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Break-through in the genetic modification of plants

A collaboration between the IBL and LUMC has resulted in the discovery that the polymerase theta enzyme is essential for the integration of Agrobacterium T-DNA into the genome of plants. The finding means a break-through for the development of more efficient systems for targeted genome modification in the production of improved, tailormade crop plants.

Agrobacterium is a plant pathogen that forms tumors on plants by injecting a piece of its tumor-inducing (Ti) plasmid into plant cells during infection. This T(ransferred )–DNA is integrated into the genome and expressed, leading to the conversion of normal cells into tumor cells. The bacterium has been disarmed and in this way this naturally occurring process of genetic modification has become the most common method for plant transformation.

An important issue with this process is that the T-DNA integrates at a random position in the plant genome. This can on the one hand be used to obtain plant mutants, but on the other hand overshadows controlled targeted integration. Therefore, already  for a long time scientists at the IBL in the group of Prof. Paul Hooykaas and elsewhere in the world have been studying the mechanism of T-DNA integration and the plant factors involved. So far, none of the studied proteins involved in various DNA repair pathways in plants, were essentially required for the T-DNA integration process. 

Recently an alternative end-joining DNA repair pathway was discovered involving polymerase theta, studied in detail by the group of Prof. Marcel Tijsterman at the LUMC. An important observation for the discovery was made by Maartje van Kregten, a previous PhD student of Hooykaas (now working as a postdoc in the group of Tijsterman). She noted a remarkable resemblance between the scars after repair of double strand breaks by polymerase theta mediated end-joining and the junctions between T-DNA and flanking plant DNA after T-DNA integration. This resulted in the hypothesis that polymerase theta is responsible for T-DNA integration. In the subsequent collaboration between both groups a break-through followed after the finding that Arabidopsis mutants of the polymerase theta homologue showed resistance for T-DNA integration.

The scientific paper describing this discovery has been published in Nature Plants (nplants.2016.164)

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