Transcomplementation and synergism in plants: implications for viral transgenes?
Summary
In plants, viral synergisms occur when one virus enhances infection by a distinct or unrelated virus. Such synergisms may be unidirectional or mutualistic but in either case synergism implies that protein(s) from one virus can enhance infection by another. A mechanistically related phenomenon is transcomplementation, in which a viral protein, usually expressed from a transgene, enhances or supports the infection of a virus from a distinct species.
To gain an insight into the characteristics and limitations of these helper functions of individual viral genes and to assess their effects on the plant/pathogen relationship, reports of successful synergism and transcomplementation were compiled from the peer-reviewed literature and combined together with data from successful viral gene exchange experiments. Results from these experiments were tabulated to highlight the phylogenetic relationship between the helper and dependent viruses and, where possible, to identify the protein responsible for the altered infection process. Analysis of more than 150 publications, each containing one or more reports of successful exchanges, transcomplementation or synergism, revealed the following: (1) diverse viral traits can be enhanced by synergism and transcomplementation. These include the expansion of host range, acquisition of mechanical transmission, enhanced specific infectivity, enhanced cell-to-cell and long-distance movement, elevated or novel vector transmission, elevated viral titre and enhanced seed transmission; (2) transcomplementation and synergism are mediated by many viral proteins including inhibitors of gene silencing, replicases, coat proteins and movement proteins; (3) although more frequent between closely related viruses, transcomplementation and synergism can occur between viruses that are phylogenetically highly divergent.
As indicators of the interoperability of viral genes, these results are of general interest, but they can also be applied to the risk assessment of transgenic crops expressing viral proteins. In particular, they can contribute to identification of potential hazards and can be used to identify data gaps and limitations in predicting the likelihood of transgene-mediated transcomplementation.
Transformation-induced Mutations in Transgenic Plants: Analysis and Biosafety Implications
Summary
Transgene insertion is infrequently, if ever, a precise event and it therefore causes various alterations to the plant genome. Mutations present at transgene insertion sites include insertion of superfluous DNA and deletion and rearrangement of host chromosomal DNA. These mutations vary in frequency depending in particular on the method of delivery. Transgene insertion sites introduced using Agrobacterium tumefaciens tend to be simpler but can be associated with very large chromosomal rearrangements, while transgenes delivered by particle bombardment appear invariably to be associated with deletion and extensive scrambling of inserted and chromosomal DNA. Nevertheless, the frequency and impact of these mutations are poorly understood, especially those caused by particle bombardment. This is exemplified by the fact that only a single functional transgene insertion site resulting from particle bombardment has been comprehensively analysed for insertion-site mutations. Additionally, most data on insertion-site mutations come from the model organism Arabidopsis thaliana, and it is not yet clear if there are important species-specific differences in patterns of insertion-site mutation.
Genetic mutations, linked and unlinked, to the transgene insertion-site also arise from procedures associated with plant transformation, such as tissue culture and infection with A. tumefaciens. These genome-wide mutations can number from hundreds to many thousands per diploid genome, and are likely to be important sources of phenotypic variation.
The potential phenotypic consequences of genetic damage from insertion-site mutations and genome-wide mutations are discussed and recommendations for safety assessments are made. A better understanding of the genetic consequences of plant transformation should improve the quality and interpretation of scientific experiments that rely on plant transformation and should advance the debate on the safety of transgenic crops.
The Mutational Consequences of Plant Transformation
Summary
This paper is a more concise version of Transformation-induced Mutations in Transgenic Plants: Analysis and Biosafety Implications (2006) BGER 23: 209-237
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