video credits: PLAMORF ERC Synergy Grant
Fast track to optimized plants that no longer carry traces of the genetic tool: Researchers have combined a classic method of plant improvement with CRISPR/Cas technology to simplify the development of new crops. Natural shoots are grafted onto rootstocks, which genetically modify the CRISPR/Cas gene scissors. Only the blueprint of this tool is then transported to the flowers as messenger RNA, where it leads to genetically adapted seeds that do not carry any foreign genes themselves. In this way, new crops could be produced without any problems, the characteristics of which correspond to those of classically bred varieties, the scientists explain.
Since the development of agriculture, humans have been adapting crops to their specific needs. However, the classic form of breeding requires a lot of time and effort: Naturally occurring mutations in certain hereditary factors have to be established over many generations through selection and crossing in new varieties. Attempts have been made for some time to accelerate this process using modern genetic engineering methods. Among other things, this could result in plant varieties that are optimized for climate-friendly energy production and as raw material suppliers for industry. Green genetic engineering has received enormous impetus in recent years, above all through the discovery of the CRISPR/Cas gene scissors: The process allows genes in plants to be modified or switched off in a targeted manner in order to give them certain characteristics.
How do you get rid of the tool?
"The components of the CRISPR/Cas gene scissors are themselves introduced as a DNA sequence into the plant DNA in the cell nucleus," explains senior author Friedrich Kragler from the Max Planck Institute of Molecular Plant Physiology in Potsdam. So far, however, the method has had a catch: the CRISPR/Cas foreign DNA introduced into the genome for the precise cuts has to be removed from the resulting plants by laborious backcrossing. Otherwise they remain transgenic and their genetic stability can also be impaired. Kragler and his team are now presenting a clever method that can be used to produce seeds for optimized plants in the first generation that no longer contain any foreign DNA.
The process is based on the combination of CRISPR/Cas technology with the age-old process of grafting, which is used to graft fruit trees, vines or roses, for example. It involves transplanting a shoot from a plant with specific characteristics onto a rootstock with other desired characteristics. With their new method, the scientists now cut off the stalk of a plant whose cell nuclei contain DNA from the gene scissors above the rootstock and then grafted the shoot of a genetically unmodified recipient plant onto it. The researchers have adapted the DNA sequence of the gene scissors in such a way that a messenger RNA is formed that can be sent from the rootstock to the genetically unmodified parts above ground.
Genetic engineering approach
This mRNA can then also serve as a blueprint for the gene scissors in the flowers of the plant. There they can also bring about the desired changes in the plant cells from which the seeds form. As a result, the team was able to show in the case of the model plant Arabidopsis thaliana that some of them already carry the desired gene cuts in the next generation. The scientists explain that the seeds and the plants that grow from them are free of any foreign DNA and cannot be distinguished from variants with naturally occurring gene mutations.
As they explain, there is now considerable potential for the development of genetically optimized plant varieties. Because many cultivated plants have so far been difficult or impossible to cross, or have very long generation times - such as tree species. Getting the foreign DNA out of these plants by crossing them would take many years. Kragler also sees promising potential in another aspect: "So far, targeted genetic engineering methods have only been established for a few, very well researched plants. However, since grafting often also works between many species that are not so closely related and the rootstocks can be easily propagated, it is conceivable that a rootstock can be used several times at the same time in order to selectively plant different species or cultivars with desired new properties The scientists are convinced that by combining the classic method with modern molecular biology technology, new varieties could be bred quickly and inexpensively in the future.
Source: Max Planck Institute for Molecular Plant Physiology, Article: Nature Biotechnology, doi: 10.1038/s41587-022-01585-8