A centuries-old horticultural technique—grafting—is gaining renewed attention as a potentially revolutionary method for gene editing a wide range of plants, particularly those that have proven difficult or impossible to modify using conventional approaches. This innovative strategy could significantly boost agricultural productivity and nutritional value while reducing the environmental impact of farming and addressing rising food prices.
The Challenge of Gene Editing Plants
The ability to precisely alter plant genetics through gene editing technologies like CRISPR offers a powerful tool for improving crop yields and resilience. However, modifying plants can be technically challenging. Unlike animal cells, plant cells possess rigid cell walls, making it difficult to introduce genetic material. Current genetic engineering techniques, such as firing DNA-coated pellets (biolistics) or using the bacterium Agrobacterium, often necessitate the regeneration of entire plants from modified cells. This process is ineffective for many important species, including cocoa, coffee, sunflowers, cassava, and avocados.
Regulatory Hurdles and Alternative Approaches
Even when gene editing does work, there’s another issue at play: regulation. In some countries, tiny, naturally occurring mutations induced by gene editing are treated like standard plant breeding, bypassing lengthy and costly regulatory trials. However, methods like biolistics and Agrobacterium often introduce extra DNA into the plant’s genome, triggering a full and more rigorous regulatory review process. Scientists are actively seeking alternative strategies that circumvent this issue, enabling gene edits without the introduction of foreign DNA.
One avenue is employing viruses to deliver RNA coding for CRISPR components. However, the Cas9 protein, a key element in the CRISPR toolkit, is relatively large, limiting the RNA sequences that can be effectively transported by most viruses.
Grafting and RNA: A New Combination
In 2023, researchers at the Max Planck Institute of Molecular Plant Physiology unveiled a promising new approach. Recognizing that plants produce a special type of RNA in their roots that can travel throughout the plant and enter cells in the shoots and leaves, they genetically engineered plants to produce such RNAs. These RNAs coded for two critical CRISPR components: the Cas protein that performs the editing and the guide RNA that directs it to the target location. They then grafted shoots of unmodified plants onto the roots of these modified plants, successfully achieving gene editing in some of the shoots and seeds.
Expanding Possibilities with Grafting
Ugo Rogo at the University of Pisa, Italy, and his colleagues believe this technique holds immense potential and have published a paper encouraging further development. “Grafting gives us the possibility to use the CRISPR system in trees or in plants such as sunflowers,” Rogo explains.
The advantage of grafting lies in its ability to connect relatively distantly related plants. For instance, tomato shoots can be successfully grafted onto potato rootstocks. This means that even if it’s impossible to genetically engineer a sunflower rootstock directly for gene editing, scientists could potentially engineer a related species to create a compatible rootstock.
A Universal Rootstock for Gene Editing
Once a suitable rootstock capable of producing the necessary CRISPR RNAs is established, it can be used to gene edit a broad spectrum of plants. “You can use the roots to deliver Cas9 and editing guides to all sorts of elite varieties,” notes Julian Hibberd at the University of Cambridge.
Ralph Bock, also at the Max Planck Institute, highlights the efficiency of this method: “Making the transgenic rootstock is not a big effort, given that it just needs to be made once, and then can be used forever and for multiple species.”
As a concrete example, only a few grape varieties, like Chardonnay, can regenerate from single cells and are amenable to genetic modification. However, once a disease-resistant Chardonnay rootstock is created through gene editing, it could be utilized for all grape varieties.
Combining Approaches for Greater Flexibility
Rogo envisions a future where grafting is combined with viral delivery, maximizing flexibility. Rootstocks could deliver the large mRNA sequences required for Cas9, while viruses could provide the smaller guide RNAs. This integrated strategy would allow the same rootstock to be employed for a diverse range of gene edits, providing an incredibly versatile tool for plant improvement.
The innovative grafting technique offers a practical and scalable solution to expand gene-editing possibilities, promising a new era of agricultural advancements and greater food security for a growing world.
