The Mutational Consequences of Plant Transformation

There is little understood about the frequency of mutations caused by plant transformation and the potential biosafety hazards they pose on crop species. Plant transformation is often used for breeding of commercial crop by inserting transgenes into plant chromosomal DNA. However, the transgene insertion is rarely perfect; it often includes deletions and rearrangements of host chromosomal DNA. Insertion sites that include Agrobacterium tumefaciens, particle bombardment, and ancillary procedures associated with plant transformation often contain extensive chromosomal rearrangements, deletion, scrambling of inserted and chromosomal DNA, and mutations.

Plant transformation is the insertion of a transgene into a plant genome, which is usually done by infecting plant cells with disarmed pathogenic organism, Agrobacterium tumefaciens, which contains the transgene, or using particle bombardment, the bombardment of cells with metal particles carrying the transgene. However, the mechanism by which transgenes insert into host DNA are barely understood. Transgene insertion usually involves tissue culture, which causes the reversion of plant tissues around hormones, antibiotics and other selective agents and allows the regeneration of an intact plant from a genetically modified cell possessing the transgene. Components of plant transformation include mutagens. It is not well known how pathogen infection and antibiotics can cause mutations in plant genomes. However, the clearest evidence implicating this comes from T-DNA tagging experiments which use plant transformation without tissue culture, therefore, transformants emerging from plant transformation procedures are exposed to various known or suspected mutagens. Insertion site mutations were found as a result of Agrobacterium mediated transformation. Agrobacterium-mediated transformation has been used to create commercial transgenic cultivars. For sbach and his colleagues examined 112 independent insertions into the A thaliana genome and found most insertions resulted in small deletions of plant DNA at the insertion site. In fact, 21% (24/112) appeared to be associated with rearrangement or deletion of plant chromosomal DNA; though two were confirmed as chromosomal translocations, the remaining 22 were never characterized. Insertion site mutations were found to be generated by particle bombardment. Most of the particle bombardment insertion events involve the insertion of multiple copies of delivered DNA, which appears to be normal. Due to the minimal number of events conclusions regarding particle bombardment insertion events are only provisional; however, it seems transgene integration resulting from particle bombardment is usually accompanied by substantial disruption of plant DNA and insertion of superfluous DNA. Deletions and rearrangements associated with transgene insertion increase the probability of changes to the plant phenotype.

Genome-wide mutations are also associated with plant transformation. Studies suggest that plant transformation procedures introduce hundreds to thousands of genome-wide mutations into the DNA of transgenic plants. Basically, extensive genetic mutations resulted from plant transformation procedures and were caused by passage through tissue culture. Insertion-site and genome-wide mutations may result in transgenic plants with unexpected traits. Transformation-induced mutations have the potentially affect any biosafety phenotype under their genetic control. Biosafety risk assessments prevent hazardous crops from being sold. Transformation-induced mutations are likely to have unpredictable on the phenotypes, in fact, their effects are less likely to be detected. The phenotypic consequences of transformation induced mutations can be avoided.

First, by ensuring transgenic plants are nearly identical to their parents, which is achieved by eliminating tissue culture, effective backcrossing, targeted insertion and possibly, and development of alternative transformation methods. Second, by improving genetic analysis and selection of transgenic plants.

In conclusion much remains to be discovered about insertion-site mutations, but it should not be overlooked that though transformation mutations magnify risks, they bring no benefits and are unnecessary for the production of transgenic crops.