Grafting trick could let us gene-edit a huge variety of plants

An ancient trick grafting one plant onto another could have very modern applications – allowing the editing of plant genes that are very difficult or impossible to edit in other ways.

“It's still in its early stages, but this technique has a lot of potential,” says Hugo Rogo at the University of Pisa in Italy.

Increasing plant productivity and nutritional value is vital to solving the problem huge damage caused by agriculture and to limit increase in food prices How World population is growing And Climate change is increasingly affecting crop yields. The exact changes you can make with CRISPR gene editing are the best way to achieve this goal.

However, editing plant genes is challenging because plants have rigid cell walls around them, unlike animal cells. Traditional plant genetic engineering involves techniques such as burning DNA-impregnated beads in plant cells (known as biolistics) or using natural genetically engineered microbe known as Agrobacterium.

These approaches rely on creating whole plants from modified cells, something that cannot be done with many plants, especially trees. For example, it does not work with cocoa, coffee, sunflower, cassava or avocado.

Even for factories where it works, there is another major problem. When gene editing is used to cause tiny mutations that are very common in nature, regulators in several countries view it as the equivalent of standard plant breeding, meaning such plants may get approval without the time-consuming and costly testing required for conventional genetically modified plants. But with the help of biolistics and AgrobacteriumPlant genomes often have extra DNA inserted into them, so they will have to go through a full regulatory process.

Plant scientists are trying to find ways to edit plants that work for more species and don't add unwanted pieces of DNA to the genome. One option is to use viruses to deliver RNA encoding parts of the CRISPR toolkit into plant cells. The problem here is that the Cas9 protein, widely used for gene editing, is very large, meaning the RNA that encodes it cannot fit into most viruses.

In 2023 Friedrich Kragler at the Max Planck Institute for Molecular Plant Physiology in Germany. introduced another approach. He knew that plant roots produce a special type of RNA that can travel throughout the plant and enter the cells of the shoots and leaves.

So his team genetically engineered plants to produce such RNAs, which encode two key components of CRISPR: the Cas protein, which does the editing, and a guide RNA, which tells where to make the editing. They then grafted shoots from unmodified plants onto the roots of modified plants and showed that this resulted in gene editing in some shoots and seeds.

Rogo and his colleagues find this approach so promising that they wrote a paper highlighting it and encouraging others to help develop it. “Grafting gives us the opportunity to use the CRISPR system on trees or plants like sunflowers,” says Rogo.

The key point of grafting is that relatively distantly related plants can be grafted onto each other – for example, tomato shoots can be grafted onto potato rootstocks. So while it is not possible to genetically engineer sunflower rootstock for gene editing, it should be possible to engineer related plants to produce a compatible rootstock.

Once a rootstock that produces the necessary RNAs appears, it can be used to edit the genes of a wide variety of plants. “You can use the roots to deliver Cas9 and editing guides to all kinds of elite varieties,” says Julian Hibberd at Cambridge University.

“Creating a transgenic rootstock does not require much effort, given that it only needs to be done once and can then be used forever and across multiple species,” says Ralph Bockwho also works at the Max Planck Institute but is not part of Kragler's team.

For example, only some grape varieties, such as Chardonnay, can regenerate from individual cells and thus be modified. But once a gene-editing rootstock is created for Chardonnay that imparts, say, disease resistance, it will work for all grape varieties and beyond.

Rogo also suggests combining vaccination with a viral approach. Rootstocks can be used to deliver large Cas9 mRNAs, while viruses provide guide RNAs. So the same rootstock can be used to make many different gene changes, he says.