A breakthrough in plant breeding! Combining grafting with CRISPR tools

Although CRISPR/Cas9 is a huge advance in plant breeding, it remains an expensive and laborious solution, making its application in most plants infeasible. Recent advances by a team of scientists at the Max Planck Institute for Molecular Plant Physiology in Germany overcome these limitations.

Recently, scientists at the Max Planck Institute for Molecular Plant Physiology are applying a breakthrough in the CRISPR tool (aka "gene scissors") to edit plant genomes, marking a change in approach. By combining grafting with "mobile" CRISPR tools, the discovery could simplify and accelerate the development of new, genetically stable commercial crop varieties.

Unmodified shoots were grafted onto roots containing mobile CRISPR/Cas9, which allowed the genetic scissors to move from the root to the shoot. It's there to edit the plant's DNA, but not to leave its mark on the next generation of plants. This breakthrough will save time and money, circumvent current limitations of plant breeding, and contribute to sustainable food solutions for multiple crops.

Many of the crops that feed the world are already threatened by heat, drought and plant pests and diseases, and climate change is exacerbating these factors. To ensure efficient and effective future crop yields for these important plants under challenging conditions, the CRISPR/Cas9 system can be used to edit plant genomes with high precision to introduce beneficial gene functions or remove unfavorable gene functions.

Commercial crop plants need to be genetically stable; they cannot contain any genetic sequences from the CRISPR/Cas9 system, and should be transgene-free. Typically, this is achieved through multiple generations of outcrossing or a tedious regeneration process. Both methods are time- and money-intensive and difficult, if not impossible, to grow in many crops. A team of scientists led by Friedrich Kragler set out to change that. As part of the EU-funded PLAMORF project and a proof-of-concept project funded by the German Ministry of Research, they are investigating the transport sequences that enable the movement of RNA from roots to shoots. The research team discovered tRNA-like sequences (TLS), which act as signals for the long-distance movement of RNA within plants. A recent breakthrough has been to combine this discovery with the CRISPR/Cas9 genome editing system. When such a TLS is added to the CRISPR/Cas9 sequence, the plant produces a "mobile" version of the CRISPR/Cas9 RNA. Then, the GMO-free, unmodified shoots were transplanted into the roots of plants containing the mobile CRISPR/Cas9 RNA, which then progressed from the roots into the shoots and eventually into the seed-producing flowers.

"The magic happens in the flowers," explains Kragler. "The CRISPR/Cas9 RNA goes in and turns into the corresponding protein, which is the real 'genetic scissors'. It edits the plant DNA in the flowers. But the CRISPR/Cas9 system itself is not integrated into the DNA. Therefore, the seeds that develop from these flowers carry only the desired edits. There is no trace of the CRISPR/Cas9 system in the next generation of plants, and it works surprisingly efficiently.”

What makes this new system even more exciting is its potential to bring together different species. The scientists showed that this method of "editing" does not only work when the roots and shoots come from the same plant—in this case, the model plant is Arabidopsis thaliana. They also grafted shoots of its commercial relative rapeseed onto Arabidopsis roots to produce mobile CRISPR/Cas9. Encouragingly, Friedrich Kragler's team also found edited canola plants.

"Our novel gene editing system can be effectively used in many breeding programs and crop plants. This includes many important agricultural plant species that are difficult or impossible to engineer with existing methods," he concludes.