized to be frequent in nature and clear evidence is found for recombination to play a key role in virus evolution (Awadalla, 2003; Chenault & Melcher, 1994; Moonan et al., 2000; Padidam et al., 1999; Revers et al., 1996; García-Arenal et al., 2001; Moreno et al., 2004). Understanding the role of recombination in generating and eliminating variation in viral sequences is thus essential to understand virus evolution and adaptation to changing environments

Knowledge about the existence and frequency of recombination in a virus population might help understanding the extent at which genes are exchanged and new virus variants arise. This information is essential, for example, to predict durability of genetic resistance because new recombinant variants might be formed with increased fitness in host-resistant genotypes. Determination of the extent and rate at which genetic rearrangement through recombination does occur in natural populations is also crucial if we use genome and genetic-mapping information to locate genes responsible of important phenotypes such as genes associated with virulence, transmission, or breakdown of resistance. Therefore, better estimates of the rate of recombination will facilitate the development of more robust strategies for virus control (Awadalla, 2003).

Safety of virus-resistant transgenic plants two decades after their introduction: lessons from realistic field risk assessment studies

Potential safety issues have been raised with the development and release of virus-resistant transgenic plants. This review focuses on safety assessment with a special emphasis on crops that have been commercialized or extensively tested in the field such as squash, papaya, plum, grape, and sugar beet. We discuss topics commonly perceived to be of concern to the environment and to human health--heteroencapsidation, recombination, synergism, gene flow, impact on nontarget organisms, and food safety in terms of allergenicity. The wealth of field observations and experimental data is critically evaluated to draw inferences on the most relevant issues. We also express inside views on the safety and benefits of virus-resistant transgenic plants, and recommend realistic risk assessment approaches to assist their timely deregulation and release.

Molecular characterization of grapevine plants transformed with GFLV resistance genes: II

A collection of 127 putatively transgenic individuals of Vitis vinifera cv. Russalka was characterized by PCR and Southern hybridization. Six different constructs containing the neomycin phosphotransferase (nptII) marker gene and sequences of the Grapevine Fanleaf Virus Coat Protein (GFLV CP) gene including non-translatable and truncated forms were transferred via Agrobacterium-mediated transformation. Detection of transgenic sequences by PCR was positive in all lines. Southern blot analysis revealed that the number of inserted T-DNA copies ranged from 1 to 6. More than 46% of the tested transgenic lines contain one copy of the inserted T-DNA, qualifying them as interesting candidates for further breeding programs. Southern data of one line indicate the presence of an incomplete copy of the T-DNA, thus confirming previous PCR results. Since many putative transgenic lines shared identical hybridization patterns, they were clustered into 39 lines and considered as having originated from independent transformation events. The detection of the tetracycline (TET) resistance genes in 15% of the lines shows that an integration of plasmid backbone sequences beyond the T-DNA borders occurred. Enzyme-linked immunosorbent assay (ELISA) performed on leaf tissue did not show any accumulation of the GFLV CP in the 39 transgenic lines analyzed. Reverse transcription polymerase chain reaction (RT-PCR) and Northern blot were carried out; RT-PCR analyses showed that the GFLV CP mRNA was expressed at variable levels.

Genetic diversity and biological variation among California isolates of Cucumber mosaic virus

Genetic diversity and biological variation were compared for California isolates of Cucumber mosaic virus (CMV). These fell into five pathotypes based on their reactions on three cucurbits including a susceptible squash, a melon with conventional resistance and a commercial CMV-resistant transgenic squash. Thirty-three isolates infected and caused symptoms on CMV-resistant transgenic squash. Forty-two isolates infected the CMV-resistant melon, but only 25 isolates infected both. Single-strand conformation polymorphism (SSCP) analysis was used to differentiate 81 California isolates into 14 groups, and the coat protein (CP) genes of 27 isolates with distinct and indistinguishable SSCP patterns were sequenced. Fourteen isolates corresponding to the different SSCP patterns were also used for phylogenetic analysis. Seventy-nine isolates belonged to CMV subgroup IA, but two belonged to CMV subgroup IB. This is the first report of subgroup IB isolates in the Americas. All CMV isolates had a nucleotide identity greater than or equal to 93.24 %. There was no correlation between CP gene variation and geographical origin, collection year, original host plant, or between the degree of CP amino acid sequence identity and the capacity to overcome transgenic and/or conventional resistance. SSCP and sequence analyses were used to compare 33 CMV isolates on CMV-resistant transgenic squash and susceptible pumpkin plants. One isolate showed sequence differences between these two hosts, but this was not due to recombination or selection pressure of transgenic resistance. CMV isolates capable of infecting cucurbits with conventional and transgenic CMV resistance were present in California, even before CMV transgenic material was available.

Risk assessment of virus-resistant transgenic plants

Virus-resistant transgenic plants (VRTPs) hold the promise of enormous benefit for agriculture. However, over the past ten years, questions concerning the potential ecological impact of VRTPs have been raised. In some cases, detailed study of the mode of action of the resistance gene has made it possible to eliminate the source of potential risk, notably the possible effects of heterologous encapsidation on the transmission of viruses by their vectors. In other cases, the means of eliminating likely sources of risk have not yet been developed. When such residual risk still exists, the potential risks associated with the VRTP must be compared with those associated with nontransgenic plants so that risk assessment can fully play its role as part of an overall analysis of the advantages and disadvantages of practicable solutions to the problem solved by the VRTP.

Transgenic or plant expression vector-mediated recombination of Plum Pox Virus

Different mutants of an infectious full-length clone (p35PPV-NAT) of Plum pox virus (PPV) were constructed: three mutants with mutations of the assembly motifs RQ and DF in the coat protein gene (CP) and two CP chimeras with exchanges in the CP core region of Zucchini yellow mosaic virus and Potato virus Y. The assembly mutants were restricted to single infected cells, whereas the PPV chimeras were able to produce systemic infections in Nicotiana benthamiana plants. After passages in different transgenic N. benthamiana plants expressing the PPV CP gene with a complete (plant line 4.30.45.) or partially deleted 3'-nontranslated region (3'-NTR) (plant line 17.27. 4.), characterization of the viral progeny of all mutants revealed restoration of wild-type virus by recombination with the transgenic CP RNA only in the presence of the complete 3'-NTR (4.30.45.). Reconstitution of wild-type virus was also observed following cobombardment of different assembly-defective p35PPV-NAT together with a movement-defective plant expression vector of Potato virus X expressing the intact PPV-NAT CP gene transiently in nontransgenic N. benthamiana plants. Finally, a chimeric recombinant virus was detected after cobombardment of defective p35PPV-NAT with a plant expression vector-derived CP gene from the sour cherry isolate of PPV (PPV-SoC). This chimeric virus has been established by a double recombination event between the CP-defective PPV mutant and the intact PPV-SoC CP gene. These results demonstrate that viral sequences can be tested for recombination events without the necessity for producing transgenic plants.

Genetically modified plants - the debate continues

The debate about the potential risks and benefits of genetically modified organisms (GMOs) has hit the headlines over the past few months. The polarization of much of the debate obscures what really constitutes ecological risk, and what methods we can apply to identify and quantify those risks. Ecological science has much to offer in this respect, including ecological theory, manipulative experiments, the application of molecular tools and the interpretation of observational data from conventional agriculture. In the current heated debate, it is perhaps belief in the scientific method, above all else, that needs to be promoted and discussed.